Pasteurization of milk and dairy products (part 1)

In short. Pasteurization means heating milk for a set time to destroy pathogenic microbes; the method was proposed by Louis Pasteur in the 19th century. There is long-time pasteurization (63–65 °C, 30–40 minutes, in a VDP batch pasteurizer), short-time pasteurization (72–76 °C, 15–20 seconds) and flash pasteurization (85–87 °C, a few seconds). Pasteurization does not destroy bacterial spores, so even pasteurized milk is not kept for long.

Milk and dairy products are well known to spoil quite quickly. Milk is also a favourable medium for the growth of various microbes and bacteria that can be dangerous to humans. In the mid-19th century, the French microbiologist Louis Pasteur proposed a new way of keeping dairy products for longer without the risk of picking up an infection from the milk. The method came to be called pasteurization, after its inventor.

What is it?

It is the process of heating liquid products (milk, beer) to a high temperature of 60 to 80 degrees for a set time. A dedicated milk pasteurization vessel is used for this (it can also be done at home).

There are three types of milk pasteurization:

• Long-time — a temperature of 63–65 degrees is used, with a treatment time of 30–40 minutes. With this type, practically all pathogenic microbes and bacteria are destroyed; only spores survive. The drawback of this method is that the unit takes up a large area — it uses a VDP pasteurizer (VDP long-time pasteurization vat);
• Short-time — a temperature of 72 to 76 degrees, with a treatment time of 15–20 seconds. The liquid is fed into the vessels by special pumps. This method destroys even the thermophilic bacteria that survive long-time treatment, but it should be noted that it also changes the composition and properties of the product — fewer beneficial substances remain;
• Flash — a temperature of 85–87 degrees is used, and treatment takes only a few seconds. The big advantage of this method is that practically all bacteria and microbes are destroyed, but again the properties of the product change — it is no longer as beneficial and becomes just an ordinary ingredient.

This type of product treatment is mandatory at all dairies and breweries — without it the product is unfit for consumption because of its high bacterial count. This applies especially to dairy products: in an unfavourable environment a great many different microbes multiply in them, and if not treated in time they can become the causative agents of dangerous diseases such as tuberculosis, typhoid, dysentery, cholera and so on. We often hear that milk should always be kept at a low temperature, since warmer air changes the chemical composition and properties of the product and increases the number of microbes and bacteria. But it should also be noted that milk should not be stored for too long, even when pasteurized: the microbes are destroyed, but the spores remain, and if the ambient temperature changes, the spores will also start to multiply actively.

Can pasteurization be used for canning?

The answer is simply no. A sealed metal container is an excellent environment for various pathogenic bacteria — mainly anaerobic ones (that is, those that multiply without oxygen, in a closed space). Tyndallization is the most suitable method here: the liquid is heated in stages, usually 4–5 times at intervals of a day (24 hours), at a temperature of around 100 degrees. During the rest periods, the bacterial spores that survived the heat treatment germinate, and the new microbes are destroyed in the next treatment. In this way the product is completely freed of microbes and bacteria.

Although progress never stands still and new food preservation methods and technologies appear every day, it is still not worth risking your health by keeping perishable products for long. It is better to buy milk more often and use it as intended than to try to keep it in the fridge in the hope that pasteurization has destroyed all the bacteria. Despite the high efficiency of this method, a small number of bacteria still remain in the product, and long storage can allow them to multiply.