Heat treatment or pasteurization is the process of heating milk from 63 ° C to a temperature close to the boiling point. This process got its name from the famous French scientist Louis Pasteur (1822-1892), who first used this method to destroy microorganisms in wine and beer.
The effect of pasteurization on microorganisms contained in milk depends on the temperature to which the milk is heated and the duration of exposure at this temperature. Pasteurization destroys microbes, and sterilization (heating milk above boiling point) also destroys spores. Boiling destroys all microflora of milk, with the exception of spores that are resistant to boiling temperatures. Pasteurization without noticeable changes in the organoleptic properties of milk (taste, smell and consistency) destroys tuberculosis, brucellosis and other pathogenic bacteria. In ordinary collected milk, 99% of bacteria die only under the condition of good, reliable sterilization of equipment, equipment, and utensils used in the pasteurization process. Thus, the addition of contaminated milk containing 1 billion bacteria to pasteurized milk (i.e., the amount that could remain inadvertently in dairy equipment) will increase the number of bacteria in milk to 1 million in 1 ml. These bacteria will actively multiply and will inevitably lead to spoilage of all milk. Pasteurization is therefore the simplest and cheapest way to disinfect milk. Milk is also pasteurized during the production of all dairy products in order to protect them in the future from undesirable processes that are caused by the activity of bacteria and especially E. coli, butyric acid bacteria, etc. When cattle are kept on pasture, the microflora of milk is destroyed by heating more completely than when kept in stalls. This is explained by the fact that when kept in stalls, bacteria enter the milk mainly from manure particles. These bacteria are more resistant to heat due to their properties. When kept on pasture, milk contains mainly bacteria that multiply on plants. Milk must be thoroughly cleaned before pasteurization. In practice, three pasteurization modes are used: with long-term pasteurization, milk is heated to 63-65 ° C and maintained at this temperature for 30 minutes; short-term pasteurization is carried out at 72-75 °C with holding for 15-20 s, which is carried out in a stream; flash pasteurization - heating milk to a temperature of 85-90 ° C without holding it. Thermal effects on milk lead to some changes in its constituent substances. When heated, the gases dissolved in milk evaporate from the milk. Due to the removal of carbon dioxide, the acidity of milk decreases by 0.5-1 °T. At temperatures above 85° casein partially changes. But milk albumin is most affected: at 60-65 °C it begins to denature. The salt composition of milk is also disrupted during pasteurization. Soluble phosphate salts become insoluble. Due to the partial coagulation of proteins and the formation of insoluble salts, a milky sediment (burnt) is deposited on the surface of heating devices (pasteurizers). Pasteurized milk is curdled more slowly by rennet. This is explained by the precipitation of calcium salts. Adding a calcium chloride solution to such milk restores its ability to coagulate. Vitamins are resistant to high temperatures, especially if milk is heated without access to oxygen. Heating to high temperatures (80-85°) gives milk a special taste and aroma, which intensifies as the temperature rises. When boiling, the composition of milk also changes. For example, the content of vitamins A and C is reduced by almost 2 times. Nutrients are lost in the range of 15 to 20% due to the formation of sediments of proteins, fat and calcium salts on the walls of the dishes. Therefore, you should not boil pasteurized milk unless absolutely necessary.
At home, we can also recommend long-term pasteurization of milk, which can be done without much difficulty. It is produced through heated water. The milk poured into the pan is stirred with a clean spoon while heating. As soon as the temperature rises to 63-65 °C, heating should be stopped and maintained for 20-30 minutes. After this, place the pan with milk in cold water.
INFLUENCE OF HEAT TREATMENT ON THE PROPERTIES OF MILK
Heat treatment of dairy raw materials is carried out for the purpose of its disinfection. It should ensure not only reliable suppression of the vital activity of microorganisms, but also the maximum possible preservation of the original properties of milk. Any thermal effect on milk disrupts its original composition and physicochemical properties. The degree of physicochemical changes in the components of milk depends mainly on the temperature and duration of heat treatment.
