Discovered by French scientist Anselme Payen in 1833, diastase is a complex of malt enzymes that degrade starch in a limited manner during malting but extensively during mashing. Extensive diastase degradation of starch during mashing occurs because starch is gelatinized and solubilized. Mashing and cooking temperatures cause gelatinization and solubilization of starches. The products of diastase degradation of starch are fermentable sugars and dextrins. Malting increases the diastase complex of cereal grains. Enzymes of the diastase complex are alpha amylase, beta amylase, limit dextrinase, alpha glucosidase, and phosphorylase. See alpha amylase. During malting, alpha amylase and limit dextrinase are produced in the aleurone layer and beta amylase is activated in the starchy endosperm.

In standard brewing and distilling practices alpha amylase, beta amylase, and limit dextrinase are mainly responsible for the production of the fermentable sugars found in brewers’ worts and distillers’ washes. Alpha amylase will attack un-gelatinized starch in a limited manner by pitting during malting. During mashing the diastase complex of enzymes will attack gelatinized/solubilized starch quickly to produce the glucose, maltose, maltotriose, maltotetrose, and dextrins found in the wort.

Alpha amylase reduces starch viscosity rapidly, producing a mixture of sugars and dextrins; beta amylase releases mainly maltose and limit dextinase debranches the amylopectin fraction of solubilized starch. The actions of alpha amylase and limit dextrinase facilitate the maltose producing action of beta amylase during mashing. This results in increased fermentability of the wort. Alpha amylase activity is analyzed as dextrinising units (DU) and beta amylase potential is analyzed as diastatic power (DP). DP levels do not reflect alpha amylase levels.