Effect of Heat Treatments on Survival and Growth of a Psychrotroph and on Nitrogen Fractions in Milk

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Abstract
Grade A raw milk which had initial psychrotrophic counts of less than 103/ml was inoculated with an antibiotic-resistant Pseudomonas sp. to a final cell concentration of 102, 104, or 106/ml. The inoculated milk was held at 4 C for 14 h and then exposed to the following time-temperature treatments: 72 C for 15 sec, 79 C for 15 sec, 88 C for 10 sec, and 95 C for < 5 sec. An uninoculated raw milk control was handled and analyzed along with inoculated samples. Aliquots of milk were analyzed for marked Pseudomonas sp., total psychrotrophic counts, numbers of Pseudomonas, and for distribution of nitrogen before and after each heat treatment and after storage of non-heat-treated raw milk and heat-treated samples for 7 and 14 days at 7 C. Psychrotrophic counts were significantly affected by heat treatment, initial cell inoculum, days stored, and plating media. Non-casein N, non-casein protein, total albumin, β-lactoglobulin, proteose-peptone, and globulin N were significantly decreased by heat treatment. Non-case...

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