Milk proteins are denatured by heat. Whey proteins are the most heat-sensitive, denatured at temperatures above 65 °C, while casein has high heat resistance. At temperatures above 100 °C, partial decomposition of lactose begins, as a result of which milk acquires a specific taste, smell and color (brown). Milk fat remains virtually unchanged when heated to 100 °C. During heat treatment, vitamins are partially destroyed, especially water-soluble ones (C, B12, thiamine, etc.), and enzymes (reductase, phosphatase, peroxidase) are also inactivated. As a result of the transition of soluble calcium and phosphorus salts to an insoluble state, mineral salts partially precipitate. Changes in milk components that cause a negative effect on nutritional value and organoleptic properties should be insignificant.
Types of heat treatment include pasteurization, sterilization and thermization.
Pasteurization of milk– This is the heat treatment of milk in order to destroy vegetative forms of microflora, including pathogenic ones. The pasteurization regime should also ensure that the desired properties of the finished product are obtained, in particular organoleptic characteristics (give taste, desired viscosity, curd density).
The effect of pasteurization, determined by the degree of death of pathogenic microflora, influences the choice of modes and methods of pasteurization. Of the pathogenic microorganisms, tuberculosis bacteria are more resistant to heat treatment. Since the work of identifying the causative agents of tuberculosis is complex, the effectiveness of pasteurization is usually determined by the death of no less persistent E. coli. The effect of pasteurization depends on the temperature (t) and duration of heat treatment (z), the relationship of which is established in the form of the following equation: ln z = 36.84 – 0.48 t, where 36.84 and 0.48 are constant values. Depending on these factors, three pasteurization modes are distinguished: long-term pasteurization - at a temperature of 60–63°C with holding time for 30 minutes; short-term – at a temperature of 74–78 °C with a holding time of 20 s; instant - at a temperature of 85–87 °C or 95–98 °C without holding.
The choice of pasteurization modes is determined by the technological conditions and properties of the product. If the product contains components with low heat resistance, long-term pasteurization should be used. The process of long-term pasteurization, although it ensures reliable destruction of pathogenic microbes and the least change in the physicochemical properties of milk, however, requires large costs associated with the use of low-performance equipment.
The most common method in the production of pasteurized milk, fermented milk products, and ice cream is short-term pasteurization. This method is also reliable for inactivating microbes and maximizing the preservation of the original properties of milk. Instant pasteurization has the same effect on microbes and milk properties as short-term pasteurization. It is recommended for the pasteurization of cream, from which butter is produced, and in the production of canned milk. Thus, all pasteurization methods make it possible to obtain a product that is harmless for direct consumption, but has a limited shelf life.
The resistance of microorganisms to heat treatment increases with increasing fat and dry matter content in products (cream, ice cream mixture), since fatty and protein substances have a protective effect on microbial cells. Therefore, for products with a high content of fat and solids, the pasteurization temperature should be increased by 10–15% compared to the pasteurization temperature of milk.
Simultaneously with the pasteurization process, to improve the organoleptic characteristics of milk and cream, they are deodorized. Changes in organoleptic characteristics occur due to the presence of volatile substances and gases in milk, especially oxygen, which cause undesirable taste and smell. Oxygen present in milk during storage contributes to the oxidation of the fat fraction and the destruction of vitamins. Vacuum deodorization units are used to remove these unwanted substances from milk. Deodorization is usually carried out at a temperature of 65–70 °C and a vacuum of 0.04–0.06 MPa for 4–5 s. Under these conditions, the milk boils, and unwanted gases and volatile substances are removed along with the vapor.
One of the types of pasteurization is thermization.
Thermization– This is the heat treatment of milk in order to increase its shelf life by reducing the overall bacterial contamination of milk. It is carried out at a temperature of 65 °C for 15 s. Thermization as a low-temperature short-term heat treatment is recommended to increase the stability of raw milk during storage. In cheese making, thermization is used to process milk with increased bacterial contamination and intended for ripening, and in the production of canned milk - to increase the heat resistance of milk.
