The survival of microbial pathogens in dairy products
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Date
2009-01
Authors
Lefoka, 'Mamajoro
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
English: The history and the background of dairy products, specifically fermented milk products and foodborne diseases were reviewed. The constructed literature review also gave details of the microbial and nutritional compostion of fermented milks and examples of African traditional fermented milks. Health benefits of these dairy products were also included. Furthermore, the most common foodborne pathogens and their survival in dairy products were discussed. The survival of Escherichia coli O157:H7, non pathogenic Escherichia coli, Listeria monocytogenes, Staphylococcus aureus, Salmonella spp. and Shigella spp in plain and fruit yoghurt during cold storage at 4 °C was investigated. The survival of these microbial pathogens ranged between 1-14 days in both types of yoghurt, with Staphylococcus aures being the most sensitive pathogen to the yoghurt enviroment. Lactic acid bacteria and yeasts counts remained fairly stable at an average of 8 log cfuml-1 and below 10cfuml-1 respectively. A reduction in pH was noticed during yoghurt storage. The high numbers of LAB and low pH played a major role in the inhibition of the food borne pathogens over time in both plain and fruit yoghurt. The survival of the same microbial pathogens in fruit yoghurt during temperature abuse at 25°C was investigated. There was a rapid die-off of the food borne pathogens at this temperature. Their survival in the yoghurt ranged between 1-3 days. There was a rapid decrease in pH and an increase in the LAB and these together with the high temperature played a major role in the higher death rate of the foodborne pathogens in the fruit yoghurt. Yeasts grew to high levels that caused the yoghurt to spoil. When comparing yoghurt stored at 4 °C and 25°C a conclusion was made: yoghurt stored at 4 °C will be of good quality but will not be regarded as safe as pathogens survival for a long period, and yoghurt stored at 25°C will be regarded as safe but will not be of good quality as high yeasts counts will result in spoilage. The microbial pathogens were also inoculated into yoghurt and their survival at 4 °C after temperature abuse at 12 °C and 37 °C for 4-6 hours was studied. The pathogenic microorganisms died-off at a higher death rate during temperature abuse at high temperatures than at a low temperature at 4 °C. At 4 °C the death rate decreased and the survival in yoghurt ranged between 1-11 days. A rapid decrease in pH and a slight increase in LAB were observed during and after temperature abuse. The yeast increased during the storage of yoghurt. This study showed that inhibition of foodborne pathogens is high at higher temperatures. The survival of the microbial pathogens in milk during the production of Sethemi at 25 °C and 37 °C was studied. The pathogens increased, growing to high counts during the first 24hrs of fermentation. After 24hrs growth of some pathogens was inhibited, however, all the pathogens could still be detected even at the end of fermentation at both temperatures. Yeasts grew to some extend and the lactic acid bacteria grew to high levels during the fermentation of milk at both temperatures. The lactose produced by LAB decreased with time and there was a slight increase in galactose. A rapid increase in lactic acid was noted after a few hours of fermentation, this caused a decline in pH which resulted in the inhibition of pathogens. The inhibition was most effective at 37 °C.
Description
Keywords
Dairy products -- Analysis, Fermented milk -- Contamination, Dissertation (M.Sc. (Microbial, Biochemical and Food Biotechnology))--University of the Free State, 2009, Sethemi, Yoghurt, Pathogens, Foodborne, Microbial, Inhibition, Dairy products