Heat Resistance of Alkaline Phosphatases Produced by Microorganisms Isolated from California Mexican-Style Cheeses

Samples of Queso Fresco produced in California were tested for microbiological quality and alkaline phosphatase activity. Total plate counts ranged from 6 to 8 logs for freshly purchased cheeses. The majority of bacteria were nonlactic psychrotrophs. Coliform and enteric counts were greater than or equal to six logs in all samples. Colonies isolated on Standard Methods Agar were inoculated into a variety of media and incubated at 20C. Each culture was tested for alkaline phosphatase activity by the Scharer test. The resistance of each phosphatase-posirive culture to a heat treatment of 63C for 30 min was also determined. The degree of enzyme inactivation varied between cultures when different growth media were used. Alkaline phosphatases sensitive to pasteurization were produced by several isolates under culture conditions. The Scharer test may not be a reliable indicator of pasteurization in Queso Fresco cheeses.