Classification of conservation methods, their purpose and essence. Food Preservation Methods
The applied methods of food preservation can be grouped into three groups: physical. chemical and microbiological.
Physical methods. The main method of preservation is the processing of hermetically sealed products by heating. Most microorganisms die at a temperature of 110...120°C, many (not forming spores) at 60...100°C. However, some heat-resistant bacteria persist when heated even up to 130°C.
Heating canned food at temperatures up to 100 ° C is called pasteurization, and at a temperature of 100 ° C and above - sterilization. The duration of heating depends on the chemical composition of the raw material (especially on acidity), its consistency, volume, type of container, etc.
Preservation of food products in a hermetically sealed container is also possible by high-frequency current sterilization (HFC), in which, as a result of the oscillatory movement of the charged particles of the product, the canned food is quickly heated and microorganisms die. At the same time, prolonged heating of raw materials is excluded and the products are of better quality. The duration of heating is 1 ... 2 minutes, sometimes several seconds. High-frequency currents pasteurize compotes and juices in glass containers when heating is required not more than 100 ° C. Sterilization of canned food by RF currents is used to a limited extent due to the complexity of the equipment.
Currently, extensive research is being carried out on the sterilization of canned food using Ionizing radiation, excluding heating of products. Microorganisms under irradiation die very quickly, and their spores lose their ability to develop.
Used to preserve juices and puree-like products. Aseptic (disinfecting) method canning. The essence of the method lies in the fact that the juice or puree is briefly heated in a stream at a temperature of 130 ... 160 ° C, cooled and poured into a sterile container under aseptic conditions. Short-term heating (from a few seconds to 2...3 minutes) kills microorganisms without changing the chemical composition of the product. The method of conservation according to the principle of action on microorganisms refers to sterilization. Jars sealed under aseptic conditions with sterile juice or puree are not subjected to further heat treatment. This method of conservation is one of the most promising. During the period of mass receipt of raw materials, it is possible to quickly preserve the products in large tanks (up to 400 m3), and later, when necessary, pack the products in small containers.
Drying makes it possible to bring the water content in the product to such an amount at which microorganisms can no longer develop. For example, the development of bacteria requires at least 30, and molds - 15% moisture. Drying is the oldest method of preservation and is still being improved. For example, freeze drying is now used (sublimation is the evaporation of ice at low negative temperatures in a vacuum). It is carried out in sublimators with subsequent final drying at a temperature of about 40°C. At the same time, dried products of the highest quality are obtained. Many microorganisms, especially their spores, remain in dried products, and if the importance of the products increases, then the microorganisms begin to develop and spoil them. Therefore, it is necessary to seal dried products (especially with a residual moisture content of 4 ... 5%) or store them in dry storages and warehouses.
Freezing fruits and berries at a temperature of - 25, - 35 ° C and subsequent storage of frozen products at - 18 ° C suspend all physiological processes and microbial activity, but do not destroy them. Therefore, in order to maintain the quality of this type of product, it is necessary to strictly observe the conditions for their storage and quickly use it for food after defrosting. The quality of frozen fruits and berries differs little from fresh ones.
Cooling- this is the processing and storage of fresh fruits and berries at a temperature of about 0 ° C. At the same time, cell juice does not freeze (berries freeze at - 0.7 ... 1.5 ° C, apples at - 1.5 ... 4 ° C, depending on the variety and duration of storage). Cooling slows down biochemical processes, stops the development of microorganisms, but does not destroy them.
Preservation of products by high osmotic pressure occurs when used in high concentrations of sugar and salt. Osmosis is the slow penetration of a solvent into a solution through a thin partition separating them. In this case, the solvent is the water of microbes and it goes through their shells into a solution of sugar or salt. Thus, in jam with a mass fraction of sugar of about 65%, such a high osmotic pressure develops, at which microorganisms are dehydrated and cannot develop. This is also observed in sienna products with a salt concentration of more than 10%.
However, if you store fruits and berries preserved with sugar in an open container and in humid conditions, then the concentration of sugar will decrease and the products may begin to deteriorate. Therefore, canned food must be corked.
The physical methods of preservation include sterilization by filtration, when thin plates are used to filter out microorganisms. Enzymes remain in the product (usually clear juice), so the use of filters alone is not enough to preserve the juice. Heating or cooling required.
