text:
Water Activity Meter Operator's Manual Version 5
for AquaLab models Series 3 and 3TE
Decagon Devices, Inc.
Decagon Devices, Inc. 2365 NE Hopkins Court Pullman WA 99163 (509) 332-2756 fax: (509) 332-5158 www.aqualab.com firstname.lastname@example.com Trademarks AquaLab is a registered trademark of Decagon Devices, Inc. Š 1990-2007 Decagon Devices, Inc. 10607-05
AquaLab Table of Contents
Contents 1. Introduction . . . . . . . . . . . . . . . . . . .1 About this Manual . . . . . . . . . . . . . . . . . . . . 1 Customer Service . . . . . . . . . . . . . . . . . . . . . 1 Phone: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Fax: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 E-mail: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Note to our AquaLab Users . . . . . . . . . . . . 3 Seller's Liability . . . . . . . . . . . . . . . . . . . . . . . 3 2. About AquaLab . . . . . . . . . . . . . . . 5 AquaLab Models and Options . . . . . . . . . . 5 AquaLab and water activity . . . . . . . . . . . . 5 How AquaLab works . . . . . . . . . . . . . . . . . . 6 AquaLab and Temperature . . . . . . . . . . . . . 7 Series 3TE . . . . . . . . . . . . . . . . . . . . . . . . . . . .9 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 3. Getting Started . . . . . . . . . . . . . . 12 Components of your AquaLab . . . . . . . . . 12 Choosing a Location . . . . . . . . . . . . . . . . . . 13 Preparing AquaLab for Operation . . . . . 13 4. The Menus . . . . . . . . . . . . . . . . . . .15 The Main Menu . . . . . . . . . . . . . . . . . . . . . . 15 Changing Languages . . . . . . . . . . . . . . . . . . 16 Normal sampling mode . . . . . . . . . . . . . . . 16 Continuous Mode . . . . . . . . . . . . . . . . . . . . 17 Temperature Equilibration Screen . . . . . . 18 i
AquaLab Table of Contents
System configuration . . . . . . . . . . . . . . . . . 18 Changing the beeper . . . . . . . . . . . . . . . . . . 19 EXIT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Adjusting for linear offset . . . . . . . . . . . . . 20 Series 3TE menu . . . . . . . . . . . . . . . . . . . . . 20
5. Linear Offset and Verification Standards . . . . . . . . . . . . . . . . . . . . . . . . 24 What is Linear Offset? . . . . . . . . . . . . . . .24 Verification Standards . . . . . . . . . . . . . . . .24 When to Verify for Linear Offset . . . . . . 26 How to Verify and Adjust for Linear Offset 26 Adjusting for linear offset . . . . . . . . . . . . . 27 6. Sample Preparation . . . . . . . . . . 32 Preparing the Sample . . . . . . . . . . . . . . . . . 32 Samples Needing Special Preparation . .34 Coated and Dried Samples . . . . . . . . . . . 34 Slow Water-Emitting Samples . . . . . . . . . 35 Propylene Glycol . . . . . . . . . . . . . . . . . . . . 36 Low Water Activity . . . . . . . . . . . . . . . . . .37 Samples not at Room Temperature . . . . . .38 7. Taking a Reading . . . . . . . . . . . . 39 Measurement Steps . . . . . . . . . . . . . . . . . .39 How AquaLab takes Readings . . . . . . . . .40 Cautions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 8. Computer Interface . . . . . . . . . . .44 AquaLink Software . . . . . . . . . . . . . . . . . . .44 Using Windows Hyperterminal . . . . . . . . .45 9. Theory: Water Activity in Products 47 ii
AquaLab Table of Contents
Water content . . . . . . . . . . . . . . . . . . . . . . .47 Water activity . . . . . . . . . . . . . . . . . . . . . . .47 Effect of Temperature on water activity 49 Water Potential . . . . . . . . . . . . . . . . . . . . . .50 Factors in determining Water Potential . 51 Osmotic effects . . . . . . . . . . . . . . . . . . . . . . 51 Matric effects . . . . . . . . . . . . . . . . . . . . . . .52 Sorption Isotherms . . . . . . . . . . . . . . . . . . .53 Relating aw to water content . . . . . . . . . .53
10. Cleaning and Maintenance . . . . 55 Tools Needed . . . . . . . . . . . . . . . . . . . . . . .55 Cleaning the Block and Sensors . . . . . . . . 56 Accessing the Block . . . . . . . . . . . . . . . . . 56 Checking Calibration . . . . . . . . . . . . . . . . .59 11. Repair Instructions . . . . . . . . . . . 61 Shipping Directions . . . . . . . . . . . . . . . . . . . 61 Repair Costs . . . . . . . . . . . . . . . . . . . . . . . . . 63 Loaner Service . . . . . . . . . . . . . . . . . . . . . . 63 12. Troubleshooting . . . . . . . . . . . . . 64 Problems and Solutions . . . . . . . . . . . . . . .64 Component Performance Screen . . . . . . .72 13. Further Reading . . . . . . . . . . . . . 74 Water Activity Theory and Measurement 74 Food Quality and Safety . . . . . . . . . . . . . .78 Water Activity and Microbiology . . . . . .79 Water Activity in Foods . . . . . . . . . . . . . . 83 Meat and Seafood . . . . . . . . . . . . . . . . . . . 83 Dairy Products . . . . . . . . . . . . . . . . . . . . . . .85 Fruits and Vegetables . . . . . . . . . . . . . . . . .86 iii
AquaLab Table of Contents
Baked Goods and Cereals . . . . . . . . . . . . 88 Beverages/Soups/Sauces/Preserves . . 90 Pharmaceuticals/Cosmetics . . . . . . . . . . . 91 Miscellaneous . . . . . . . . . . . . . . . . . . . . . . . 92 Appendix A . . . . . . . . . . . . . . . . . . . 94 Preparing Salt Solution . . . . . . . . . . . . . . .94 Appendix B . . . . . . . . . . . . . . . . . . . 96 Declaration of Conformity . . . . . . . 97 Certificate of Traceability . . . . . . . 98
AquaLab Introduction
1. Introduction Welcome to Decagon's AquaLab Series 3, the industry standard for measuring water activity (aw). AquaLab is the quickest, most accurate, and most reliable instrument available for measuring water activity. Whether you are researching or working on the production line, AquaLab will suit your needs. It is easy to use and provides accurate and timely results. We hope you find this manual informative and helpful in understanding how to maximize the capabilities of your AquaLab.
About this Manual Included in this manual are instructions for setting up your AquaLab, verifying the calibration of the instrument, preparing samples, and maintaining and caring for your instrument. Please read these instructions before operating AquaLab to ensure that the instrument performs to its full potential.
Customer Service If you ever need assistance with your AquaLab, or if you just have questions, there are several ways to contact us:
AquaLab Introduction
Phone: Our toll-free telephone number is available to our customers in the US and Canada, Monday through Friday, between 8 a.m. and 5 p.m. Pacific time at 1-800-755-2751. For our customers outside of the US and Canada, our regular telephone number is (509) 332-2756. Fax: Our fax number is (509) 332-5158. When you fax us, please include your AquaLab's serial number, your name, address, phone and fax number along with a description of your problem. E-mail: If you need technical support, you can send us messages via e-mail at email@example.com. Again, please include as part of your message your AquaLab's serial number, your name, address, phone, fax number, and return e-mail address.
If you have a question about your application with AquaLab, please send your message with the above information to firstname.lastname@example.com.
Warranty AquaLab has a 30-day satisfaction guarantee and a threeyear warranty on parts and labor. To validate your warranty, please complete and return the warranty card 2
AquaLab Introduction
included with this manual. You can return your warranty information by fax, e-mail, phone or by mailing the postage-paid card. Please include all the information requested on the card. It is necessary for Decagon to have your current mailing address and telephone number in case we need to send updated product information to you.
Note to our AquaLab Users This manual is written to aid the end user in understanding the basic concepts of water activity, enabling them to use our instrument with confidence. Every effort has been made to ensure that the content of this manual is correct and scientifically sound.
Seller's Liability Seller warrants new equipment of its own manufacture against defective workmanship and materials for a period of three years from date of receipt of equipment (the results of ordinary wear and tear, neglect, misuse, accident and excessive deterioration due to corrosion from any cause are not to be considered a defect); but Seller's liability for defective parts shall in no event exceed the furnishing of replacement parts F.O.B. the factory where originally manufactured. Material and equipment covered hereby which is not manufactured by Seller shall be covered only by the warranty of its manufacturer. Seller shall not be liable to Buyer for loss, damage or injuries to persons (including death), or to property or things of whatsoever kind (including, but not without limitation, loss of 3
AquaLab Introduction
anticipated profits), occasioned by or arising out of the installation, operation, use, misuse, nonuse, repair, or replacement of said material and equipment, or out of the use of any method or process for which the same may be employed. The use of this equipment constitutes Buyer's acceptance of the terms set forth in this warranty. There are no understandings, representations, or warranties of any kind, express, implied, statutory or otherwise (including, but without limitation, the implied warranties of merchantability and fitness for a particular purpose), not expressly set forth herein.
AquaLab About AquaLab
2. About AquaLab AquaLab is the fastest instrument for measuring water activity, giving readings in five minutes or less. Its readings are precise, providing ±0.003 accuracy. The instrument is easy to clean and checking calibration is simple.
AquaLab Models and Options AquaLab comes in two models to suit the needs of different users. Here is a brief description of each: Series 3: Our standard model, adequate for most water activity needs. Series 3TE: User-selectable internal temperature control model, uses thermoelectric (Peltier) components to maintain internal temperature.
AquaLab and water activity Water activity (aw) is a measurement of the energy status of the water in a system. It indicates how tightly water is "bound", structurally or chemically, within a substance. Water activity is the relative humidity of air in equilibrium with a sample in a sealed measurement chamber. The concept of water activity is of particular importance in deter5
AquaLab About AquaLab
mining product quality and safety. Water activity influences color, odor, flavor, texture and shelf-life of many products. It predicts safety and stability with respect to microbial growth, chemical and biochemical reaction rates, and physical properties. For a more detailed description of water activity as it pertains to products, please refer to Chapter 9, titled "Theory: Water Activity of Products� of this manual.
How AquaLab works AquaLab uses the chilled-mirror dewpoint technique to measure the aw of a sample. In an instrument that uses the dewpoint technique, the sample is equilibrated with the headspace of a sealed chamber that contains a mirror and a means of detecting condensation on the mirror. At equilibrium, the relative humidity of the air in the chamber is the same as the water activity of the sample. In the AquaLab, the mirror temperature is precisely controlled by a thermoelectric (Peltier) cooler. Detection of the exact point at which condensation first appears on the mirror is observed with a photoelectric cell. A beam of light is directed onto the mirror and reflected into a photodetector cell. The photodetector senses the change in reflectance when condensation occurs on the mirror. A thermocouple attached to the mirror then records the temperature at which condensation occurs. AquaLab then signals you by flashing a green LED and/or beeping. The final water activity and temperature of the sample is then displayed. 6
AquaLab About AquaLab
In addition to the technique described above, AquaLab uses an internal fan that circulates the air within the sample chamber to reduce equilibrium time. Since both dewpoint and sample surface temperatures are simultaneously measured, the need for complete thermal equilibrium is eliminated, which reduces measurement times to less than five minutes.
