When milk turns sour. Reasons for the rapid souring of milk in cows. Favorable conditions for bacterial growth

Since ancient times, milk has been the product with which animals, mammals, and people begin their diet. This is not only tasty, but also an incredibly healthy drink, which was created by nature itself; it can be called a real elixir of growth. In most newborns of any kind - kittens, puppies, babies - the stomach is simply not adapted to digest any other food except milk, which contains a huge amount of nutritious and beneficial substances that help a small, fragile organism grow and develop faster.

Today, milk consists of simply a huge variety of products, and milk production itself has grown into a fairly large industrial sector.

The Institute of Nutrition of the Russian Academy of Medical Sciences has even developed approximate norms for the consumption of dairy products for a person per year, and in terms of milk (in kilograms), this turns out to be as much as 392 kg of milk! Of course, this is not milk in its pure form; These include cheeses, butter, cottage cheese, and other fermented milk products. However, according to statistics, the average person in Russia consumes about 200 kilograms of milk per year, which is almost half the recommended amount.

Composition of milk

Before talking about what makes milk sour, you need to understand what it consists of, what is contained in this drink.

The content of various substances in milk largely depends on its origin. Cow's milk is the most popular type of milk of all; It is this milk that is produced in the largest volumes in the livestock sector of the entire world agriculture. As for the composition of this type of milk, you can find an incredible amount of vitamins in it: this includes almost the entire line of B vitamins, as well as vitamins E, D, PP, H, ascorbic acid, nucleic acids, lactose, monounsaturated fatty acids and amino acids. And of course, milk contains a large amount of calcium, which is simply necessary for human health, for the development of the body and strong bones. Cow's milk contains this macronutrient in the most optimal form for easy absorption by the body. On average, this amount is 100-150 mg%.

Cow's milk also contains magnesium, sulfur, chlorine, sodium, potassium, and in addition to them - a variety of citrates, chlorides and trace elements. For the most part, the amount of all vitamins and minerals, as well as the percentage of milk fat, largely depends on many factors, these are:

cow diet;
her state of health;
age of the cow;
season;
other factors affecting cow lactation.

By the way, milk producers resort to some kind of trick to “adjust” a huge amount of milk to a certain percentage of fat content, which is indicated on the packaging. Milk of different fat contents is simply mixed together until you get, for example, 3.2%, which is indicated on the package. The manufacturer also usually writes that the fat content of milk may fluctuate slightly.

From all of the above, it becomes clear that the benefits of milk are beyond doubt, however, this product is not equally beneficial for all people. Thus, milk is simply contraindicated for people with lactose intolerance (but lactose-free milk can also be found on store shelves, although it will cost much more). People suffering from diseases of the intestinal tract, diseases of the pancreas or liver should also consume milk with caution, or better yet, abandon it altogether in favor of fermented milk products.

Touching on the topic of fermented milk products, you first need to decide what lactic acid bacteria are and why they are needed, what function they perform. As already mentioned, milk is simply a storehouse of various useful substances. Milk also contains a truly wonderful substance - lactose, in other words, it is milk sugar. It is lactose that is the substance that prevents milk from immediately turning sour, protecting it from such a fate. But if fresh milk is left for some time, it will acquire a sour taste, which means it will begin to sour. Why is this happening? The thing is that lactic acid bacteria live on the surface of the mammary glands. This is where the droplets of milk are retained. Once only one droplet sours under the influence of lactic acid bacteria and gets into the rest of the milk, the milk will begin to sour. Of course, this will not be noticeable right away, since one sour drop is diluted in a large amount of “good” milk. But lactic acid bacteria tend to multiply quickly, and if you just leave the milk unboiled, it will turn sour completely.

To prevent milk from turning sour too quickly, simply boil it, and after the boiled milk has cooled, pour it into a clean container and put it in the refrigerator. When boiling, milk may “run away”. To prevent this from happening, grease the inside of the top 5 centimeters of the pan with butter.

Lactic acid bacteria themselves are the type of bacteria that ferment carbohydrates to form lactic acid. Of course, milk contains not only lactose, but also many other substances that other bacteria “feed” on, but it is the lactic acid bacteria that bring benefits; the rest of the bacteria can bring mainly only diseases.

