Chemical Changes at Mashing
During mashing the grist is suspended in water and the mixture heated. Many of the components of wort arise at this time by simple solution. Others are formed as a result of reactions between malt-derived enzymes and their substrates, which become possible because of the provision of an aqueous environment and the presence of an appropriate combination of pH, temperature, and ions—cations and anions. The mashing process is conducted over a period of time at various temperatures in order to activate various enzymes. Enzymes are biological catalysts responsible for initiating specific chemical reactions.
The acid rest is responsible for reducing the initial mash pH (4.8–5.2) for traditional decoction mashing of lager beers. Acidification of the mash is primarily done by the enzyme phytase, which is active at 30 to 53 degrees C (86–127°F) and breaks down insoluble phytin, a complex organic phosphate containing both calcium and magnesium, to phytic acid. Phytic acid has a strong affinity for calcium ions, and it forms calcium phosphate and releases hydrogen ions in the process.
Although the term “protein rest” has long been adopted for low-temperature mashing systems, most of the protein degradation occurs during the malting process, not mashing.
Proteinase is responsible for degrading albumins already dissolved in the wort, along with insoluble globulins, into simpler medium-sized proteins (peptones and polypeptides).
The enzyme peptidase is responsible for degrading medium-sized proteins (peptones and polypeptides) into smaller proteins (peptides and amino acids).
By far the most important change brought about in mashing is the conversion of starch molecules into fermentable sugars and unfermentable carbohydrates called dextrins. The principal enzymes responsible for starch conversion are alpha- and beta-amylase. Together, alpha- and beta-amylase are capable of converting only 60 to 80 percent of the available starch to fermentable sugars, which is mostly maltose and maltotriose (Dougherty, 1977).
Alpha-amylase very rapidly reduces insoluble and soluble starch by splitting complex starch molecules into many shorter chains (i.e., partially-fermentable polysaccharide fractions—dextrins and maltotriose) that can be attacked by beta-amylase.
Beta-amylase is the other mash enzyme capable of degrading starch. Beta-amylase is more selective than alpha-amylase since it breaks off two sugars at a time from the starch chain.
Stages of Starch Breakdown
The primary function of mashing is the conversion of unfermentable starch into fermentable sugars such as glucose, maltose, and maltotriose. Starch degradation during the mashing occurs in three distinct stages and can be distinguished in the enzymatic breakdown of starch: gelatinization, liquefaction, and saccharification.
So, given that alpha- and beta-amylase have different optimum temperatures the brewer will often slowly ramp the temperature of the mash from 50 to 78 degrees C (120–172°F). This ensures that each enzyme becomes activated at its optimum temperature as the process ensues. In addition, the ramping of the temperature allows the brewer to verify that each of the steps has been reached.
Iodine Test for Saccharification
During mashing, complex carbohydrates need to be hydrolyzed into sugars that can be consumed by yeast, such as glucose, maltose, and maltotriose. This protocol describes how to check if the starch has been sufficiently hydrolyzed.
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