Milk is sterile at secretion in the udder but is contaminated by bacteria even before it leaves the udder. In normal case the bacteria at this point are harmless and few in number but milk is a good growth nutrient for them. So the control for pathogens is really important, specially in raw milk products but bateria are used as well starter cultures in the production of cultured dairy products such as yogurt.
Typical pathogens in milk and diary products: Bacillus cereus, Listeria monocytogenes, Yersinia enterocolitica, Salmonella spp., Escherichia coli O157:H7, Campylobacter jejuni
Coliforms: Are used as indicator system for fecal contamination and they are closely associated with the presence of pathogens but not necessarily pathogenic themselves.
Spoiling bacteria (degradation of protein, carbohydrates and fats): Most psychrotrophs are destroyed by pasteurization temperatures, however, some like Pseudomonas fluorescens, Pseudomonas fragi can produce proteolytic and lipolytic extracellular enzymes which are heat stable and capable of causing spoilage. Some species and strains of Bacillus, Clostridium, Corynebacterium, Arthrobacter, Lactobacillus, Microbacterium, Micrococcus , and Streptococcus can survive pasteurization and grow at refrigeration temperatures