Chemical methods. Based on the use of various chemicals that have a detrimental effect on microorganisms. First of all, these include antiseptics - substances that inhibit the development of microorganisms. The most common antiseptic is sulfur dioxide (sulphurous anhydride), or 0.1 ... 0.2% sulfurous acid. This method of preservation is called sulfitation. Sulfur dioxide has a strong effect on bacteria, less on mold and yeast. It is poisonous, therefore, sulphated raw materials are a semi-finished product and are used for processing after removing sulfur dioxide by heating (desulphization). Sulfitation is mainly used for preserving puree processing of raw materials before drying, etc.
To preserve acidic juices, benzoic acid is used in the form of a sodium salt, which is highly soluble in water. 0.05 ... 0.1% sodium benzoate has a detrimental effect on yeast and mold, weaker on bacteria. This preservative is harmless to humans.
In recent years, 0.05 ... 0.1% sorbic acid has been widely used as an antiseptic, which inhibits the development of molds and yeasts in an acidic environment. It is successfully used in combination with sugar, for example, in the production of pureed berries. This acid is also harmless to humans.
In addition to antiseptics, wine (ethyl) alcohol, acetic or lactic acid are used to preserve food. A high concentration of acids makes the product unfit for consumption, so they are used for preparing semi-finished products or used in combination with other canning methods. For example, in the production of marinades, low doses of acetic acid and sterilization in hermetically sealed containers are used.
microbiological methods. During fermentation, salting, urinating products, as well as in the production of wine, microbiological processes occur, as a result of which a preservative is formed - lactic acid or alcohol.
However, to preserve products only with lactic acid or alcohol, their high concentration is required, which cannot be formed as a result of microbiological processes. Therefore, a combination of physical (storage at low temperature) and chemical (use of alcohol or salt) methods of preservation is also used here.
Canned food that is produced in jars without sterilization is called preserves. They are preserved through the use of preservatives (sugar, salt, acetic acid, etc.) or storage at low temperatures.
Control questions
1. What are the main causes of food spoilage? What is the role of microorganisms and enzymes in this? 2. What are the food preservation methods? 3. What methods of food preservation exist? 4. What is the essence of food preservation by sterilization, freezing, drying, HDTV processing? 5. What antiseptics are used in food preservation? 6. When is it necessary to combine different preservation methods?
Chemicals used in food preservation must be harmless and not alter the taste, color or odor of the product.
Currently, the following chemicals are allowed for canning in the Republic of Belarus: ethyl alcohol, acetic, sulphurous, benzoic, sorbic acids and some of their salts, boric acid, urotropine, some antibiotics, etc.
Preservation with ethyl alcohol. This method of preservation is based on the destructive effect of alcohol on microorganisms. Ethyl alcohol is used as a preservative in the production of semi-finished fruit juices. At concentrations of 12-16%, ethyl alcohol delays development, and at 18% it completely suppresses the vital activity of microflora. Juices with an alcohol concentration of 25-30% are used in the production of alcoholic beverages, and with a concentration of 16% - in the production of soft drinks.
Pickling. This is a preservation method based on increasing the acidity of the medium by adding acetic acid. At concentrations of 1.2-1.8%, acetic acid inhibits the activity of many microorganisms, and primarily putrefactive ones. Microorganisms are sensitive to changes in the pH of the medium, because. this leads to a change in the surface amphoteric structures of the cell and, as a result, to disruption of cellular balance and subsequent cell death.
To enhance the preservative effect, pickling is sometimes combined with other types of preservation: pasteurization, salting, storage at low temperatures. In pasteurized pickled products, the content of acetic acid is reduced to 0.8-1.2%, which favorably affects their taste.
In the production of pickled products, table vinegar is usually used, containing 3-6% acetic acid, or food vinegar essence with an acetic acid content of 70-80%. For the production of marinades, biochemical vinegar (alcohol, wine, fruit and berry, etc.) is more desirable, since vinegar from the essence has a sharp taste. In addition to vinegar, salt, spices, and sugar are added to the marinade filling.
