Effect of different amounts of pH, salt and temperature on survival rate of Lactobacillus casei in milk during storage time

Subject Areas : Veterinary Clinical Pathology

حمید Mirzaei ¹

1 - Department of Food Hygiene, Faculty of Veterinary Medicine, Islamic Azad University, Tabriz Branch, Tabriz, Iran

Received: 2008-05-10 Accepted : 2008-08-25 Published : 2008-05-21

Keywords: pH, Milk, salt, temperature, Survival rate, Lactobacillus casei,

Abstract :

      Considering that one of the most important points of probiotic products is the survival rate of live microbes during storage time, so the aim of this study was to determine the effect of different amounts of pH, salt and temperature on survival rate of Lactobacillus casei in milk during 30 days of storing. For this purpose, at first four erlene Meyers with capacity of 2 litters were selected and 1.5 litter of sterilized milk was added to each of them. Two percent of yoghourt starter was inoculated to each of the first, second and third erlenes and 2% sterilized milk was inoculated to the forth erlene as control group and samples were incubated at 42 ˚C until the pH of first to third samples reached 3.5, 4.5 and 5.5 respectively. Then the erlenes were heated at 90 ˚C for 10 minutes to inactivate the yoghourt starter bacteria and then 500 ml of each sample in first, second, third and forth erlenes was transported to three sterilized caped glass containers and so  four groups consisting of 3 glasses were produced and in each collection 0,  2% and 4% of salt was added to first, second and third samples respectively and then 5 ml of McFarland solution containing 3× 109 cfu/ml of Lactobacillus casei was inoculated to each of the samples. This Procedure was repeated 12 times and obtained samples were stored at about 4˚C in the first six repetitions and at about 25˚C in the second six repetitions. The number of Lactobacillus casei in each of samples was counted soon after production, 4, 8, 12, 16 and 30 days after storing by pour plate method in MRS agar and obtained means were analyzed statistically. According to repeated measures analysis of variance and one way analysis of variance, the mean number of live Lactobacillus casei in samples with pH of 6.5 and 5.5 was significantly more than their numbers in the samples with pH of 4.5 and 3.5 and in the samples stored at 4˚C the number was more than those stored at 25˚C (P<0.01). Whereas salt concentration did not show significant effect on survival rate of Lactobacillus casei in milk. These results showed that milk with pH of 6.5 and 5.5 containing maximum salt of 4%  at 4˚C for about 30 days and at 25˚C for about 15 days is a good environment for storage and transport of  Lactobacillus casei to the consumers.

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