Microbial Determinations

To assess the presence and extent of microbial contamination in food, it is standard practice to perform several types of microbial determinations. These determinations are important because microorganisms from foods can cause such diseases as staphylococcal food poisoning, salmonellosis, typhoid fever, cholera, and gastroenteritis.

The standard plate count. One method for determining the number of bacteria in foods is the standard plate count. The procedure is performed by taking a gram of food sample and diluting it in 99‐ml bottles of sterile water or buffer in the method described in the chapter on aquatic microbiology. The number of bacterial colonies is multiplied by the dilution factor to determine the number of bacteria per gram of food.

Coliform and fungal determinations. In food, it is valuable to assess the number of coliform bacteria. These Gram‐negative intestinal rods do not cause disease but are valuable indicators of fecal contamination. Presumably, when coliform bacteria are present, the food has been contaminated with fecal matter and is unfit to consume. The coliform most commonly sought is Escherichia coli Various types of bacteriological media, such as violet red bile agar, are available for cultivating this bacteria, and the standard plate count technique can be performed to assess the number of coliform bacteria per gram of food.

Fungi can be assessed in foods by using a medium such as Sabouraud dextrose agar. This medium encourages fungal spores to germinate and form visible masses of filaments called mycelia. A count of the resulting mycelia gives an estimate of the fungal contamination of the food.

The phosphatase test. To determine contamination in milk, the plate count technique can be used, but a more rapid test is the phosphatase test (Figure ). Phosphatase is an enzyme destroyed by the pasteurization process. However, if the test for phosphatase shows that it is present after the milk has been treated, then the pasteurization has been unsuccessful. Testing for phosphatase is a more rapid and efficient method for determining contamination than the plate count technique.

Figure 1

The phosphatase test. The substrate sodium diphenyl phosphate is added to a pasteurized milk sample, and the tube is incubated. Then the dye reagent is added. If phosphatase is present, the tube contents turn blue. However, they remain clear if phosphatase was destroyed during the pasteurization process.