Various quality aspects of pasteurized and ultra-pasteurized fluid milk during storage.
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Eight dairy processing plants located in the west, midwest, and southern regions of the U. S. were surveyed in order to determine pasteurized milk quality. Whole and 2% milks were obtained in duplicate from local grocery stores. Four of the milks obtained were ultra pasteurized (UP), 6 came from organic certified dairies. One set of milk samples was initially evaluated for sediment, freezing point, lab pasteurization counts (LPC), HR-3 test results, coliforms, standard plate counts, protein, fat, and somatic cells. Milks were evaluated for flavor using a 10 point scale (10 meaning no defect) by a 5-member trained panel. Milks were stored at 7°C for two weeks and then evaluated again for flavor, and SPC and coliform counts were conducted. Coliforms were found in 2 samples upon arrival. Remaining fresh samples contained less than 10 CFU /mL. Standard plate counts ranged from less than 100 to 300,000 CFU/mL on fresh samples. No sediment was found in any of the samples. Samples received a mean score of 7 for sensory evaluation upon arrival with the most common criticism being cooked which dropped to 3 after 2 weeks. Common criticisms of samples were cooked, rancid, unclean, and fermented fruity. Region of the U. S. the samples were received from showed no differences in the quality of the samples.Keywords:
Pasteurization
Plate count
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This study was designed to monitor some bacteriological and physical quality of milk from two dairy factories before and after pasteurization, and also to assess the compliance of this quality detected with the standard set by the Sudanese Standards and Meteorology Organization (SSMO). Two hundred and Twenty raw and pasteurized milk samples were collected from dairy factories and sale points in Khartoum State. The average total bacterial counts before pasteurization was1×10 5 -9×10 6 cfu/ml and after pasteurization it was 0-9×10 4 cfu/ml. Most of the market samples in day 1,3 and day 6 showed range of 0-1×10 4 cfu/ml. Staphylococci counts before pasteurization were found in range of less than 1×10 4 cfu/ml and after pasteurization were in the range of 0-9×10 cfu/ml. The Coliform counts before pasteurization were in the range of 1×10 3 -9×10 3 cfu/ml and immediately after pasteurization the range was reduced to 5- 1×10 2 cfu/ml. E.coli was identified from24%of the samples before pasteurization. Before pasteurization acidity was found to be 0.17%, directly after pasteurization the acidity was 0.15-0.16% but it was 0.16-0.17% in market samples.
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Dairy industry
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Pasteurization
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The microbial quality of farm buffalo milk was evaluated during this study. A total of 75 milk samples 25 each of fresh, bulk and transportation tank milk were screened for total viable count, coliform count, yeast and mould, and aerobic spore-former count. The results showed that the average TVC (5.51 log CFU/mL), coliform (5.45 log CFU/mL), yeast and mould (3.69 log CFU/mL) and spore former count (2.66 log CFU/mL) was significantly higher (P<0.05) in transportation tank milk samples, followed by bulk tank samples (5.29, 4.37, 3.60 and 2.59 log CFU/mL) and fresh milk samples (4.74, 3.51, 3.48 and 1.76 log CFU/mL). It was concluded from the findings of present investigation that average total viable count, coliform, yeast and mould, and aerobic spore former counts were higher in transportation tank milk as compared to fresh and bulk tank milk samples.
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Most probable number
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A total of 216 raw milk samples with a variety of Standard Plate Counts and psychrotrophic bacteria counts were laboratory-pasteurized, stored at 7 C, and then evaluated for flavor after 1 and 2 weeks. Results showed that milk with counts of >1,000,000/ml before heating frequently developed objectionable flavors after pasteurization and subsequent storage. The most common defect was a bitter flavor which appeared within 2 weeks after pasteurization in nearly all samples which as raw milk had counts exceeding 10,000,000/ml. This off-flavor developed in spite of small numbers of organisms in the pasteurized product and in the absence of post-pasteurization contamination.
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The quantitative determination of nitrate-reducing microorganisms in food is important because of their role in the formation of N-nitroso compounds and methemoglobin. The total counts of microorganisms and the counts of nitrate-reducing microorganisms were assayed in various milk products and in human milk by the method of the most probable numbers using nitrate broth medium. The mean value and standard deviation of the nitrate-reducing microorganisms were the following: in pasteurized milk 3.4 +/- 1.1 log/ml (in spring and summer 4.2 +/- 1.1, and in winter 2.8 +/- 0.6 log/ml), in kefir "Tallinn' 2.9 +/- 1.1 log/ml, in fat kefir 2.9 +/- +/- 0.7 log/ml, in sour milk 3.9 +/- 0.5 log/ml, in fresh milk 4.6 +/- 0.6 log/ml and in human milk 2.7 +/- +/- 1.1 log/ml. The amount of nitrate-reducing microorganisms in pasteurized milk of the spring-summer period and in fresh milk was significantly higher than in human milk. The mean count of nitrate-reducing microorganisms in human milk is regarded as a maximum permitted count of these organisms in milk for bottle-fed sucklings. For them and for atrophic gastritis patients pasteurized or fresh milk should be boiled.
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Kefir
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On two separate occasions pasteurized milk samples were taken from a creamery on the day before and the day after the down‐day (when no milk is processed and the plant is cleaned). On each occasion, three lots of eight samples were taken throughout the processing period. All samples were subjected to a preincubated count (incubation at 6°C for five days followed by a plate count (2l°C/25h)). The results showed that (a) within each lot of eight there was a large sample to sample variation in bacterial numbers; (b) bacterial numbers tended to increase the later the samples were taken in the day; (c) samples taken before the down‐day were more heavily contaminated than samples taken after the down‐day; (d) nearly all the bacteria isolated were found to be Gram negative rods, primarily Pseudomonas spp.
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Incubation period
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The consumption of raw milk in Italy is allowed only “after boiling”. The aim of this research was to bet-ter understand how the heat treatment of raw milk performed at home by consumers assures their mi-crobiological safety. 50 samples of raw milk (each sample 500 ml) provided to consumers who regularly buy raw milk from self-service automatic vending machines were followed from delivery till to after do-mestic heat treatment. Heating was performed by consumers according to their habits. The 50 samples were exposed to different heat treatments of which the mildest was at 68.5 °C and the most intense was at 97.8 °C before switching off. The average of temperatures used was 89.5 °C and the mode was 93.2 °C. According to the different parameters of heat treatment observed, 35 samples of raw milk and 35 samples of heated milk were selected for microbiological and process indicator analyses. Total Microbial Count (TMC), total and fecal coliforms, Escherichia coli, Staphylococcus aureus and Bacillus cereus loads were determined. E. coli was isolated only from one sample of raw milk. No B. cereus nor S. aure-us were found in all samples. After heat treatment, 4 samples showed a residual TMC ranging between 1,7 CFU/ml and 3,2 CFU/ml, whilst the count of total and fecal coliforms were irrelevant. The test for alkaline phosphatase has showed negative in all samples of heated milk, while the test of lactoperoxi-dase was positive in 3 samples. Results indicated that the microbiological risk attributable to the consumption of home heated raw milk is low, if the consumer applies regularly a good heating process.
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