Fermented milk products contain a huge number of living bacteria and various other microorganisms. If, before the milk begins to sour, it is processed in a special way, then fermented milk products can be obtained. Different nations have their own types of such products, sometimes repeated, but still bearing different names. Below is a description of most types of fermented milk products from different nations.

Curdled milk. It is prepared by fermenting pasteurized milk with a starter made from pure cultures of lactic acid plants. If in the same way you ferment not milk, but its fattest part - cream, then you will get sour cream.
Ryazhenka. This product resembles sour cream in taste and consistency, but still has a unique taste and differs from sour cream in color - more creamy than white. Ryazhenka has a sweetish taste, which makes it reminiscent of baked milk. Usually on the shelves you can find fermented baked milk of the same fat content - 6%. Yes, and fermented baked milk is prepared a little differently - for this you need to use lactic acid streptococcus. It is also worth noting that fermented baked milk is much higher in calories than other types of yogurt.
Katyk, matsun or matsoni. Yes, all this is one product, which is named differently by different people. Most people know it under the name matsoni. This is a type of sour milk that is prepared from camel, goat, sheep and cow's milk.
Dzhugurt- squeezed sour milk, which looks similar to sour cream. This product is produced mainly in the North Caucasus.
Ayran. A fairly common and popular drink in the countries of Central Asia, which is prepared from different types of milk - both cow, goat or sheep.
Kurunga- also refers to fermented milk drinks drunk in Northeast Asia.
Shubat- also a fermented milk drink that is loved in Asia. Made from camel milk. The main difference from other fermented milk drinks is that shubat tends to foam a lot.
Cheese. Prepared from sour milk, it is a nutritious, high-calorie and healthy product.
Cottage cheese- one of the most famous fermented milk products. It is prepared by fermenting milk, after which the whey is removed.

How does milk sour?

As mentioned above, milk sours under the influence of lactic acid bacteria. When they multiply in milk, various chemical reactions begin to occur, one of which is protein folding. As a result, the milk thickens (sours) and separates into two parts. The first part is liquid, this is whey, and the second is actually the sour and thickened milk itself.

Temperature is an important factor for bacterial growth, which results in milk turning sour. For example, at room temperature it will take a couple of days for milk to completely sour, but in the refrigerator, on the contrary, fresh milk can be stored for up to 5-7 days.

To prevent the milk from turning sour so quickly, you can carry out a special heat treatment. For example, during pasteurization, milk must be heated to approximately 60-80 degrees Celsius. The milk should remain at this temperature for about half an hour to an hour. As a result, the shelf life of milk increases by several days, mainly due to the fact that lactic acid bacteria die.

There is a more radical method of preserving the freshness of milk - sterilization. This is the method used by most milk producers. This method involves bringing milk to a boiling point. If sterilized milk is stored in an airtight container where neither air nor bacteria can enter, the milk can be stored for up to several months. Of course, even this method has a disadvantage, which for some consumers is quite significant - the benefits of milk whose shelf life is extended in this way are greatly reduced.

If lactic acid bacteria act during natural souring of milk, then during artificial souring you can use various acids, for example, vinegar. When vinegar gets into milk, milk protein is released, and accordingly, milk sours. At the same time, artificial souring occurs much faster than natural, it takes not several days, but only a few seconds.

If you need to make homemade kefir or yogurt, which means artificially fermenting milk, you can buy a special starter at the pharmacy. A package of this starter will cost about 60 rubles.

There is a very widespread belief among people that milk turns sour much faster during a thunderstorm. Of course, only if it is not in the refrigerator at that moment. According to legend, this is due to the fact that it is during a thunderstorm that a certain effect of long-frequency magnetic pulses can occur. There is another version, according to it, during a thunderstorm, milk protein and calcium interact in a special way, which leads to this result.

If, however, you did not have time to process the milk in time and it turned sour, you can make very tasty pancakes from it. To do this, take:

200 ml sour milk
a glass of flour
3 eggs
sugar and salt to taste
half a teaspoon of soda
a quarter cup of vegetable oil

Mix all the ingredients listed and beat with a whisk or mixer at low speed to prevent lumps from appearing. Lightly oil the pan and cook the pancakes as usual.

How to choose milk in a store

It has long been no secret to anyone that a huge number of artificial substitutes have appeared for almost any product, including milk. How can you “by eye”, right in the store, determine whether real milk is on the shelf or not?

