A protein rest is done before a saccharification rest (resting your mash in the 148-158 °F range for ~60-90 minutes) by bringing your mash to 122-131 °F for ~20 minutes. Most malts do not require the use of a protein rest, as they have been well modified (a high degree of of breakdown during malting of the protein-starch matrix in the malted grain).
Under-modified barley malt, or grists with a large percent (more than 25%) of wheat, oatmeal, unmalted grain, flaked barley or rye, will benefit from a protein rest. It will break down proteins and make the starches more accessible for conversion as well as making for a more effective runoff by breaking the large, gelatinous proteins into smaller chains.
It also reduces chill haze-causing albumins and creates amino acids that the yeast can use for their growth and development. Using a protein rest on a grist that does not require one (with well modified malts), will contribute to a thin, watery beer.
You can perform a protein rest in a couple different ways:
- If your mash tun can be directly heated, you can apply heat to move the mash from the initial protein rest to the saccharification rest, but be sure to stir the mash as you do so to prevent scorching.
- The other way to do this multi-rest mashing is to start with a thicker mash, giving yourself room to add a small amount of boiling water to raise the mash temperature to the desired saccharification rest temperature.