How the Mash Works
The Protein Rest and Modification
Modification is the term that describes the degree of breakdown during malting of the protein-starch matrix (endosperm) that comprises the bulk of the seed. Moderately-modified malts benefit from a protein rest to break down any remnant large proteins into smaller proteins and amino acids as well as to further release the starches from the endosperm. Fully-modified malts have already made use of these enzymes and do not benefit from more time spent in the protein rest regime. In fact, using a protein rest on fully modified malts tends to remove most of the body of a beer, leaving it thin and watery. Most base malt in use in the world today is fully modified. Less modified malts are often available from German maltsters. Brewers have reported fuller, maltier flavors from malts that are less modified and make use of this rest.
Malted barley also contains a lot of amino acid chains which form the simple proteins needed by the germinating plant. In the wort, these proteins are instead utilized by the yeast for their growth and development. Most wort proteins, including some enzymes like the amylases, are not soluble until the mash reaches temperatures associated with the protein rest (113-131°F). The two main proteolytic enzymes responsible are peptidase and protease. Peptidase works to provide the wort with amino acid nutrients that will be used by the yeast. Protease works to break up the larger proteins which enhances the head retention of beer and reduces haze. In fully modified malts, these enzymes have done their work during the malting process.
The temperature and pH ranges for these two proteolytic enzymes overlap. The optimum pH range is 4.2 - 5.3 and both enzymes are active enough between 113 - 131°F that talking about an optimum range for each is not relevant. This optimum pH range is a bit low with respect to most mashes, but the typical mash pH of 5.3 is not out of the ballpark. There is no need to attempt to lower the mash pH to facilitate the use of these enzymes. The typical Protein Rest at 120 - 130°F is used to break up proteins which might otherwise cause chill haze and can improve the head retention. This rest should only be used when using moderately-modified malts, or when using fully modified malts with a large proportion (>25%) of unmalted grain, e.g. flaked barley, wheat, rye, or oatmeal. Using this rest in a mash consisting mainly of fully modified malts would break up the proteins responsible for body and head retention and result in a thin, watery beer. The standard time for a protein rest is 20 - 30 minutes.
The other enzymes in this temperature regime are the beta-glucanases/cytases - part of the cellulose enzyme family, and are used to break up the beta glucans in (un)malted wheat, rye, oatmeal and unmalted barley. These glucan hemi-celluloses (i.e. brambles) are responsible for the gumminess of dough and if not broken down will cause the mash to turn into a solid loaf ready for baking. Fortunately, the optimum temperature range for the beta glucanase enzyme is below that for the proteolytics. This allows the brewer to rest the mash at 98 -113°F for 20 minutes to break down the gums without affecting the proteins responsible for head retention and body. The use of this rest is only necessary for brewers incorporating a large amount (>25%) of unmalted or flaked wheat, rye or oatmeal in the mash. Sticky mashes and lauters from lesser amounts can usually be handled by increasing the temperature at lautering time (Mashout). See Chapter 17 - "Getting the Wort Out - Lautering" for further discussion.