Right from infancy, we are familiar with milk and products as food that nourishes us and also, we love many delicacies made out of it. However, not many are aware of the invisible world of microbes in them. We can’t see the microbes unless we use microscope, however, we can see large number of them clumped together when bread spoils with either green or black mould.
How do microbes get into milk? Dairy farm environment is rich with microbes and many of these get into the cow udders and contaminate the milk. So, milk while milking is already having many microbes and in addition gets more microbes added to it as it is being milked and stored until processing. There are all kinds of microbes that can get into milk and as milk is a rich source of nutrients not just for humans but for other animals and even microbes so they can grow rapidly in milk if not controlled or destroyed.
There are many different types of microbes including moulds, yeasts and bacteria; some are beneficial for health, some are useful in fermentation and there are some that cause spoilage, while a few are dangerous as they cause food poisoning or diseases. Hence, milk should be handled carefully as it can undergo spoilage very quickly. The trick is hygiene, use of heat to destroy spoilage or disease-causing microbes, adding beneficial and useful organisms and processing under controlled conditions like fermentation which permits growth of desirable microbes.
Beneficial Microbes (https://my.clevelandclinic.org/health/treatments/14598-probiotics)
There are some bacteria belonging to Lactobacillus, Bifidobacter and others that are known to provide health benefits when consumed. These are safe for human and can withstand the stomach environment and go through the gut and lodge in large intestine where they use soluble fibre for their growth and produce compounds like short chain fatty acids that have health benefits and also discourage the growth of pathogenic organisms. During diarrhea, these microbes, which are also called probiotics, can be used to help control it. These are also beneficial when a person is lactose intolerant as these microbes from curd or yogurt not only help digest lactose but also have lactase enzyme which can also hydrolyse lactose to it no longer causes problem.
Milk contains a large number of bacteria that can use lactose, the milk sugar, for their growth and convert it into lactic acid. Under uncontrolled conditions this will spoil the milk, souring it. However, when it is pasteurised or heated and these fermenting microbes are added back, they produce acid without other spoilage organisms growing. The process will produce dahi (curd) or yogurt with good acidity and excellent flavour from the fermenting organisms. Fermenting organisms chosen can work well at higher temperature where unwanted microbes do not grow. Dahi can be further hung in cloth to drain off whey to prepare shrikhand.
Cheese making is slightly different, wherein pasteurised milk is fermented with microbes to produce some acidity to which rennet extract is added. This extract is commonly obtained from calf stomach but in India this is banned so microbial rennet is used for coagulation of acidified milk. This enzyme destabilises the casein, milk protein, so it precipitates. This is then drained to remove whey and solidify the cheese. (http://milkfacts.info/Milk%20Processing/Cheese%20Production.htm)
There are some cheeses that are further ripened using microbes such as mould (blue cheese), or bacteria (Swiss or Brie cheese). Blue cheese gets its colour from mould growth which has bluish green colour while bacteria growing in Swiss cheese produce carbon dioxide which expands and makes holes in cheese.
Paneer is a kind of unfermented and unripened cheese. The acidification and coagulation is done by addition of acid, very commonly lemon juice or vinegar. Boiling helps both getting rid of spoilage organisms as well as hardening the curdled milk.
Undesirable Microbes (https://www.cdc.gov/foodsafety/rawmilk/raw-milk-questions-and-answers.html#print)
As milk is rich nutrient source, undesirable microbes can contaminate and grow in it if uncontrolled. Normally these are commonly present in dairy farm environment. Hygienic practices using clean hands and equipment as well as ensuring clean, healthy animals without any diseases will reduce the chances of undesirable contamination. Many harmful bacteria including Salmonella, E. coli, Listeria, Campylobacter, Cryptosporidium and Brucella may be present. Pasteurisation is a very efficient method of destroying these along with other unwanted microbes. Many microbes may not cause diseases but may cause production of acid and undesirable smells along with loss of nutrients, so their removal is also desirable.
Thus, microbes are always there in milk and dairy products and are difficult to remove once they are allowed to grow to large numbers. Some of them are useful either for health or for making some fermented products. Others could be either harmful or spoilage and need to be avoided and/or controlled. Proper precautions like hygiene and good handling or milk and dairy products would allow us to enjoy them and also derive health benefits from them.