Characteristics of Milk

Spoilage microorganisms


Lactic acid bacteria

•Spoilage microorganisms: Spoilage microorganisms cause degradation of proteins, fats, and carbohydrates and the milk’s texture, color, taste, and smell is deteriorated. Psychrotrophs (bacteria that grow at refrigeration temperatures) are usually involved in spoilage reactions. Most are destroyed by pasteurization, but can still produce heat stable lipolytic and proteolytic enzymes that can cause spoilage.

•However, some strains and species of Clostridium, Bacillus, Arthrobacter, Cornebacterium, Microbacterium, Micrococcus, Streptococcus, and Lactobacillus can survive pasteurization and grow at refrigeration temperatures and cause spoilage.

•Coliforms: Coliforms are closely associated with the presence of pathogens, but they are not always pathogenic themselves, thus they are called indicator organisms. They are killed by proper pasteurization treatment. Their presence indicates improper or inadequate pasteurization or it indicates contamination after pasteurization. Escherichia coli is an example of a coliform. They are facultative anaerobes and grow best at 37oC or 98.6oF. Coliforms are able to degrade milk proteins and ferment lactose, causing milk to be spoiled quickly.

•Lactic acid bacteria: Lactic acid bacteria can produce chemical changes that are desirable in the production of fermented dairy products because they are able to ferment lactose to lactic acid. They are naturally present in the milk and are used as starter cultures in the production of cultured dairy products, such as yogurt and cottage cheese. Lactic acid bacteria include lactococci, lactobacilli, and Leuconostoc.

Somatic Cells and Microorganisms



Significance in cultured products


Bovine milk contains about 10^4 to 10^7 cells/ml. These numbers are usually reported as somatic cell counts. Somatic cell counts are a mixture of epithelial cells (2%) and leukocytes (98%). Many enzymes, such as catalases and proteases, are found in leukocytes and many nucleic acids originate from these cells. Also, they have been used as a source of deoxyribonucleic acid and as a substrate for the polymerase chain reaction.

The proteases contained in somatic cells are capable of hydrolyzing B-Casein and they could cause proteolysis in aged cheeses if not destroyed during processing. These proteases damage raw milk quality during storage and have an adverse effect on pasteurized fluid milk and milk during cheese making. U.S. Dairy Herd Improvement Association Programs routinely use somatic cell counts using electronic cell counters. High somatic cell counts are associated with reduced milk yields and an increasing incidence of mastitis.

p.54 – 55


 Early, Ralph. The Technology of Dairy Products. New York, NY: Thompson Science, 1998.

 Jensen, Robert G. Handbook of Milk Composition. San Diego, CA: Academic Press, Inc., 1995. Pages: 54,55,82,83

 Dairy Processing Handbook: Page 19


 “Milk” by Stuart Patton



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