Heat Resistant Psychrotrophic Bacteria in Raw Milk and Their Growth at 7 C

Microbiological data are presented for 109 raw milk samples, and for the same samples after heat treatment at 80 C for 12 min, and upon subsequent storage at 7 C for 7, 14, and 28 days. The Standard Plate Count of the raw milk averaged 110,000/ml with 65% of these organisms being penicillin-resistant. Immediately after heat treatment, 87% of the samples had psychrotrophic spore counts< 10/ml. After 14 and 28 days of storage, 50 and 83% of the samples had psychrotrophic counts ≥ 100,000/ml. It was concluded that growth of heat-resistant psychrotrophic organisms may cause spoilage of heated milk. No relationship could be demonstrated between gram-negative counts of raw milk, or initial mesophilic spore counts of heated milk, and bacterial numbers in heated, stored milk.