Pay attention to the composition. Good milk cannot boast of a long composition; usually in this section only “pasteurized milk” or “normalized milk” can be written. The packages on which you see a long list of ingredients, in which you can find milk powder, vegetable fats, margarine, oils and similar products, cannot possibly contain real milk.
Look at the expiration date. For real milk, it usually lasts 3-5 days, after which the milk will begin to sour right in the package. Some manufacturers, despite the excellent composition indicated on the packaging, can store milk for up to six months, and most likely it is sterilized milk. As mentioned above, although this milk is real, it is no longer as healthy as just pasteurized milk.
Price. Nowadays, sad as it may be, good, real milk cannot cost less than 60 rubles per liter. Of course, many retail chains have their own milk production and sell it as real milk, for only 20-30 rubles per liter. But, as practice shows, such milk neither in properties nor in taste resembles the real thing, the one that costs 60 rubles and more. Most likely, if you want to cook porridge, when it boils, it will curdle or flakes will appear, despite the fact that today’s date is written on the packaging where the production date is. Even when boiling, it may not change in appearance, but the taste will clearly resemble the taste of kefir. Of course, such milk simply cannot be called real, tasty and healthy.

Why does store-bought milk sometimes not turn sour?

Sometimes you may notice that if you leave store-bought milk with a long shelf life in the refrigerator and “forget” about it, it will not turn sour. Many immediately blame the milk producer, saying that supposedly natural milk cannot but turn sour in a few days, even in the refrigerator. They are partly right, but we should not forget that when milk is sterilized, all lactic acid bacteria disappear from it, and without them, milk simply cannot sour.

But no matter how sterilized the milk is, in an open package after a few days in or outside the refrigerator, the milk may begin to deteriorate, or simply “go rotten,” even if there are still a couple of months before the expiration date. This “behavior” of milk may even frighten the average person, but this is not the fault of the manufacturer.

The thing is that if lactic acid bacteria are mainly to blame for the souring of milk, then milk begins to “go rotten” due to the fact that bacteria that have entered the milk from the air cause the destruction of milk protein, or, in other words, its rotting. This process is popularly called “rancidity.”

Thus, even if the milk you purchased in the store is in no hurry to sour even when opened, this does not always mean that it is not of high quality or not real.

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

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

Causes of sour milk

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

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

How to increase the shelf life of milk?

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

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

These technologies differ, first of all, in temperature and time regimes. Initially, pasteurization was called heating 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 expiration dates for the same product.

Not to sour for a long time: an advantage or a disadvantage?

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

The natural process is the colonization by microflora or the development of microflora that was in the milk and remains. It could have remained if the pasteurization regime was insufficient. And get there - from containers, equipment, hands, gauze, dishes, in contact with anything. It's called secondary seeding. Most “samokvass” get it this way.
The next reason is the ingress of any substances from the outside that can shift the balance. This is primarily acid and enzymes. If they somehow get into the milk, it turns sour.

If pasteurized milk does not sour, this indicates the presence of inhibitory 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 during treatment. After the quarantine period has expired, she is returned to the herd. But if for some reason she still has antibiotic substances in her blood, then the milk from one such cow contaminates the entire batch with antibiotics.

Since drinking pasteurized milk is not a long-lasting 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 yeast 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 sour on its own, but becomes somehow greenish and smells bad. At the same time it remains absolutely liquid. Or it doesn’t change in appearance at all, but becomes bitter. This is a sign of the presence of antibiotics.

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

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

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

Why does sterilized milk not turn sour, but go rotten?

Does it spoil quickly - does it mean it's natural?

The expiration date and the “naturalness” of milk are in no way related to each other. It all depends on the production technology. Expiration dates are determined by safety. And the raw materials can be anything. By the way, making low-quality milk with a short shelf life is even more profitable. And people are more willing to buy up.

What kind of milk should be boiled?

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

Gomov Ilya

The research work was carried out by a 2nd grade student. The work is educational in nature. In the course of work, the student learns not only to conduct experiments, but also to formalize his research.

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Ministry of Education and Science of the Russian Federation

Municipal autonomous educational institution "Lyceum No. 6"

Educational and research work

Why does milk turn sour?

