the beneficial effects of including whole grains in the diet have been demonstrated by several authors . cereal food manufactures have responded to these advantages with the development of new whole grains and rich fiber products , most of them being in the form of flakes or blends such as muslin . beside that , the demand of precooked cereal products , in the form of snacks , breakfast cereals , and flours , has increased during the last decades . market globalization and changes on food consumption model of developing countries have been the main factors for this increasing demand . the most used processes for the production of precooked cereal foods are flaking , puffing , and extrusion . extrusion cooking is considered not only an htst process , but also a versatile one to obtain a wide range of ready - to - eat cereal foods . it is considered a staple food for more than 3,000 million people and provides them with about 60% of their daily energy intake . the unique properties of rice , such as its hypoallergenicity , bland taste , white color , and easy digestion , make it a very desirable grain for new food development [ 69 ] . rice is most consumed as grain in its three traditional forms : white , parboiled , and brown rice . it is well known that much of the vitamins and minerals are lost during the polishing step of white rice processing . even though protein content of rice is lower than that in maize and wheat , it is richer in lysine . on the other hand , an increase of demand of nongluten products based on rice has been verified in the last decades , and there is a need of novel formulas in order to diversify the use of rice other than the traditional one . rice varieties containing higher amylose content are normally more resistant to hydration   and give more viscous flour dispersion during water cooking . it has been reported that flour from rice containing less than 20% amylose is desirable to use in bread without gluten formulas since it produces much less retrogradation , but other authors   have shown that the use of rice varieties , having medium and high amylose content ( 1827% ) , gave good results for gluten - free bread . moreover , juliano   pointed out that waxy rice is preferred for cakes and pudding , because its flour is easy to hydrate and gives to the products better stability . on the other hand , high amylose rice variety is recommended for the production of noodles and is preferred for the production of expanded snacks products [ 15 , 18 , 19 ] . the advantages of extrusion as a cooking process are well known and have been discussed by several authors [ 20 , 21 ] . the wide range of degree of cooking of starchy materials that can be obtained by extrusion is remarkable , since it is possible to produce samples with low degree of cooking ( including loss of crystallinity and granule destruction ) to fully cooked samples ( highly destructed granules and total loss of native crystallinity ) [ 18 , 22 , 23 ] . water dispersion prepared with precooked flour obtained by extrusion under condition of high degree of cooking has much less viscosity than dispersion prepared with flour precooked by other processes , which can be considered a nutritional advantage . the effects of extrusion variables on structural changes and product properties of starchy materials have been extensively studied by several authors [ 2327]and particularly for rice [ 68 , 19 , 2832 ] . however , studies regarding the processing effect and product properties from brown rice having different amylose content are scarce . in the present work , two commercial rice varieties with different amylose content were selected to analyze the effect of extrusion variables on energy consumption and whole grains extrudate properties , using surface response methodology . two commercial long rice varieties , paso 144 ( 28% amylose ) and fortuna ( 16% amylose ) , with length / width ratio of 3.5 and 2.3 , respectively , were provided by molino trimacer ( santa fe - argentina ) in the form of dehulled ( or brown ) grain . brown rice was milled to obtain grits with a particle size between 1.190 and 0.420  mm , using a buhler miag , roll mill , according to a milling diagram [ 24 , 33 ] . moisture content , crude protein , fat content ( expressed as petroleum ether extract ) , and ash content were determined by aoac methods . the extrusion process was carried out with a brabender 20 dn single screw extruder , using a 4  :  1 compression ratio screw , a 3/20  mm ( diameter / length ) die , and a screw speed of 150  rpm . the effects of grits moisture ( m ) and extrusion temperature ( t ) were analyzed by surface response methodology using a factorial design 3 with triplicate of the central point , resulting in 11 runs for each rice variety . the levels of each variable were as follows : t : 160175190c and m : 1416.519% . rice grits samples were conditioned by adding water to reach the moisture level corresponding to each experimental sample , 1  h before each run . each extruded sample was obtained as soon as the stationary condition was reached , with