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

Abstract

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.

Prediction of the shelf‐life of pasteurized milk at different storage temperatures

“…Identification and estimation of the relative proportions of bacterial types present was carried out according to the method previously described by Griffiths & Phillips (1988).…”

“…The initial psychrotroph count was determined by a Most Probable Number technique (Oblinger & Koburger 1975) using 50, 10 and 1 ml volumes of the freshly pasteurized milks (Griffiths & Phillips 1988).…”

“…Results obtained with pre-incubation at 12 and 15°C were similar to those obtained for pre-incubation followed by selective enumeration using CPN (Tables 2 and 3). Slightly better results were achieved for a pre-incubation temperature of 15°C rather than 12°C and was probably the result of the slightly faster growth rates achieved by the indigenous microflora at the former temperature (Griffiths & Phillips 1988). There was a good correlation between the 21°C pre-incubation count determined on CPN and the non-selective count obtained after pre-incubation at 15°C (r = 0.80, n = 46, P < 0*001).…”

Modelling the growth of Pseudomonas fluorescens and Candida sake in ready-to-eat meals

“…Correlation coefficients obtained indicated that their relationship was linear ðr 2 > 0:93Þ. An accurate estimation of the lag time is very important for two reasons, the first being the extension of shelf life of chilled foods (Griffiths & Philips, 1988), and second being its role in the mathematical modelling of growth data.…”

Short communication: Postpasteurization hold temperatures of 4 or 6°C, but not raw milk holding of 24 or 72 hours, affect bacterial outgrowth in pasteurized fluid milk

“…Similarly, the effect of low temperature storage on pasteurized fluid milk has also been studied (Muir et al, 1987;Phillips and Griffiths, 1987;Griffiths and Phillips, 1988). Although there is a consensus that lower storage temperatures lead to slower and reduced bacterial growth over time, the fluid milk manufacturer can do very little to control this factor once the milk has left the processing facility.…”