Completed: Gomov Ilya Igorevich,

2nd grade student B

Supervisor : Gordenkova Nina Vasilievna,

primary school teacher

Tambov

2016

Introduction………………………………………………………………………………3

Literature review……………………………………………………4-6

History of milk…………………..................................4

1.2. Composition of milk……………………………………………………….4

1.3 The benefits and harms of milk…………………………………….…………4-5

1.4. Lactic acid bacteria……………………………………………..5

1.5. Lactic acid products……………………………………………………..5-6

2. Research methodology……………………………………………………………6-7

2.1. Stages of the study…………………………………………………….6

Research results………………………………………………………7

Conclusions………………………………………………………………………………….8

Conclusion………………………….……………………………………..9

Resources used……………………………………………………………..10

Applications…………………………………………………………………………………11

Introduction: Both people and mammals - from mice to elephants and whales - begin their diet with milk. This is a wonderful, tasty and healthy drink, a real elixir of growth, which was invented by nature itself. In a newborn, no matter who he is - a boy or a girl, a kitten or a puppy - until a certain age, the stomach cannot digest any food other than milk. But it contains all the useful and nutritious substances a small creature needs to grow faster and develop properly.

Relevance: It is important to be attentive to yourself, to your health, to be able to protect your body from harmful influences, because human life is priceless, and muscles, skin, bones, and teeth are built from the nutrients that are in milk. In addition, milk is also a medicine, because thanks to the range of beneficial substances it contains, doctors prescribe it to many patients. Children should drink milk every day.

Hypothesis:

Target:

Tasks :

Subject of study: milk

Study location: apartment (kitchen)

February 2016

Research methods:

Literature review

1.1History of milk

Since ancient times milk used to feed children unable to digest solid food and to feed the sick. The composition of cow's milk contains many useful elements, is a rich source of calcium and vitamins, and is perfectly absorbed by the body. Milk was drunk in ancient times, as evidenced by milk vessels found during archaeological excavations in the caves of primitive people. In an ancient temple discovered near Babylon, wall paintings were found depicting the process of milking a cow. It is believed that this temple is approximately five thousand years old. There are many references to milk in the Bible. Abel, the son of Adam, tended sheep and probably drank milk. Sayings about milk. (see Appendix No. 1)

1.2.Composition of milk

The composition of cow's milk is largely determined by its origin. It has a multicomponent composition in the form of a polydisperse system with a liquid consistency. Cow's milk is the most popular type of milk of all mammals, and accounts for the largest share of all types of milk produced by the livestock industry in world agriculture. Cow's milk consists of protein, fats, carbohydrates, water, ash, organic acids, minerals and vitamins. It contains almost the entire line of B vitamins, vitamins E, D, H, ascorbic acid, beta-carotene, vitamin PP, choline, nucleic acids, monounsaturated fatty acids, lactose, essential amino acids. Of the mineral elements, perhaps the most valuable and essential element of the composition is the macroelementcalcium . It is included in the chemical composition of cow's milk in an optimal form for easy absorption by the body, in an average amount of 100 to 150 mg%. It also includes potassium, phosphorus, chlorine, sulfur, magnesium, sodium, various chlorides and citrates, and a number of trace elements. The amount of mineral elements and vitamins, as well as the percentage of fat content in milk may vary depending on the time of year, housing conditions, health and diet of cows, age and other factors affecting lactation.

1.3 The benefits and harms of milk

The benefits of cow's milk for the body are undeniable - it is the richest source of vitamins and minerals. The lactose contained in the composition is good for the heart. However, milk should be consumed with caution. It is contraindicated for people with lactose intolerance. People with diseases of the gastrointestinal tract, liver or pancreas should also refrain from consuming dairy and choosedairy products .

1.4. Lactic acid bacteria

Fresh milk contains many important nutrients. But the most remarkable substance contained in milk is milk sugar. Its name is lactose. It is lactose that “protects” milk from souring. If fresh (that is, just milked) milk sits for some time, it begins to acquire a sour taste. Why is this happening? Who “steals” milk sugar? (These are lactic acid bacteria - big “gourmets”.) They love various sugars and cannot live without many vitamins and some other compounds. Where do lactic acid bacteria get into milk? It turns out that they live in those places where milk is formed - on the surface of the mammary glands, where droplets of milk are retained. Only a small drop that remains outside “sours”; it will be “diluted” in sterile milk and the sour taste will not be noticeable. But if you wait a little, the bacteria will have time to multiply and will certainly make themselves felt.LACTIC ACID BACTERIA, a group of microorganisms that ferment carbohydrates producing mainly lactic acid. The classification of LACTIC ACID BACTERIA is not sufficiently developed.Milk contains not only milk sugar, but many other substances that feed other types of bacteria. However, lactic acid bacteria are beneficial. Other types of bacteria can cause various types of illness.

