Milk is a very valuable food.No wonder our ancestors called the home cow "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 because of the propensity of the drink to ferment. To understand why milk turns sour, let's see what it consists of.
Milk is designed to feed the young.mammals. It contains the full range of nutrients necessary for the growth and development of newborns. Nutrient absorption is very high and is close to 95%.
Moreover, the milk of animals of different species significantlydiffers in caloric content and quantitative content of biological substances. Let us dwell on the composition of cow's milk. It contains the following substances:
There is also a certain amount of milk.bacteria that belong to the 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 caused by them, differ.
Lactic acid, propionic acid bacteria, kefir fungi and milk yeast are involved in the “useful” fermentation of milk.
Lactic acid microbes are found in milk.natural conditions and are the main "culprits" of why sour milk. The biology of bacteria is based on the processing of lactose into lactic acid. As a result, the acidity of the drink increases, and protein-casein is rolled. Some types of lactic acid bacteria are introduced into milk specifically for the production of yogurt, cottage cheese, fermented milk cheeses, sour cream and acidophilus. This group includes: acidophilic, Bulgarian and cheese lactic acid sticks; lactic streptococci.
Propionic acid bacteria contribute to the milk in the production of cheese. As a result of the processing of milk sugar, propionic and acetic acids are formed and carbon dioxide is released.
Along with lactic fermentation, alcohol fermentation can also occur in milk. It is called specific yeast and is used in the production of kefir.
For sour milk whole milkenough to leave in a warm place for 1-2 days. But for the manufacture of other products necessary microorganisms contribute to the prepared substrate. This explains why milk will turn sour with this or that result.
In addition to the desired fermentation, somemicroorganisms cause butyric fermentation. That is why sour milk is bitter. Spore-forming butyric acid bacteria convert dairy sugar into carbon dioxide, butyric acid and hydrogen. As a result, the milk gets a bitter taste and unpleasant smell. Such fermentation occurs mainly in sterilized and pasteurized milk, as well as in cheeses. The fact is that the butyric microbes transfer the boiling point for a long time (up to 30 minutes) and remain the only inhabitants able to ferment the product.
Another reason why milk sours andacquires an unpleasant taste; there may be putrefactive bacteria that develop in fresh milk in case of its contamination and in violation of storage conditions. Putrefactive bacteria affect the product at temperatures below + 10 ° C, lactic acid viable at + 10 ° C - + 20 ° C. Unlike lactic acid, putrefactive microbes do not die during pasteurization, therefore milk from a bag often “gets rotten” rather than fermented. In this case, the microorganisms break down proteins and milk fats, which leads to the appearance of degradation products with a characteristic rancid or rotten odor.
The speed of fermentation of milk depends on several factors.
It should be noted that for fresh milkripening is a natural process, at room temperature it begins approximately 12-24 hours after sowing from one cow. Mixed milk sours 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.
Milk is pasteurized in several ways:
Такое молоко сохраняет значительную часть vitamins and enzymes, and most bacteria die. "In the ranks" are only heat-resistant microbes. This explains why milk does not turn sour for a long time. Pasteurized milk is stored in the refrigerator for 2 weeks. Also, this product is used to introduce various microorganisms and create directional fermentation.
The most optimal way to keep the maximumuseful substances - ultrapasteurization. With this technology, milk is exposed to high temperature (+ 135 ° C) for 3-4 seconds. Then the product is cooled to + 4 ° C and packaged in sterile packaging. Unlike conventional pasteurization, persistent spore forms (including putrefactive bacteria) die. Ultrapasteurized milk can be stored in the refrigerator for up to two months.
Sterilization kills all microorganisms.Such milk is sterile, packaged in aseptic containers, it has a shelf life of up to 12 months. Everyone knows why homemade milk does not sour after boiling, because bacteria die. But at home it is not possible to carry out high-temperature processing and provide a working area and aseptic packaging free from bacteria. But in industrial conditions, milk is sterilized at a temperature of +120 - + 150 ° С for 20-30 minutes.
Другой причиной, почему не скисает коровье milk, there may be metabolic disorders in the body of cows. With an incorrect ratio in the feed of sugar and protein, with protein overfeeding, a disease called “ketosis” occurs. Ketone milk is very harmful for the human body, it practically does not get sour, and sour cream with a bitter taste is obtained from the separated cream.
Dairy products are known since ancient times.Each culture has its own ways of preparing this wonderful and healthy food. They differ mainly in the initial composition of milk and injected ferment.
Хранят молоко и молочнокислые продукты в чистой closed dishes, observing the temperature conditions and terms indicated on the packaging by the manufacturer. Adhere to the recommendations, and the question “Why does milk sour quickly?” Will not arise. If there are no clear instructions on the storage mode, 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.