AquaLab and Temperature For the reasons noted above, temperature control is unnecessary for most applications with AquaLab. The change in aw due to temperature change for most materials is less than 0.002 per degree Celsius. Thus, AquaLab is ideal for the measurement of samples at room temperature. However, samples that are not at room temperature during the read cycle will equilibrate to the temperature of AquaLab before the aw is displayed. Large temperature differences will cause longer reading times, since a complete and accurate reading will not be made until the sample and the instrument are within 2 degrees of each other. To better help you control the temperature difference between your sample and the instrument, you can access a sample equilibration screen at the main menu that can shows the difference in temperature between the sample and chamber block (see chapter 4). Though temperature control is usually unnecessary for most applications, there are some instances where it is desired. For this reason, Decagon offers a temperature7
AquaLab About AquaLab
controlled AquaLab model: the Series 3TE. There are several advantages in having a temperature-controlled model. Here are a few major reasons: 1. Research purposes. To study the effects of temperature on the aw of a sample, comparison of the aw of different samples independent of temperature, accelerated shelf-life studies or other water activity studies where temperature control is critical. There are many shelf-life, packaging, and isotherm studies in which the added feature of temperature control would be very beneficial. 2. To comply with government or internal regulations for specific products. Though the aw of most products varies by less than ± 0.002 per °C, some regulations require measurement at a specific temperature. The most common specification is 25°C, though 20°C is sometimes indicated. 3. To minimize extreme ambient temperature fluctuations. If the laboratory and AquaLab temperatures fluctuate by as much as ± 5°C daily, water activity readings will vary less than ± 0.01aw. Such variations in ambient temperatures are uncommon. As stated above, this much uncertainty in sample aw is sometimes acceptable, so there may be no need for temperature control. However, if your lab temperature varies to this degree and you require better than 0.01aw precision, you may want a temperature-
AquaLab About AquaLab
controlled model. If your application meets the criteria listed above, you may want to use the Series 3TE. If you already own a Series 3 model and wish to utilize the temperature-control options of the Series 3TE, contact Decagon for information on upgrading your system. Here is a description of the AquaLab Series 3TE: Series 3TE The AquaLab Series 3TE is an AquaLab that has thermoelectric components installed that allow the instrument to maintain a set chamber temperature. The 3TE has an extra option in the System configuration menu (see chapter 3) that allows the user to set the temperature.
Limitations AquaLab's only major limitation is its ability to accurately measure samples with high concentrations of certain volatiles such as ethanol or propylene glycol, which can cocondense on the surface of the chilled mirror. Not all volatiles react this way, but it is important to note that some volatiles can affect the performance of your instrument. The extent of the effect is both concentration- and matrixdependent; thus, just because a product contains some ethanol or proplylene glycol does not necessarily mean the readings will be erroneous. Therefore, if your sample contains propylene glycol or a high concentration of other volatiles, it is still possible to make accurate readings. Refer 9
AquaLab About AquaLab
to Chapter 6's section titled "Propylene Glycol" or call Decagon for more details.
AquaLab About AquaLab
Features Function Keys
LED indicator light
LCD Sample drawer
Front view of AquaLab Lid thumb-screw
RS-232 interface
Power cord Plug ON/OFF Switch
fuse well
Back view of AquaLab
AquaLab Getting Started
3. Getting Started Components of your AquaLab Your AquaLab should have been shipped with the following items: •
AquaLab water activity meter
Power cord
RS-232 interface cable
100 disposable sample cups
Operator's Manual
Quick Start guide
Cleaning Kit
3 vials each of the following verification solutions: 0.984 aw 0.5 molal KCl 0.760 aw 6.0 molal NaCl 0.500 aw 8.57 molal LiCl 0.250 aw 13.41 molal LiCl
AquaLab Getting Started
Choosing a Location To ensure that your AquaLab operates correctly and consistently, place it on a level surface. This reduces the chance that sample material will spill and contaminate the inside of the instrument. To protect the internal electrical components, and to avoid inaccurate readings, place your AquaLab in a location where the temperature remains fairly stable. This location should be well away from air conditioner and heater vents, open windows, outside doors, refrigerator exhausts, or other items that may cause rapid temperature fluctuation.
Preparing AquaLab for Operation After finding a good location for your AquaLab, plug the power cord to the back of the unit. Before turning it on, pull open the sample drawer (turn the knob to the "OPEN/LOAD" position). An empty disposable sample cup is usually placed upside-down in the drawer to protect it during shipment. Remove this sample cup and turn the instrument on. The ON/OFF switch is located on the lower right corner of the AquaLab's back panel. The following screens will appear on the LCD (model Series 3TE used here as an example):
AQUALAB TE Series 3B
v 3.0
AquaLab Getting Started
(Note: the "v.3.0" shown the above illustration is an example showing the version of operating code used in the instrument. We periodically update the code and add new features, so if your AquaLab shows a version number higher than this, don't be alarmed.) Then:
0.000 aw
This is the main menu, displaying the water activity (aw) on the top left portion of the screen, and the sample temperature in the lower right. In order to provide the most accurate readings, your AquaLab should ideally be allowed a warm-up period of at least 15 minutes after turning it on. This allows the air inside the AquaLab to equilibrate to the temperature of its surroundings. The Series 3TE model allows you to equilibrate the sensor block and drawer assembly to a set temperature. For instructions on how to adjust the temperature setpoint in the TE models, see Chapter 4.
AquaLab Menus
4. The Menus The Main Menu
0.000 aw
Each time you turn on your AquaLab, it will come to this screen. If this screen doesn't appear, refer to Chapter 13 for troubleshooting instructions. As mentioned earlier, the water activity and sample temperature are displayed on the screen. On each side of the LCD there are two buttons. Each button performs a different function. Following is a description of the modes and options you may use, and the buttons used to set them. continuous/normal mode
0.000 aw system setup
language selection
temperature equilibration screen
diagram of button options from main menu
AquaLab Menus
Changing Languages The AquaLab comes to you with English as the default on-screen user language. If you prefer not to use English, you can change it to one of a variety of other languages: German, French, Spanish, Italian, Swedish, Danish, Norwegian, Czech, Portuguese, Japanese, Polish or Finnish. This is done simply by pressing the upper right button of the instrument while the drawer knob is in the OPEN/ LOAD position. You will see the following screen:
English -exitPress the upper right key again, and the next language option (German) will appear:
Deutsch -zur端ckEach time you press the right button, the display will scroll to the next language option. Select the desired language, then press the lower left button to exit.
Normal sampling mode The first time you turn on the AquaLab, it will be in normal sampling mode. In this mode, the sample is measured once, after which the instrument notifies you that it is finished. Here is a brief summary of the normal sampling 16
AquaLab Menus
mode features: •
single sampling (not continuous).
instrument may or may not beep after measurement is made, depending on how it is set up. (Factory default is 4 beeps)
green LED blinks until you open the sample drawer.
Continuous Mode Continuous mode reads your sample continuously until you turn the knob to the OPEN/LOAD position. It will read the sample, display the water activity and sample temperature, then begin another read cycle without turning the knob or breaking the seal of the cup. Between samples, it will signal you with the green LED flash, accompanied by the beeper, if it is enabled. To toggle between the normal and continuous modes, press the top left button. The display will show a small "c" to the left of the water activity readings: "c" for continuous mode c
0.000 aw
main menu with continuous mode enabled If you press the upper left button again, the "c" will disap17
AquaLab Menus
pear and you will be back in normal sampling mode.
Temperature Equilibration Screen To see the temperature difference between your sample and the AquaLab, press the lower right button at the main menu. This screen can only be accessed when the drawer knob is in the OPEN/LOAD position. The following screen will appear:
Ts = 23.1 Ts - Tb = 0.125 This screen shows the temperature difference between the sample (Ts) and the chamber block (Tb), allowing you to quickly check if the sample is too hot, which may cause condensation inside the chamber. Press the lower right button to exit.
System configuration If you press the bottom left button while in the main menu, it will bring you to the system configuration menu.
0.000 aw
system configuration menu button
This menu allows you to make minor system changes. You 18
AquaLab Menus
can change the beeper signal after each sample, enter the linear offset adjustment menu, and adjust the temperature setpoint (TE model only) from this menu. Beeper Icon
Beeper Linear offset menu Mode indicator (continuous shown)
+ set T
temp. set (TE model only)
System configuration menu Changing the beeper When you are sampling, the AquaLab has two ways of notifying you that the water activity reading is complete: the beeper and a flashing green LED, located on the left front corner of the AquaLab's case. In normal sampling mode, when a sample is started, the LED will flash once, and when it is finished it will flash continuously until the knob is moved to the OPEN/LOAD position (if not operating in continuous mode). You cannot turn off or change the LED flashing functions.
There are three beeper options, represented by three
AquaLab Menus
No beeping.
Beeps four times, then stops. Beeps until drawer is opened.
definition of beeper icons The beeper can be turned off completely, it can beep momentarily when the sample is finished and then stop, or it can beep continuously until the knob is turned to the OPEN/LOAD position. After you have adjusted the beeper setting, it will remain as you have set it until you change it again, and will not be affected by turning the instrument on or off. EXIT You may press the EXIT button (the lower left button) to exit back to the main menu at any time. Adjusting for linear offset When you need to adjust for linear offset, press the upper right button in the system configuration menu, and you will be brought to the linear offset menu. For more details on linear offset and how to verify for it, please refer to the next chapter. Series 3TE menu If you have an AquaLab Series 3TE, you have the ability to manually set your AquaLab's sample chamber tempera20
AquaLab Menus
ture. To access this menu, press the lower left button in the main menu, and then the lower right button next to the "set T" in the system configuration menu. The following screen will appear:
0 Adjust setpoint + -Exit-
Adjusting the Setpoint Temperature Use the buttons next to the + and - buttons to adjust the target setpoint temperature (displayed in the lower right corner). If you press either button it adjusts in increments of 0.1°C. Note: Holding down the button for 2 seconds will scroll the values. The target setpoint temperature roughly corresponds to the temperature at which you wish the chamber to read. Therefore, adjust the setpoint to the temperature that you want, then begin measurements to see how close your AquaLab comes to your desired temperature (this works best by putting the AquaLab in continuous mode). After several samples, it should show consistent temperature readings. At this point, make any needed adjustment to the setpoint index number to reach your desired temperature. You will be able to adjust the index number between 50° and 15.1°C. If you press the - button after you reach 15.1°, it will disable the temperature control function, displaying "off " until you raise the index number again. Note: 21
AquaLab Menus
To make aw measurements at temperatures below 15°C, disable the temperature control feature and place the entire instrument into a temperature-controlled chamber. Setting Temperature Equilibration Time In the upper left corner, before the words "Adjust Setpoint" there is a number (default is 0). This value can be one of three integers: 0, 1, or 2, and is adjusted by pressing the upper left button. This number sets the level of temperature equilibration desired after the knob is turned to the READ position and before the AquaLab begins to make measurements. A setting of 0 begins measurement immediately (as long as sample is not above 4°C of block temperature), 1 requires it to be within 1 degree, and 2 requires it to be within 1/2 of a degree. Note: If the equlibrium level is set to 0, the AquaLab will begin taking readings when the temperature is within 4 degrees, but you will not get a final reading until the temperature has stabilized enough for the instrument to make an accurate final value. Important tips with the Series 3TE •
Before sampling, wait for approximately 30 minutes to let the chamber's temperature to stabilize after turning it on.
The AquaLab may have a difficult time measuring the temperature of a sample that is being cooled if the ambient temperature is high.
AquaLab Menus
When changing samples, the opening and closing of the drawer may cause the temperature in the chamber to fluctuate. Therefore, make sure to take more than one reading per sample to ensure that the temperature and aw are stable between readings.
Never place a hot or warm sample in a cooled chamber, because condensation will form inside the chamber, causing errors.
AquaLab Linear Offset and Verification Standards
5. Linear Offset and Verification Standards What is Linear Offset? AquaLab uses the chilled mirror dewpoint technique for measuring water activity. Because this is a primary measurement method of relative humidity, no calibration is necessary; however, it is important to check for linear offset periodically. The components that the instrument uses to measure aw are subject to changes that may affect AquaLab's performance. These changes are usually the result of chamber contamination. When this occurs, it changes the accuracy of your readings. This is what is called a "linear offset." Therefore, frequent linear offset verification can assure you that your AquaLab is performing correctly. Linear offset can be checked by using a salt solution and distilled water.
Verification Standards Verification standards are specially prepared salt solutions that have a specific molality and water activity that is constant and accurately measurable. The verification standards that were sent with your initial shipment are very accurate and readily available from Decagon Devices.
