Mashing is the process that converts starch from grain into sugar that the yeast can use. Starch consists of long chains of sugar units. In the mashing reaction, called hydrolysis, water molecules are inserted between the sugar units, breaking them off from the chain. The hydrolysis reaction goes under the influence of enzymes provided by the malt. The enzymes prepare the water molecules and organize the starch molecules to allow hydrolysis to occur under moderate conditions.

Mashing is initiated by mixing the milled malt (grist) with warm water (140-158 F = 60-70 C). In modern brewing, mashing typically takes about an hour, but certain European beers are mashed by a multistep process that can take six hours.

This topic is covered in The Chemistry of Beer: The Science in the Suds, Chapter 7.