1.4. Lactic acid products

Fermented milk products contain a huge number of living bacteria and other microorganisms. It turns out that milk can be pre-treated before it starts to sour. In this way, lactic acid products can be obtained.

Curdled milk If you take the fatty part of milk - cream, then the same types of lactic acid bacteria will turn it into sour cream.

Ryazhenka

Matsoni, matsun, katyk

Dzhugurt

Kurunga

Ayran

"Sour milk"

Chal (shubat)

Cottage cheese - another fermented milk product. Cottage cheese is a protein product obtained by fermenting milk and then removing the whey.

Cheese - a highly nutritious food product made from curdled (sour) milk.

2. Research methodology

2.1. Research stages

Stage 1 Stage 2

2.2. Description of experiments

Experience 1. Observing droplets of fat

Fat floats in milk in the form of many small droplets. Since fat is lighter than water, it gradually floats to the top - a layer of cream is formed. By churning the cream, you get butter: droplets of fat from the beating merge together and separate from the water.

Experiment 2. Souring milk

I decided to check where milk sours faster - at room temperature or in the refrigerator. All the milk in the room turned sour on the second day. In the refrigerator, unboiled village milk turned sour on the 5th day of the experiment, but boiled milk did not sour on the 5th day.

Experiment 3 Action of acid

Using the example of experiment No. 2, I saw that unboiled and boiled milk begins to sour from the second day of the experiment. I decided to speed up this process. In the word lactic acid products. Two roots, milk and sour. And I thought that if you add acid, the milk will turn sour. I added a little acetic acid to one glass and lemon juice to the other. Milky flakes have formed on the surface of the milk. From the literature I found out that casein (Latin caseus - cheese) - complex protein , formed from the precursor of casein - caseinogen during milk curdling. Casein (caseionogen) is present in milk in bound form as a calcium salt (calcium caseinate).It is dissolved in milk like sugar in water. But it is worth adding acid to the milk so that the casein is released, taking the fat with it. This experience helped me see the effects of acid. This means that lactic acid bacteria also separate casein, resulting in lactic acid products.

Experiment No. 4 Milk fermented with a piece of rye bread

In the room, in a glass of village milk, in which there was a piece of bread, the milk turned sour the next morning. The boiled milk turned sour later. In the refrigerator, pasteurized unboiled and boiled milk with pieces of rye bread on the 4th day of the experiment became rancid, but not sour. From this experience we can conclude that the presence of yeast under certain conditions (temperature) also causes milk to sour. My grandmother in the village uses this method to quickly sour milk. And then she gets cottage cheese.

3.Research results

Conclusions:

Milk is contraindicated for people with lactose intolerance.

Conclusion

Resources used:

1. Sushansky A. G., Liflyandsky V. G. Encyclopedia of healthy nutrition. T. I,. Nutrition for health / St. Petersburg: “Publishing House “Neva””; M.: “OLMA-PRESS”, 1999

2. Sharygina A. M., Kalinina L. V. General technology of milk and dairy products, M.: Koloss, 2007

3. https://yandex.ru/images/

4. https://ru.wikipedia.org/wiki/Milk

5.http://sostavproduktov.ru/produkty/molochnye/moloko/korove

7. http://www.woman-lives.ru/

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Appendix No. 1

Interesting facts about milk

The physician Hippocrates called milk a medicine.

Academician Pavlov called milk food prepared by nature itself.

Avicena argued that milk is the best food for people.

But is it always good when milk turns sour? No! If you need to preserve milk unchanged, how can you ensure that lactic acid bacteria do not multiply and “change” the taste of the milk? To destroy microbes, you can simply boil milk.

One of the most common technologies is milk pasteurization. The milk is kept at 61–63°C for 30 minutes or at 72–73°C for only 15 seconds. This does not impair the taste of the product, but kills pathogenic bacteria.

