A Protein Rest is a period of enzyme activity during mashing when excess protein is removed and digested. It is typically part of a series of temperature holds arranged in a sequence to ensure progressive digestion of beta-glucans, proteins, and starches.

Not all mashes require a protein rest, but if poorly modified malt or high protein adjuncts are used, excess protein is likely to be released into the wort, leading to possible hazes in finished beer.

Removing protein from wort involves both precipitation and enzyme digestion. A temperature of 119^oF (43^oC) is optimal for these and results in the removal of intact proteins in a precipitate and their breakdown into polypeptides and amino acids.

This digestion is most effectively achieved using natural protease enzymes derived from the barley malt. Whereas the endo-proteinases act best at relatively low temperatures of around 119^oF (43^oC), the exo-peptidases (notably carboxypeptidase) are much more heat tolerant and are able to act at the higher temperatures used for saccharification.

It is the endo-peptidases whose survival would be facilitated at the lower temperatures, but there is a school of thought that says these enzymes cannot act because they are blocked by endogenous inhibitors from the grist. Accordingly it has been suggested that the term “protein rest” is a misnomer and that it should be called a “beta-glucan rest” because it is the breakdown of beta-glucans by heat-sensitive beta-glucanases that is much more relevant at these lower temperatures. Many brewers seek a middle ground between the optimum temperatures for peptidase and proteinase activity, settling on a rest temperature of 122^oF (50^oC), with the typical stand lasting from 10 to 20 minutes. Yet other brewers feel that given today’s well-modified malts, the protein rest is an unnecessary anachronism. As in many other areas of brewing, actual practice is often assembled from a blend of theory, observation, and tradition, with each brewer deciding individually upon what he thinks works best for his beer and his brewing system.