protein rest. (pro'-teen rehst)
A stage of the mashing process during which complex proteins are decomposed by proteolytic enzymes to progressively less complex fractions. The stability of finished beer is largely established during the protein rest. Nutrients required for proper fermentation also are developed during this process. The proteolytic enzymes become active at temperatures from 113-140 °F (45-60 °C). The protein rest should be employed for a period of twenty to thirty minutes when using undermodified malts.
It is important to note that some unbroken protein is necessary for head retention. There are head enhancing agents that can be added, heading malts and high alpha hops (which are also bittering) that will improve head retention. There is also the option of cutting down on or cutting out protein rest time.