Milk and dairy products present an ideal environment for growth of beneficial, pathogenic and spoilage microorganisms (MO). In our thesis, we investigated how these affect raw and pasteurised milk, yoghurt, kefir, cream and butter. By reviewing the literature, we realised that the most common beneficial MO are lactic acid bacteria (LAB), for example Lactobacillus, Lactococcus, Leuconostoc, Streptococcus and Enterococcus. The most technologically important function of LAB is lactose fermentation, which is important in the production of fermented dairy products. Beside technologically important LAB, there are also health beneficial probiotics (for example Bifidobacterium spp.). The most common pathogens in milk and dairy products are Listeria monocytogenes, Salmonella spp., Staphylococcus aureus, Campylobacter spp., Escherichia coli, Enterobacter and Yersinia enterocolitica, mainly as a result of poor health control of cows and staff. However, consumption of such raw milk and products can lead to disease outbreak. The presence of spoilage MO is due to poor hygiene of milking equipment, insufficient heat treatment, and careless and unhygienic processing of raw milk. Spoilage is evident as uncharacteristic taste, smell, colour and aroma. The most common spoilage MO are psychrotropic bacteria, which are heat-sensitive, but they produce proteolytic and lipolytic enzymes.
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