Alpha Amylase is a major mash enzyme of critical concern to brewers in their production of fermentable wort. It digests starch, a large polymer of glucose, into smaller units, exposing it to further digestion by beta amylase. Together these two amylases produce the spectrum of wort sugars essential in the production of a beer. Alpha amylase is an endo enzyme mainly digesting the alpha 1–4 bonds of starch at points within the chain, not at the ends.
To focus on the use of alpha amylase in brewing, it is necessary to look at the needs of a successful mash, in particular the spectrum of sugars required in the final wort. Ideally these should be a suitable balance of simple fermentable sugars—glucose, maltose, and maltotriose—and larger unfermentable dextrins roughly in a 3:1 proportion. Unlike wine, where virtually all of the sugars are fermented, beer is distinct in having residual sugars to provide sweetness, body, and mouthfeel. Dextrins contribute strongly to this and give beer a major part of its character.
A starch molecule is, in essence, a group of glucose molecules linked together. Enzymes break those links. Alpha amylase contributes to the digestion of starch by breaking internal bonds between glucose molecules. As a result it opens up the starch molecule, breaking it into a range of intermediate sizes. Beta amylase further digests these intermediate molecules mostly into maltose—a sugar of two glucose units—but also to glucose itself and to the three-glucose molecule maltotriose. The major limitation to this digestion is the side bonds of starch amylopectin, which are not digested by either alpha or beta amylase. The parts of the starch molecule containing these side bonds form the basis of the important unfermentable dextrins produced by mashing.
The alpha amylase used in the mash comes from the malt, where it is entirely produced in the aleurone layer during malting.
Levels of alpha amylase are typically high in pale malt but are virtually zero in roasted malt due to heat degradation. Levels vary according to malt variety and to malting conditions. Generally six-row barleys have higher levels than two-row barleys due to grains being smaller with less endosperm in proportion to aleurone.
Alpha amylase is not restricted to barley but occurs in most organisms from bacteria to humans. Salivary amylase, ptyalin, is a well-known amylase that initiates starch digestion in the mouth of mammals.
Enzymes tend to have specific temperature and pH ranges at which they will be active—this range is referred to as “optima.” Alpha amylase has a significantly different temperature and pH optima than beta amylase. For alpha amylase the temperature optima is higher at around 70°C compared to 60°C–65°C for beta amylase as the enzyme may be stabilized by calcium ions. The pH optima of alpha amylase is also higher at 5.3–5.7 compared to 5.1–5.3 for beta amylase. These differences can result in different wort sugar profiles from mashes conducted at different temperatures and are one means of varying beer character by control of mash conditions.
In traditional breweries, all the enzymes needed for brewing are contained within the natural ingredients out of which the beer is made. However, exogenous alpha amylase is available in purified form from enzyme suppliers and may have different properties according to its origin. These are widely used in the production of “light beers.”
Commercial alpha amylases may also cause problems if they are impure and contain beta amylase and proteases or if they contain toxins from their bacterial or fungal growth. However, their use is growing in many food industries and will continue to have application in brewing, particularly if novel ingredients are sourced for future beers.