torque and mass output simultaneously measured . these values were used to determine the specific mechanical energy consumption ( smec ) [ 23 , 24 ] using the following formula : smec ( j  g ) = k  t  n  qa , where k is : 61.6 10 , t is torque in brabender units ( bu ) , n is screw rpm , and qa ( g / min ) is the mass output , referring to feeding moisture level . all extruded samples were air - dried in an oven at 50c until a moisture content of 6% was reached , this moisture level being considered adequate for texture evaluation . each dried sample was divided into several portions and kept in plastic bags hermetically sealed until their evaluation . diameters were measured with a vernier caliper on ten pieces of sample , and axial expansion ( e ) was determined as the ratio e = d  d , where d is the extrudate diameter ( average of ten determinations ) and d is the die diameter . extrudate specific volume ( sv ) was obtained by calculating the volume / weight ( d    b ) ratio ( as cm / g ) , corresponding to an extrudate piece of about 15  cm long . an amount ( 150  g ) representative of each extrudate sample was first ground with a laboratory hammer mill ( retsch - muhle - germany ) using a 2  mm sieve and then with a cyclotec mill ( ud corp boulder colorado - usa ) using a 1  mm sieve . water solubility was done by dispersing 2.5  g of flour in 50  ml water , agitating during 30  min . and centrifuging at 2000  g ; soluble solids were obtained after evaporation in an oven at 105c and calculated as soluble solids 100  g of flour ( d    b ) . water absorption was determined using bauman method and expressed as ml of water / g of sample . one extruded sample from each rice variety was selected to evaluate flour dispersion viscosity , and rheograms were carried out for two solids concentrations ( 8 and 11% , w / w ) , at 60c , using an rv3 haake rotovisco viscometer ( germany ) . in every case , power law parameters k and n  (  = kg ) were estimated by regression . analysis of variance was carried out using the software statgraphics plus 3.0 , and the statistical differences among samples were determined using the lsd test . the values are similar to those reported in the literature for brown rice . 
 table 2 shows results of specific mechanical energy consumption ( smec ) , expansion , specific volume ( sv ) , water absorption ( wa ) , and solubility ( s% ) corresponding to the eleven experimental extruded samples and for the two rice varieties .  
 table 3 shows the degree of significance ( p value ) , corresponding to each term of the regression model obtained for each response . the lack of fit was not significant ( p > 0.5 ) , except for wa corresponding to paso 144 . it is observed that , for all responses , one or more terms of the regression model were significant ( p < 0.05 ) . linear terms m and t were significant in all cases , except for m for wa corresponding to paso 144 ( p < 0.6779 ) and t for solubility corresponding to both rice varieties ( p < 0.2544 for f and p < 0.1337 for p ) . quadratic terms ( m and t ) were significant only in the following cases : ( a ) m in expansion and wa for fortuna variety and in expansion for paso 144 variety ( b ) t in sv for fortuna and also in s% for paso 144 . smec surface response corresponding to each rice variety is observed in figures 1(a ) and 1(b ) . the surface corresponding to fortuna is planar , but that of paso 144 shows a curvature in the t direction . according to the anova results ( table 2 ) , only the linear terms ( h and t ) were significant ( p < 0.05 ) , but , in the case of paso 144 , the degree of significance of the term t is not negligible in both cases , smec was inversely related to both h and t , since as h and t increased , friction level in the extruder decreased and consequently smec decreased [ 23 , 36 ] . several authors working with others materials , such as wheat , quinoa , or barley , have found similar effect [ 32 , 3739 ] . smec values of both varieties were in similar range , although those corresponding to fortuna tend to be slightly higher , particularly at low m level , which is in agreement with the results of water solubility ( table 2 ) which is discussed later on this paper . since starch fraction of fortuna rice variety reached higher degree of cooking during extrusion ( higher s ) , energy dissipation ( smec ) moreover , the presence of fiber and amylose content would reduce smec , since when low and high amylose polished rice varieties are extruded , higher level of solubility was obtained . expansion response surfaces corresponding to each rice variety are observed in figures 2(a ) and 2(b ) . in the case of fortuna , the curvature in m direction is explained by the significance of the term m ( p < 0.0448 ) . while in the case of paso 144 , the expected plane is modified by the influence of the term m  t , whose significance is not negligible ( 0.0614 ) . both varieties show similar tendency as that of smec , supporting the idea that expansion rate is directly related to the degree of friction inside the extruder [ 23 , 40 ] . as it is expected , expansion values corresponding to both varieties are higher at the lower level of m , since this extrudate property is positively correlated with the elastic component of the melt coming out from the die , which would decrease as t or m increases . this inverse relationship between e and m was also found by hagenimana et al .   