FDA (21CFR 131.110) describes milk as “the lacteal secretion, practically free from colostrums, obtained by the complete milking of one or more healthy cows.” Milk obtained aseptically from the udders of healthy cows contains a very low number (<1,000/ml) of bacteria. However, since milk is an excellent growth media, microorganisms introduced during production and handling may multiply rapidly under conditions of marginal refrigeration (slow cooling and storage between 40˚F and 45˚F). The number of microorganisms in Grade A raw milk depends upon the sanitary conditions in which it was produced, rate of cooling, and storage temperature. Life begins at 40˚F for spoilage bacteria. Thus, rapid cooling and storage at temperatures between 32˚F and 38˚F may mask the presence of undesirable microorganisms associated with unsanitary conditions. If the temperature of milk is allowed to get above 40˚F, psychrotrophic type bacteria are capable of rapid growth and of reducing milk quality.

Regulatory Standards: Microbiological standards for raw milk based upon total viable bacteria or standard plate count (SPC) are listed in the Grade A Pasteurized Milk Ordinance (PMO). Current standards are:

Individual Producer Samples (<100,000 SPC/ml), and Commingled (more than one producer) Samples (<300,000 SPC/ml).

Regulatory standards establish minimum requirements for assuring a safe product to protect public health. The PMO standards have served the industry well for many years. However, regulatory standards should not be confused with quality standards. The true microbiological quality of raw milk is determined by the type, number, and ability of the organisms present to grow, multiply and alter the components and characteristics of milk.

Psychrotrophic Bacteria: These bacteria not only exist at cold temperatures, but can actually grow slowly; causing off-flavors and other defects. Many dairy authorities (Standard Methods for the Examination of Dairy Products) believe that psychrotrophic bacteria are the single most important group of organisms from a quality control point of view. Psychrotrophic bacteria commonly found in raw milk are Gram negative and destroyed by pasteurization. They may gain access to pasteurized milk through defective gaskets/systems or by spills of raw milk. As spilled milk stands on the plant floor, the organisms can become airborne and be deposited on milk-contact surfaces, eventually contaminating the pasteurized product.

In addition, some species of psychrotrophic bacteria, notably Pseudomonas, can produce proteolytic and lipolytic enzymes, which can cause a range of off flavors and physical defects. Once begun, this process cannot be halted or reversed by pasteurization, exposure to cold or by other means. Less common is the presence of Gram positive thermoduric, psychrotrophic spore formers, usually of the genus Bacillus. These organisms are capable of surviving pasteurization and causing spoilage several days after packaging. With extended shelf-life goals, these spore formers may play an even bigger role.

Stress is good. Rapid cooling and low temperature storage of raw milk will mask unsanitary production and handling conditions by not allowing undesirable bacteria to grow or multiply. The Standard Plate Count (SPC) test performed by regulatory authorities to ensure that milk meets the minimum Grade A standards does not accurately determine the number of psychrotrophic bacteria present or their ability to multiply under favorable conditions. This fact was recognized by Dr. C. K. Johns of the Canadian Department of Agriculture more than five decades ago. Dr. Johns developed a simple stress test, commonly known as the Preliminary Incubation (PI) test. The PI test measures the true microbiological quality of raw milk.

The PI test procedures are found in the 15th edition of Standard Methods, but are not included in the latest editions. Samples are incubated at 12.8˚C (55˚F) for 18 hours. SPCs are performed according to Standard Methods before and after PI. Interpretation: PI counts of ≥100,000/ml indicate gross contamination while PI counts that are double or more than the SPC, but are <100,000/ml, indicate psychrotrophic growth. The PI test should be performed on plant silo as well as tanker samples.

Progressive Standard: The standard should eventually be lowered to ≤50,000/ml.

The PI test is a valuable quality improvement tool. Use effectively.