Dairy processors do an excellent job of processing and bottling milk so that low initial bacterial numbers are observed. In fact, the standard plate count (SPC) of freshly pasteurized milk typically runs approximately 500 cfu/mL (or even lower).
Dairy processors do an excellent job of processing and bottling milk so that low initial bacterial numbers are observed. In fact, the standard plate count (SPC) of freshly pasteurized milk typically runs approximately 500 cfu/mL (or even lower). However, these low numbers on fresh milk products do not assure that the shelf-life of the milk will be as long as it should be. The simple reason is that there is no indication as to the type of bacteria present. If all or most of the bacteria are psychrotrophic, i.e. capable of fairly rapid growth at refrigeration temperatures, the counts could reach a million or more per mL within 10-12 days at 7°C (45°F). The reason why this number is so critical is that normally a count of at least a million is required before an off-flavor is detected. The average generation time (the time required for a bacterial population to double in number) of the most common psychrotrophs is approximately nine hours at 7°C (45°F); therefore, it only requires a few days for "several hundred" bacteria to become a serious shelf-life issue.
Stress testing provides a better method for evaluating the quality and shelf-life of pasteurized products. The purpose of this test is to allow time for any bacteria that may be present to grow to estimable numbers. In cases where most of the bacteria are psychrotrophs (either Gram-negative rods like Pseudomonas or Gram-positive rods like Bacillus), resultant counts of stress testing can better indicate the quality and shelf-life of fluid milk.