Ideally, microbial contamination of raw milk and milk products should be addressed primarily through preventive measures on the farm and throughout processing. However, far too many contamination sources exist to prevent entry of all bacteria. Therefore, milk handling and processing strategies are designed to reduce and control bacterial numbers in processed products to protect milk quality and milk safety. The first of these measures involves efficient cooling of milk to 4°C immediately following milking. Reduced temperatures inhibit growth of mesophils and thermophils and reduce the activity of degradative enzymes. Modern dairy farms use refrigerated bulk storage tanks which maintain milk at 4°C or below. As bulk tank milk pick-up typically occurs daily or every other day, product from multiple milkings is frequently mixed and stored in the same tank. To prevent fresh, warm milk from the most recent milking from raising the temperature of milk already present in the bulk tank, many farms employ pretank cooling systems to reduce product temperature before addition to the tank.
Heat treatment plays a critical role in controlling bacterial numbers in processed milk products. The three basic approaches to heat treatment of raw milk, pasteurization, ultrapasteurization and UHT, differ primarily in their underlying purpose. Pasteurization aims to eliminate the non-spore-forming pathogen most resistant to thermal destruction, currently recognized as being Coxiella burnetii, and concurrently reduce nonpathogenic bacterial numbers in milk. Ultrapasteurization adds the additional goal of increasing product shelf life through further reduction in total bacterial numbers. UHT processing aims to achieve microbial sterility to create a shelf-stable fluid milk product.
The PMO lists seven time and temperature combinations (Table 3) which are acceptable for milk pasteurization; these temperatures increase by 3°C if the milk product contains added sweeteners or greater than 10% fat. Two particular time and temperature combinations have become standard in the United States: low-temperature long-time (LTLT) and high-temperature short-time (HTST). In LTLT, or ''vat,'' pasteurization, which is commonly used for milk intended for manufactured products such as cheese and yogurt, milk is held at a minimum of 63°C for 30 min. In HTST pasteurization, which in the United States is currently most commonly used for fluid milk products, milk is held at a minimum of 72°C for 15 s. In ultrapasteurization, milk is held at a minimum of 138°C for at least 2 s, and in UHT processing, milk is held at 140- 150°C for a few seconds (Bylund, 1995; U.S. Public Health Service, 1995). UHT processing involves the additional step of aseptic packaging in which heat-treated milk is cooled and packaged directly into sterilized containers under aseptic conditions. Typical shelf lives for heat-treated fluid milk are 14-21 days for HTST; 40-60 days for ultrapasteurized
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