Effect of Temperature at the Shelf Life of Yogurt and the Role of <i>Listeria monocytogenes</i> and <i>E. coli</i> Inoculated in Unflavoured and Flavoured Yogurts

Abstract

E. coli and L. monocytogenes are all dairy product related pathogens. The presence of these pathogens can lead to contamination. To increase the shelf life of yogurt we have to monitor the temperature at which it is stored, the change in microbial counts, pH, acidity, sensory attributes and percentage of free whey. Adaptation Test Acid demonstrates that the microorganisms such as Salmonella spp., E. coli, and L. monocytogenes are frequently identified to have a higher survival rate in meals when compared with non-adopted ones. When they are exposed to unfavorable growth circumstances such as severely acidic environments. In this study, we evaluated the survival of wild and adapted L. monocytogenes strains, inoculated at the same concentration around 4 log cfu/g; a slow decrease in the loads was observed until d 28 in unflavored inoculated with the wild strain. Three different experiments are performed on yogurt to evaluate the difference between flavored and unflavored yogurt, shelf life of them at different temperatures and to determine the role of E. coli and Listeria monocytogenes and see what changes it brings to the composition of yogurt. First trial is performed at three different temperatures. These are at 4, 8 and 20°C. Both flavored and unflavored yogurt sample showed low viable counts at 4°C until the end of trial. While performing this trial at 4°C the loads are lower in strawberry yogurts as compared to that of unflavored yogurt because potassium sorbate is present in fruit pure and anti-microbial activity is exerted by that. In second trial, E. coli and Listeria monocytogenes are added to the yogurt sample at two concentrations which 2 and 5 log cfu/g which will show a rapid decrease in acidic conditions in both flavored and unflavored yogurt samples. Listeria monocytogenes is very resistant in this case and presence of it can always be seen until the end period. In the third trial, the adaption of the yogurt sample is monitored after the inoculation of microorganisms in it. Between the wild acid adapted strains of L. monocytogenes no statistically significant difference is detected and that must be because to the quick adaption after the inoculation. Pasteurization is one of the main processes which are used to make the dairy products pathogen free and the basic functioning of it is through temperature and discussed in the paper.