Mashing is the process in which the milled grain is mixed with water. This activates enzymes that were already present in the barley seed or have been formed during the malting process. These enzymes work best in particular temperature and pH ranges. By varying the temperature of the mash, the brewer has control over the enzyme activity. In essence, mashing needs to been seen as an extension of the malting process. The following sections describe the enzymes that are more or less important for the mashing process.
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Starch converting enzymes (Beta Amylase, Alpha Amylase, Limit Dextrinase)
Starch conversion is the most important aspect of mashing. In barley starch makes up 63% - 65% of the dry weight. Brewer's yeast can only ferment monosacharides (glucose, ???), disacharides (maltose, ???) and trisacharides (matotriose, ???). The latter is only completely fermentable by lager yeas strains (s. uvarum). But the malt contains mainly starch which is a polysaccarid (very large chanis of glucose) that is insoluble in water.
In order for that starch to be converted into water soluble sugars (fermentable and unfermentable), two processes need to happen. First the starch is gelatenized to become water soluble. For starch found in barley and wheat malt this happens above 140 *F (60 *C). Other starches (rice for example) gelatinizes only above 194 *F (90 *C) and require boiling (see cereal mash) before they can be converted by enzymes. Secondly the amylase enzymes break the long chained starch into shorter chains.
The starch found in malt is composed from Amylase and Anylopectin. Amylase is a single chain of glucose molecules liked between their 1st and 4th carbon atom (called 1-4 link). It
This is from How to brew and needs replacing or a acknowledgement adding... It's just an example and place holder really.
Beta amylase works by hydrolyzing the straight chain bonds, but it can only work on "twig" ends of the chain, not the "root" end. It can only remove one (maltose) sugar unit at a time, so on amylose, it works sequentially. (A maltose unit is composed of two glucose units, by the way.) On an amylopectin, there are many ends available, and it can remove a lot of maltose very efficaciously (like a hedge trimmer). However, probably due to its size/structure, beta cannot get close to the branch joints. It will stop working about 3 glucoses away from a branch joint, leaving behind a "beta amylase limit dextrin."