Fruits, vegetables, mushrooms, fish, etc. are marinated. Prepared fresh, blanched or fried foods are poured with marinade filling, jars are rolled up and pasteurized at a temperature of 90-100 ° C. When marinades are stored, they ripen, which lasts from 20 days to 2 months. In the process of ripening, acetic acid, sugar and salt diffuse into products, under the action of acids about 75% of sucrose is converted into invert sugar, and the taste of the product improves. Store marinades at low temperatures (from 0 to 4 ° C), as many molds absorb acetic acid and can cause food spoilage.
acid preservation. The preservation of food with sulfurous acid, its salts, and sulfurous anhydride is called sulfitation. Sulfurous acid is a strong antiseptic, inhibits the activity of molds and bacteria; yeast, especially wine races, are more resistant to its action. This acid is used for the preservation of fruits, berries, fruit and vegetable semi-finished products. The effectiveness of sulfuric acid depends on the temperature and pH of the medium. With increasing acidity, the degree of dissociation of sulfurous acid decreases, and thus more undissociated molecules with a preservative effect are retained.
Sulfitation is carried out in various ways. For disinfection of premises, barrels, tanks, gaseous sulfur dioxide is used, which is formed during the combustion of sulfur. Sulfur dioxide can be supplied from steel cylinders in which liquefied gas is under pressure. Sulfitation is also carried out with a 5-6% aqueous solution or with the help of salt solutions that release sulfur dioxide.
In addition, sulfur dioxide can be used by placing sodium bisulfite in boxes of grapes (or other berries). Slowly decomposing during storage and reacting with the water released by the grapes, sodium bisulfite forms a small amount of sulfur dioxide, which is quite sufficient to prevent spoilage of the berries.
Sulfurous acid inactivates enzymes, inhibits the processes of respiration of fruits and vegetables, thereby extending their shelf life and protecting them from browning.
When sulphited products are heated, sulfurous acid is rapidly decomposed, releasing gaseous sulfur dioxide. This property of sulfurous acid is based on the process of its removal from the product - desulfurization. Sulphated products are used only for further processing after removal of sulfurous acid. Sulfur dioxide acts on the respiratory organs and causes irritation of the mucous membrane, therefore, in high concentrations, it is dangerous to humans.
The most commonly used salts of sulfurous acid include sodium bisulfite (NaHSO3), potassium bisulfite (KHSO3), sodium pyrosulfate (Na2S2O3), sodium sulfite (Na2SO3) and potassium sulfide (K2SO3).
The residual content of sulfur dioxide in dried vegetables and fruits should not exceed 0.01-0.06%, in fruit and berry purees - 0.2, in juices - 0.12-0.15%.
Benzoic acid (С6Н5СООН) and sodium benzoic acid are used for preserving С6Н5СООН) fruit and berry semi-finished products, juices, sprats.
Benzoic acid is poorly soluble in water, so its salt, sodium benzoic acid (C6H5COOHa), is usually used for preservation. This acid inhibits the vital activity of yeast, has a less intense effect on butyric acid bacteria, has little effect on acetic acid, and has almost no effect on the development of lactic acid bacteria and molds. The strongest antiseptic effect of benzoic acid and benzoic acid sodium is manifested in an acidic environment at pH 2.5-3.5. The disadvantage of benzoic acid as a preservative is its negative effect on the taste of the preserved product; under its action, turbidity of plant materials containing protein also occurs. Therefore, the amount of benzoic acid added to food products is strictly regulated and does not exceed 70–100 mg per 100 g of product.
Sorbic acid (C6H8O2) and its salts are strong antiseptics and are harmless. They are used to preserve fruit juices, purees, marinades and other low pH products.
Sorbic acid refers to unsaturated and is a white or slightly yellowish odorless crystals with a slightly acidic taste. It inhibits the activity of fungi and yeasts and has little or no effect on bacteria. This acid is sparingly soluble in cold water, so it is often used in the form of water-soluble salts - sodium or potassium sorbate. The advantage of sorbic acid over other preservatives is that it does not change the taste and smell of canned foods.
The amount of sorbic acid allowed for the preservation of various products is not the same and ranges from 0.05--0.1% (soft drinks, juices) to 0.5% (semi-smoked sausages).
Fruit and berry juices with 0.05% sorbic acid are stored for 8 months without the use of cold. Paper is impregnated with sorbic acid and introduced into the composition of films used for food packaging. Loaves of semi-smoked sausages are treated with a solution of sorbic acid to lengthen their shelf life. Small additions of sorbic acid inhibit alcoholic fermentation in the production of semi-sweet wines.
