Skim Milk Agar (SM agar for Skim Milk Agar) is used in food bacteriology for the enumeration of aerobic bacteria in dairy products. It is also used for the enumeration of psychrotrophic microorganisms. This medium is used to determine proteolysis by microorganisms and is recommended for the culture and confirmation of Pseudomonas aeruginosa in recreational water analysis as well as in the food and dairy industry.

The hydrolysis of casein (a primary milk protein) is often used to assess the proteolytic activity of organisms. The enzyme caseinase hydrolyses casein protein. Milk agar contains casein and glucose as carbon sources to promote growth. A yeast extract is added to the medium as a source of vitamins. Microorganisms such as Pseudomonas aeruginosa hydrolyze the casein to form soluble nitrogen compounds that form a clear zone surrounding the colonies. Clearer areas are observed on milk agar if the bacteria produce acid from fermentable carbohydrates in the medium.