"Psychrotrophs" and milk quality

J Russell Bishop, Ph.D., Dept. of Food Science & Technology, Virginia Tech University, Blacksburg, Va.

When considering the quality of refrigerated milk and dairy products, the concern is almost exclusively with microorganisms that grow at storage temperatures. These bacteria arc referred
to as "psychrotrophs," or psychrotrophic bacteria. They are defined based on:

1) optimum growth temperature of 21°C or 70°F,
2) ability to grow at low temperatures;
3) method of enumeration; and
4) criteria which are independent of the incubation temperature.

Psychrotrophs are generally found in water and soil. They are introduced into milk when these sources become established on milk contact surfaces, equipment, flooring, and drains in the milking parlor and processing plant. Milk produced or processed under sanitary conditions usually contains less than 10 percent of the total microflora as psychrotrophs.  But milk produced or processed under unsanitary conditions can contain more than 75 percent psychrotrophic bacteria.

While psychrotrophs have been reported to survive laboratory pasteurization in very small numbers, abundant evidence shows that they rarely survive in commercial operations (except when the raw milk is very heavily contaminated). Consequently, their presence in a pasteurized product almost certainly represents post-pasteurization contamination. This can occur at or beyond the pasteurizer through leaking valves, hairline or pinhole cracks in plates, contaminated pasteurized storage tanks, etc.

In the past, a laboratory pasteurized count (LPC) has been used, or overused, to assess product quality. The method enumerates those bacteria capable of surviving pasteurization (thermoduric bacteria). The LPC method provides very little information about the ultimate quality of the product, however, because it is psychrotrophs that cause degradation of dairy quality and very few psychrotrophs survive pasteurization. Therefore, it becomes obvious that the greatest iced is for the enumeration of psychrotrophic bacteria.

The standard enumeration method for psychrotrophic bacteria (called Psychrotrophic Bacteria Count, or PBC) involves pour-plating a sample in tryptone yeast extract agar, and incubating for 10 days in 7°C (45°F). A more rapid technique (called modified Psychrotrophic Bacteria Count, or mPBC), developed by OIiveira and Parmelee, enumerated psychrotrophic bacteria at 21°C (70°F) for 25 hours.