Milk Microbiology

The Milk Microbiology section contains information relating to microbial concerns in milk. A brief overview of dairy microbiology is presented below as an introduction this section. The topics covered are:

Dairy Microbiology

Milk is virtually sterile when it is synthesized in a healthy cow's udder (mammary gland). Cows, like humans, are natural reservoirs of bacteria. Many of these bacteria are not harmful to humans, but some may be harmful to humans even though the cows are not affected and appear healthy.

Milk may become contaminated with bacteria during or after milking. The mammary glands of cows (and humans) can become inflamed due to a bacterial infection called mastitis. During a mastitis infection, very high numbers of bacteria present can be in the udder and in the milk. Some disease causing organisms (pathogens) can be shed through cow feces and may contaminate the outside of the udder and teats, the farm environment (bedding, for example) and the milking equipment. Although optimal growth conditions for bacteria are different for different organisms, milk contains important nutritional components for mammal growth, and, therefore, it is also an ideal medium for the growth of many different bacteria. Temperature plays an important role in bacterial growth. Many bacteria prefer to grow at body temperature (86-98°F, 30-37°C), but will grow at lower temperatures (such as refrigerator temperature) at slower rates.

The area of dairy microbiology is large and diverse. The bacteria present in dairy products may cause disease or spoilage. Some bacteria may be specifically added to milk for fermentation to produce products like yogurt and cheese. A detailed discussion of fermentation bacteria is outside the scope of this web site, although these organisms are discussed briefly in the section on yogurt production and cheese production. This section of the web site is dedicated to the discussion of pathogens because of their importance in human health.

Human illness from milkborne pathogens is usually associated with consumption of raw milk or products made from raw milk such as fresh cheeses. In the past 20 years, foodborne illnesses from dairy product consumption have been predominantly associated with Salmonella enterica, Listeria monocytogenes, Campylobacter jejuni, and Escherichia coli O157:H7. These organisms have been isolated from bulk tank samples at rates ranging from 0.87% of samples taken for E. coli O157:H7 analysis in Ontario, Canada to 10% for E. coli O157:H7 in Wisconsin (Jayarao et al., 2001 and 2006; McManus and Lanier, 1987; Padhye and Doyle, 1991; Steele et al., 1997; Van Kessel et al., 2004). Because there is a risk of pathogen contamination in milk produced from healthy cows under sanitary milk conditions, pasteurization of milk prior to consumption will destroy pathogens and provide protection for illness associated with consumption of dangerous microbes. Occasionally, human illness has been linked to pasteurized milk products but these cases usually have been a result of contamination of the product after pasteurization or improper pasteurization.