Abstract:

The microbial, sensory and chemical characteristics: vitamins A and D, Tyrosine Value (TV) and Acid Degree Values (ADV), of 2% Ultra Pasteurized (UP) fluid milk from 2 plants in New York State (NYS) were studied over 10-week periods. Four batches of 5 randomly selected samples were obtained from each of the UP-processing plants at approximately 3 months intervals. The samples were stored at 7+/-l°C and analyzed during the first, fourth, seventh and tenth weeks of post processing storage. Vitamin A and D contents were determined using HPLC, Tyrosine Value as measured by Juffs’ method and ADV using the modified Copper Soap method. Raw skim and whole milk used as raw materials for 2% UP were analyzed in the first week for the same chemical characteristics as the 2% UP milk. Microbial analysis on the raw milk samples included Standard Plate Counts (SPC), Psychrotrophic Plate counts (PPC), Coliform Counts, Gram-negative counts and Laboratory Pasteurized Counts (LPC). The 2% UP samples were examined for SPC, PPC and Heat-Resistant Spore Formers (HRSF). The microbial quality of the raw milk varied among batches and factories but in all cases, there was no growth on plates inoculated with the 2% UP milk samples over the 10 weeks of storage. This implies that the processing conditions adequately destroyed the microbes present in the raw milk and no post-processing contamination occurred. The 2% UP milk was judged by a panel of 10 trained panelists as fair (mean score 7.0-7.5) throughout the 10 weeks. An increase (30 to 50%) was observed in the Tyrosine Values of the 2% UP milk over the 10-week period. The Tyrosine Values encountered (65-350 fig/ml) however, were below the levels at which flavor would be affected (1500p.g/ml). Similar trends (increase with time over the 10-week period) were observed in the ADV but still the ADVs observed (0.5 - 0.7 meq FFA/liter) were below the levels at which rancidity could be detected (1.0 - 1.5 meq/liter). During the first week, Vitamin A and D levels were within the legal fortification requirements of 2000-3000 lU/Qt and 400 RJ/Qt. respectively. An increase was observed at weeks 4, 7, and 10 possibly due to interference from degradation products associated with the vitamins. In the tenth week the vitamin A and D levels returned to approximately that observed in the first week, possibly as the degradation products separated out and away from the vitamins.