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.