AquaLab Linear Offset and Verification Standards
These particular standards are accurate and easy to use. Most importantly, they greatly reduce preparation errors. Because of these reasons, we recommend using the standards provided by Decagon for the most accurate verification of your AquaLab's performance. Performance Verification Standards come in four water activity levels: 1.000, 0.984, 0.760, 0.500, and 0.250 aw. The standards are produced under a strict quality assurance regime. The accuracy of the standards is verified by an independent third party and are shelf stable for one year. Verification Standard @25°C
Water Activity
Distilled Water
1.000 ±0.003
0.5m KCl
0.984 ±0.003
6.0m NaCl
0.760 ±0.003
8.57m LiCl
0.500 ±0.003
13.41m LiCl
0.250 ±0.003
See Appendix B for water activity values for these standards at different temperatures. If for some reason you cannot obtain Decagon's verification standards and need to make a saturated salt slush for verification, refer to Appendix A. 25
AquaLab Linear Offset and Verification Standards
When to Verify for Linear Offset Linear offset should be checked against a known verification standard daily, either once per shift or before each use. Linear offset should never be verified solely against distilled water, since it does not give an accurate representation of the linear offset. For batch processing, the instrument should be checked regularly against a known standard of similar aw. It is also a good idea to check the offset with a standard of similar aw when the general water activity range of your sample is changing. Checking the aw of a standard solution will alert you to the possibility of contamination of the unit or shifts in the linear offset from other causes.
How to Verify and Adjust for Linear Offset To verify for linear offset of your AquaLab, do the following: 1. Choose a verification standard that is close to the aw of the sample you are measuring. Make sure that your standard is at ambient temperature (or the instrument set temperature for TE models) before you load it into the sample drawer, and that your AquaLab has warmed up long enough to make accurate readings. 2. Empty the whole vial of solution into a sample cup and place it in the AquaLab's sample drawer.
AquaLab Linear Offset and Verification Standards
3. Carefully slide the drawer closed, being especially careful so the solution won't splash or spill and contaminate the chamber. 4. Turn the drawer knob to the READ position to make an aw reading. Make two readings. The aw readings should be within ± 0.003 of the given value for your salt solution. See Appendix B for the correct water activity value for standards at temperatures other than 25°C. 5. If your AquaLab is reading within ±0.003 of the salt solution, prepare a sample cup half full of steam-distilled water or a second verification salt and make two readings—the first reading may be low. The second aw reading for distilled water should be 1.000 ±0.003. If your salt reading is correct and your distilled water reading is not, it is probably due to contamination of the sensor chamber. For cleaning instructions, see Chapter 10. After cleaning, repeat these instructions. 6. If you consistently get readings that are outside of the aw of your salt solution by more than ±0.003, a linear offset has probably occurred. In this case, adjust the reading on the salt solution to its correct value. This is done by the following: Adjusting for linear offset
1. Once you are certain that a linear offset has occurred, 27
AquaLab Linear Offset and Verification Standards
choose a verification standard that is close to the aw of the sample you are measuring. Each of the verification standards supplied by Decagon has its aw labeled. Before you begin sampling, make sure that your standard is at ambient temperature before you load it into the sample drawer, and that your AquaLab has warmed up long enough to make accurate readings. Refer to Appendix B for a table of correct aw values at different temperatures. 2. Enter the system configuration menu by pressing the lower left button in the main menu. Press the upper right button in the system configuration menu to enter the linear offset menu. You will be guided through the linear offset routine. The following screen will appear:
Change the offset? yes no 3. If you wish to continue, press the button next to "yes." To return to the main menu, press the button next to "no." After selecting "yes," the following screen will appear:
place standard in drawer and read 28
AquaLab Linear Offset and Verification Standards
4. Empty the whole vial of solution into a sample cup and place it in the AquaLab's sample drawer. 5. Carefully slide the drawer closed, being especially careful so the solution won't splash or spill and contaminate the chamber. 6. Turn the drawer knob to the READ position to make an aw reading. Note: If you decide at this point not to continue with the linear offset program, just return the knob to the OPEN/LOAD position and you will be returned to the main screen. After it has finished sampling the verification standard, it will bring you to the following screen:
0.760 exit
adjust + -
use these buttons to adjust the value
7. At this screen, adjust the water activity value to its proper value for the particular solution you are measuring. Press the upper right button to move the value up, the lower right button to move it down. When it is at the value you want, press the Exit button. The value will be stored. Note: This is the only menu where these buttons can change the lin29
AquaLab Linear Offset and Verification Standards
ear offset, so you won't hurt anything by pressing these buttons in other menus. 8. Re-measure the sample of your verification standards again in the normal sampling mode. It should read the proper value at a given temperature for your particular standard (see Appendix B). 9. Prepare and read a sample of distilled water or second verification standard. Read it twice. Distilled water should be within ¹0.003 of 1.000. 10. If the water activity value of distilled water is correct, resume sampling. If, after adjusting for linear offset and cleaning the chamber, you still are getting incorrect readings when reading verification standards, contact Decagon at 509-332-2756 (1-800-755-2751 in US and Canada) for further instructions.
AquaLab Linear Offset and Verification Standards
Measure Verification Standard Correct
Not Correct
Measure dH2O or 2nd Verification Standard Correct
OK to Sample
Adjust for Offset
Not Correct Clean
This flowchart is a graphical representation of the directions given above for checking for linear offset.
AquaLab Sample Preparation
6. Sample Preparation Your AquaLab will continually provide accurate water activity measurements as long as its internal sensors are not contaminated by improperly-prepared samples. Careful preparation and loading of samples will lengthen time between cleanings and will help you avoid cleaning and downtime.
Preparing the Sample To prepare a sample, follow these steps: 1. Make sure that the sample to be measured is homogeneous. Multi-component samples (e.g., muffins with raisins) or samples that have outside coatings (like deep-fried, breaded foods) can be measured, but may take longer to equilibrate. For samples like these, AquaLab may take more than five minutes to give an accurate reading, or may require multiple readings of the same sample. Measuring the aw of these types of product is discussed in detail later in this chapter (see Materials Needing Special Preparation). 2. Place the sample in a disposable sample cup, completely covering the bottom of the cup, if possible. AquaLab is able to accurately measure a sample 32
AquaLab Sample Preparation
that does not (or cannot) cover the bottom of the cup. For example, raisins only need to be placed in the cup and not flattened to cover the bottom. A larger sample surface area increases instrument efficiency by providing more stable infrared sample temperatures. It also speeds up the reading by shortening the time needed to reach vapor equilibrium. 3. Do not fill the sample cup more than half full. Overfilled cups will contaminate the sensors in the sensor chamber! Filling the sample cup will not make the readings faster or more accurate. There only needs to be enough sample in the cup to allow the water in the sample to equilibrate with the water in the vapor phase and not change the moisture content of the sample. There is a minimum amount of sample needed; therefore, covering the bottom of the sample cup is normally enough. 4. Make sure that the rim and outside of the sample cup are clean. Wipe any excess sample material from the rim of the cup with a clean tissue. Material left on the rim or the outside of the cup will contaminate the sensor chamber and be transferred to subsequent samples. The rim of the cup forms a vapor seal with the sensor block when the drawer knob is turned to the READ position. Therefore, any sample material left on the cup rim will be transferred to the block, preventing this seal and contaminating future samples.
AquaLab Sample Preparation
5. If a sample will be read at some other time, put the sample cup's disposable lid on the cup to restrict water transfer. For long-term storage, seal the lid by placing tape or Parafilm completely around the cup/lid junction. It is necessary to seal the cup if it will be a long time before the measurement is made.
Samples Needing Special Preparation AquaLab reads most materials in less than five minutes, depending on which mode you are operating in. Some samples, however, may require longer reading times, due to the nature of the material you are sampling. These materials need additional preparation to ensure quick, accurate readings. To find out whether special sample preparation is necessary, take a reading and see how long it takes to find the water activity. If it takes longer than six minutes, remove the sample and take a reading of a verification standard. This will ensure that the sample itself is causing the long read time, and that there is not a problem with your instrument. If the verification standard also takes longer than six minutes to sample, refer to Chapter 12 of this manual for more information. Coated and Dried Samples Samples with coatings such as sugar or fat often require longer reading times, because it takes longer for them to equilibrate. If this is the case for your samples, don't worry 34
AquaLab Sample Preparation
that something is wrong with your instrument; it simply means that your particular sample takes longer than most to equilibrate water with its outside environment. To reduce the time needed to take an aw reading for coated or dried samples, one thing you can do is crush, slice, or grind the sample before putting it in the sample cup. This increases the surface area of the sample, thus decreasing reading times. Keep in mind, however, that modifying some samples may alter their aw readings. For example, a candy may have a soft chocolate center and a hard outer coating. The aw reading for the center and the outer coating are different, so one would need to evaluate which part of the sample needed to be measured before crushing it. When the candy is crushed, for example, the aw will represent the average water activity of the entire sample; whereas leaving the candy whole will give a reading for the coating, which may act as a barrier to the center. Note: if you crush, grind, or slice your sample, be consistent in the method you use in order to obtain reproducible results. Slow Water-Emitting Samples Some extremely dry, dehydrated, highly viscous water-inoil (butter), high fat, or glassy compositions may have increased read times, due to their moisture sorption characteristics. AquaLab may require up to ten minutes to reach an accurate measurement of aw, and nothing can be 35
AquaLab Sample Preparation
done to decrease the reading times of these types of samples. For faster reading, it is important to have the water activity of the chamber at or below the water activity of these type of samples. This causes the sample to release water to the vapor phase and equilibrate with the chamber. If the water activity of the headspace is greater than this type of sample, a long period of time will be required to reach equilibrium and the aw of the sample may be affected. Propylene Glycol AquaLab will give accurate readings on most alcohols. However, samples with propylene glycol in concentrations higher than 10% will give inaccurate aw values for consecutive samples. This is because propylene glycol condenses on the mirror during the reading process, but it does not evaporate from the mirror as water does. As a result, the first reading will be accurate, but subsequent readings will not unless the propylene glycol is cleared out of the chamber after each reading. You can clean the propylene glycol out of the chamber by running a sample of the activated charcoal (included as part of the AquaLab cleaning kit) after each propylene glycol-bearing sample. Once you have finished sampling, clean the chamber to make sure all residue is eliminated (see Chapter 10).
As another solution, Decagon offers a special sample block designed for measuring volatiles such as propylene 36
AquaLab Sample Preparation
glycol and ethanol. The Volatiles Sensor Block attaches and operates in a similar manner as the chilled-mirror dewpoint block that is currently in your AquaLab. When it is installed, the AquaLab will sense that the Volatiles Block is attached, and will make measurements accordingly. The only difference in operation is a lower accuracy of ±0.01 aw. All other operations and features will be the same, including measurement times and adjusting for linear offset. Contact Decagon for more details about this accessory.
Low Water Activity Samples that have an aw of less than about 0.03 cannot be accurately measured with the normal AquaLab Series 3 model. Samples with such low aw values are rare. When a sample's aw value is below its ability to accurately measure, your AquaLab will display an error message indicating the last reading it could make on that particular sample. For example, say you are measuring a dry sample and the following screen appears:
aw < 0.031
This screen indicates that the last water activity reading the AquaLab measured on this sample was 0.031 at 24.7° C. Therefore, the actual water activity of the sample is lower than the instrument's components can measure.
AquaLab Sample Preparation
If your sample is not extremely dry but is still getting the error message, refer to Chapter 12 for other possible explanations. Samples not at Room Temperature Samples that are 4 degrees colder or warmer than the instrument (chamber) temperature will need to equilibrate to ambient temperature before a fast, accurate reading can be made. Rapid changes in temperature over short periods of time will cause the aw readings to rise or fall until the temperature stabilizes. When the temperature stabilizes within one or two degrees of the chamber temperature, you can proceed with normal measurements.
