Microbiology Of Fluid Milk Products

A. Flavored Milks

The microbiology of flavored milk differs from that of unflavored milk in that conventionally pasteurized chocolate milk typically spoils faster than conventionally pasteurized unflavored milk. Douglas et al. (2000) found that after 14 days at 6°C, chocolate milk samples had higher standard plate counts and higher psychrotrophic plate counts than unflavored milk samples from the same raw milk batch (P < .001). Further experiments indicated that the chocolate powder, and not the additional sucrose, contributed to the increased bacterial growth. The chocolate powder did not introduce additional microbes into the milk. Rather microbes already present in the raw milk grew faster owing to the presence of the chocolate powder. Rosenow and Marth, (1987) in comparing growth of Listeria monocytogenes in skim, whole, and chocolate milk and in whipping cream also found that chocolate milk consistently produced the highest bacterial numbers by a factor of 10 or more.

B. Unflavored Milks

A wide variety of unflavored fluid milk products exist, including skim (< 0.5% fat), 1% fat, 2% fat, and whole milk; low-lactose (< 30% normal milk) and low-

sodium (< 100 mg/L) milk; and half-and-half (10.5-18.0% fat), light cream (1830% fat), light whipping cream (30-36% fat), and heavy cream (> 36% fat) (US Public Health Service, 1995). Studies indicate that the microbiology of many of these products is quite similar. Brown et al. (1984) compared the shelf lives of skim (0.1% fat), semiskim (1.6% fat), and whole (3.8% fat) milk at 4 and 7°C and with and without Pseudomonas contamination and found no difference in the rate at which samples reached 107 cfu/mL. Similary, Rosenow and Marth (1987) found no difference in the growth rate of L. monocytogenes in skim and whole milk and in whipping cream. The genera of spoilage bacteria found in pasteurized heavy cream and their lipolytic and proteolytic activities are comparable to the genera found in pasteurized milk, suggesting that fat standardization has little impact on the microbiology of the resulting cream and milk (Phillips et al. 1981).

Although the microbiology of various fluid milk products is similar, spoilage from nonmicrobial factors may vary from product to product. Recent data suggest that UHT-processed skim and whole milk behave differently during their respective shelf lives. López-Fandiño et al. (1993) found increased activity of both native and bacterially produced proteases in UHT-processed skim milk as compared to UHT-processed whole milk.

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