Preservation with boric acid, borax and urotropin. Boric acid (H3BO3), borax (Na2B4O7 * 10H2O) 0.3% concentration and urotropine are used to preserve granular sturgeon caviar. Boric acid is also used as a preservative in the production of melange.
Ethylene-releasing drugs - 2-chloroethylphosphonic acid - etrel and its derivatives: hydrel, dehydrel, etc. - are inhibitors of growth processes and are used to prevent the germination of potatoes, root crops and onions during storage, they increase their resistance to phytopathogenic microorganisms. Vegetables are treated with a 0.5% aqueous solution of the drug before laying them for long-term storage. The preparations are harmless: in a neutral environment they break down to ethylene, phosphoric acid residues, nitrogen and chloride ions.
To prevent the germination of vegetables, it is allowed to use an aqueous solution of the sodium salt of maleic acid hydroside, which is used to treat vegetables 2-4 weeks before harvesting, as well as methyl ester of naphthylacetic acid in the form of dust, which is used to pollinate potatoes in the spring.
Preservation with antibiotics. Antibiotics that can be used in the food industry, along with a pronounced antimicrobial effect, should have a low resistance to the external environment, and also be easily inactivated during the heat treatment of products. Currently, chlortetracycline (biomycin), nystatin and nisin are used in the food industry.
Chlortetracycline (biomycin) when heated forms an isomer of isochlortetracycline, which is harmless to the human body and has a bacteriostatic property. This antibiotic acts on mucus-forming microorganisms. In the food industry, it is used to treat meat (the surfaces of carcasses are irrigated or administered intravenously 1 hour before the slaughter of the animal) and fish transported over long distances. For the treatment of cod fish, biomycin ice is used, i.e. ice containing chlortetracycline in an amount not exceeding 5 g per 1 ton of product.
Nystatin is an antibiotic that acts on yeast and fungi that cause mold in meat. In the food industry, it is usually used in combination with chlortetracycline for more efficient processing of meat carcasses. The concentration of chlortetracycline in solutions should not exceed 100 mg, and the concentration of nystatin - 200 mg per 1 liter of water.
Nisin is used in the production of dairy and fruit and vegetable canned food. It is a polypeptide formed during the metabolism of lactic streptococci. The composition of nisin includes various amino acids: methionine, leucine, valine, lysine, histidine, proline, glycine, serine, etc. Nisin inhibits the growth of various staphylococci, streptococci, clostridia, etc. In the human body, nisin is quickly destroyed without having a negative effect. An important feature of nisin is its ability to reduce the resistance of packs of heat-resistant bacteria to heat, which makes it possible to reduce the sterilization regime.
Phytoncides are antibiotics of plant origin. Of these, allyl mustard oil, extracted from mustard seeds, is most applicable for canning. The introduction of this antibiotic into marinades in an amount of 0.002% allows you to keep them for a year without pasteurization, but on condition that the jar is hermetically sealed.
Canning gases. Ozone, which has disinfecting and deodorizing properties, is used to maintain quality while extending the shelf life of food products. Being a strong oxidizing agent, ozone inhibits or stops the development of bacteria and molds and their spores both on the surface of the product and in the air. The effectiveness of ozone depends on the concentration, relative humidity of the air, as well as on the initial microbial burden of the product.
Ozone is recommended to be used for disinfection and deodorization of air in refrigerators, for disinfection of vehicles, equipment and containers. Ozonation should be carried out with high concentrations of ozone (25–40 mg/m3) for 12–48 hours, which makes it possible to reduce the contamination of chambers by more than 90%.
For the processing of food products (meat, sausages, cheeses), the ozone concentration should not exceed 10 mg/m3, since its higher content causes a deterioration in their presentation, taste and nutritional value.
Carbon dioxide in high concentrations suppresses or completely stops the vital activity of many microorganisms.
The effectiveness of CO2 impact on microorganisms depends on its concentration in the atmosphere, air temperature and type of microorganisms. Mold development is retarded at a CO2 concentration of about 20%, and at 40–50%, their growth almost completely stops. Bacteria are more resistant to the action of CO2. Some anaerobic hydroforming bacteria are able to grow at 60 - 80% CO2.
