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Use high pressure to extend shelf life of pasteurized fluid milk products.

The shelf life of commercial pasteurized milk is limited to a range of 10 to 14 days because of post-pasteurization contamination that's caused by psychrotrophic microorganisms. Although ultra-high-temperature (UHT) treatments significantly increase the product's shelf life, they also create a noticeable flavor difference in comparison to pasteurized milk.

Nonthermal processes such as high-pressure processing (HPP) can reduce microbial counts in foods. HPP may extend the shelf life of milk by reducing psychrotrophic bacteria counts without creating the off-flavors that result from traditional heat treatments. HPP also usually does not alter a product's physical properties.

The objective of scientists at The Ohio State University was to extend the shelf life of pasteurized milk by using high-pressure treatments that would eliminate post-pasteurization contamination from psychrotrophic bacteria. Commercial samples of pasteurized 2% milk, skim and chocolate milk were high-pressure-treated at 400 MPa and 800 MPa below 30 C for 10 minutes in a cold isostatic press. The samples contained psychrotrophic bacteria counts ranging from [10.sup.4] cfu per mL to [10.sup.7] cfu per mL.

Other commercial samples of pasteurized 2% milk and chocolate milk containing less than 40 cfu per mL were high-pressure-treated at 400 MPa, 600 MPa and 800 MPa below 30 C for 10 minutes. The samples were enumerated for psychrotrophic growth on days 0, 10, 20 and 30, using the International Dairy Federation enumeration method.

HPP reduced the psychrotrophic bacteria counts by 5 logs for skim and chocolate milk and by 2 logs for 2% milk. The average particle size and color of the milk were not significantly altered by the treatments. HPP treatments of 400 MPa, 600 MPa and 800 MPa extended the shelf life of pasteurized milk more than 10 days beyond the shelf life of untreated pasteurized milk. HPP resulted in a longer extension for fresh milk than for milk near its expiration date.

Further information. Jim Harper, Department of Food Science and Technology, The Ohio State University, 2015 Fyffe Court, 329 Parker Food Science Building, Columbus, OH 43210; phone: 614-292-7798; fax: 614-292-0218; email:
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Publication:Microbial Update International
Date:Oct 1, 2005
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