The benefits of storing food in the cold have long been known. At temperatures just below zero, bacteria continue to multiply, but very slowly. Therefore, milk can be frozen and stored and stored in this form for quite a long time.

When there were no refrigerators, milk was stored in the winter in the form of frozen circles and was even sold in this form in markets. The first mention of frozen milk dates back to 1792

Other known methods of storing milk include drying. In spray installations, milk is dried at a temperature of 150-180 °C.

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Appendix No. 3

Curdled milk is a fermented milk drink made from pasteurized milk by fermenting it with starter prepared from pure cultures of lactic acid plants.

Ryazhenka - cream color, taste and consistency resembles sour cream, but has a unique taste. The sweetish taste is reminiscent of stewed milk. The fat content of fermented baked milk is 6%. To prepare it, pure cultures of lactic acid streptococcus are used. The calorie content of fermented baked milk is significantly higher than the calorie content of yogurt of other varieties.

Matsoni, matsun, katyk- in fact, these are different names for approximately the same type of southern sour milk, produced from cow, buffalo, sheep, camel or goat milk.

Dzhugurt produced in the North Caucasus (mainly in Kabardino-Balkaria). This is pressed sour milk, similar in appearance to sour cream or pasta.

Kurunga is a fermented milk drink, widespread in Northeast Asia among the Buryats, Mongols, Tuvans and other peoples.

Ayran - a very common drink among the peoples of Central Asia, the Caucasus, Tatarstan, and Bashkiria. Prepared from cow, goat, sheep milk.

"Sour milk" - Bulgarian sour milk, prepared using the “Bulgarian stick”, discovered at the beginning of this century by Stamen Grigorov

Chal (shubat) - fermented milk, highly foaming drink with a pure fermented milk taste and yeasty smell, prepared from camel milk.

2. Research methodology

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I. I. Gomov

2nd grade student B

Why does milk turn sour?

Hypothesis: milk turns sour because bacteria and other microorganisms multiply in it

Target: find out why milk turns sour

Tasks :

Study scientific literature and online resources on the topic.
Conduct research on the process of milk souring.
Research what can enhance and slow down the souring of milk.
Draw conclusions, summarize and systematize the material.

Subject of study: milk

Study location: apartment (kitchen)

Time frame for the study: February 2016

Research methods:

Theoretical (analysis of facts from literary and Internet sources, compilation of material, generalization);
Empirical (experiment and observation, photography)

Since ancient times milk used to feed children unable to digest solid food and to feed the sick. The composition of cow's milk is largely determined by its origin. It has a multicomponent composition in the form of a polydisperse system with a liquid consistency. The benefits of cow's milk for the body are undeniable - it is the richest source of vitamins and minerals. However, milk should be consumed with caution. Fresh milk contains many important nutrients. But the most remarkable substance contained in milk is milk sugar. Its name is lactose. It is lactose that “protects” milk from souring. Where do lactic acid bacteria get into milk? It turns out that they live in those places where milk is formed - on the surface of the mammary glands, where droplets of milk are retained. Only a small drop that remains outside “sours”; it will be “diluted” in sterile milk and the sour taste will not be noticeable. But if you wait a little, the bacteria will have time to multiply and will certainly make themselves felt. Fermented milk products contain a huge number of living bacteria and other microorganisms. It turns out that milk can be pre-treated before it starts to sour. In this way, lactic acid products can be obtained.Stages of the study: Stage 1- study of literature and Internet resources. Stage 2 – conducting experiments and research.For the experiments, I took village milk, unboiled and boiled. I poured it into glasses (200 ml volume) and observed it for five days. At the same time, one part of the glasses with milk was in the room, and the other in the refrigerator. I added a piece of rye bread to one of the glasses, and acid to the other two. In one acetic acid, in the other lemon juice.

3.Research results

1. Fat floats in milk in the form of many small droplets.

2. Milk sours faster in warm conditions.

3. Using acid, you can separate casein (a complex protein)

4. The presence of yeast accelerates the process of milk souring.

Conclusions:

Milk is a valuable source of nutrition.
Lactic acid bacteria are the source of sour milk. They are useful for intestinal function.
It is contraindicated for people with lactose intolerance.