However, for the storage of food products, the concentration of CO2 should not exceed 20 -22%, since a higher content of carbon dioxide causes a deterioration in their quality. Therefore, it is advisable to use CO2 in combination with cooling. In this case, the shelf life of meat, fish, poultry and sausages at a temperature of 0 ° C and 10-20% CO2 increases by 2-3 times compared to conventional cold storage.
In the canning industry, refrigeration storage of grape juice in tanks with a capacity of 20-50 tons in a CO2 atmosphere has become widespread.
1. Physical preservation methods
2. Physical and chemical methods of preservation
3. Biochemical preservation methods
4. Chemical preservation methods
5. Combined preservation methods
Canning as a method of processing products allows you to arrange to eliminate seasonality in the consumption of certain products, expand the range of food products, and increase the degree of readiness of products for eating.
The action of preservative factors is aimed at slowing down or stopping enzymatic processes, as well as suppressing the vital activity or destroying microorganisms that cause food spoilage.
Preservation methods, depending on the nature of the factors that have a preservative effect, are divided into physical, physico-chemical, biochemical, chemical and combined.
I. Toward physical methods include conservation based on the action of high and low temperatures, mechanical sterilization, processing of products with ionizing radiation, ultraviolet rays, and ultrasound.
There are two methods of high-temperature processing - pasteurization and sterilization.
At pasteurization the product is heated to a temperature of 63-70 ° C and maintained at this temperature for some time.
With such treatment, enzymes are inactivated, vegetative forms of microorganisms die, but spores remain in a viable state and, when favorable conditions arise, begin to develop. Therefore, pasteurized products (milk, cottage cheese, etc.) are stored at low temperatures for a limited period of time.
Distinguish long(at a temperature of 63-65 ° C for 30-40 minutes), short(at a temperature of 85-90 ° C for 0.5-1 min) and instant pasteurization (at a temperature of 98 ° C for a few seconds).
Sterilization - is the heating of food at temperatures above 100 ° C. During sterilization, not only vegetative forms of microorganisms die, but also most spores. Therefore, sterilized products are well preserved even at room temperature, but sterilized products have a reduced nutritional value, since vitamins, proteins and other biologically active compounds are destroyed at high temperatures.
Most often, sterilization is carried out at a temperature of 113-120 ° C for 15-60 minutes. Also used aseptic sterilization - short-term heating (within a few seconds) to a temperature of 150 ° C, rapid cooling to a temperature of 30-40 ° C and aseptic filling (packaging) into a sterile container.
Low temperature processing does not lead to irreversible inactivation of enzymes and death of microorganisms. The nutritional value of chilled and frozen foods does not change significantly.
Cooling - this is the processing and storage of food at a temperature close to cryoscopic (freezing temperature of cell sap). For most products, this temperature is around 0°C.
A variation of the method is hypothermia , which is carried out at lower temperatures than cooling, but at higher than freezing. In a supercooled state, meat and fish are transported over long distances, eggs are stored (at a temperature of -1°C to -2°C).
Freezing - processing and storage of food products at a temperature of -6 ... -8 ° C and below. Frozen foods last from a few months (fruits and vegetables) to a year or more (meat). The quality of frozen food depends on the speed of freezing and the method of defrosting. At quick freezing(at a temperature of -30…-40°C) small ice crystals are formed in the product, which are evenly distributed and do not destroy cell walls (the loss of cell juice during defrosting is minimal). At slow defrosting at a temperature of 0 to 4°C, the crystals thaw out gradually, and the cell colloids have time to bind the resulting moisture. This technology of freezing and thawing allows you to maximize the quality, including the nutritional value of the product.
Freezing is carried out in different ways:
With the use of natural cold (-6 ... -8 ° C and below);
With the use of quick freezers and chambers at a temperature of -18 ... -30 ° С;
Using the fluidization method (freezing in a fluidized bed) - an intense flow of cold air is fed through the product layer from bottom to top, the product passes into a suspended state, mixes well, quickly releases heat, individual particles do not stick together;
In an atmosphere of liquid nitrogen at temperatures from -80 to -190°C, etc.
In terms of nutritional value, frozen foods are inferior to chilled ones.
