In the refrigerator, it stays fresh for up to 2-3 days, at room temperature it deteriorates much faster. Untreated cow's milk, which has been kept at temperatures up to 20 degrees C, becomes sour in about 10-20 hours.

But why does milk turn sour? How does this process take place? And can you drink it stale? We will answer these and other questions.

How and why does the souring process take place?

Milk contains a special milk sugar called lactose. There is not much of it, only 4.7%. Lactose is an excellent breeding ground for bacteria and various microorganisms.

This drink also contains proteins:

• albumen,
• casein,
• globulin.

In addition to lactose and proteins, milk also contains lactic acid bacteria that enter it from the udder of the cow, from its milk ducts and the environment. There are very few of them in a fresh product, then they begin to multiply and their number increases.

In the course of their life, these bacteria feed on lactose and secrete acid, that is, they process milk sugar into lactic acid. As a result, the acidity of the drink rises, and proteins, in particular casein, are curtailed.

It is with the help of lactic acid bacteria that many fermented milk products are made:

• cottage cheese,
• yogurt
• sour cream
• fermented baked milk and others.

In production, they are specially added to fresh milk to make the desired product.

There are different types of lactic acid bacteria: they belong to the Lactobacillaceae family, which in turn is divided into Lactobacilleae and Streptococceae. Most often, the first type of bacteria, lactobacilli, is used to ferment milk, since they act much faster.

Why does milk sour more slowly in the refrigerator, and faster in the room?

Why does milk, which was stored at room temperature, turn sour faster, and when the drink is stored as in the refrigerator, it may not deteriorate for up to several days?

Low temperature slows down fermentation

The explanation is simple: the optimum temperature for the life of lactic acid bacteria is from +25 to 30 ° C. If it drops to +15, then fermentation slows down. Therefore, when storing milk in the refrigerator, the process of reproduction of bacteria is suspended, and therefore the process of souring.

Excess bacteria due to unsanitary conditions

Why else can the milk souring process be accelerated? If the milkmaid did not monitor the cleanliness, worked in unsanitary conditions, then a large number of bacteria could get into it. Sometimes the dirty container in which the drink was stored is to blame.

If there are a lot of bacteria, milk turns sour much faster than usual, even when stored in the refrigerator.

Why does a store drink last much longer than a natural one?

Because store milk is pasteurized before being sold. It is heated to a temperature of 60 to 80 ° C, which causes the bacteria to die. Therefore, such a product is stored much longer.

It turns out that such milk should not turn sour at all. But this does not happen, since when heated, spores of microorganisms survive, which begin to develop as soon as conditions become favorable. Such processing of dairy products extends their shelf life, but does not destroy nutrients.

Why does milk turn sour in a thunderstorm? And is it true?

There is a belief that milk sour during a thunderstorm much faster than usual, and it doesn't matter if it was in the refrigerator or just stood in the kitchen. I would like to say that this is a myth, another superstition that it is time to forget about. But the facts suggest otherwise.

Indeed, milk turns sour during a thunderstorm much faster than on other days. So far, it has not been possible to reliably accurately identify the cause of the acceleration of the activity of lactic acid bacteria, but there are several hypotheses explaining this phenomenon.

Special electromagnetic impulses

Scientists believe that long-wave electromagnetic pulses, which were called spherixes, are to blame for accelerating the fermentation process of milk. They can be detected even at a distance of several hundred kilometers from lightning discharges. However, it is not yet clear exactly how these impulses affect the drink.

It is interesting that on the days of spherix activity, not only milk is curdled, but other compositions also undergo changes: for example, blood thickens in people, and the permeability of gelatin changes at printing enterprises.

Favorable conditions for the growth of bacteria

There is another explanation for this phenomenon. Scientists believe that lactobacilli begin to convert milk sugar into acid faster during a thunderstorm due to more favorable conditions. Everyone knows that when it rains, the humidity and the ambient temperature increase.

Can I drink a sour drink?

We figured out why and how milk turns sour. But is it worth drinking, and isn't it harmful? You should not refuse a sour product, since it is no less useful than a fresh one, but it is absorbed many times better (sour milk - by 90%, fresh - by 32%).