Conclusion

I did this work together with my teacher, N.V. Gordenkova. I consider it relevant, since it is necessary to know about the benefits and harms of milk. It is interesting to study what you don’t know and find out the reason for what is happening. In the course of my work, I deepened my knowledge of the presence of lactic acid bacteria, which are necessary for the formation of lactic acid products.

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Annotation.

The research work “Why milk turns sour” is designed for students in grades 2-4. During the work, children learn to observe and draw conclusions by comparing observations. Work teaches children to observe, draw conclusions, and defend their point of view. Develops curiosity and interest.

Milk is a very valuable food product. It’s not for nothing that our ancestors called the domestic cow “nurse.” Thanks 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 drink’s tendency to ripen. To understand why milk sours, let's figure out what it consists of.

What substances are contained in milk

Milk is intended to feed young mammals. It contains the full range of nutrients necessary 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'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 that protect newborns from infectious diseases.

Milk also contains a certain amount of bacteria, which are classified as normal microflora. They are the answer to the question “why does milk sour?” The biology of microbes and, as a result, the type of fermentation they cause varies.

Bacteria causing desired fermentation

Lactic acid and propionic acid bacteria, kefir grains and milk yeast participate in the “useful” fermentation of milk.

Lactic acid microbes are naturally found in milk and are the main “culprits” 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 added to milk specifically to produce yogurt, cottage cheese, fermented 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 cheese production. 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, alcoholic fermentation can also occur in milk. It is caused by specific yeasts 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 added to the prepared substrate. This explains why milk sours with one or another result.

Tasteless curdled milk

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 into carbon dioxide, butyric acid and hydrogen. As a result, milk acquires a bitter taste and unpleasant odor. This fermentation occurs mainly in sterilized and pasteurized milk, as well as in cheeses. The fact is that butyric acid microbes tolerate boiling temperatures 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 or if storage conditions are violated. Putrefactive bacteria exert their effect on the product at temperatures below +10°C, lactic acid bacteria are viable at + 10°C - +20°C. Unlike lactic acid microbes, putrefactive microbes are not killed during pasteurization, so milk from a package often “goes rotten” rather than fermented. In this case, microorganisms break down the proteins and fats of the milk, which leads to the appearance of decay products with a characteristic rancid or rotten odor.

Why does milk turn sour quickly?

The speed of milk fermentation depends on several factors.

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

Milk bought in a store quickly sours even when stored in the refrigerator due to violations of production technology. This could 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-quality packaging, and so on.

It should be noted that for fresh milk, ripening 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 temperature treatment of the product, but differ in the mode of exposure.

Pasteurization

Pasteurization of milk is carried out in several ways:

Maintain 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 part of the vitamins and enzymes, and most bacteria die. Only heat-resistant microbes remain “in service.” This explains why milk does not sour for a long time. Pasteurized milk can be stored in the refrigerator for 2 weeks. This product is also used to introduce various microorganisms and create targeted fermentation.

The most optimal way to preserve maximum nutrients 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 sterile packaging. Unlike conventional pasteurization, persistent spore forms (including putrefactive bacteria) die. UHT milk can be stored in the refrigerator for up to two months.

Sterilization

Sterilization kills all microorganisms. This milk is sterile, packaged in aseptic containers, and 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 it is not possible to carry out high-temperature treatment 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. 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.

Doesn't milk from a domestic cow turn sour?

Another reason why cow's milk does not sour may be metabolic disorders in the cow's body. If the ratio of sugar and protein in food is incorrect, protein overfeeding causes a disease called “ketosis.” Ketone milk is very harmful to the human body; it is practically not fermented, and the separated cream produces sour cream with a bitter aftertaste.

Milk fermentation products

Fermented milk products have been 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 the milk and the starter added.

Curdled milk is easy to prepare at home. To do this, add starter 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 intestinal diseases. 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 grains added to the original product cause lactic acid and alcohol 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 to do 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.

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

Cheeses, cottage cheese, and cultured butter are also produced using a variety of starter cultures.

Store milk and lactic acid products in clean, sealed containers, observing the temperature conditions and terms specified on the packaging by the manufacturer. Follow 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 a temperature of +4°C - it is suitable for almost all dairy products. Remember that dairy foods require careful handling, and spoiled products can cause serious poisoning.