High-water activity samples that are warmer than the chamber temperature can cause condensation inside the measuring chamber, which will adversely affect subsequent readings. A warning message appears (Sample too hot) if the sample temperature is more than 4° C above chamber temperature. If this message appears, immediately remove the sample from the instrument, place a lid on the cup, and allow the sample to cool to within 4° C of the instrument before measuring. Samples that are lower than 4° C of the instrument's temperature will cause long read times. The sample temperature must be within one or two degrees of the chamber temperature before fast, accurate readings can be made. Refer to Chapter 12 for additional information. 38
AquaLab Taking a Reading
7. Taking a Reading Measurement Steps Once you have prepared your sample, you are ready to take readings. The process is simple: 1. Turn the sample drawer knob to the OPEN/LOAD position and pull the drawer open. 2. Place your prepared sample in the drawer. Check the top lip of the cup to make sure it is free from sample residue (remember, an over-filled sample cup may contaminate the chamber's sensors). 3. Carefully slide the drawer closed, being especially careful if you have a liquid sample that may splash or spill and contaminate the chamber. 4. Turn the sample drawer knob to the READ position to seal the sample cup with the chamber. The following screen will appear:
chamber sealed measurement started
AquaLab Taking a Reading
This will start the read cycle. In about 40 seconds, the first aw measurement will be displayed on the LCD. Length of read times may vary depending on temperature differences between the chamber and your sample, and other properties of your sample. For TE models: If you have the temperature equilibrium menu set at 1 or 2, then an equilbration screen appears:
Ts = 23.1 Ts - Tb = 0.125 Note: Samples that have a large difference in aw from previous samples may need extra time to reach equilibrium, since some of the previous sample's atmosphere stays in the chamber after measurement.
How AquaLab takes Readings AquaLab's reading cycle continues until the rate of change of three consecutive readings are less than 0.0005 of each other. The instrument crosses the dew threshold numerous times to ensure the accuracy of readings. When the instrument has finished its read cycle, the aw is displayed, accompanied by the LED flash and beeper (if you have the beeper enabled).
AquaLab Taking a Reading
Cautions •
Never leave a sample in your AquaLab after a reading has been taken. The sample may spill and contaminate the instrument's chamber if the instrument is accidentally moved or jolted.
Never try to move your instrument after a sample has been loaded. Movement may cause the sample material to spill and contaminate the sample chamber.
Take special care not to move the sample drawer too quickly when loading or unloading liquid samples, in order to avoid spilling.
If a sample has a temperature that is four degrees higher (or more) than the AquaLab's chamber, the instrument will display:
sample too hot alerting you to cool the sample before reading. Although the instrument will measure warmer samples, the readings may be inaccurate. Warm samples can cause condensation in the chamber if they have a high water activity. •
The physical temperature of the instrument 41
AquaLab Taking a Reading
should be between 5°- 43°C. Between these ambient temperatures, AquaLab will measure samples of similar temperature quickly and accurately. The AquaLab Series 3TE has temperature control capabilities that enable it to read samples at temperatures different from ambient temperature, but no higher than 43°C. •
If you are sampling and a triangular warning symbol appears in the top right corner, this indicates
0.853 aw
that the mirror has become too dirty to give accurate measurements, and you need to clean the mirror and chamber before continuing to sample. For more details about this symbol, please refer to Chapter 12. For cleaning instructions, refer to Chapter 10. •
If a sample has an aw lower than about 0.03, AquaLab will display a message, accompanied by the flashing light, notifying you that your sample is too dry to be accurately measured by the AquaLab. Following is an example:
aw < 0.031
AquaLab Taking a Reading
This message will stay on the screen until you open the sample drawer. If you know that your sample's water activity is above what the screen is telling you, your instrument's sensors may have been contaminated and will need to be cleaned (see Chapter 10) or serviced (see Chapter 11).
AquaLab Computer Interface
8. Computer Interface Your AquaLab was shipped to you with a standard RS-232 interface cable. Using this, you can use your computer's terminal program to send water activity data to your computer for further analysis and storage.
AquaLink Software An optional program that is available for use with your AquaLab is AquaLink. AquaLink is a Windows-based program designed for collection and graphing of data from all AquaLab models. It logs water activity, temperature, time of measurement, and a time and date stamp (from your computer's clock). It also has sample identification and comments fields that you can use to help annotate the data that your AquaLab is taking. AquaLink takes the data and displays it in a real-time graph. You can control the increments of each axis to customize the graph as you wish. If you are interested in purchasing a copy of AquaLink, contact Decagon or your authorized distributor. Here is a sample picture of the AquaLink program:
AquaLab Computer Interface
AquaLink screen
Using Windows Hyperterminal To use Hyperterminal with your AquaLab, follow these steps: 1. Press the Start button and select Programs > Accessories > Hyperterminal and click on the Hyperterminal icon. 2. At the prompt, choose a name for this program (AquaLab is a good one) and choose an arbitrary icon above to represent it. In future downloads, you will be able to click on this icon in have it already set up for you to download. Click the OK button. 45
AquaLab Computer Interface
3. A pop-up menu labeled "Connect To" will appear. Click on the scroll bar on the bottom of the screen labeled "Connect Using" and select the COM Port your RS-232 cable is connected to. 4. A pop-up menu labeled "COM Properties" will appear, showing the port settings for the COM port you selected. Make sure the settings are the following: Bits per second, 9600; 8 databits, no parity, 1 stop bit, and flow control set to hardware. Click OK. 5. Plug your RS-232 cable to the COM port you selected and connect it to your AquaLab. Begin sampling. AquaLab's data will be displayed on screen as it samples. It will be output in the following format: time (since chamber was closed), sample temperature, and aw. Here is an example: time since temp chamber was (°C) closed
3.1, 24.3, 0.862
6. When you are finished sampling, you can print the data in the terminal session, or cut and paste it to a spreadsheet or text editor. To save the data, go into the Transfers menu and select "Capture text," and designate where it should be saved. 46
AquaLab Theory
9. Theory: Water Activity in Products Water is a major component of foods, pharmaceuticals, and cosmetics. Water influences the texture, appearance, taste and spoilage of these products. There are two basic types of water analysis: water content and water activity.
Water content The meaning of the term water content is familiar to most people. It implies a quantitative analysis to determine the total amount of water present in a sample. The primary method for determining water content is by loss on drying, but secondary methods such as infrared, NMR, or Karl Fisher titration are also used. Moisture content determination is essential in meeting product nutritional labeling regulations, specifying recipes and monitoring processes. However, water content alone is not a reliable indicator for predicting microbial responses and chemical reactions in materials. The limitations of water content measurement are attributed to differences in the intensity with which water associates with other components.
Water activity Water activity is a measure of the energy status of the water in a system, and thus is a far better indicator of perishabil47
AquaLab Theory
ity than water content. Figure 1 shows how the relative activity of microorganisms, lipids and enzymes relate to water activity. While other factors, such as nutrient availability and temperature, can affect the relationships, water activity is the best single measure of how water affects these processes.
Fig. 1: Water Activity Diagram—adapted from Labuza Water activity of a system is measured by equilibrating the liquid phase water in the sample with the vapor phase water in the headspace and measuring the relative humidity of the headspace. In the AquaLab, a sample is placed in a sample cup which is sealed against a sensor block. Inside the sensor block is a fan, a dew point sensor, a temperature sensor, and an infrared thermometer. The dew point sensor measures the dew point temperature of the air, and the infrared thermometer measures the sample tempera48
AquaLab Theory
ture. From these measurements the relative humidity of the headspace is computed as the ratio of dew point temperature saturation vapor pressure to saturation vapor pressure at the sample temperature. When the water activity of the sample and the relative humidity of the air are in equilibrium, the measurement of the headspace humidity gives the water activity of the sample. The purpose of the fan is to speed equilibrium and to control the boundary layer conductance of the dew point sensor. In addition to equilibrium between the liquid phase water in the sample and the vapor phase, the internal equilibrium of the sample is important. If a system is not at internal equilibrium, one might measure a steady vapor pressure (over the period of measurement) which is not the true water activity of the system. An example of this might be a baked good or a multi-component food. Initially out of the oven, a baked good is not at internal equilibrium; the outer surface is at a lower water activity than the center of the baked good. One must wait a period of time in order for the water to migrate and the system to come to internal equilibrium. It is important to remember the restriction of the definition of water activity to equilibrium.
Effect of Temperature on water activity Temperature plays a critical role in water activity determinations. Most critical is the measurement of the difference 49
AquaLab Theory
between sample and dew point temperature. If this temperature difference were in error by 1°C, an error of up to 0.06 aw could result. In order for water activity measurements to be accurate to 0.001, temperature difference measurements need to be accurate to 0.017°C. AquaLab's infrared thermometer measures the difference in temperature between the sample and the block. It is carefully calibrated to minimize temperature errors, but achieving 0.017°C accuracy is difficult when temperature differences are large. Best accuracy is therefore obtained when the sample is near chamber temperature. Another effect of temperature on water activity occurs with samples are near saturation. A sample that is close to 1.0 aw and is only slightly warmer than the sensor block will condense water within the block. This will cause errors in the measurement, and in subsequent measurements until the condensation disappears. A sample at 0.75 aw needs to be approximately 4°C above the chamber temperature to cause condensation. The AquaLab warns the user if a sample is more than 4°C above the chamber temperature, but for high water activity samples the operator needs to be aware that condensation can occur if a sample that is warmer than the block is put in the AquaLab.
Water Potential Some additional information may be useful for understanding what water activity is and why it is such a useful 50
AquaLab Theory
measure of moisture status in products. Water activity is closely related to a thermodynamic property called the water potential, or chemical potential (µ) of water, which is the change in Gibbs free energy (G) when water concentration changes. Equilibrium occurs in a system when µ is the same everywhere in the system. Equilibrium between the liquid and the vapor phases implies that µ is the same in both phases. It is this fact that allows us to measure the water potential of the vapor phase and use that to determine the water potential of the liquid phase. Gradients in µ are driving forces for moisture movement. Thus, in an isothermal system, water tends to move from regions of high water potential (high aw) to regions of low water potential (low aw). Water content is not a driving force for water movement, and therefore can not be used to predict the direction of water movement, except in homogeneous materials.
Factors in determining Water Potential The water potential of the water in a system is influenced by factors that effect the binding of water. They include osmotic, matric, and pressure effects. Typically water activity is measured at atmospheric pressure, so only the osmotic and matric effects are important. Osmotic effects Osmotic effects are well known from biology and physical 51
AquaLab Theory
chemistry. Water is diluted when a solute is added. If this diluted water is separated from pure water by a semi-permeable membrane, water tends to move form the pure water side through the membrane to the side with the added solute. If sufficient pressure is applied to the solutewater mixture to just stop the flow, this pressure is a measure of the osmotic potential of the solution. Addition of one mole of an ideal solute to a kilogram of water produces an osmotic pressure of 22.4 atm. This lowers the water activity of the solution from 1.0 to 0.98 aw. For a given amount of solute, increasing the water content of the systems dilutes the solute, decreasing the osmotic pressure, and increasing the water activity. Since microbial cells are high concentrations of solute surrounded by semi-permeable membranes, the osmotic effect on the free energy of the water is important for determining microbial water relations and therefore their activity. Matric effects The sample matrix affects aw by physically binding water within its structure through adhesive and cohesive forces that hold water in pores and capillaries, and to particle surfaces. If cellulose or protein were added to water, the energy status of the water would be reduced. Work would need to be done to extract the water from this matrix. This reduction in energy status of the water is not osmotic, because the cellulose or protein concentrations are far too low to produce any significant dilution of water. The reduction in energy is the result of direct physical binding 52
AquaLab Theory
of water to the cellulose or protein matrix by hydrogen bonding and van der Waal forces. At higher water activity levels, capillary forces and surface tension can also play a role.
Sorption Isotherms Relating aw to water content Changes in water content affect both the osmotic and matric binding of water in a product. Thus a relationship exists between the water activity and water content of a product. This relationship is called the sorption isotherm, and is unique for each product. Figure 1 shows a typical isotherm. Besides being unique to each product, the isotherm changes depending on whether it was obtained by drying or wetting the sample. These factors need to be kept in mind if one tries to use water content to infer the stability or safety of a product. Typically, large safety margins are built in to water content specifications to allow for these uncertainties.