and have shown that a maximum of e exists at intermediate t ( 130150c ) . however , this temperature range is lower than that we used . specific volume has been proposed as a good indicator of degree of cooking ( dc ) for cereal extruded products , because dc is directly related to granule structure destruction . in most of the cases , response surfaces corresponding to each rice variety are observed in figures 3(a ) and 3(b ) . in the case of fortuna , the curvature in t direction is explained by the significance of the term t ( p < 0.0415 ) , which also can explain the high dependence of this response on temperature . in the case of paso 144 , the expected planar surface is modified by the term m whose significance is not too low ( p < 0.0968 ) . sv decreases as m increases , while an inverse effect is observed for t. this tendency could be explained taking into account that as m increases and t decreases , dc decreases and consequently sv decreases . similar tendency was also found for other materials such as barley and corn - bean mixtures [ 6 , 23 , 36 ] . moreover , gonzlez et al . have found that as the extrusion temperature increases , extrudate structure became lighter , as a consequence of the reduction of wall thickness of the pores . it is interesting to point out that the effects of t on sv is higher for fortuna than for paso 144 . this is in agreement with the results obtained by gonzlez et al .   which have shown that rice variety , containing lower amylose content , are more sensible to thermal effects than those of higher amylose content . water absorption values corresponding to paso are higher than those of fortuna . according to the anova results , the lack of fit corresponding to paso 144 was significant ( p < 0.0396 ) ; thus the regression model does not explain adequately the response variability , suggesting that some other factor , not controlled during the experiment , played some role .   for fortuna rice , wa was directly related to both variables and the highest value corresponded to the sample obtained at 190c and 19% ( figure 4 ) . there is an important difference in the tendency observed for the moisture effect , because other responses evaluated , decreased as m increased . this would suggest that wa is more affected by thermal effect than that of friction . it is known that as extrusion conditions minimize friction effects , destruction of granular structure is also minimized and consequently wa is improved . s values corresponding to paso were significantly lower than those of fortuna , which is in agreement with the results obtained by other authors , who have found an inverse relation between amylose content and solubility [ 6 , 8 , 19 , 29 , 32 ] . moreover , it is also important to point out that as dc increases , s increases as a consequence of starch granule destruction , implying a reduction of wa . solubility ( s ) response surfaces corresponding to each rice variety are observed in figures 5(a ) and 5(b ) .   in the case of fortuna , the surface is almost a plane and s is only affected by m , suggesting that friction effects are the most important ones . as it was observed by other authors [ 23 , 42 ] , s is directly related with smec and sv . again , the lower amylose content of this variety could explain the high sensitivity to friction .   in the case of paso 144 , the surface shows a curvature in t direction , as it is expected , since t term was highly significant ( p < 0.00112 ) . as it was observed for wa , s decreases as m increases and similar explanation could be done regarding friction effects . the highest s values corresponded to samples obtained at low m level ( 14% ) and at 160c and 190c . the existence ( at any m level in the range of this study ) of minimum values of s , at intermediate t level ( 175c ) , is difficult to explain . s results suggest that the behavior of both varieties inside the extruder would be different . on one hand , fortuna shows much higher s values than paso 144 , indicating that a higher degree of cooking was reached by fortuna rice . moreover , in the extrusion of rice grits obtained by milling white rice ( without the germ and the bran ) and for similar extrusion conditions , s values were much higher than those obtained in this study , suggesting that lower degree of cooking is obtained when grits from whole grains is used in comparison with those coming from degermed and dehulled grains . regarding that , pan et al .   reported that the addition of oil and bran to rice grits caused a reduction of energy consumption , indicating a reduction of degree of cooking .   