Pasteurization refers to a one-time process of heating products that have a liquid consistency. The technology is named after Louis Pasteur (1822-1892), a French microbiologist who proposed to disinfect beer and wine by destroying unwanted microorganisms from the drinks. Subsequently, this method gained great popularity and began to be used to disinfect products and increase their shelf life.
Milk pasteurization process
The process of pasteurization of milk affects the microorganisms contained in its composition, depending on the degree of temperature treatment and the duration of heating. The process of pasteurization of products differs from the sterilization process in that in the first case only the destruction of microbes occurs, and in the second - spores. Pasteurization does not involve boiling, which eliminates absolutely all milk microflora, but cleans the product from pathogenic microorganisms (for example, tuberculosis or brucellosis bacteria) at a temperature slightly lower than the boiling point. At the same time, the basic properties of milk (consistency, taste, smell) remain practically unchanged.
It is possible to achieve complete (99%) destruction of microorganisms by ensuring the sterility of dishes, equipment and equipment used for pasteurization of collected milk. If contaminated milk gets into contact with already pasteurized milk, you should expect spoilage of the entire amount of the product. Poorly sterilized equipment used for milk pasteurization can contain up to 1 billion bacteria, which, by actively multiplying, are capable of increasing the number of pathogenic microbes in the total mass of the product to 1 million/ml in a fairly short period of time.
Pasteurization is considered the least expensive and accessible way to disinfect the resulting milk for further consumption or production of various dairy products based on it. The use of milk that has undergone pasteurization guarantees the high quality of the finished products, prevents their souring caused by butyric acid bacteria, and the occurrence of undesirable processes associated with the active proliferation of E. coli or other microorganisms.
The microflora of livestock kept on pastures can be destroyed as much as possible by pasteurizing it. When receiving milk from stalled animals, there is a high probability that manure particles, which are resistant even to high temperatures, will get into the product. On the contrary, milk collected from pasture-raised animals contains mainly plant bacteria. This fact determines the mandatory procedure for cleaning milk before placing it in pasteurizers.
Pasteurization methods
There are 3 methods of pasteurization:
This method is called long-term pasteurization. Milk must be heated to a temperature of 63-65°C and pasteurized for half an hour;
This method, otherwise called short-term pasteurization, involves heating the product to a temperature of 72-75°C. After holding for 15-20 seconds, the heat treatment should be stopped;
During flash pasteurization, milk should be brought to a temperature of 85-90°C without further holding.
Heat treatment (at 80-85°C) changes the taste and aroma of milk. When exposed to temperature, some elements contained in milk change their physical and chemical properties, and accordingly the composition of the product changes slightly.
For example, the gases contained in milk evaporate, the acidity of the product decreases slightly (by 0.5-1 °T), changes also affect the salt composition (phosphate salts become insoluble). Heating devices used in the production of pasteurized milk may become coated with burnt deposits caused by milkstone deposits. Calcium salts slow down the coagulation of milk, which requires the addition of an artificial solution of calcium chloride. With flash pasteurization (when the temperature is above 85°C), casein begins to change. And albumin protein tends to denature already at 60-65°C. Vitamins are the most resistant to heat treatment, especially when the access of oxygen to the pasteurizer is limited.
Experts do not recommend boiling pasteurized milk, as this changes the composition of the milk, reducing the content of nutrients and vitamins A and C. You can pasteurize milk at home in a water bath. When heating the water in a saucepan to 63-65°C, turn off the heat, let the milk stand for 20-30 minutes, and then cool it by placing it in cold water. During the heating process, it is necessary to constantly stir the milk.
A unique food product. It has a balanced composition, which contains proteins and vitamins, fats and carbohydrates, and mineral elements. Its taste is familiar to everyone since childhood. Of the variety of products sold in stores, pasteurized milk is considered the healthiest. What is it, does it retain its composition after processing, can it cause harm to health? The answers to these questions are in this article.
What's happened pasteurized milk?
