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Milk is an animal product, and contains bacteria both from the cow and from the environment. Microbes like bacteria can cause food poisoning, depending on the species. Raw milk can, therefore, be risky, so farmers generally pasteurize the product before sale to prevent possible food poisoning from milk. When milk is past its use-by date, it can harbor heavy bacterial growth that can also cause foodborne illness.
Cows produce milk from their udders and dairy farmers typically use milking machines, placed on the udders, to collect the milk. Although a perfectly healthy cow produces almost sterile milk, a cow that has even a minor infection of the udder called mastitis may shed bacteria into the milk. In addition, milking machines, and the cow itself, do not have surfaces that are completely free from bacteria.
All of the bacteria that are present on machinery and animals in a well-kept milking shed can get into the milk, so collecting virtually sterile milk is impractical. Not all of the bacteria that can get into milk can cause food poisoning from milk in humans, but some, like E. coli and Salmonella, can. It is also virtually impossible to know what specific types of bacteria are present in each particular batch of milk.
Pasteurization is a process that kills most of the bacteria present, and all of the dangerous bacteria that can cause food poisoning from milk. This involves heating the milk either at a low temperature like 145 degrees Fahrenheit (about 63 degrees Celsius) for 30 minutes, or at a high temperature like 162 degrees Fahrenheit (about 72 degrees Celsius) for a short time of 15 seconds. When pasteurization is complete, a milk drinker can be sure that no dangerous pathogens that can cause food poisoning from milk are present in the finished product.
This is not the case with raw milk, which does not undergo any heat treatment. This type of milk still contains all the contaminants from the cow, from the milking machines, and from the environment, such as splatter on the cow's skin from other cow's feces. Raw milk can be considered a high risk food product, which can be a source of food poisoning when it contains bacteria like Campylobacter, Salmonella and Staphylococcus aureus.
Listeria monocytogenes is another bacterial species that can be present in raw milk, and as well as foodborne illness, it can cause miscarriages in pregnant women. Mycobacterium tuberculosis, the cause of the potentially lethal disease tuberculosis, can also be present in unpasteurized milk. Avoiding raw milk can therefore protect you from getting milkborne illnesses from these particular species. You can also check that the milk you buy states that it is pasteurized on the label, and if the label doesn't state this, you can ask the seller to confirm it, as food safety laws vary from country to country, or even state to state.
Pasteurized milk can become contaminated after processing, but this is unlikely. Do not drink milk with a cap that a bird has pecked open, for example, or milk that is not properly sealed. As well as contamination, you should only drink milk that is within its use-by date.
This is an important consideration, because all of the bacteria that pasteurization does not kill grow over time, even in the refrigerator, and sour the milk past this date. Milk should always be kept at refrigerator temperatures, as this helps to slow down the decomposition of the milk. Pasteurized milk is a low-risk food product once you follow these guidelines.