dedicated vessels. Mash tuns have a powerful stirring mechanism, known as a mash rake, to keep the temperature of the mash uniform. They also have an efficient heating method, often based on steam, that will not scorch the malt. This heating method is combined with proper insulation enabling mash to maintain rest temperatures for up to one hour inside the vessels. A spray ball for clean-in-place (CIP) operation helps with periodic deep cleaning. Sanitation is not a major concern before wort boiling, so a rinse-down is usually all that is necessary between batches. Smaller breweries often use a boil kettle or a lauter tun for mashing. Using a lauter tun limits the breweries to single-step infusion mashing, however, because such a vessel is not completely appropriate for the lautering process[cite?]. Mashing-in Mixing of the strike water used for mashing in and milled grist must be done in a way that minimizes clumping and oxygen uptake. This was traditionally done by first adding water to the mash vessel and then introducing the grist from the top of the vessel in a thin stream, but this led to a lot of oxygen absorption and loss of flour dust to the surrounding air. A premasher, which mixes the grist with mash-in temperature water while it's still in the delivery tube, reduces oxygen uptake and prevents dust from being lost. Mashing in – sometimes called "doughing-in" – is typically done between , but for single-step infusion mashes, mashing in must be done between for amylases to break down the grain's starch into sugars. The weight-to-weight ratio of strike water and grain varies from one-half for dark beers in single-step infusions to one-quarter or even one-fifth ratios that are more suitable for light-colored beers and decoction mashing, where much of the mash water is boiled off. Enzymatic rests In step infusion and decoction mashing, the mash is heated to different temperatures to allow specific enzymes to work optimally. The table at right shows the optimal temperature ranges for key enzymes and what materials those enzymes break down. There is some contention in the brewing industry as to the optimal temperatures for these enzymes, as it is often very dependent on the pH of the mash and its thickness. A thicker mash acts as a buffer for the enzymes. Once a step is complete, the enzymes active in that step are denatured by the increasing heat