A Survey of the Number and Types of Aerobic Mesophilic Spores in Milk Before and After Commercial Sterilization

SUMMARY: Milk, before and after commercial sterilization, supplied by ten different sterilizing depôts, was examined over a period of 18 months to determine the aerobic mesophilic spore content. The numbers present in raw bulk tanker milk exhibited a seasonal variation, the counts being higher in winter than in summer. This was not found in sterilized milk. Spore counts in raw milk ranged from 0—700/100 ml and in sterilized milk, with one exception, from 0—1·1/100 ml. During the last 12 months of the experiment the types of aerobic mesophilic spore forming organisms causing spoilage were isolated and identified, using a simplified method of identification. Eight different species were isolated from raw milk and seven from sterilized milk. The incidence of the different types in raw milk was: B. licheniformis , 62%; B. brevis , 15%; B. subtilis , 7%; others 9% and unidentified, 7%. In sterilized milk this was: B. subtilis , 44%; B. licheniformis , 26%; others, 17% and unidentified, 13%. There was no apparent seasonal variation in types.