In this article we will discuss about the measures to minimize the contamination of milk.
Due to moderate pH (6.4-6.6), good quantity of nutrients, high water content etc. make milk an excellent nutrient for the microbial growth. It is mainly the udder interior, teats surrounding environment and manual milking process, make the source of contamination. These organisms mainly are micrococci, streptococci, and the diphtheroid Corynebacterium Boris.
An inflammatory disease of mammary tissues called mastitis is due to many microorganisms such as Staphylococcus aureus, Escherichia coli. Streptococcus agalactiae, S. uberis, Pseudomonas aeruginosa and Corynebacterium pyogenes. This disease causes economic loss in the dairy industries.
Severely contaminated teats give upto 105 colony forming unit (CFU) in 1 ml in the milk. Sometimes the animal feed and manure can be a source of infection in human beings. Such pathogens are E. coli. Salmonella and Bacillus species. Clostridium butyricum can get into milk from silage fed to cows. This causes the problem called late blowing in some cheese.
Measures to Minimize the Contamination of Milk:
The dairy industry needs enough cleaning, preventing muddy area wherever possible not to allow to leave urine and faeces, shaving udders and trimming tails. There should be a regular washing of teats with warm water containing some disinfectants. Such precautions are to be taken to minimize milk contamination so as to improve the microbiological quality of milk.
During the storage of milk at low temperature in refrigerated holding tanks until its use temperature remains below 70°C and most of the organisms grow at such a temperature.
In raw milk, Gram-negative rods of the genera Pseudomonas, Acinetobacter, Alcahgenes, Flavobacterium, Aerobacter spp. and Gram-positive Bacillus spp. are dominant microorganisms. The presence of psychrotrophs is a traditional test for the microbiological quality of milk based on the reduction of a redox dye such as methylene blue. Psychrotrophs reduce the dye poorly.
Heat treatment is based on thermal destruction of microorganisms. To protect against milk- borne diseases in human beings, heat treatment and particularly pasteurization was found most suitable.
The following types of heat treatment applied in the milk are given below in the Table 21.1:
A simple phosphatase test is recommended to determine whether milk has been properly pasteurized or not. Milk has the alkaline phosphatase inactivated by the time/temperature combinations applied during pasteurization. To determine about the complete pasteurization of milk, if it is free from microorganisms contaminating raw milk, a chromogenic substrate is added.
The alkaline phosphatase present in the milk will hydrolyse the substrate producing a colour which can be compared to standards to determine whether the milk is acceptable or not. The pasteurized milk should have less than one coliform ml-1 and after 5 days storage at 6°C, its count at 21°C should be less than 105 cfu/ml.
Sometimes, raw milk may also contain a number of organisms called thermoduric that can survive mild pasteurization treatments. These are generally Gram-positive bacteria namely Micro-bacterium, Micrococcus, Enterococcus, and Lactobacillus, but Gram-negative, Alcaligenes tolerans may also survive.
The spoilage of pasteurized milk is also due to the growth of psychrotrophic Gram-negative rods such as Pseudomonas, Alcaligenes and Psychrobacter which may contaminate the milk after pasteurization.
The pasteurized milk produced under proper manufacturing process should keep for more than 10 days under refrigeration. The spoiled milk imparts off odours and flavours, sometimes shows clotting due to proteolytic activity. The souring of milk also indicates milk spoilage due to the growth of lactic acid bacteria.
The thermoduric Bacillus cereus causes a flavour defect in the milk by showing appearance of bitty cream phenomenon produced by the lecithinase activity. This enzyme hydrolyse the phospholipids associated with the milk fat globules to produce small proteinaceous fat particles which float on the surface of hot drinks and adhere to the surfaces of crockery and glasses.