While the sorption isotherm is often used to infer water activity from water content, one could easily go the other direction and use the water activity to infer the water content. This is particularly attractive because water activity is much more quickly measured than water content. This method gives particularly good precision in the center of the isotherm. In order to infer water content from water activity, one needs an isotherm for the particular product; produced, ideally, using the process that brings the prod53
AquaLab Theory
uct to its final water content. For example, if one were using the AquaLab to monitor the water content of dried potato flakes, one would measure the water activity and water content of potato flakes dried to varying degrees using the standard drying process for those flakes. An isotherm would be constructed using those data, and the water content would be inferred using the measured water activity of samples and that isotherm. The importance of the concept of water activity of foods, pharmaceuticals, and cosmetics cannot be overly emphasized. Water activity is a measure of the energy status of the water in a system. More importantly, the usefulness of water activity in relation to microbial growth, chemical reactivity, and stability over water content has been shown.
AquaLab Cleaning and Maintenance
10. Cleaning and Maintenance The accuracy of your AquaLab is vitally dependent on keeping your instrument clean. Dust and sampling debris can contaminate the sampling chamber and must therefore be regularly cleaned out. To clean your instrument, carefully follow these instructions.
Tools Needed •
thin plastic rod or other non-metal implement
Distilled Water
Isopropyl Alcohol
Lint-free or sizing-free tissues (Kimwipe®) or
AquaLab Cleaning Kit
Note: Kimwipes® are ideal because they don't leave a lint residue, like most tissues. They also don't have any other compounds in the tissue that may contaminate the sensors in the AquaLab's block. Also, never use cotton swabs to clean the block sensors. Most cotton 55
AquaLab Cleaning and Maintenance
swabs contain adhesives and other compounds that are released and transferred to the mirror and other surfaces, contaminating them.
Cleaning the Block and Sensors Accessing the Block
1. Turn the power off on your AquaLab (switch in back.) 2. Remove the case lid screw located on the back panel. Carefully remove the lid by pulling the back of the lid upward and then sliding the lid back (away from the front of the case) and off. 3. Unscrew the two thumbscrews that secure the sensor block. 4. Unplug the cable with the 20-pin socket that attaches the block to the main circuit board by releasing the two locking levers that are on either side of the socket. 5. Carefully lift the block straight up from its mount. Turn the block over to expose the chamber cavity as shown in the illustration:
AquaLab Cleaning and Maintenance
optical sensor
chamber fan View of inside block chamber Mirror: 1. Wash hands with soap and water (to prevent oils from contaminating the Kimwipe tissue and being transferred to the mirror). 2. Wrap 1.5 — 2cm wide strips of Kimwipe around the plastic spatula 3. Cleaning your AquaLab sensor block is a three-step procedure. First, clean the sensors and block using an isopropyl alcohol-moistened Kimwipe tissue. Follow this with a Kimwipe tissue moistened with either Decagon's cleaning solution or distilled water. Finally, wrap a new, dry Kimwipe around the spatula, and use it to thoroughly
AquaLab Cleaning and Maintenance
wipe the cleaning solution from the mirror, thermopile and chamber surfaces. Optical Sensor: You will probably clean the optical sensor while you are cleaning the mirror, since they face each other in a very small gap. Thermopile Sensor: Clean the thermopile in the same manner as described above for the mirror. If the thermopile is very dirty, clean it with isopropanol, followed by multiple distilled watermoistened tissues. This sensor must be free of all dirt, smudges, and lint for the AquaLab to measure accurately. Inside Chamber: Clean all other portions of the chamber block with a Kimwipe moistened with distilled water. Note: Be extremely careful not to damage the fan blades in the chamber. The fan blades are very fragile; if one of them breaks, your instrument won't work properly. Therefore, take extra care when cleaning this portion. Inside Case: 1. Clean the sample drawer and drawer base. Remove any debris that may have collected inside the case.
AquaLab Cleaning and Maintenance
2. Check once more to make sure there is no debris in the sample chamber cavity. 3. Replace the block, and insert the ribbon cable socket into to the 20-pin plug on the block. Lock it in place with the locking levers. 4. Screw the thumb-screws on the block back in until they are hand-tight. 5. Replace the AquaLab case lid and secure the lid screw. 6. Plug the AquaLab's power cord back in.
Checking Calibration After you have cleaned the chamber and other parts of your AquaLab, it is important to check the instrument's performance in order to correct for any linear offset that may have occurred during cleaning procedures. Before you check the instrument, however, we recommend that you run a sample of the activated charcoal pellets that come with your AquaLab cleaning kit. This cleans the air inside the chamber, helping it come back to a stable sampling environment. Check the response of your instrument by measuring the water activity of both a verification standard and a distilled water sample.
AquaLab Cleaning and Maintenance
If a linear offset has occurred, refer to chapter 5 for directions on how to correct for linear offset. If, after adjusting for linear offset your instrument is still not reading samples correctly, contact Decagon for technical support.
AquaLab Repair Instructions
11. Repair Instructions If your AquaLab ever needs to be sent in for service or repair*, call Decagon at (509) 332-2756 or (US and Canada) 1-800-755-2751, or fax us at (509) 332-5158. We will ask you for your address, phone number, and serial number. For non-warranty repairs, we will also ask for a payment method (such as a purchase order or credit card number), a repair budget, and billing address. *Note: If you purchased your AquaLab from one of our international distributors, please contact them before contacting Decagon. They may be able to provide you with local service and help you remedy the problem.
Shipping Directions When you ship your instrument back to us, please include with it a document with the complete shipping address, name and department of the person responsible for the instrument, and (most importantly) a description of the problem. This information will better help our technicians and our shipping department to expedite repair on your instrument and ship it back to you in good time.
AquaLab Repair Instructions
Following are steps that will help in safely shipping your instrument back to us: 1. If possible, ship your AquaLab back to us in its original cardboard box with foam inserts. If this is not possible, use a box that has at least 4 inches of space between your instrument and each wall of the box. 2. Put your instrument in a plastic bag to avoid disfiguring marks from the packaging. 3. Don't ship your AquaLab to us with the power cord; we have plenty here to use with your instrument. 4. If you aren't using the foam inserts, pack the box moderately tight with packing material, like styrofoam peanuts. 5. Tape the box in both directions so it cannot be broken open in shipment. 6. Include necessary paperwork so your repair can be processed quickly. This should include your name, address, serial number, phone and fax numbers, purchase order, and a description of the problem. Ship to: Decagon Devices Inc. ATTN: Repair Department 2365 NE Hopkins Court 62
AquaLab Repair Instructions
Pullman, WA 99163
Repair Costs Manufacturer's defects and instruments within the threeyear warranty will be repaired at no cost. For non-warranty repairs, costs for parts, labor, and shipping will be billed to you. We have a $65 minimum repair charge. An extra fee will be charged for rush work. Decagon will provide an estimated repair cost, if requested.
Loaner Service We have loaner instruments that can be provided while your instrument is being serviced. There is, however, a limited number of loaner instruments. They are granted on a "first-come-first-served� basis. This service is in place to help you if your AquaLab needs service during critical operations.
AquaLab Troubleshooting
12. Troubleshooting AquaLab is a high performance instrument, designed to have low maintenance and few problems if used with care. Unfortunately, sometimes even the best operators using the best instruments encounter technical difficulties. Here is a list of some problems that may occur. If you have encountered a problem that isn't addressed here, or if these remedies still don't resolve your problem, contact Decagon at 1-800-755-2751 or (509) 332-2756 (for those not in the US or Canada).
Problems and Solutions The following table is a brief guide to help you quickly define solutions to your problems. For more detailed descriptions of these problems and their solutions, see the explanations below the table. Table 1: Troubleshooting Problem
1.Won't turn on
Possible Solutions
• Power cord disconnected • Blown fuse 64
AquaLab Troubleshooting
Table 1: Troubleshooting Problem
2. Long read time
Possible Solutions
• Dirty sample chamber • Product desorbs slowly • Propylene glycol • Broken chamber fan blade
3. aw readings on verification standards are too high/low to adjust
• Dirty thermopile • Dirty mirror • If using a saturated salt solution, it may not be in equilibrium
4. "Sample too hot"
• Sample not in thermal equilibrium with AquaLab.
5. "aw < 0.0...."
• Sample too dry to accurately measure • Dirty chamber or mirror
6. Triangle appears in upper right corner
• Mirror is dirty
AquaLab Troubleshooting
Table 1: Troubleshooting Problem
7. "Block failure" appears on screen after turning on AquaLab
Possible Solutions
• Block is disconnected from motherboard • Block memory component has failed
1. PROBLEM: AquaLab won't turn on. SOLUTION: •
Check to make sure your power cord is securely attached the back of the instrument, and into the power outlet.
A power surge may have caused a fuse to blow. To change the fuses, follow these instructions:
1. Unplug the power cord from the wall and the instrument. 2. Locate the panel where the power cord plugs in. The fuse box is on the right side of that panel. Press in on the release tab and pull the fuse-holder out. 3. Pull the broken fuse(s) out and replace with a 1.25
AquaLab Troubleshooting
Amp 250V fuse. Caution: Do not use any other kind of fuse or you will risk damage to your instrument as well as void your warranty. 4. Replace the fuse-holder and push it into the fuse-well until the release tab snaps in place. 5. Re-connect the power cord and turn your instrument on. If the fuse blows again, a failed component may be causing the problem. Contact Decagon to make arrangements for repairs. 2. PROBLEM: Readings are slow or inconsistent. SOLUTION: •
The sample chamber may be dirty. Refer to Chapter 11 of the manual for directions on cleaning the sample chamber.
Some products absorb or desorb moisture very slowly, causing measurements to take longer than usual, and nothing can be done to speed up the process. Refer to Chapter 6 for further explanation.
Your sample may contain propylene glycol. This compound is known to cause unstable readings, because it condenses on the surface of the chilled mir67
AquaLab Troubleshooting
ror and alters readings. Please refer to the propylene glycol section in chapter 6 for hints on reducing difficulties with measuring samples with propylene glycol. If you have further questions regarding the measurement of propylene glycol, contact Decagon. Note: As yet, propylene glycol is the only volatile know to act unpredictably. Glycerol and other alcohols used to flavor foods can usually be measured without problems; however, some aromatics may also caused unstable readings. If this seems to be a problem for you, contact Decagon. •
A fan blade may be broken inside the block. If even salt standards take a long time to read, and the sample chamber is clean, you may have a broken chamber fan blade. This is especially likely if you have just cleaned the chamber. If you suspect this may have happened, contact Decagon for details on replacement.
3. PROBLEM: Water activity readings on verification standards are too high/low and a linear offset adjustment cannot be made any higher/lower. SOLUTION: •
The thermopile in your chamber, which measures sample temperature, may have become contami-
AquaLab Troubleshooting
nated. Refer to Chapter 11 for directions on cleaning. •
The chamber mirror may be dirty. Refer to Chapter 11 for directions on cleaning.
If you weren't using Decagon's performance verification standards, high readings may indicate that the salt solution you are using is not in equilibrium.
4. PROBLEM: Message on screen displays the following:
sample too hot SOLUTION: •
Your sample's temperature is too high for the instrument to equilibrate with it in a reasonable amount of time. The instrument and sample need to be in temperature equilibrium before accurate samples can be made. Therefore, very cold samples will take a very long time to measure due to the same reason. To avoid this problem, make sure to only measure samples that are at the same temperature as the instrument.
AquaLab Troubleshooting
Message on screen displays the following (example):
aw < 0.028
The sample is too dry for the instrument to read accurately. If your sample has a water activity that is less than below the detection limits of the instrument, this message will come up. Essentially, it means that there is not enough sample moisture to condense on the mirror and provide a reading.
The mirror may be dirty. Try cleaning the mirror and chamber and measuring the sample again.