Sour milk is very useful (no less than fermented milk products): thanks to its regular intake, the immune system is strengthened, the intestinal microflora and the work of the digestive tract are improved. Therefore, it is advised to drink it for those who suffer from constipation. In addition, it contains vitamins of group B, E, A, D and a lot of calcium.

And even if you do not like sour milk, you should not throw it away - you can make an excellent dough from it, for example, for pancakes, donuts or pancakes.

Video: what can you quickly and tasty cook from sour milk?

Igor Nikolaev

Reading time: 3 minutes

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A person is allergic to ordinary milk. And in sour milk, protein and lactose are modified. The digestive system is more prepared for such a product. Many people prefer lactic acid drinks because of their taste and health benefits. But sometimes the nutrient liquid turns sour at the wrong time and becomes unusable. Why does a cow's milk turn sour sometimes too quickly?

Cow's milk can stay fresh at room temperature for less than a day. When placed in the refrigerator, it can last for three days, sometimes up to a week. Do not allow milk to sour leukocytes, immunoglobulins and other substances. They have bactericidal qualities that prevent harmful bacteria from growing.

After the specified period, the milk begins to turn sour. The fact is that it contains lactic acid bacteria.

There are not very many of them in the milk itself, but they fall into the can from the udder, from the environment and begin to act. For them, the product creates a wonderful environment. The higher the temperature, the more actively they reproduce with the help of protein, fats and lactose.

List of reasons for sour milk

It is worth boiling the milk, it is curdled, divided into lumps and whey in a saucepan. But if the process occurs earlier than the prescribed shelf life, the product acquires a sour taste and smell, there are explanations for this.

Diseases

Changes in the composition, consistency or reactions of milk are often due to inflammation or infection. Some of them are acute, some are more secretive:

1. mastitis affects the souring process of the product. Inflammation of the udder causes the growth of harmful bacteria. They enter the milk and ferment. It has to be treated with antibiotics. You cannot drink milk at this time;
2. ovarian follicular cyst causes the product to quickly curdle when heated and sour, acquire a bitter taste. When the cow is sick, milk is donated in smaller volumes. The cyst must be cured with the help of a specialist;
3. ketosis is distinguished among the reasons. This is a metabolic pathology due to improper feeding. An excess of acetone and acetic acid is found in milk during this period. For the animal's recovery, the diet is changed and vitamins are introduced.

Storage and milking methods

One of the simple factors that affect the rapid souring of milk is the storage temperature.

Lactic acid bacteria enjoy life in warmth - from 25 to 30 degrees. As soon as the temperature drops to plus fifteen degrees, fermentation will slow down.

Thus, in the refrigerator, this process and the oxidation of the product is slower than at room temperature.

Another reason why a cow's milk quickly turns sour is elementary unsanitary conditions:

1. hand washing and clean clothes should be the first rules for milking;
2. milk container is provided not only clean, but also dry;
3. the udder is cleaned with warm water and wiped dry with soft tissues;
4. in the pen, cleaning of dirt and manure is mandatory, and a regular change of litter.

If these rules are not followed, then foreign objects and dust can get into the milk, with a huge amount of bacteria. They cause its early sourness. After the completion of the process, the liquid must be poured into clean containers, straining through cheesecloth. The latter is recommended to be boiled or simply changed every three days.

Cow diet

Burenka's food affects the quality of milk. The healthier, fresher and more complete the diet, the richer and richer the composition of the product will be.

For example, in the second half of summer, some herbs begin to bloom. Why does a cow have a lot of milk in this season, but sometimes it quickly disappears? Many of the plants affect the taste of milk, helping to reduce its shelf life. These include fresh beet tops. Before giving to the cow, the tops are dried for several days. Then the oxalic acid evaporates.

It is advisable to keep track of what the cow eats in the pasture. In addition to the fact that heavy foods, chunks of vegetables and other food cause digestive problems, some contribute to sour milk.

Prevent sourness

The only sure way to keep milk from souring quickly is boiling. Sterilization of the product allows it to be stored for seven days.