This is a heat-treated product. It increases shelf life. In the process of industrial pasteurization, milk is heated to 60 degrees for 1 hour or to 80 degrees for half an hour. Under the influence of this temperature, all pathogenic bacteria and microbes that are always present in whole milk are destroyed. During pasteurization, up to 90 or even 99% of microorganisms die (data on this issue vary). Using pumps, milk is cleaned of foreign impurities, then pumped into a separator, where the cream is separated. Then the product is cooled and placed in a special container, packaged and sent to the refrigerator.
The shelf life of the product ultimately increases to a week if stored in a sealed container in the refrigerator. Then the milk sours and curdled milk is formed. At room temperature the product can be stored for only a few hours.
What is the difference UHT milk?
Recently, on store shelves you can find not only pasteurized milk, but also ultra-pasteurized milk. It is produced using a similar technology, but heats up to a higher temperature of 135 degrees in just 2-3 seconds. The conducted studies confirm that such a short processing time is sufficient for high-quality purification of milk from harmful bacteria. Then it is immediately cooled to +4 degrees.
Ultra-pasteurization extends the shelf life of the product to 6 weeks or more, even when stored at room temperature. After opening the package, it remains fresh for 3-4 days, then it sours, like all other milk.
Video: What is hidden under the symbols on milk. Sterilized, pasteurized milk.
Useful properties and composition
Pasteurized milk is inferior in its characteristics to whole milk. Still, it can be called a useful product. Of all the modern methods of processing milk, pasteurization is considered the best, since it allows you to remove all foreign impurities and destroy harmful microbes and fungal spores. Therefore, such a product does not require additional boiling before use.
The calorie content of pasteurized milk is low. 100 grams of a product with 2.5% fat content contains 54 Kcal, and 3.5% fat content contains 60 Kcal. By drinking 1 glass, a person receives:
- almost 50% of the daily value of calcium;
- other minerals - copper, iodine, strontium;
- vitamins D, group B.
milk protein;
A pasteurized product is perfect for people who cannot stand the taste of steamed food. It does not contain preservatives, is safe for health and is suitable for baby food. The temperature at which pasteurization is carried out allows the majority of vitamins and microelements to be preserved.
Pasteurized protein milk is considered especially useful - it is obtained by mixing whole and skim milk powder. It contains 1% fat and 4.3-4.5% protein.
There is also skim pasteurized milk, in which the fat content is only hundredths of a percent. This product is useful for people intolerant to animal fats.
Sterilization occurs at a much higher temperature - up to 150 degrees. Thus, the raw materials are processed within half an hour.
There are 3 main differences between the two types of product:
- Pasteurized milk retains beneficial lactic acid bacteria, while sterilized milk does not contain any beneficial microflora.
- Pasteurized milk in a hermetically sealed box is stored for about a week (ultra-pasteurized - 2 months or more). Sterilized products do not lose their quality for a year after production, if the original packaging is not opened during this time.
- Sterilized milk has less nutritional value than pasteurized milk.
In terms of practicality, sterilized milk wins. It lasts much longer. In terms of composition, a pasteurized product is still healthier.
Comparison of pasteurization and ultra-pasteurization
After pasteurization, some bacteria still survive and are particularly heat resistant. Ultra-pasteurization destroys all harmful microorganisms, but beneficial substances remain: the treatment is carried out at a higher temperature, but only for 2-4 seconds. Under such conditions, milk sugar (lactose) is not destroyed, and the original properties of calcium and other mineral salts, vitamins, and enzymes are preserved.
Experts from the US Institute of Food Technology call ultra-pasteurization one of the highest achievements of the 20th century. The results of many studies show that such short-term treatment allows you to preserve much more valuable bacteria and vitamins. Many of these beneficial substances cannot withstand prolonged heat during the pasteurization process and are destroyed.
How to make milk stay fresh and healthy longer?
If there is a need to extend the shelf life of pasteurized milk, you can freeze it. In the freezer the product will retain its beneficial qualities. Only you should freeze it only once, and then you need to boil it. It is worth boiling pasteurized milk even if you are going to feed it to a small child.