Mechanical sterilization is a preservation method based on the use of decontaminating filters, the pore size of which is smaller than the size of microorganism cells. It is used for liquid products: clarified juices, grape wines, beer, etc. The advantage of the method is the maximum preservation of nutritional value - organoleptic properties, biologically active substances.
Canning with ionizing radiation is under development and is used for the processing of containers and packaging materials.
ultraviolet irradiation used for sterilizing the surface of meat carcasses, sausages, for processing warehouses. Mechanism: denaturation of nucleic acids and nucleoproteins of microorganisms (l=250-280 nm).
Ultrasound (oscillations with a frequency above 20 kHz) are used for preserving milk, grape must, disinfecting water, and sterilizing canned food. Microorganisms die as a result of cavitation - under the influence of a sound wave, small gaps form in the liquid, leading to disruption of all vital processes of the cell.
Preservation by ultra-high and ultra-high frequency currents (UHF and microwave) based on rapid uniform heating of the product to a temperature of 100°C and above. With such heating, the nutritional value is well preserved and the sterilization effect is quickly achieved. The method is used for sterilization of juices, milk, disinfection of grain, cereals, flour.
II. Physico-chemical methods of conservation are based on increasing the osmotic pressure, either by dehydrating the product or by increasing the concentration of solids. Under conditions of high osmotic pressure, with a lack or absence of free water, microorganisms cease their vital activity, and enzymes are inactivated.
The physico-chemical methods of preservation include drying, concentration, preservation with salt (salting) and sugar.
Drying is a preservation method based on the removal of moisture from the product to a residual content of 3 to 25%. During drying, aromatic substances partially evaporate, the color and consistency of the product changes, vitamins and other thermolabile compounds are destroyed. Before use, dried products are, as a rule, pre-treated to restore the primary structure. The shelf life of dried products is from 6 months. up to 2 years in conditions of low relative humidity, as they are highly hygroscopic.
Ambassador used to preserve fish, meat, vegetables and other products. There are dry, wet and mixed salting, cold (-10 ... 0 ° C), chilled (0 ... 5 ° C) and warm (10 ° C and above).
Due to diffusion processes during salting, the structure of the product changes, new taste and aromatic properties are formed. The nutritional value of the product is reduced as cell sap with nutrients diffuses into the brine.
Sugar canning is used in the production of jam, marmalade, jam, syrups. The method is most often combined with heat treatment.
III. Biochemical methods are based on the preservative effect of substances formed as a result of biochemical reactions. Biochemical methods include pickling vegetables or urinating fruits and berries.
The methods are based on the preservative effect of lactic acid, which is formed and accumulated during lactic acid fermentation, and ethyl alcohol, which is formed and accumulated during alcoholic fermentation. When fermenting vegetables, table salt is added at a concentration of 2-6% (when urinating fruits - salt and sugar) to isolate cell sap, rich in sugar and being a substrate for the development of lactic acid bacteria. Fermentation is carried out in 2 stages: at the first stage (fermentation stage) maintain a temperature of 18-25°C for intensive reproduction of lactic acid bacteria. On the second stage, the product is cooled to 0-5°C and kept at this temperature for the accumulation of lactic acid.
Lactic acid inhibits the vital activity of putrefactive bacteria, but does not affect the development of acetic acid bacteria, but does not affect the development of acetic acid bacteria and molds, therefore fermented and soaked products are stored at a low temperature (0-2 ° C) under anaerobic conditions.
IV. Chemical preservation methods based on the bactericidal action of chemicals (preservatives). Organic acids (acetic, sorbic, benzoic, sulfurous, etc.) are used as preservatives. So, for example, preservation with acetic acid at a concentration of 1.2-1.8% is called pickling. Sorbic acid and sodium sorbate are used as a preservative in the manufacture of cakes.
V. Combined preservation methods are based on the combined action of several preservative factors: smoking, drying, making preserves.
Canning methods
When canning at home, the following canning methods are most easily implemented and are most commonly used.
1. Sterilization. With this method of preservation, microorganisms die, and enzymes are destroyed as a result of heating food products placed in hermetically sealed jars. Sterilization is usually carried out at a temperature of 100–120 °C. Raw materials are filled with water or other filling.
2. Cooking fruits with sugar. When cooking sweet fruits, a product with a high sugar concentration is obtained - 60–65% and more (jam, jam, marmalade, etc.).