People have long noticed that milk becomes sour during a thunderstorm, but scientists do not yet know why milk turns sour faster in bad weather. It is believed that the fermentation process is influenced by electromagnetic waves, but this has not been proven.

The process of fermentation or curdling of milk is familiar to everyone from childhood. But many people mistakenly believe that milk turns sour from high temperatures. This is not so, or rather not quite so. The fermentation process is accelerated by the high temperature, but even in the refrigerator, the milk will sooner or later turn sour.

Reasons for sour milk

It's all about streptococci, of which there are a lot in milk. It is they who react with milk sugar, which provokes the release of lactic acid. Since these bacteria resemble a stick in shape, they are called "milk" or "Bulgarian" sticks. Bulgarian - because they were first investigated by Ivan Mechnikov, who studied Bulgarian fermented milk products and their effect on the body.

He was not the only one who wondered why the milk was sour. The French chemist Louis Pasteur, back in 1857, proved in his research that fermentation is caused by bacteria that are present in the product. His early research is related to wine (this is not surprising, since France is a world wine producer). Pasteur proved that yeast microorganisms are the main reason why wine turns into vinegar. A good way to prolong the freshness of food is to reheat it to a temperature of 50-60 degrees and hold it for a while. Today this method is mainly used for juices, milk and beer, and we call it "pasteurization".

How do you increase the shelf life of milk?

Drinking milk is the simplest of all dairy products. Ways to increase its shelf life - the use of certain technological modes.

The basic technology for the production of drinking milk is heat treatment, which ensures the disinfection of milk. These are pasteurization, sterilization and ultra-pasteurization.

These technologies differ, first of all, in temperature and time regimes. Initially, pasteurization was called heating up to 100 degrees, and sterilization above 100.

The second aspect of the safety of milk (and any food product) is packaging. Depending on the packaging and closure used, there can also be completely different shelf life for the same product.

For a long time not to sour: advantage or disadvantage?

Milk souring is coagulation or denaturation as a result of a shift in electrical and ionic equilibrium. But the reasons for the shift may be different.

• The natural process is the colonization of microflora or the development of microflora that was in the milk and remained. She could have stayed with an insufficient pasteurization regime. And to get there - from containers, equipment, hands, gauze, dishes, in contact with anything. Secondary seeding is called. Most of the "samokvass" get it that way.
• The next reason is the ingress of any substances from the outside that are capable of shifting the balance. This is primarily acid and enzymes. If they somehow get into milk, it turns sour.

If pasteurized milk does not turn sour, this indicates the presence of inhibiting substances, including antibiotics.

An exception may be pasteurized filtered milk. It is sealed using nitrogen. And after the expiration date, it may not turn sour, but become "carbonated".

Antibiotics are often used to treat cows. Moreover, the cow is quarantined for the duration of the treatment. After the quarantine time has expired, she is returned to the herd. But if, for some reason, she still has antibiotic substances in her blood, then milk from one such cow infects the entire batch with antibiotics.

Since drinking pasteurized milk is not a long-term product, this usually goes unnoticed. But some still "run into" pasteurized milk with antibiotics.

• First of all, it is noticed by those who make homemade yoghurts and other fermented milk products. The leaven in milk with antibiotics cannot develop, and it simply does not turn sour.
• Another sign is when, after the expiration date, pasteurized milk does not turn sour by itself, but becomes somehow greenish and smells bad. At the same time, it remains absolutely liquid. Or outwardly it does not change at all, but becomes bitter. This is a sign of the presence of antibiotics.

However, it makes no sense to give up on this or that brand. Because the next batch of the same milk may already be flawless. Antibiotics in milk are the exception to the rule.

But these are not the only inhibitors. The most common, in addition to antibiotics (especially at home), is the ingress of detergents. Surfactants in detergents prevent the development of microflora. Therefore, in a well-washed, but insufficiently rinsed container, the milk will not turn sour.

Antibiotics can only get into milk from raw materials; no one adds them to milk especially during production.