You can pasteurize farm milk at home. This will help extend the shelf life, because such a product sours very quickly. You will need a large saucepan, sterilized bottles or jars with tight lids and a funnel. The procedure is as follows:
Pour milk into a saucepan.
Boil.
Cool.
Pour into jars.
Close tightly and put in the refrigerator.
Pasteurized milk, prepared at home, can stand in the cold with the lid tightly closed for about a week. All this time it will remain fresh, all beneficial bacteria, vitamins, and microelements will be preserved in it.
Possible harm
Potential harm to the product mainly stems from the fact that chemicals may be added to extend shelf life.
As a result of pasteurization, up to 90% of the vegetative forms of bacteria that live in milk die. The problem is that only those microorganisms that are in an active state are destroyed. Their spores remain viable (although they cannot withstand ultra-pasteurization). After entering the human body, when more or less favorable conditions appear, they will begin to multiply quickly. Therefore, pasteurized milk should be stored correctly - at a cool temperature and no longer than the period indicated on the package. Otherwise, consuming the product may result in poisoning and other negative reactions of the body.
We can conclude that pasteurized milk is not completely neutralized. It would be more correct to call it a product with an extended shelf life. If storage conditions are met and the milk itself is of high quality, then it carries no more health risks than fresh milk.
Hidden threats to children
Pasteurized milk is optimal for baby food. It does not contain harmful preservatives that provoke allergic reactions, including diathesis.
There is also a warning for parents. It is recommended for children to prepare porridge with pasteurized milk only from the age of 6-7 years. After 1 year, the baby can drink it, but not earlier.
It is better to boil pasteurized milk for a child. During the heat treatment at the factory, some microorganisms covered with a resistant film are not destroyed. They are safe for adults, but children's bodies are more sensitive.
How to choose healthy milk in the store?
There are many types of pasteurized milk. Therefore, in the store you should first look at the production date and expiration date. If it has expired or will expire soon, it is better to refuse the purchase.
Plastic bags are not suitable for long-term storage of the product. They are also not strong enough. Plastic transfers foreign taste and odor to milk. Glass bottles do not have this disadvantage, but it is better to choose products in cardboard bags. They are great for long-term storage.
You should also study the composition. If it says whole milk, it is a natural product that has been heat-treated. Whole milk can be diluted with reconstituted milk - made from dry powder. It is not necessarily of poor quality, there are simply fewer useful substances in such a product.
There is an easy way to check the quality of milk. You need to drop a drop of it into a glass of water. If it sinks to the bottom, the milk is whole; if it spreads, it is diluted.
Good pasteurized milk does not have sediment, but this can only be checked at home after opening the package.
Antibiotics are often found in imported milk. If it does not sour for a long time, it means that these substances are in the product, as well as acidity stabilizers. It is clearly not worth buying it - it will not bring any health benefits.
The technological scheme for the production of all types of pasteurized milk includes the following operations: acceptance and assessment of the quality of raw materials, cleaning and normalization, heat treatment and homogenization, cooling, packaging, bottling, labeling and storage.
Receiving milk at dairy plants is carried out according to quality. The quality of milk at the time of delivery and acceptance must meet the requirements of GOST R 52054-2003 “Natural cow’s milk - raw materials”.
Upon acceptance, they are assessed by organoleptic characteristics, fat content, acidity and temperature. To produce pasteurized milk, the natural milk used must be at least 2nd grade. Requirements for the quality of raw milk are given in table. 1, where KMAFAnM is the number of mesophilic aerobic microorganisms and facultative anaerobic microorganisms; CFU—colony-forming units; Coliform bacteria are coliform bacteria.
Table 1. Quality indicators of raw milk
At quality assessment An average sample is taken from a batch of milk - a part of the product taken from each package in one container, and from road or railway tanks - from each compartment separately. Before taking the average sample, the milk is mixed until completely homogeneous. A label is placed on the container with the average sample of milk, indicating the sender and the date of receipt.
