Growth of certain psychrophilic bacteria in pasteurized milk as influenced by previous excessive psychrophilic growth in the raw milk

Undoubtedly some of the most objectionable bacteria encountered in milk are the psychrophiles. At the turn of the century it was established that these bacteria were capable of multiplying extensively at temperatures near freezing and were found in milk but little attention was paid to them because milk was normally consumed before these slow growing species were able to bring about milk spoilage (6). In recent years psychrophiles have received considerable attention because of adoption of processing and marketing procedures which tend to increase the time between processing and consumption of milk (74). The importance of psychrophiles is linked to their ability to grow at refrigeration temperatures and has been accentuated by advancements in refrigeration, increased emphasis on flavor and longer holding times (76). Psychrophiles were invariably present in farm milk supplies, and attained exceedingly large numbers in bulk raw milk at creameries (68). Like many organisms, the psychrophiles constituted a part of the normal microflora of raw milk supplies and resulted in spoilage whenever the milk was subjected to conditions that favored their growth. Their numbers depended upon the sanitary conditions under which milk was produced, handled, and processed, the holding temperature, and the time that elapsed before final consumption. The purpose of this investigation was to determine the significance of excessive psychrophilic bacterial growth in raw milk, held at refrigeration temperatures, to subsequent growth of post-pasteurization contaminants in the pasteurized finished product under normal market conditions. It was felt that experimental data of this nature might be of value to processors, public health officials, and all parties concerned in locating sources of contamination, and deciding upon proper corrective measures to prevent extensive psychrophilic growth, and subsequent spoilage of refrigerated market milk prior to consumption.