Incidence and identification of psychrotrophic (7 degrees C-tolerant) Bacillus spp. in German HTST pasteurized milk

The psychrotrophic (7 degrees C-tolerant) Bacillus flora of German commerci al HTST pasteurized milk was analyzed using the most probable number method . For reliable results, a volume of 100 ml had to be included and an incuba tion period of 3 weeks was necessary. Psychrotrophic aerobic sporeformers w ere detected in all samples. In both winter milk and summer milk, B. circul ans represents the predominant species, followed by B. weihenstephanensis. While the number of psychrotrophic bacilli showed no significant seasonal v ariation, the variety of different strains isolated from summer milk was la rger compared to winter milk. The number of morphotypes within one species can differ significantly between batches of milk. This phenomenon might be a hitherto unrecognized factor contributing to the sudden appearence and di sappearence of rapid spoilage of pasteurized milk. In prethermised HTST pas teurized milk, the level of psychrotrophic bacilli was lower than in non-pr ethermized milk, but B. circulans and B. weihenstephanensis were also found to be the predominant species.