Milk purification and normalization
What arrives at the plant contains mechanical inclusions, therefore centrifugal milk purification is used in milk separators, which is carried out simultaneously with normalization. Cleaning, normalization, homogenization, pasteurization and cooling occur in the flow on plate pasteurization and cooling units complete with a homogenizer.
After cleaning and cooling (to 2-4 °C), milk, if necessary, is stored in intermediate containers for no more than 48 hours.
Cleaning and normalization are carried out at 40±5 °C on milk separators and cream separators. Normalization is carried out in-line by mixing cream and skim milk in such proportions as to ensure the desired fat content of the milk.
The normalized milk mixture enters the homogenizer - a high-pressure plunger pump at a temperature of at least 60 °C. At a pressure of 12.5±2.5 MPa in the homogenizer, the fat globules are crushed, and the milk fat, destabilized as a result of mechanical and thermal effects, acquires a protein-lecithin shell. The size of fat globules decreases by 10 times during homogenization. Thanks to the homogenization of milk during the sales period, the formation of a cream plug on the surface of the milk slows down.
Thermal treatment of milk
In the production of milk and dairy products, the following types of heat treatment are used: thermization, pasteurization, heating, sterilization and ultra-high temperature treatment of milk (UHT treatment).
Thermization - the process of heat treatment of raw milk, which is carried out at a temperature of 60 to 68 ° C with a holding time of up to 30 s, while maintaining the activity of milk alkaline phosphatase.
Pasteurization - heat treatment of milk at temperatures below its boiling point, carried out in order to neutralize milk microbiologically, inactivate enzymes, and give milk a certain taste and smell. Pasteurization of milk weakens or destroys some defects in the taste and smell of milk, and in combination with cooling and aseptic filling, it eliminates secondary contamination with microorganisms and prevents spoilage of the product during storage. Possible bacterial contamination during technological processing of milk is clearly visible.
The critical temperatures for the death of pathogenic microorganisms are lower than those of lactic acid bacteria, especially thermophilic bacteria; tuberculosis bacteria are the most resistant. Enzyme destruction temperatures are also different. Thus, phosphatase is inactivated at 72-74 °C, native lipase - at 74-80 °C, bacterial lipase - at 85-90 °C.
The pasteurization temperatures of milk and mixtures are set taking into account the critical temperatures of the death of microorganisms, the inactivation of enzymes, as well as in order to impart certain properties to the milk, on which the yield and quality of the product depend.
Currently, two types of pasteurization are used:
- low-temperature - carried out at a temperature not higher than 76 ° C and is accompanied by inactivation of alkaline phosphatase;
- high-temperature - carried out under various modes (temperature, time) at temperatures from 77 to 100 ° C and is accompanied by inactivation of both phosphatase and peroxidase.
Heating milk- the process of aging milk, carried out at a temperature of 85-99 ° C for at least 3 hours or at a temperature of 105 ° C for at least 15 minutes. At the same time, the biological value of milk decreases, but it acquires characteristic organoleptic characteristics - a nutty taste and smell, a creamy or light brown tint.
Sterilization - heat treatment of milk at temperatures above 100 °C. In this case, all types of vegetative microorganisms and their spores are completely destroyed, and enzymes are inactivated.
The following types of sterilization are used in the dairy industry: sterilization in containers at a temperature of 115-120 ° C with a holding time of 30 and 20 minutes; ultra-high temperature treatment (UHT treatment or ultra pasteurization) at a temperature within 140 °C with a holding time of 2 s.
UHT processing followed by aseptic packaging ensures that the product meets the requirements of industrial sterility and is carried out in a flow in a closed system with a holding time of at least 2 with one of the following methods:
- by contact of the processed product with a heated surface at a temperature of 125 to 140 ° C;
- by directly mixing sterile steam with the product being processed at a temperature of 135 to 140 ° C.
After heat treatment, the milk is cooled to 4-6 °C, checked for quality and packaged in small or large containers.
The finished product is stored in refrigerators at a temperature of 0-8 ° C and a relative humidity of 85-90%. The shelf life of most types of pasteurized milk is no more than 36 hours from the end of the technological process.