Quantitative Study of Growth of Some Dairy Psychrophiles

Summary and Conclusions:

Samples of pasteurized skim milk were steamed for 25 minutes at approximately 100° C. and inoculated with pure cultures of Pseudomonas fluorescens, A and B, at different levels, Pseudomonas fragi, Brevibacterium lipolyticum. The fifth culture was the normal flora of commercially pasteurized skim milk treated as a pure culture. All samples were stored at 4° C. Bacterial growth was determined using agar plates incubated at 25° C. for 3 days.

The generation time was calculated for all cultures which ranged from 5.55 hours for Pseudomonas fragi to 11.24 hours for Brevibacterium lipolyticum. The generation time was found the same (7.22 hours) for Pseudomonas fluorescens cultures A and B regardless of differences in inoculum size for the two cultures.

In the pure cultures, bacterial count after 7 days of storage was the highest for Pseudomonas fragi, 3.0 x 10⁷, and the lowest for Brevibacterium lipolyticum, 1.9 x 10⁵ cells per ml. However, the mixed flora culture obtained a slightly lower count, 1.7 x 10⁵ cells per ml.

The lag phases for the pure cultures studied ranged from 1 day for Pseudomonas fragi to 3 days for both cultures of Pseudomonas fiuorescens. The mixed flora culture showed still a longer lag phase of 4.2 days.

Of all cultures studied, Pseudomonas fragi was found to be the most actively growing culture. It had the shortest generation time, the highest counts after 7 days of storage, and the shortest lag phase.

Major is listed as Dairying.