6. PROBLEM: A small triangle appears in the upper right corner after sampling:
0.853 aw
SOLUTION: The mirror needs to be cleaned, along with the rest of the sample chamber, or a volatile contaminant is interfering with the dewpoint determination. This triangle is a
AquaLab Troubleshooting
mirror performance indicator. The performance of the mirror is measured on a 0 to 1 scale, with 1 being the cleanest. When the AquaLab senses that the mirror performance has dropped to unacceptable levels, it will display the triangular warning sign after the sample has been measured. To see what the mirror performance value is, press the upper right button when the triangle appears, and it will show you the value. At this point, you should stop sampling and clean the chamber. If the triangle is still on the screen after cleaning, the mirror is most likely still dirty or a volatile in your sample is contaminating the mirror. Contact Decagon for more assistance. 7. PROBLEM: The following screen comes up after turning on the machine:
block failure SOLUTIONS: •
The block is not plugged in to the motherboard. Open the case and check to make sure that the small ribbon cable that connects the block to the motherboard is snapped and locked in place.
One or more components has failed on the
AquaLab Troubleshooting
block's circuit board. If the block is properly plugged in to the motherboard and this message appears, it is likely that one or more of the components have failed on the block's circuit board. If you press -exit- at this prompt, the instrument will not have values for the component that has failed, which will lead to incorrect readings. If this message appears and you continue to sample, Decagon cannot be liable for errors in reading that may occur. Contact Decagon for a solution to this problem.
Component Performance Screen If, after cleaning your instrument and reading the other troubleshooting hints, you have reason to believe that one of the components of your AquaLab may be causing measurement error, you can access a screen that will display values for component performance. This is done either by holding down the lower right button while turning on the instrument, or by pressing the upper left button while in the linear offset adjustment main menu (Change linear offset? Y/N). After it initializes, the following screen will appear:
sensors -Exit-
3.21 23.5
0.030 1100
This screen gives you four values. The top left value is the value the thermocouple is reading. It is basically the differ72
AquaLab Troubleshooting
ence in temperature between the block and the mirror. It should typically have a value of 3, ±0.3. If this is zero, there is something wrong with the thermocouple. The top right value is the value read by the thermopile, which is the temperature difference between the block and what it "sees" below it (the sample, when reading). This value should be around zero, but will change when you change the drawer position. The bottom left value is the block temperature. This value should be around ambient temperature. The bottom right value is the mirror reflectance voltage, in units of millivolts. This value should normally be between 400 and 2400mV, and should be steady. You can't change anything in this screen, but it is here to give you an indication of the component performance. If you notice that any of these values are not what they should be, contact Decagon for further instruction. Press the button next to -Exit- to get back to the main menu.
AquaLab Further Reading
13. Further Reading Water Activity Theory and Measurement Bousquet-Ricard, M., G. Qualyle, T. Pharm, and J.C. Cheftel. (1980). Comparative study of three methods of determining water activity in intermediate moisture foods. Lebensmittel-Wissenschaft und-Technologie. 13:169-173. Chirife, J., G. Favetto, C. Ferro-Fontan, and S. Resnik. (1983). The water activity of standard saturated salt solutions in the range of intermediate moisture foods. Lebensmittel-Wissenschaft und-Technologie. 16:36-38. Duckworth, R. (1975). Water Relations of Foods. Academic Press, New York. Gomez-Diaz, R. (1992). Water activity in foods: Determination methods. Alimentaria. 29:77-82. G贸mez, R. and Fernandez-Salguero J. (1992). Water activity and chemical composition of some food emulsions. Food Chemistry. 45:91-93. Greenspan, L. (1977). Humidity fixed points of binary saturated aqueous solutions. Journal of Research of the National Bureau of Standards - A.Physics and Chemistry. 81A:89-96. 74
AquaLab Further Reading
Karmas, E. (1981). Measurement of moisture content. Cereal Foods World. 26(7):332-334. Kitic, D., D.C. Pereira-Jardim, G. Favetto, S.L. Resnik, and J. Chirife. (1986). Theoretical prediction of the water activity of standard saturated salt solutions at various temperatures. Journal of Food Science. 51:1037-1042. Labuza, T.P. and R. Contreras-Medellin. (1981). Prediction of moisture protection requirements for foods. Cereal Foods World. 26(7):335-343. Labuza, T.P., K. Acott, S.R. Tatini, and R.Y. Lee. (1976). Water activity determination: A collaborative study of different methods. Journal of Food Science. 41:910-917. Prior, B.A. (1979). Measurement of water activity in foods: A review. Journal of Food Protection. 42(8):668674. Reid, D.S. (1976). Water activity concepts in intermediate moisture foods. In: Intermediate Moisture Foods. Davies, R., G.G. Birch, and K.J. Parker (ed.) Applied Science Publishers, London. pp. 54-65. Richard, J. and T.P. Labuza. (1990). Rapid determination of the water activity of some reference solutions, culture media and cheese using a new dew point apparatus. Sciences des Aliments. 10:57-64. Roa, V. and M.S. Tapia de Daza. (1991). Evaluation of water activity measurements with a dew point electronic humidity meter. Lebensmittel-Wissenschaft und-Technologie. 24(3):208-213. 75
AquaLab Further Reading
Roos, K.D. (1975). Estimation of water activity in intermediate moisture foods. Food Technology. 29:26-30. Scott, V.N. and D.T. Bernard. (1983). Influence of temperature on the measurement of water activity of food and salt systems. Journal of Food Science. 48:552-554. Snavely, M.J., J.C. Price, and H.W. Jun. (1990). A comparison of three equilibrium relative humidity measuring devices. Drug Development and Industrial Pharmacy. 16(8):1399-1409. Stamp, J.A., S. Linscott, C. Lomauro, and T.P. Labuza. (1984). Measurement of water activity of salt solutions and foods by several electronic methods as compared to direct vapor pressure measurement. Journal of Food Science. 49:1139-1142. Stoloff, L. (1978). Calibration of water activity measuring instruments and devices: Collaborative study. Journal of Association of Official Analytical Chemists. 61:1166-1178. Troller, J.A. (1983). Methods to measure water activity. Journal of Food Protection. 46:129-134. Troller, J.A. and J.H.B. Christian. (1978). Water Activity and Food. Academic Press, New York. Troller, J.A. and V.N. Scott. (1992). Measurement of water activity (aw) and acidity. In: Compendium of Methods for the Microbiological Examination of Foods. Vanderzant, C. and D.F. Splittstoesser (ed.) American Public Health Association, Washington, D.C. pp. 135-151. 76
AquaLab Further Reading
van den Berg, C. (1985). Water activity. In: Concentration and Drying of Foods. MacCarthy, D. (ed.) Elsevier, London. pp. 11-35. van den Berg, C. (1991). Food-water relations: Progress and integration, comments and thoughts. In: Water Relationships in Foods. Levine, H. and L. Slade (ed.) Plenum Press, New York. van den Berg, C. and Bruin. (1981). Water activity and its estimation in food systems: Theoretical aspects. In: Water Activity: Influences on Food Quality. Rockland, L.B. and G.F. Stewart (ed.) Academic Press, New York. pp. 1-61. Vega, M.H. and G.V. Barbosa-Canovas. (1994). Prediction of water activity in food systems: A review on theoretical models. Revista Espanola De Ciencia Y Tecnologia De Alimentos. 34:368-388. Vega, M.H., B. Romanach, and G.V. Barbosa-Canovas. (1994). Prediction of water activity in food systems: A computer program for predicting water activity in multicomponent foods. Revista Espanola De Ciencia Y Tecnologia De Alimentos. 34:427-440. Vos, P.T. and T.P. Labuza. (1974). Technique for measurements of water activity in the high aw range. Journal of Agricultural and Food Chemistry. 22:326327. Voysey, P. (1993). An evaluation of the AquaLab CX-2 system for measuring water activity. Digest, Microbiology Section. 124 77
AquaLab Further Reading
Food Quality and Safety Brandt, L. (1996). Bound for success. Controlling water activity gives technologists the edge in developing safe, shelf-stable foods. Food Formulating. September:41-48. Chirife, J. and B.M. Del-Pilar. (1994). Water activity, glass transition and microbial stability in concentrated/ semimoist food systems. Journal of Food Science. 59:921-927. Chirife, J. and M.P. Buera. (1995). A critical review of some non-equilibrium situations and glass transitions on water activity values of foods in the microbiological growth range. Journal of Food Engineering. 25:531-552. Chirife, J. and M.P. Buera. (1996). Water activity, water glass dynamics, and the control of microbiological growth in foods. Critical Reviews in Food Science and Nutrition. 36(5):465-513. Franks, F. (1982). Water activity as a measure of biological viability and quality control. Cereal Foods World. 27(9):403-407. Franks, F. (1991). Water activity: a credible measure of food safety and quality? Trends in Food Science and Technology. March:68-72. Hardman, T.M. (1988). Water and Food Quality. Elseiver Press, London. Kress-Rogers, E. (1993). Food quality measurement. Food Industry News. September:23-26. Levine, H. and L. Slade. (1991). Water Relationships in 78
AquaLab Further Reading
Foods. Plenum Press, New York. Mannheim, C.H., J.X. Liu, and S.G. Gilbert. (1994). Control of water in foods during storage. Journal of Food Engineering. 22:509-532. McMeekin, T.A. and T. Ross. (1996). Shelf life prediction: Status and future possibilities. International Journal of Food Microbiology. 33:65-83. Nelson, K.A. and T.P. Labuza. (1994). Water activity and food polymer science: Implications of state on arrhenius and WLF models in predicting shelf life. Journal of Food Engineering. 22:271-289. Rockland, L.B. and G.F. Stewart. (1981). Water Activity: Influences on Food Quality. Academic Press, New York. Rockland, L.B. and S.K. Nishi. (1980). Influence of water activity on food product quality and stability. Food Technology. 34:42-59. Seow, C.C., T.T. Teng, and C.H. Quah. (1988). Food Preservation by Moisture Control. Elsevier, New York. Taoukis, P., W. Breene, and T.P. Labuza. (1988). Intermediate moisture foods. Advances in Cereal Science and Technology. 9:91-128.
