What kind of bacteria cause spoilage in milk?

Milk is a nutrient-rich fluid secreted by all mammalian species to meet the nutritional needs of their newborns, since become an important part of the human diet. Because of its complex content, high water activity, and neutral pH, milk is very susceptible to microbial infection, resulting in the spoilage of raw milk and milk products.

Spoilage refers to any undesired alteration or decrease in the quality of milk. These alterations can take the form of a disagreeable look, colour, odour, or taste, among other things. Milk has a complicated biochemical makeup and a high water activity, making it an ideal culture medium for the growth and multiplication of bacteria that can cause major health problems. Milk spoilage can be caused by both primary and secondary reasons.

Primary factor includes microbes or enzymes that degrade milk quality by decomposing elements such as protein, fat, and carbohydrates.

A secondary reason is the addition of an undesirable chemical or toxin to the milk, rendering it unfit for human consumption. Furthermore, it covers factors that are naturally present in food, like as moisture and oxygen concentration, which promote microbial growth.

Bacteria can enter milk from a number of places, including personnel, diseased cows’ udders, faeces, and dust in barns, milk containers, and other equipment. Some microorganism when present in milk, can cause sickness.

Spoilage of milk and milk products results from growth of fermentative bacteria when storage temperatures are sufficiently high for psychrotrophs. Heat-resistant proteinases of psychrotrophic bacteria cause spoilage in processed milk because of enzyme-retaining activity after the heat treatment.

Raw Milk Spoilage

Because of its high moisture content, near-neutral pH, and vitamin content, raw milk is a great medium for microbes. Lactose, nitrogenous compounds (such as proteins, amino acids, ammonia, urea, and others), unsaturated fatty acids, triglycerides, and minerals all contribute to microbial deterioration of raw milk.

The bacteria that cause spoiling in raw milk are predominantly aerobic Gram-negative psychrotrophic rods such Alcaligenes, Flavobacterium, Pseudomonas, and a few coliforms. Pseudomonas spp. account for 65–70% of psychrotrophic microbes in raw milk.

Pasteurised Milk Spoilage

Psychrotolerant bacteria, mainly nonsporeforming Gram-negative rods or Gram-positive spore-forming bacteria, degrade pasteurised milk. Gram-negative rods, such as Pseudomonas spp., spoilage pasteurised milk by insufficient heating or, more commonly, postprocessing contamination.
Most acid-forming bacteria are killed by pasteurisation, but heat-resistant thermoduric bacteria are not (such as Bacillus, Clostridium, Corynebacterium, Enterococcus, Lactobacillus, Micrococcus, and Streptococcus).Because some species are also psychrotrophic, these thermoduric bacteria can develop at low temperatures. Coliforms, Alcaligenes, Flavobacterium, Pseudomonas, and other bacteria can infiltrate milk after it has been pasteurised. Pasteurized milk has a short shelf life in the refrigerator, owing to the accumulation of psychrotrophic pollutants