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Acid rest is a step that can be used in the mashing process to reduce the pH of the mash. It is performed by holding the mash at an initial temperature of 95 Fahrenheit (35 Celsius).
At this temperature rest, the enzyme phytase is able to convert the protein phytin present in the malt into calcium, magnesium phosphate, and phytic acid. This step is traditionally used to enable the brewing of pale lagers with water with low levels of carbonate and calcium.