beside that , dc is , in general , directly related to t ; in our case , it is true only at t higher than 175c and for any moisture level . it seems that an increase of t from 160c to 175c produces a reduction of dc ( lower s ) , so the effect of the reduction of friction caused by this increase of t would be higher than the thermal effect on dc . then , above 175c the relative importance of these two effects is inversed and the expected effect of t is verified . these results would indicate that the presence of germ ( oil ) and bran ( fiber ) could reduce the dc reached by the extrudate . this was confirmed by sample observation under microscope , which revealed the presence of native starch in all paso samples . even though no quantitative determination was done , the proportion of native starch present in samples corresponding to paso 144 was much higher than that corresponding to fortuna ; moreover , no native starch was observed in fortuna samples obtained at low - level moisture ( 14% ) . this incomplete cooking process is attributed to perturbation of the particles transport inside the extruder that not only could retard the cooking process of starchy particles caused by friction , but also could broaden the residence time distribution of particles inside the extruder . so , some particles would reach the die very fast , without suffering much change ( remaining in native state ) . this interference on the cooking process of starch caused by the presence of oil and fiber could be overlapped by changing the extrusion condition in order to increase dc , for example , reduce die diameter , increase screw compression ratio , reduce moisture content , and reduce particle size . since the milled extrudate could be used as a base for gruel and cream soup formulations , dispersion viscosity becomes an important factor to be taken into account . the evaluation of dispersion viscosity was done on the samples obtained at 14% and 160c , since these extrusion conditions permitted the highest dc . viscosity was obtained from rheograms at 135  s shear rate according to prez et al . since this level of shear rate would be in range of that exerted inside the mouth when a fluid food is ingested .   table 4 shows the results of extrudate flour viscosity evaluation : power law parameters ( k and n ) and regression coefficient obtained from the rheograms made for each solid concentration ( 8% and 11% ) and for each rice varieties . it is observed that power law model adjusted adequately the rheograms data ( r > 0.994 ) . dispersion corresponding to fortuna variety showed lower viscosity than paso , indicating that samples having lower degree of cooking give thicker dispersion . beside that , a direct correlation was observed between water absorption and viscosity ( r > 0.945 ) , as it was expected , since water absorption is an indicator of dispersion thickness . these results are in agreement with those obtained in other works [ 3 , 43 ] . low amylose variety showed values of smec , sv , and s higher than those of higher content ; on the contrary , wa was lower , indicating that the last one reached lower degree of cooking . regarding the extrusion conditions , 14% m and 175c could be selected to obtain a good expanded product , with high expansion and intermediate degree of cooking . since degrees of cooking reached by both rice samples ( under the same extrusion condition ) were lower than those obtained using grits from polish rice , it is suggested that the presence of germ and bran interferes the cooking process by both decreasing friction level and by broadening residence time distribution .   the higher protein , fiber , and mineral content of brown rice in comparison to polish rice implies nutritional advantages , so the use of brown rice to produce snacks products is highly recommended .
the utilization of whole grains in food formulations is nowadays recommended . 
 extrusion cooking allows obtaining precooked cereal products and a wide range of ready - to - eat foods . 
 two rice varieties having different amylose content ( fortuna 16% and paso 144 , 27% ) were extruded using a brabender single screw extruder . 
 factorial experimental design was used to study the effects of extrusion temperature ( 160 , 175 , and 190c ) and grits moisture content ( 14% , 16.5% , and 19% ) on extrudate properties . 
 specific mechanical energy consumption ( smec ) , radial expansion ( e ) , specific volume ( sv ) , water absorption ( wa ) , and solubility ( s ) were determined on each extrudate sample . in general , 
 fortuna variety showed higher values of smec and s ( 703409 versus 637407  j / g ; 33.021.0 versus 20.111.0% , resp . ) 
 than those of paso 144 ; on the contrary sv ( 8.643.47 versus 8.274.53  ml / g ) and wa tended to be lower ( 7.75.1 versus 8.46.6  ml / g ) . 
 both varieties showed similar values of expansion rate ( 3.602.18 ) . 
 physical characteristics depended on extrusion conditions and rice variety used . 
 the degree of cooking reached by paso rice samples was lower than that obtained for fortuna . 
 it is suggested that the presence of germ and bran interfered with the cooking process , decreasing friction level and broadening residence time distribution .