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Time-temperature relationships were developed for heat inactivation of alkaline phosphatase in pooled human milk and colostrum. Heat inactivation of the enzyme in human milk proceeds at the same rate as in cows' milk. The heat sensitivity of the enzyme is slightly greater in human colostrum than in human milk.

Seven pooled human milks, each from at least 6 different breast-feeding mothers (4 weeks post-partum) were studied for microbial population and flora changes during storage at 37°C. Coagulase-negative Staphylococcus spp. constituted a considerable part (39-100%) of the microflora in 6 of 7 pools of fresh milk, with coryneform bacteria, Lactobacillus , Micrococcus and Streptococcus spp. isolated less frequently. Storage of raw milk for 8 h caused a shift in flora favoring the Lactobacillus spp. After 24 h, coagulase-negative Staphylococcus were dominant in 4 of 7 pools, coryneform bacteria in 2 of 7, and Lactobacillus and Micrococcus spp. each in 1 of 7 pools. Low levels of bacteria were detected in 2 of 7 pasteurized pools, consisting of Staphylococcus spp. or yeasts. Freezing of the milk before storage at 37°C had little effect on the aerobic plate counts.

Lipolysis was quantitated during storage of fluid and freezedried human whole and skim milks. Fatty acid accumulation was faster in whole fluid milk stored for 1 week at 4°C than in frozen (-20°C) samples stored for 180 d. The rapid accumulation of fatty acids during 24 h of storage at 4°C was enhanced in previously frozen milk samples. While freeze-dried whole milk showed no lipolysis when stored at -20°C, accumulation of free fatty acids was rapid in samples stored at room temperature. Fluid and freeze-dried skim milk samples exhibited no appreciable lipolysis.

Bile salt-stimulated lipase activity was monitored in fresh human milk and skim milk during refrigerated (4°C) and frozen (-20°C) storage, and in the lyophilized milks stored at -20°C and at room temperature. Following a sharp initial drop to approximately 77% of the original lipase activity, lipase was relatively stable in frozen or freeze-dried milks during 180 d of storage at -20°C. Activity losses were greatest (P < .05) in freeze-dried whole milks and skim milks stored at room temperature, approximating a 30% loss during 30 d of storage. Lipase activity was stable during refrigerated (4°C) storage of whole milk for 1 week.

Time-temperature relationships for heat-inactivation of bile salt-stimulated lipase activity in human milk and colostrum were systematically measured using a pH-stat assay procedure with triolein as substrate. The enzyme was not affected in either menstruum at 45°C for 40 min. The enzyme was destroyed almost instantaneously at 60°C, and was slightly more heat-sensitive in colostrum than in milk. The bile salt-stimulated lipase(s) in human milk was more heat sensitive than lipase in bovine milk.

Survival and/or growth of Pseudomonas fluorescens , Micrococcus luteus , Micrococcus sp. 102 and Staphylococcus aureus was studied in raw, frozen, pasteurized or heat-sterilized pooled human skim milks. Growth response of P. fluorescens at 25°C was essentially the same in the untreated and treated milks with an increase in count/ml of 4 log cycles during a 24-h incubation period. Counts (per ml) of S. aureus in raw, pasteurized and frozen milks increased approximately 3 log cycles. Growth of S. aureus in heat-sterilized milk was somewhat less. Micrococcus sp. 102 could not be recovered from raw and frozen milk after incubation at 37°C for 14 and 24 h, respectively. The count (per ml) of this organism in pasteurized milk decreased approximately 2 log cycles during the 24-h incubation period, whereas counts in sterilized milk increased approximately 2 log cycles. In no instance did M. luteus survive in raw, pasteurized or frozen milks at 37°C even for short periods after inoculation. The organism survived in heat-sterilized milk for 14 h at 37°C, but then decreased in numbers until the organism could be recovered from only 1 of 5 pools after 48 h of incubation.

Time-temperature relationships for heat-inactivation of the bile salt-stimulated lipase activity were compared in whole human milk and in the same product fortified to 9 mM/ml with sodium taurocholate. Heat treatments were varied from 45 to 70°C for times ranging from 15s to 40 min. Enzyme activity was more heat stable in human milk fortified with taurocholate than in control samples. The temperature required for the onset of heat inactivation at 30-min holding time was increased from 45°C for control samples to 60°C following addition of taurocholate. A temperature differential of approximately 12°C was required in the fortified milks to produce inactivation equivalent to that observed in the control milks over the heating range studied.