Psychrotrophic Microorganisms: Agar Plate Methods, Homogenization, and Dilutions

Isolation and enumeration of the microorganisms present in foods usually demands preliminary treatment of samples to release into a liquid medium those microorganisms present that may be included within the food. In a mixing procedure known as “stomaching,” the food sample (see Note 1 ) and diluent are put into a sterile plastic bag that is vigorously struck on its outer surfaces by paddles inside a stomacher, the compression and shearing forces break up solid pieces of food. After samples are removed for analysis, the bag and its remaining contents can be discarded and the equipment is ready for use. If a stomacher is not available, an electric blender with cutting blades revolving at high speed can be used (see Note 2 ).