Is the milk you drink pasteurized?
A long time ago, both scientists and ordinary residents knew that milk would spoil and become kefir after a long time.
But scientists know a little more, and they find yeast cells or bacteria in kefir that can cause items to become acidic.
In fact, whether it is the acidity of milk or the deterioration of beer, or even the alcohol found in fruits, these are the credit of yeast cells and bacteria.
However, limited by the limited technology at that time, many people believed that these microorganisms were the result of food spoilage, that is, because the food deteriorated, so the microorganisms were produced.
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It wasn't actually scientific, but it was just the dominant idea at the time.
When people first associated the observed microbes with the deterioration of such foods, a new doctrine arose, which is called "vitality theory".
However, scholarship in this area is often considered wrong, and some even say that such doctrines have set science back 50 years.
And the center of the vortex is Pasteur.
Louis Pasteur was actually a Frenchman who lived from 1821 to 1895 and was both a microbiologist and a famous chemist.
At the time, he asked this question:
If fermentation is derived from bacteria, where do bacteria come from?
The story dates back to September 1854, when Pasteur was still the head of the chemistry department and had a keen interest in the alcohol industry.
There is a very important process in the production of alcohol, that is, fermentation.
In studying the fermentation process, Pasteur discovered that there was a spherical body in the fermentation broth that was much smaller than yeast.
After these globular bodies are cultured, they slowly become yeasts.
Alcohol and carbon dioxide are produced during yeast propagation, which are the consequences of yeast breaking down sugar.
And the whole process does not require oxygen at all, and can be carried out under anaerobic conditions.
So fermentation is not the result of bacteria, but the bacteria in the process of their own growth caused by fermentation.
In order to test his conjecture, Pasteur did many experiments, and gradually proved this idea through his own research in the 1860s.
After that, he found that heating the liquid to 55 ° C could kill all the microorganisms in the liquid.
After that, the sterilized milk has not deteriorated for a long time, both in storage and in transit.
He demonstrated the important role of bacteria in fermentation through a series of experiments.
If the bacteria are destroyed, then there is no fermentation.
It should be known that between 1856 and 1860, people's nature of fermentation was still in the exploratory stage, and at that time he had already proposed that microbial metabolic activity was the basis of fermentation, which could be regarded as a new theory.
In 1857, he published an article on fermentation, "A Record of Lactic Fermentation", which has always been regarded as a microorganism and a classic paper recognized by the academic community.
This also opened a door to the microbial community, allowing countless scholars to emerge their own ideas, and people's understanding of the world has been deepened by an inch.
Of course, to this day, this experiment is no longer an experiment, but a very mature technology, which has been improved and applied to the preservation and transportation of milk and other foods.
If you go to the supermarket to buy milk, it is likely that this method is used, which is called "pasteurization".
In fact, pasteurization is to use the characteristics of many pathogens that are not heat-resistant, and completely kill these heat-intolerant bacteria at the appropriate temperature and holding time.
In general, the most suitable growth temperature for bacteria is 28 ° C - 37 ° C.
However, if the liquid is heated until 68 ~ 70 ° C, and maintained for 30 minutes, it can effectively kill the general bacteria.
However, there are some bacteria that can continue to survive at such a high temperature, but I did not expect that pasteurization also has a magic weapon, that is, sharp cooling.
After maintaining a high temperature for 30 minutes, it is usually necessary to rapidly cool the liquid to 4 to 5 ° C.
This drastic change in heat and cold causes other related bacteria to die.
But does pasteurization really sterilize?
Obviously, if you want to kill all bacteria and meet the medical sterilization standards, you need more stringent means.
At present, according to the current technology, after pasteurization, although a small number of heat-resistant bacteria or bacterial spores are retained, the number of these bacteria is relatively small and will not develop into the number of food spoilage in a short period of time.
Therefore, as long as the pasteurized milk is still placed in a 4 ° C environment, it can also be retained for 3 to 10 days, and it can not exceed 16 days at most.
The next time you drink milk and beer, will you remember pasteur's story?
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