Water Activity and Microbiology Beuchat, L.R. (1981). Microbial stability as affected by water activity. Cereal Foods World. 26(7):345-349. Chen, H.C. (1995). Seafood microorganisms and seafood safety. Journal of Food and Drug Analysis. 3:133144. 79
AquaLab Further Reading
Farber, J.M., F. Coates, and E. Daley. (1992). Minimum water activity requirements for the growth of Listeria monocytogenes. Letters In Applied Microbiology. 15:103-105. Garcia de Fernando, G.D., O. Diaz, M. Fernandez, and J.A. Ordonez. (1992). Changes in water activity of selected solid culture media throughout incubation. Food Microbiology. 9:77-82. Gibson, A.M., J. Baranyi, J.I. Pitt, M.J. Eyles, and T.A. Roberts. (1994). Predicting fungal growth: The effect of water activity on Aspergillus flavus and related species. International Journal of Food Microbiology. 23:419-431. Goaleni, N., J.E. Smith, J. Lacey, and G. Gettinby. (1997). Effects of temperature, water activity, and incubation time on production of aflatoxins and cyclopiazonic acid by an isolate of Aspergillus flavus in surface agar culture. Applied and Environmental Microbiology. 63:1048-1053. Hocking, A.D. and B.F. Miscamble. (1995). Water relations of some Zygomycetes isolated from food. Mycological Research. 99:1113-1118. Hocking, A.D., B.F. Miscamble, and J.I. Pitt. (1994). Water relations of Alternaria alternata, Cladosporium cladosporioides, Cladosporium sphaerospermum, Curvularia lunata and Curvularia pallescens. Mycological Research. 98:91-94. Houtsma, P.C., A. Heuvelink, J. Dufrenne, and S. Notermans. (1994). Effect of sodium lactate on toxin 80
AquaLab Further Reading
production, spore germination and heat resistance of proteolytic Clostridium botulinum strains. Journal of Food Protection. 57:327-330. Kuntz, L.A. (1992). Keeping microorganisms in control. Food Product Design. August:44-51. Li, K.Y. and J.A. Torres. (1993). Water activity relationships for selected mesophiles and psychrotrophs at refrigeration temperature. Journal of Food Protection. 56:612-615. Marauska, M., A. Vigants, A. Klincare, D. Upite, E. Kaminska, and M. Bekers. (1996). Influence of water activity and medium osmolality on the growth and acid production of Lactobacillus casei var. alactosus. Proceedings of the Latvian Academy of Sciences Section B Natural Exact and Applied Sciences. 50:144-146. Miller, A.J. (1992). Combined water activity and solute effects on growth and survival of Listeria monocytogenes Scott A. Journal of Food Protection. 55:414-418. Nakajo, M. and Y. Moriyama. (1993). Effect of pH and water activity on heat resistance of spores of Bacillus coagulans. Journal of the Japanese Society for Food Science and Technology. 40:268-271. Nolan, D.A., D.C. Chamblin, and J.A. Troller. (1992). Minimal water activity levels for growth and survival of Listeria monocytogenes and Listeria innocua. International Journal of Food Microbiology. 16:323-335. 81
AquaLab Further Reading
Petersson, S. and J. Schnuerer. (1995). Biocontrol of mold growth in high-moisture wheat stored under airtight conditions by Pichia anomala, Pichia guilliermondii, and Saccharomyces cerevisiae. Applied and Environmental Microbiology. 61:1027-1032. Pitt, J.I. and B.F. Miscamble. (1995). Water relations of Aspergillus flavus and closely related species. Journal of Food Protection. 58:86-90. Quintavalla, S. and G. Parolari. (1993). Effects of temperature, aw and pH on the growth of Bacillus cells and spore: A response surface methodology study. International Journal of Food Microbiology. 19:207-216. Saad, R.R. (1992). Effect of water activity on growth and lipids of xerophilic fungi, Aspergillus repens and Aspergillus amstelodami. Zentralblatt Fuer Mikrobiologie. 147:61-64. Santos, J., T.M. Lopez-Diaz, M.L. Garcia-Lopez, M.C. Garcia-Fernandez, and A. Otero. (1994). Minimum water activity for the growth of Aeromonas hydrophila as affected by strain, temperature and humectant. Letters In Applied Microbiology. 19:76-78. Tapia de Daza, M.S., Y. Villegas, and A. Martinez. (1991). Minimal water activity for growth of Listeria monocytogenes as affected by solute and temperature. International Journal of Food Microbiology. 14:333-338. Tokuoka, K. and T. Ishitani. (1991). Minimum water activ82
AquaLab Further Reading
ities for the growth of yeasts isolated from highsugar foods. Journal of General and Applied Microbiology. 37:111-119. Ucar, F. and I. Guneri. (1996). The effect of water activity (aw), pH and temperature on the growth of osmophilic yeasts. Turkish Journal of Biology. 20:37-46. Wijtzes, T., P.J. Mcclure, M.H. Zwietering, and T.A. Roberts. (1993). Modelling bacterial growth of Listeria monocytogenes as a function of water activity, pH and temperature. International Journal of Food Microbiology. 18:139-149. Zwietering, M.H., T. Wijtzes, J.C. De-Wit, and R.K. Van'T. (1992). A decision support system for prediction of the microbial spoilage in foods. Journal of Food Protection. 55:973-979.
Water Activity in Foods Meat and Seafood Chen, N. and L.A. Shelef. (1992). Relationship between water activity, salts of lactic acid, and growth of Listeria monocytogenes in a meat model system. Journal of Food Protection. 55:574-578. Clavero, M.R.S. and L.R. Beuchat. (1996). Survival of Escherichia coli O157:H7 in broth and processed salami as influenced by pH, water activity, and temperature and suitability of media for its recovery. Applied and Environmental Microbiology. 62:2735-2740. Duffy, L.L., P.B. Vanderlinde, and F.H. Grau. (1994). 83
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Growth of Listeria monocytogenes on vacuumpacked cooked meats: Effects of pH, a w, nitrite and ascorbate. International Journal of Food Microbiology. 23:377-390. Fernandez-Salguero J., R. G贸mez, and M.A. Carmona. (1994). Water activity of Spanish intermediatemoisture meat products. Meat Science. 38:341346. G贸mez, R. and Fernandez-Salguero J. (1993). Note: Water activity of Spanish intermediate moisture fish products. Revista Espanola De Ciencia Y Tecnologia De Alimentos. 33:651-656. Hand, L. (1994). Controlling water activity and pH in snack sticks. Meat Marketing and Technology. May:55-56. Lee, M.B. and S. Styliadis. (1996). A survey of pH and water activity levels in processed salamis and sausages in Metro Toronto. Journal of Food Protection. 59:1007-1010. Luecke, F.K. (1994). Fermented meat products. Food Research International. 27:299-307. Minegishi, Y., Y. Tsukamasa, K. Miake, T. Shimasaki, C. Imai, M. Sugiyama, and H. Shinano. (1995). Water activity and microflora in commercial vacuumpacked smoked salmons. Journal of the Food Hygienic Society of Japan. 36:442-446. Rocha-Garza, A.E. and J.F. Zayas. (1996). Quality of broiled beef patties supplemented with wheat germ protein flour. Journal of Food Science. 84
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61:418-421. Shimasaki, T., K. Miake, Y. Tsukamasa, M.A. Sugiyama, Y. Minegishi, and H. Shinano. (1994). Effect of Water Activity and Storage Temperature on the Quality and Microflora of Smoked Salmon. Nippon Suisan Gakkaishi. 60:569-576. Untermann, F. and C. Muller. (1992). Influence of aw value and storage temperature on the multiplication and enterotoxin formation of staphylococci in drycured raw hams. International Journal of Food Microbiology. 16:109-115. Williams, S.K., G.E. Rodrick, and R.L. West. (1995). Sodium lactate affects shelf life and consumer acceptance of fresh (Ictalurus nebulosus, marmoratus) fillets under simulated retail conditions. Journal of Food Science. 60:636-639. Dairy Products Fresno, J.M., M.E. Tornadijo, J. Carballo, P.J. Gonzalez, and A. Bernardo. (1996). Characterization and biochemical changes during the ripening of a Spanish craft goat's milk cheese (Armada variety). Food Chemistry. 55:225-230. Hong, Y.H. (1991). Physical and chemical properties of the process cheese on the domestic market. Korean Journal of Animal Science. 33:387-391. Kombila, M.E. and C. Lacroix. (1991). The effect of combinations of salt, lactose and glycerol on the water activity (AW) of cheese spreads. Canadian Institute 85
AquaLab Further Reading
of Food Science and Technology Journal. 24:233238. Pisecky, J. (1992). Water activity of milk powders. Milchwissenschaft. 47:3-7. Tornadijo, E., J.M. Fresno, J. Carballo, and S.R. Martin. (1993). Study of Enterobacteriaceae throughout the manufacturing and ripening of hard goats' cheese. Journal of Applied Bacteriology. 75:240246. Valik, L. and F. Gorner. (1995). Effect of water activity adjusted with different solutes on growth and Lactic acid production by Lactobacillus helveticus. Folia Microbiologica. 40:472-474. Vivier, D., M. Rivemale, J.P. Reverbel, R. Ratomahenina, and P. Galzy. (1994). Study of the growth of yeasts from feta cheese. International Journal of Food Microbiology. 22:207-215. Vivier, D., R. Ratomahenina, and P. Galzy. (1994). Characteristics of micrococci from the surface of Roquefort cheese. Journal of Applied Bacteriology. 76:546-552. Fruits and Vegetables Ayub, M., R. Khan, S. Wahab, A. Zeb, and J. Muhammad. (1995). Effect of crystalline sweeteners on the water activity and shelf stability of osmotically dehydrated guava. Sarhad Journal of Agriculture. 11:755-761. Beveridge, T. and S.E. Weintraub. (1995). Effect of 86
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blanching pretreatment on color and texture of apple slices at various water activities. Food Research International. 28:83-86. Hubinger, M., F.C. Menegalli, R.J. Aguerre, and C. Suarez. (1992). Water vapor adsorption isotherms of guava, mango and pineapple. Journal of Food Science. 57:1405-1407. Jimenez, M., M. Manez, and E. Hernandez. (1996). Influence of water activity and temperature on the production of zearalenone in corn by three Fusarium species. International Journal of Food Microbiology. 29:417-421. Kiranoudis, C.T., Z.B. Maroulis, E. Tsami, and K.D. Marinos. (1993). Equilibrium moisture content and heat of desorption of some vegetables. Journal of Food Engineering. 20:55-74. Makower, B. and G.L. Dehority. (1943). Equilibrium moisture content of dehydrated vegetables. Industrial and Engineering Chemistry. 35(2):193-197. Maltini, E., D. Torreggiani, B.R. Brovetto, and G. Bertolo. (1993). Functional properties of reduced moisture fruits as ingredients in food systems. Food Research International. 26:413-419. Marin, S., V. Sanchis, and N. Magan. (1995). Water activity, temperature, and pH effects on growth of Fusarium moniliforme and Fusarium proliferatum isolates from maize. Canadian Journal of Microbiology. 41:1063-1070. Monsalve-Gonzalez, A., G.V. Barbosa-Canovas, and R.P. 87
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Cavalieri. (1993). Mass transfer and textural changes during processing of apples by combined methods. Journal of Food Science. 58:1118-1124. Tapia de Daza, M.S., C.E. Aguilar, V. Roa, and R.V. Diaz de Tablante. (1995). Combined stress effects on growth of Zygosaccharomyces rouxii from an intermediate moisture papaya product. Journal of Food Science. 60:356-359. Zeb, A., R. Khan, A. Khan, M. Saeed, and S.A. Manan. (1994). Influence of crystalline sucrose and chemical preservatives on the water activity and shelf stability of intermediate banana chips. Sarhad Journal of Agriculture. 10:721-726. Zhang, X.W., X. Liu, D.X. Gu, W. Zhou, R.L. Wang, and P. Liu. (1996). Desorption isotherms of some vegetables. Journal of the Science of Food and Agriculture. 70:303-306. Baked Goods and Cereals Aramouni, F.M., K.K. Kone, J.A. Craig, and D.-Y.C. Fung. (1994). Growth of Clostridium sporogenes PA 3679 in home-style canned quick breads. Journal of Food Protection. 57:882-886. Cahagnier, B., L. Lesage, and M.D. Richard. (1993). Mould growth and conidiation in cereal grains as affected by water activity and temperature. Letters In Applied Microbiology. 17:7-13. Clawson, A.R. and A.J. Taylor. (1993). Chemical changes during cooking of wheat. Food Chemistry. 47:337-
AquaLab Further Reading
343. G贸mez, R., Fernandez-Salguero J., M.A. Carmona, and D. Sanchez. (1993). Water activity in foods with intermediate moisture levels: Bakery and confectionery products: Miscellany. Alimentaria. 30:55-57. Harris, M. and M. Peleg. (1996). Patterns of textural changes in brittle cellular cereal foods caused by moisture sorption. Cereal Chemistry. 73:225-231. Michniewicz, J., C.G. Biliaderis, and W. Bushuk. (1992). Effect of added pentosans on some properties of wheat bread. Food Chemistry. 43:251-257. Ramanathan, S. and S. Cenkowski. (1995). Sorption isotherms of flour and flow behaviour of dough as influenced by flour compaction. Canadian Agricultural Engineering. 37:119-124. Roessler, P.F. and M.C. Ballenger. (1996). Contamination of an unpreserved semisoft baked cookie with a xerophilic Aspergillus species. Journal of Food Protection. 59:1055-1060. Seiler, D.A.L. (1979). The mould-free shelf life of bakery products. FMBRA Bulletin. April(2):71-74. Sumner, S.S., J.A. Albrecht, and D.L. Peters. (1993). Occurrence of enterotoxigenic strains of Staphylococcus aureus and enterotoxin production in bakery products. Journal of Food Protection. 56:722724. Tesch, R., M.D. Normand, and M. Peleg. (1996). Comparison of the acoustic and mechanical signatures of two cellular crunchy cereal foods at various water 89
AquaLab Further Reading
activity levels. Journal of the Science of Food and Agriculture. 70:347-354. Weegels, P.L., J.A. Verhoek, A.M.G. de Groot, and R.J. Hamer. (1994). Effects of gluten of heating at different moisture contents: I. Changes in functional properties. Journal of Cereal Science. 19:31-38. Beverages/Soups/Sauces/Preserves Carson, K.J., J.L. Collins, and M.P. Penfield. (1994). Unrefined, dried apple pomace as a potential food ingredient. Journal of Food Science. 59:1213-1215. Durrani, M.J., R. Khan, M. Saeed, and A. Khan. (1992). Development of concentrated beverages from Anna apples with or without added preservatives by controlling activity of water for shelf stability. Sarhad Journal of Agriculture. 8:23-28. Ferragut, V., J.A. Salazar, and A. Chiralt. (1993). Stability in the conservation of emulsified sauces low in oil content. Alimentaria. 30:67-69. Ibarz, A., J. Pagan, and R. Miguelsanz. (1992). Rheology of clarified fruit juices: II. Blackcurrant juices. Journal of Food Engineering. 15:63-74. Kusumegi, K., T. Takahashi, and M. Miyagi. (1996). Effects of addition of sodium citrate on the pasteurizing conditions in "Tuyu", Japanese noodle soup. Journal of the Japanese Society for Food Science and Technology. 43:740-747. Sa, M.M. and A.M. Sereno. (1993). Effect of temperature on sorption isotherms and heats of sorption of 90
AquaLab Further Reading
quince jam. International Journal of Food Science and Technology. 28:241-248.
