They may contaminate milk at the farm or at the plant. “Chief sources of thermoduric bacteria in milk are poorly cleaned equipment including old rubber parts, areas of milkstone build-up, separators and other difficult to clean or neglected areas (soil build-up). ![]() Gram-positive cocci are responsible for premature spoilage by causing ropiness or acid souring of the milk. The second origin may be from a post-pasteurization contaminant, such as the condensation in compressed air. First, they may be present in raw milk as thermoduric organisms, meaning they survive pasteurization. There are two possible environmental contamination sources. The second psychrotrophic bacteria are gram-positive cocci. The presence of gram-negative rods shortens shelf-life by causing bitter, fruity, putrid flavors and ropiness in pasteurized milk. ![]() If these bacteria are detected, you can be assured the source is located somewhere in the process after the heating section of the pasteurizer through the packaged product. They are very psychrotrophic, meaning they proliferate at low refrigeration temperatures. Their presence in pasteurized milk is due to post-pasteurization contamination as they generally do not survive the pasteurization process. The first psychrotrophic bacteria are gram-negative rods. The three most common types of psychrotrophic bacteria that impact the shelf-life of pasteurized milk by adversely affecting flavor and quality are: “Pasteurized milk having previously supported growth of psychrotrophs to levels of 5.5logcfuml−1 has been described as having an inferior flavor.” (Source: Encyclopedia of Dairy Sciences) Three most common types of psychrotrophic bacteria in milk Contamination rates as low as a single psychrotrophic bacterium per liter can cause quality defects in pasteurized milk.Ĭontinuous inline sampling and testing must be designed to detect and identify these very low levels of bacterial contamination. The growth rate of the contaminants is the primary factor affecting quality when psychrotrophic bacteria determine shelf-life. However, these bacteria grow at refrigeration temperatures, and depending on the species, may also survive pasteurization. Psychrotrophic contamination is the primary reason for the premature spoilage of pasteurized milk. Martin, P.Torres-Frenzel and M.Wiedmann) Psychrotrophic bacteria in pasteurized milk Premature microbial spoilage of dairy products, including fluid milk, cheese, and cultured products, is a primary contributor to dairy food waste.” (Source: Controlling dairy product spoilage to reduce food loss and waste, N.H. “Estimates indicate that in the United States, approximately a quarter of dairy products are lost at the production level or wasted at the retail or consumer level annually. Monitoring and controlling milk spoilage require the due diligence of a quality control program that includes continuous testing and inline sampling to detect and identify sources of contamination that impact milk quality. ![]() In those posts, I discussed the collective efforts of the dairy industry and regulatory involvement in increasing the shelf-life of pasteurized milk. Refer to my earlier posts, How Bacterial Generation Times Impact Fluid Milk Quality and Shelf-life and Heat-Resistant Psychrotrophic Bacteria and Their Effect on the Quality of Pasteurized Milk. Bacteria in pasteurized milk is the leading cause of spoilage and reducing shelf-life.
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