Why sterilized milk does not turn sour, but rotten

Quickly deteriorates - does it mean natural?

The shelf life and "naturalness" of milk are not related in any way. It all depends on the production technology. Shelf life is dictated by safety. And the raw material can be anything. By the way, making low-quality milk with a small shelf life is even more profitable. And people are more willing to buy up.

What kind of milk should you boil?

Milk should be boiled if it is not manufactured. That is, if you buy draft milk, farm milk, market milk, from vending machines, you must boil it!



Milk is a very valuable food product. It is not for nothing that our ancestors called the domestic cow “the nurse”. Due to its unique properties, it is a source of many useful substances and the basis for the production of a large group of dairy and fermented milk products. Such production is possible due to the tendency of the drink to ferment. To understand why milk turns sour, let's figure out what it consists of.

What substances are contained in milk

Milk is intended for feeding young mammals. It contains the full spectrum of nutrients needed for the growth and development of newborns. The digestibility of nutrients is very high and approaches 95%.

Moreover, the milk of animals of different species differs significantly in calorie content and quantitative content of biological substances. Let us dwell in more detail on the composition of cow's milk. It contains the following substances:

• Water - 87.5%.
• Fat - 3.5%.
• Proteins - casein, albumin, globulin - 3.3%.
• Milk sugar - lactose - 4.7%.
• Macro and microelements (mineral part) - 1%.
• Vitamins.
• Enzymes.
• Antibodies that protect newborns from infectious diseases.

Also, there is a certain amount of bacteria in milk, which are classified as normal microflora. They are the answer to the question "why does milk turn sour?" The biology of microbes and, as a result, the type of fermentation they cause varies.

Bacteria causing the desired fermentation

Lactic acid, propionic acid bacteria, kefir fungi and milk yeast are involved in the "useful" fermentation of milk.

Lactic acid microbes are found naturally in milk and are the main culprits in why milk turns sour. The biology of bacteria is based on their processing of lactose into lactic acid. As a result, the acidity of the drink rises, and the casein protein is curtailed. Some types of lactic acid bacteria are introduced into milk specifically to obtain curdled milk, cottage cheese, sour milk cheeses, sour cream and acidophilus. This group includes: acidophilus, Bulgarian and cheese lactic acid sticks; lactic acid streptococci.

Propionic acid bacteria are introduced into milk during the production of cheese. As a result of the processing of milk sugar, propionic and acetic acids are formed and carbon dioxide is released. Simultaneously with lactic acid fermentation in milk, alcoholic fermentation can also occur. It is caused by specific yeast and is used in the production of kefir.

To obtain curdled milk, it is enough to leave whole milk in a warm place for 1-2 days. But for the manufacture of other products, the necessary microorganisms are introduced into the prepared substrate. This explains why milk turns sour in one way or another.

Tasteless curdled milk

In addition to the desired fermentation, some microorganisms cause butyric fermentation. This is why sour milk is bitter. Spore-forming butyric acid bacteria convert milk sugar into carbon dioxide, butyric acid and hydrogen. As a result, milk acquires a bitter taste and an unpleasant odor. This fermentation occurs mainly in sterilized and pasteurized milk, as well as in cheeses. The fact is that butyric acid microbes tolerate the boiling point for a long time (up to 30 minutes) and remain the only inhabitants capable of fermenting the product.

Another reason why milk turns sour and takes on an unpleasant taste may be putrefactive bacteria that develop in fresh milk if it is contaminated and if the storage conditions are violated. Putrefactive bacteria affect the product at temperatures below + 10 ° С, lactic acid bacteria are viable at + 10 ° С - + 20 ° С. Unlike lactic acid, putrefactive microbes do not die during pasteurization, so the milk from the bag often "goes bad" and does not ferment. In this case, microorganisms break down the proteins and fats of milk, which leads to the appearance of decay products with a characteristic rancid or rotten odor.

Why does milk turn sour quickly

The rate of fermentation of milk depends on several factors.

• The optimum temperature for the development of lactic acid bacteria is from + 30 ° C to + 40 ° C. At this temperature, milk turns sour very quickly. So store the product in the refrigerator at + 4 ° C.