Pharmaceuticals/Cosmetics Ahlneck, C. and G. Zografi. (1990). The molecular basis of moisture effects on the physical and chemical stability of drugs in the solid state. International Journal of Pharmaceutics. 62:87-95. Cochet, N. and A.L. Demain. (1996). Effect of water activity on production of beta-lactam antibiotics by Streptomyces clavuligerus in submerged culture. Journal of Applied Bacteriology. 80:333-337. Costantino, H.R., R. Langer, and A.M. Klibanov. (1994). Solid-phase aggregation of proteins under pharmaceutically relevant conditions. Journal of Pharmaceutical Sciences. 83:1662-1669. Enigl, D.C. and K.M. Sorrels. (1997). Water Activity and Self-Preserving Formulas. In: Preservative-Free and Self-Preserving Cosmetics and Drugs: Principles and Practice. Kabara, J.J. and D.S. Orth (ed.) Marcel Dekker, pp. 45-73. Hageman, M.J. (1988). The role of moisture in protein stability. Drug Development and Industrial Pharmacy. 14(14):2047-2070. Heidemann, D.R. and P.J. Jarosz. (1991). Performulation studies involving moisture uptake in solid dosage forms. Pharmaceutical Research. 8(3):292-297. Friedel, R.R. and A.M. Cundell. (1998). The application of water activity measurement to the microbiological 91
AquaLab Further Reading
attributes testing of nonsterile over-the-counter drug products. Pharmacopeial Forum. 24(2):60876090. Kontny, M.J. (1988). Distribution of water in solid pharmaceutical systems. Drug Development and Industrial Pharmacy. 14(14):1991-2027. Zografi, G. (1988). States of water associated with solids. Drug Development and Industrial Pharmacy. 14(14):1905-1926. Zografi, G. and M.J. Kontny. (1986). The interactions of water with cellulose- and starch-derived pharmaceutical excipients. Pharmaceutical Research. 3(4):187-193.
Miscellaneous Bell, L.N. and T.P. Labuza. (1992). Compositional influence on the pH of reduced-moisture solutions. Journal of Food Science. 57:732-734. Bell, L.N. and T.P. Labuza. (1994). Influence of the lowmoisture state on pH and its implication for reaction kinetics. Journal of Food Engineering. 22:291312. Bell, L.N. (1995). Kinetics of non-enzymatic browning in amorphous solid systems: Distinguishing the effects of water activity and the glass transition. Food Research International. 28:591-597. Brake, N.C. and O.R. Fennema. (1993). Edible coatings to inhibit lipid migration in a confectionery product. Journal of Food Science. 58:1422-1425. 92
AquaLab Further Reading
Dole, M. and L. Faller. (1950). Water sorption by synthetic high polymers. Journal of the American Chemical Society. 12:414-419. Fernandez-Salguero J., R. G贸mez, and M.A. Carmona. (1993). Water activity in selected high-moisture foods. Journal of Food Composition and Analysis. 6:364-369. Lomauro, C.J., A.S. Bakshi, and T.P. Labuza. (1985). Evaluation of food moisture sorption isotherm equations. Part I: Fruit, vegetable and meat products. Lebensmittel-Wissenschaft und-Technologie. 18:111-117. Lomauro, C.J., A.S. Bakshi, and T.P. Labuza. (1985). Evaluation of food moisture sorption isotherm equations. Part II: Milk, coffee, tea, nuts, oilseeds, spices and starchy foods. Lebensmittel-Wissenschaft und-Technologie. 18:118-124. Yasuda, H., H.G. Olf, B. Crist, C.E. Lamaze, and A. Peterlin. (1972). Movement of water in homogeneous water-swollen polymers. In: Water Structure at the Water Polymer Interface. Jellinek, H.H.G. (ed.) Plenum Press, New York/London.
AquaLab Appendix A
Appendix A Preparing Salt Solution If you choose to mix a saturated salt solution for use as a verification standard, we recommend that you use the approved AOAC method. This method is as follows: 1. Select a reagent-grade salt and place it in a test container to a depth of about 4cm for more soluble salts (lower aw), to a depth of about 1.5 cm for less soluble salts (high aw), and to an intermediate depth for intermediate salts. 2. Add distilled water in increments of about 2mL, stirring constantly. 3. Add water until the salt can absorb no more water, evidenced by the presence of free liquid. Keep the amount of free liquid to the minimum needed to keep the solution saturated with water. If you plan on using this solution over a long term period, seal the solution well to prevent losses from evaporation. Below is a table of saturated salt solutions and their respective water activities at various temperatures. Please note that these values are based on averaged published data, and the standard errors shown reflect Greenspan's standard error for each salt solution, not the AquaLab's accuracy in measuring the salt. AquaLab
AquaLab Appendix A
measures all samples with an accuracy of ±0.003 aw. 4. Saturated salt solutions are very temperature-sensitive and their values are not as accurate as the verification standards offered by Decagon. Table 2: Water Activity of Selected Salt Solutions
Saturated Solution Lithium Chloride Magnesium Chloride Potassium Carbonate Magnesium Nitrate Sodium Chloride Potassium Chloride Potassium Sulfate
aw at 20° C
aw at 25° C
0.113 ± 0.003
0.113 ± 0.003
0.331 ± 0.002
0.328 ± 0.002
0.432 ± 0.003
0.432 ± 0.004
0.544 ± 0.002
0.529 ± 0.002
0.755 ± 0.001
0.753 ± 0.001
0.851 ± 0.003
0.843 ± 0.003
0.976 ± 0.005
0.973 ± 0.005
Adapted from Greenspan (1977). Rounded to nearest thousandth.
AquaLab Appendix A
Appendix B Table 3: Temperature Correction of Decagon's Verification Standards Temp. (°C)
0.5m KCl
6.0m NaCl
8.57m LiCl
13.41m LiCl
Aqualab will measure these standards to ±0.003aw
AquaLab Declaration of Conformity
Declaration of Conformity Application of Council Directive:
Standards to which conformity is declared:
Manufacturer's Name:
Decagon Devices, Inc. 2365 NE Hopkins Court Pullman, WA 99163 USA
Type of Equipment:
AquaLab water activity meter.
Model Number:
Series 3, Series 3TE
Year of First Manufacture:
This is to certify that the AquaLab water activity meter, manufactured by Decagon Devices, Inc., a corporation based in Pullman, Washington, USA meets or exceeds the standards for CE compliance as per the Council Directives noted above. All instruments are built at the factory at Decagon and pertinent testing documentation is freely available for verication. This certification applies to all AquaLab Series 3 models, including, but not limited to, the Series 3 and 3TE.
AquaLab Certificate of Traceability
Certificate of Traceability Decagon Devices, Inc. 2365 NE Hopkins Court Pullman WA 99163 tel: (509) 332-2756 fax: (509) 332-5158 email@example.com This is to certify that AquaLab water activity meters are manufactured utilizing temperature standards with calibration traceable to the National Institute of Standards and Technology (NIST).
AquaLab Index
Index A accessories 12 accuracy 5 AquaLab and chilled mirror dewpoint technique 6 and temperature 7 limitations of 9 series 3 5 series 3TE 5, 9 theory 47 B beeper 19 changing block sensors 56 block failure 66, 71 buttons for linear offset settings 28 for menu selection 15 C c for continuous mode 17 cautions 41 CE compliance 97 charcoal 36, 59 checking calibration 59 chilled-mirror technique 6 cleaning 55 inside case 58 99
AquaLab Index
optical sensor 58 sensor block 56 thermopile 58 tools needed for 55 coated samples shortening read time for 34 communication settings 46 component performance indicator 72 components 12 computer interface 44 continuous sampling mode 17 cotton swabs not used for cleaning 55 customer service 1 Czech 16 D Danish 16 data download format 46 Declaration of Conformity 97 dehydrated samples 35 distilled water 26 dried samples shortening read time for 34 dry samples 42, 70 E e-mail address 2 error messages 64 "sample too dry" 70 "sample too hot" 69 triangle on screen 70 100
AquaLab Index
exit 20
F fan
inside sample chamber 7 fax number 2 features 11 flashing. See LED French 16 further reading 74 fuse changing 66 G German 16
H Hyperterminal using for downloading 45 I Italian 16 L languages changing 16 LED 19 linear offset causes for 24 definition 24 how to adjust for 26, 26–31 menu 20 when to verify for 26 101
AquaLab Index
loaners 63 location for sampling 13 low water activity 37, 42
M main menu 14, 15 maintenance 55 manual 1 menus main menu 15 system configuration 18 molality of verification standards 25 N NIST traceability 98 Norwegian 16 O osmotic effects 51 P peltier cooler 6 pharmaceuticals 91 Portuguese 16 preparing salt solutions 94 propylene glycol 36, 67 cleaning out of chamber 36 R read time 102
AquaLab Index
affected by sample temp. 38 long read time 7, 34, 65, 67 readings cautions 41 how AquaLab takes 39, 40 taking readings 39 references 74 baked goods and cereals 88 beverages, soups, sauces, preserves 90 dairy products 85 food quality and safety 78 meat and seafood 83 microbiology 79 pharmaceuticals 91 water activity theory 74 relative humidity 5, 49 repair costs 63 instructions for 61 shipping 61 S salt standards. See verification standards sample slow water-emitting 67 sample cups cleaning 33 filling level 33 sample equilibration screen 7, 23 sample preparation 32 sample too dry 42, 70 sample too hot 38, 41, 65 samples 103
AquaLab Index
coated 34 dehydrated 35 dried 34 low water activity 37 multi-component 32 needing special preparation 34 not at room temperature 38 slow water-emitting 35 surface area of 35 viscous 35 sampling modes continuous 17 normal 15 saturated salts 94 seller's liability 3 series 3 5 series 3TE 5, 9 important tips 22 shipping for repair 61 sorption isotherms relating water activity to water content 53 Spanish 16 spilling the sample 41 Swedish 16 T technical support 1 telephone number 2 temperature effects on water activity 49 hot samples 41 of instrument 41 samples not at room temp. 38 104
AquaLab Index
temperature control 7 reasons for 8 theory water activity 74 time format in data download 46 long read times 67 toll-free number 2 triangle mirror performance indicator 65, 70 troubleshooting 64
V verification standards 24 aw readings too high/low for 68 compared to saturated salts 95 for linear offset 28 long read times for 68 types 25 volatiles 9, 36 special sampling block for 36 W warm-up 14 warranty 2 warranty card 2 water activity adjusting for offset 29 Aqualab and 5 definition 5, 47 displayed 14 effect on food 6, 47 low 42 105
AquaLab Index
of verification standards 25 stability diagram 48 theory 47 water content definition 47 methods for determining 47 vs. water activity 47, 53 water potential factors in determining 51 matric effects 52 osmotic effects 51 relation to water activity 50