Casease is a bacterial enzyme that breaks down casein, the principal protein of milk. Certain bacteria produce it during fermentation and spoilage, hydrolysing casein into smaller peptides and amino acids. In cheesemaking, controlled proteolysis by rennet and starter-culture enzymes develops flavour and texture during ripening; excessive or uncontrolled casein breakdown causes bitterness and defects. In the laboratory, casease activity is detected by the clearing of opaque casein in agar plates, a classic test for identifying proteolytic bacteria. The enzyme illustrates how the same biochemical action is desirable in ripening cheese but a spoilage mechanism in fresh milk.