• Milk bought in a store quickly turns sour even when stored in a refrigerator due to a violation of production technology. These can be: non-observance of the sanitary regime on the farm during milking and transportation, failure in the process of sterilizing the product, violation of the integrity of the packaging, poor-quality packaging, and so on.

It should be noted that for fresh milk, fermentation is a natural process, at room temperature it begins about 12-24 hours after milk from one cow. Mixed milk turns sour faster. To extend the shelf life, such technological methods as pasteurization and sterilization are used. They are based on the temperature treatment of the product, but differ in the mode of exposure.

Pasteurization

Milk pasteurization is carried out in several ways:

• Withstand 30 minutes at + 65 ° C.
• At a temperature of + 75 ° C for 15 to 40 seconds.
• Temperature + 85 ° С, processing time 8-10 seconds.

Such milk retains a significant part of vitamins and enzymes, and most bacteria die. Only heat-resistant microbes remain "in service". This explains why milk does not turn sour for a long time. Pasteurized milk will keep in the refrigerator for up to 2 weeks. Also, such a product is used to introduce various microorganisms and create directional fermentation.

The most optimal way to preserve the maximum of nutrients is ultra-pasteurization. With this technology, milk is exposed to high temperatures (+ 135 ° C) for 3-4 seconds. Then the product is cooled to + 4 ° C and packed in sterile packaging. Unlike conventional pasteurization, persistent spore forms (including putrefactive bacteria) die. UHT milk can last up to two months in the refrigerator.

Sterilization

Sterilization kills all microorganisms. Such milk is sterile, packed in aseptic containers, it has a shelf life of up to 12 months. Everyone knows why homemade milk does not turn sour after boiling, because bacteria die. But at home, there is no way to carry out high-temperature processing and provide a bacteria-free working area and aseptic packaging.

But in an industrial environment, milk is sterilized at a temperature of +120 - + 150 ° C for 20-30 minutes. Such a product is of less value because most of the vitamins and enzymes are destroyed. Also, lactic acid derivatives cannot be prepared from it.

Does milk from a domestic cow turn sour?

Another reason why cow's milk does not turn sour may be metabolic disorders in the cow's body. With the wrong ratio of sugar and protein in feed, with protein overfeeding, a disease occurs, which is called "ketosis". Ketone milk is very harmful to the human body, practically does not ferment, and sour cream with a bitter aftertaste is obtained from the skimmed cream.

Milk fermentation products

• Fermented milk products have been known since ancient times. Each culture has its own way of preparing this wonderful and healthy food. They differ mainly in the initial composition of the milk and the leaven that is introduced.

• Sour milk is easy to make at home. To do this, a sourdough is added to warm boiled milk - a spoonful of yogurt or sour cream. Kept in a warm place for about a day. Ryazhenka is a Ukrainian curdled milk made from baked milk.

• Acidophilus is recommended for people with bowel disease. The product perfectly restores the microflora of the gastrointestinal tract.

• Fruit and berry fillings are often added to yogurt.

• Kefir comes in different varieties. Kefir fungi introduced into the original product cause lactic acid and alcoholic fermentation. The alcohol content in the drink ranges from 0.2% to 0.6%, depending on the duration of ripening. You can make kefir at home, but for this you need to get a culture of the fungus. In ancient times, kefir was so prized that the leaven was vigilantly guarded from prying eyes and passed from mother to daughter as a dowry.

• Kumis is prepared by the peoples of Central Asia from mare's or camel's milk. It has healing properties in diseases of the stomach and intestines, tuberculosis.

• Cheese, cottage cheese, sour butter are also obtained with the participation of a variety of starter cultures.

Store milk and lactic acid products in a clean, sealed container, observing the temperature regime and terms indicated on the packaging by the manufacturer. Follow the recommendations and the question "why does milk quickly turn sour?" will not arise. If there are no clear instructions on the storage regime, focus on the temperature of + 4 ° C - it is suitable for almost all dairy products. Remember that dairy food requires careful attention, and spoiled food can cause serious poisoning.