Differentiation of organisms based on coagulation and proteolysis of casein

Proteolytic bacteria use the enzyme caseinase to hydrolyze casein and form soluble nitrogenous compounds displayed as a clear zone around colonies. This clearing is much more pronounced on agar containing milk if the bacteria are able to produce acid from fermentable carbohydrates in the medium. Moreover, the hydrolysis of casein is often used to evaluate proteolytic activity of psychrotrophic microorganisms of significance to food, water, and dairy industries.