A diastase is any one of a group of enzymes that catalyze the breakdown of starches into maltose. Diastase only facillitates and does not cause this breakdown. However, the speed of this reaction is negligibly small without the presence of diastase. Diastases are present in malted grains and also in the pancreas, and they are very important in the process of brewing beer.
According to Princeton University, Diastases were the first enzymes discovered. Diastases are important in the brewing process because there are no sugars present at the beginning of it. Malted grain is initially composed primarily of starch, which yeast cannot ferment.
Two diastases are key in the brewing process: alpha and beta amylase. Alpha amylase is most viable at temperatures between 131 and 150 degrees Fahrenheit, and it converts starches to maltose. Beta amylase is most viable at temperatures between 154 and 162 degrees Fahrenheit, and it produces maltose along with other sugars that are less fermentable. Brewers use a process called mashing, which involves mixing milled grain with water in these temperature ranges. This produces a liquid full of fermentable sugars called "wort."
Brewers often use the properties of these two enzymes to help control the character of their beers. Mashing at a higher temperature associated with beta amylase tends to produce a partially unfermentable wort, and the end result is a sweeter beer with a thicker body. Meanwhile, a lower temperature mash in alpha amylase's optimum range produces a highly fermentable wort and an accordingly drier beer. However, choosing a mash temperature is not a black and white issue. These enzymes are viable outside of their optimum temperatures, so it is possible to produce wort along a spectrum of fermentability.