A protein-digesting enzyme (see protease) that exhibits maximum activity and stability in acid conditions (pH 2.0–5.0) and is inactivated at pH values above 6.0. Acid proteases have a low isoelectric point and are low in basic amino acids. Two types are widely used in the food and beverage industries: those from Aspergillus, which resemble pepsin; and those from Mucor, which resemble rennin.