Milk is a very valuable food product. No wonder our ancestors called the domestic 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 sour-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 found in milk?
Milk is intended for feeding young mammals. It contains the full range 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 on the composition of cow's milk. It contains the following substances:
- Water - 87.5%.
- Fats - 3.5%.
- Proteins - casein, albumin, globulin - 3.3%.
- Milk sugar - lactose - 4.7%.
- Macro- and microelements (mineral part) - 1%.
- Vitamins
- Enzymes.
- Antibodies protecting newborns from infectious diseases.
Also in milk there is a certain amount of bacteria that are attributed to normal microflora. They are the answer to the question "why sour milk?". The biology of microbes and, as a consequence, the type of fermentation caused by them differ.
Desired fermentation bacteria
Lactic acid, propionic acid bacteria, kefir fungi and milk yeast are involved in the “useful” fermentation of milk.
Lactobacillus microbes are found in milk under natural conditions and are the main “culprits” of why sour milk. The biology of bacteria is based on their processing of lactose into lactic acid. As a result, the acidity of the drink increases, and casein protein is curtailed. Some types of lactic acid bacteria are added to milk specifically for yogurt, cottage cheese, sour-milk cheeses, sour cream and acidophilus. This group includes: acidophilic, Bulgarian and cheese lactic acid bacilli; lactic streptococci.
Propionic acid bacteria are introduced into milk in the production of cheeses. As a result of the processing of milk sugar, propionic and acetic acids are formed and carbon dioxide is released.
Along with lactic acid fermentation , alcohol fermentation can also occur in milk. It is caused by specific yeast and is used in the production of kefir.
To get whole milk sour milk, just leave it 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 with one result or another.
Tasty yogurt
In addition to the desired fermentation, some microorganisms cause butyric acid fermentation. That's why sour milk is bitter. Spore-forming butyric acid bacteria convert milk sugar to carbon dioxide, butyric acid and hydrogen. As a result, milk acquires a bitter taste and an unpleasant odor. Such fermentation occurs mainly in sterilized and pasteurized milk, as well as in cheeses. The fact is that oily microbes tolerate 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 turns sour and gets an unpleasant taste may be putrefactive bacteria that develop in fresh milk in case of contamination and in violation of storage conditions. Putrefactive bacteria exert their effect on the product at temperatures below + 10 ° C, lactic acid is viable at + 10 ° C - + 20 ° C. Unlike lactic acid, putrefactive microbes do not die during pasteurization; therefore, milk from the bag often “spoils” and does not ferment. In this case, microorganisms break down proteins and fats of milk, which leads to the appearance of decay products with a characteristic rancid or rotten odor.
Why milk turns sour fast
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, the milk turns sour very quickly. So keep the product in the refrigerator at + 4 ° C.
- Milk purchased at the store quickly turns sour even when stored in the refrigerator due to a violation of production technology. This may be: non-compliance with the sanitary regime on the farm during milking and transportation, failure in the process of sterilization of the product, violation of the integrity of the packaging, poor packaging and so on.
It should be noted that for fresh milk, fermentation is a natural process, at room temperature it starts about 12-24 hours after being milked from one cow. Mixed milk turns sour faster. Techniques such as pasteurization and sterilization are used to extend the shelf life. They are based on the thermal processing of the product, but differ in the mode of exposure.
Pasteurization
Pasteurization of milk is carried out in several ways:
- Stand for 30 minutes at + 65 ° C.
- At a temperature of + 75 ° C for 15 to 40 seconds.
- Temperature + 85 ° C, processing time 8-10 seconds.
Such milk retains a significant portion of vitamins and enzymes, and most bacteria die. “In service” only heat-resistant microbes remain. This explains why milk does not sour for a long time. Pasteurized milk is stored in the refrigerator for 2 weeks. Also, such a product is used to introduce various microorganisms and create directed fermentation.
The most optimal way to maintain a maximum of useful substances is ultra-pasteurization. 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 a sterile package. Unlike conventional pasteurization, persistent spore forms (including putrefactive bacteria) die. Ultra-pasteurized milk can be stored in the refrigerator for up to two months.
Sterilization
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 the 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 under industrial conditions, milk is sterilized at a temperature of +120 - + 150 ° C for 20-30 minutes.
Such a product is of less value, because most vitamins and enzymes are destroyed. Also, it is impossible to prepare lactic acid derivatives from it.
Doesn't sour milk from a domestic cow?
Another reason why cow's milk does not sour, may be metabolic disorders in the body of Burenka. With the wrong ratio of sugar and protein in the feed, with protein overfeeding, a disease called "ketosis" occurs. Ketone milk is very harmful to the human body, practically does not ferment, and sour cream with a bitter aftertaste is obtained from the separated cream.
Milk Fermentation Products
Sour-milk products are known since ancient times. Each culture has its own ways of preparing this wonderful and wholesome food. They differ mainly in the initial composition of milk and introduced sourdough.
- Yogurt is easy to cook at home. To do this, add ferment in warm boiled milk - a spoonful of yogurt or sour cream. Maintain in a warm place for about a day. Ryazhenka is a Ukrainian yogurt 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 fillers are often added to yogurt.
- Kefir comes in many varieties. Kefir fungi introduced into the initial 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 maturation. You can cook kefir at home, but for this you need to get a fungus culture. In ancient times, kefir was so appreciated that the leaven was vigilantly protected from prying eyes and passed from mother to daughter as a dowry.
- Koumiss is prepared by the peoples of Central Asia from mare or camel milk. It has healing properties in diseases of the stomach and intestines, tuberculosis.
- Cheeses, cottage cheese, butter are also obtained with the participation of a variety of starter cultures.
They store milk and lactic acid products in a clean, sealed container, observing the temperature regime and the terms indicated on the packaging by the manufacturer. Follow the recommendations, and the question "why quickly sour milk?" will not arise. If there is no clear indication of the storage mode, focus on a temperature of + 4 ° C - it is suitable for almost all dairy products. Remember that dairy foods require careful attention, and spoiled foods can cause serious poisoning.