Principle And Interpretation
Skim Milk Agar is used for the demonstration of coagulation and proteolysis of casein (1). The medium is recommended by
APHA (2) for cultivation and enumeration of microorganisms encountered in dairy industry (3). Addition of skim milk to any
nutrient-rich medium creates favourable conditions for growth of organisms, which are encountered in milk. The number of
bacteria isolated thus is more than the number of organisms isolated on a regular medium (4). Proteolytic bacteria hydrolyze
casein to form soluble nitrogenous compounds indicated as clear zone surrounding the colonies. More clear zones are seen on
milk agar if, the bacteria produce acid from fermentable carbohydrates in the medium.
Casein enzymic hydrolysate provides amino acids and other complex nitrogenous substances. Yeast extract supplies vitamin B
complex. Addition of skim milk in the medium makes the conditions optimal for microorganisms encountered in milk. Glucose
acts as the carbon source.