Enzymes that work on proteins and peptides by hydrolysing the peptide bond. Broadly divided into two groups: proteases, which randomly degrade the peptide bond of proteins to create various peptides (also called proteinase); and peptidase, which breaks down proteins and peptides from the extremity into amino acid units. They are further classified by active site and the range of pH at which they are most effective.

In sake brewing, acid protease and acid carboxypeptidase (which are effective in an acidic environment) carry out important tasks. Acid protease, as well as producing peptides by breaking down the proteins in rice, also releases α-amylase which is kept ineffectively adsorbed within the protein of rice. This increases rice solubility.

Measuring acid protease is done by reacting the enzyme sample and casein solution together for a fixed time, then measuring colour development of the solubilized peptides with a phenol reagent and converting the figure so gained into tyrosine.