Saccharification (Starch conversion) is the most important aspect of mashing. In barley starch makes up 63% - 65% of the dry weight. Starch is a polysaccharide (very large chains of glucose) which is insoluble in water. Brewer's yeast, however, can only ferment monosaccharides (glucose, fructose), disaccharides (maltose, sucrose) and trisaccharides (matotriose). The latter can only be completely fermented by lager yeast strains (s. uvarum).
Saccharification during the mash is achieved by the activation of malt enzymes at the correct temperatures and moisture levels. To be susceptible to digestion by enzymes, the starches in barley malt must first be gelatinized. Barley malt starches gelatinize at temperatures between 61°C and 65°C (142°F and 149°F). Most adjunct starches, such as corn grits or rice, require higher temperatures for gelatinization and are therefore cooked separately before being added to the mash for saccharification. Once the starches are gelatinized, they are broken down by beta amylase and alpha amylase into sugars, principally maltose.
Alpha amylase is primarily responsible for the hydrolysis of starches into dextrins, and beta amylase digests dextrins into fermentable sugars. The enzymes themselves are rapidly denatured by higher temperatures. At 65°C (149°F), beta amylase is almost completely deactivated with 30 minutes, whereas alpha amylase survives somewhat longer. The time period and temperature(s) at which the mash is held to effect saccharification is called a “saccharification rest.” This temperature is a compromise between the higher temperatures required for starch gelatinization and the lower temperatures that will preserve the activity of the malt enzymes. This rest usually lasts from 30 to 60 min, depending on the enzymatic power of the malt used. Lower saccharification temperatures will favor the production of fermentable sugars by beta amylase, whereas higher temperatures will favor the production of unfermentable sugars and dextrins by alpha amylase. It is therefore possible to manipulate the sugar profile and fermentabilty of the wort through the temperature of the saccharification rest. This will, in turn, help determine the residual sweetness and body of the resulting beer.
Distillers try to convert all starches to fermentable sugars to increase alcohol yield. Unfermented sugars in a wash do not carry over to the final product and therefor are wasted.