Modelling the relation between bacterial growth and storage temperature in pasteurized milks of varying hygienic quality

The shelf‐life of pasteurized milk was mainly determined by the level of contamination with Gram‐negative psychrotrophic bacteria. The length of lag phase of the bacteria was also important, although the generation times of the naturally contaminating flora seemed to be of little relevance except for milks where the shelf‐life exceeded 10 days at 6°C. The effect of temperature on growth of the contaminants could be accurately determined by the ‘square root’ plot but the conceptual minimum temperature for growth (To) varied. The variation was related to the quality of the pasteurized milk. Effects of temperature on generation time, length of lag phase and shelf‐life were most marked at temperatures below 15°C. The microflora of pasteurized milk varied significantly with storage temperature. At refrigeration temperatures, spoilage was mainly due to the growth of Pseudomonas spp. Enterobacteriaceae and Gram‐positive bacteria assumed greater importance in the spoilage of milks stored at temperatures above 10°C. Milks of good quality also contained Bacillus spp and this group of bacteria were not detectable in milks with short shelf‐lives.