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 look at what it consists of.
What substances are contained in milk
Milk is intended to feed young mammals. It contains a full spectrum of nutrients necessary for the growth and development of newborns. Nutrient absorption is very high, approaching 95%.
Moreover, the milk of animals of different species differs significantly in calorie content and quantitative content of biological substances. Let's take a closer look at 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 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 differ.
Bacteria causing desired fermentation
Lactic acid, propionic acid bacteria, kefir fungi and lactic yeast are involved in the "useful" fermentation of milk.
Lactic acid microbes are naturally found in milk and are the main "culprits" of 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 increases, and the casein protein coagulates. Some types of lactic acid bacteria are introduced into milk specifically for the production of curdled milk, cottage cheese, sour-milk cheeses, sour cream and acidophilus. This group includes: acidophilic, Bulgarian and cheese lactic acid sticks; lactic acid streptococci.
Propionic acid bacteria are introduced into milk during cheese production. 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, alcoholic fermentation can also occur in milk. It is calledspecific yeast and is used in the production of kefir.
To get 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 with one result or another.
Tasteless curdled milk
In addition to the desired fermentation, some microorganisms cause butyric fermentation. That 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. Such fermentation occurs mainly in sterilized and pasteurized milk, as well as in cheeses. The fact is that butyric microbes endure 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 acquires an unpleasant taste may be putrefactive bacteria that develop in fresh milk if it is contaminated and if storage conditions are violated. Putrefactive bacteria have their effect on the product at temperatures below +10°C, lactic acid bacteria are viable at + 10°C - +20°C. Unlike lactic acid, putrefactive microbes do not die during pasteurization, so milk from a bag often “rots”, 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 rancidor rotten smell.
Why milk turns sour quickly
The speed of milk fermentation depends on several factors.
- The optimum temperature for the development of lactic acid bacteria is from +30°С to +40°С. At this temperature, milk sours very quickly. So store the product in the refrigerator at +4°C.
- Milk bought in the store quickly turns sour even when stored in the refrigerator due to a violation of production technology. These can be: non-compliance with the sanitary regime on the farm during milking and transportation, failure in the sterilization process of 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 approximately 12-24 hours after milking from one cow. Mixed milk sours faster. To extend the shelf life, technological methods such as pasteurization and sterilization are used. They are based on the temperature treatment of the product, but differ in the mode of exposure.
Pasteurization
Pasteurization of milk is carried out in several ways:
- Kept 30 minutes at +65°C.
- At +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. "In the ranks" are only heat-resistant microbes. This explains why milk does not sour for a long time. Pasteurized milk is stored in the refrigerator for 2weeks. Also, such a product is used to introduce various microorganisms and create directed fermentation.
The most optimal way to preserve the 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, resistant spore forms (including putrefactive bacteria) die. UHT milk can be stored in the refrigerator for up to two months.
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 the bacteria die. But at home, there is no way to carry out high-temperature processing and provide a bacteria-free work area and aseptic packaging. But in industrial conditions, milk is sterilized at a temperature of +120 - +150 ° C for 20-30 minutes.
This product has 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 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 separated cream.
Milk fermentation products
Fermented milk products have been known since ancient times. Each culture has its own way of preparing this wonderful and he althy food. They differ mainly in the initial composition of milk and the starter introduced.
- Curdled milk is easy to make at home. For this, sourdough is added to warm boiled milk - a spoonful of yogurt or sour cream. Keep in a warm place for about a day. Ryazhenka is Ukrainian curdled milk made from baked milk.
- Acidophilus is recommended for people with bowel problems. The product perfectly restores the microflora of the gastrointestinal tract.
- Yoghurt is often filled with fruit and berry fillers.
- 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 maturation. You can make kefir at home, but for this you will need to get a culture of the fungus. In ancient times, kefir was so valued that the leaven was vigilantly guarded from prying eyes and passed from mother to daughter as a dowry.
- Kumiss is prepared by the peoples of Central Asia from mare's or camel's milk. Has healing properties fordiseases of the stomach and intestines, tuberculosis.
- Cheese, cottage cheese, sour cream butter are also produced with the participation of various starter cultures.
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 does milk turn sour quickly?" will not occur. If there are no clear instructions on the storage mode, focus on a temperature of + 4 ° C - it is suitable for almost all dairy products. Remember that dairy foods require careful handling, and spoiled foods can cause serious poisoning.