3. Evaporation (concentration). In this way, fruit and vegetable juices or puree masses are processed. The juice (puree) is heated to remove much of the water it contains. Concentrated products are also an unfavorable environment for the development of microorganisms. Jam, vegetable puree and other products are prepared in this way.
4. Salting and pickling. This method of preserving fruits and vegetables is based on creating conditions under which fermentation proceeds with the formation of lactic acid.
5. Drying. This method removes most of the water it contains from the product. Dried foods are an unsuitable environment for the development of microorganisms due to their low moisture content.
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When preserving, methods are used that ensure the death of microorganisms, or transfer them to a state of anabiosis. Under the influence of conservation, the activity of microbial enzymes is also suppressed. Canning allows you to create stocks of perishable food products, move them over long distances, regardless of climatic conditions, and provide the necessary range of food products throughout the year.
Technological progress in canning technology has made it possible to put into practice methods that ensure high stability of food products during long-term storage while maintaining their nutritional, taste and biological properties.
The classification of preservation methods is presented in the table.
The thermal method is the most widely used. This method of preservation is based on the death of various types of microorganisms under the influence of temperature. Vegetative forms of microorganisms are mainly inactivated at t° 60-70° for 1-10 minutes, with the exception of thermophilic bacteria that can survive at t° 80°. Spores are resistant to high temperature, for inactivation of which heating above 100 ° is required at an exposure of 30 minutes to 2-3 hours.
Sterilization ensures the release of the preserved food product from vegetative forms of microorganisms and from spores. Sterilization uses modes with t° 108-120° for 40-90 minutes.
Preservation of liquid food products - milk, vegetable and fruit juices, beer - is produced by pasteurization. At the same time, the food product is freed from viable pathogenic microorganisms of the intestinal group, Mycobacterium tuberculosis and some other microorganisms. There are low pasteurization, which is carried out at t° 65° for 20 minutes, and high - at t° 85-90° for no more than 1 minute. This ensures a sufficient effect with a minimum change in the nutritional and taste properties of the pasteurized products.
Low temperature is the best preservative factor that ensures the preservation of perishable food products with the least changes in natural properties and the least loss of biologically active food components - vitamins, enzymes and others. Under the influence of low temperatures (-20° and below), most microorganisms stop their development, with the exception of psychrophiles, fungi and mold, which remain viable at t° - 20° and below. The low temperature used during preservation does not kill microorganisms, but only stops their growth. Pathogenic microorganisms such as salmonella and staphylococcus survive in frozen foods for several months.
Preservation at low temperature is carried out by cooling or freezing the food product. Cooling is the effect of low temperature on a food product with bringing the temperature in its thickness from 4 to 0 °. When cooled, the food product, without being frozen, retains its nutritional, taste and biological properties. The meat is most often preserved by cooling. Chilled products are stored in refrigerators at t° from 0 to 2° and relative humidity not higher than 85%. Chilled meat can be stored without signs of spoilage for up to 20 days.
Freezing significantly disrupts the structure of cells and tissues of frozen products, which after thawing differ sharply from fresh products (Fig., a). During slow freezing, large ice crystals form in the cells of the preserved food product (Fig., c, d), which destroy the membranes and cellular elements. During the thawing process, water does not return to the colloids and the product undergoes dehydration; while protein and other nutrients are lost. The rapid freezing method helps to maintain the high quality of products during thawing. In this case, a large number of small crystals are formed (Fig., b); when they are thawed, water easily returns to the colloids from which they were formed. Fast freezing gives minimal loss of vitamins and ensures the least development of microorganisms in products.
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The quality of frozen food depends on the thawing method. Rapid thawing of frozen meat is accompanied by significant losses of nutritive, extractive and biologically active substances. Therefore, frozen meat should be thawed slowly.
Dehydration
Dehydration (drying) - preservation based on the termination of the vital activity of microorganisms when the moisture content in the food product is less than 15%. When preserved by drying, microorganisms do not die, but go into a state of anabiosis; when the product is moistened, they become viable again. Drying at normal atmospheric pressure can be natural and artificial. Canning Natural drying methods include sun-drying (to produce dry fruit) and curing (for long-term preservation of fish products).
Artificial chamber drying can be jet, spray and film. With the jet method, drying is carried out in drying chambers in which food products are subjected to continuous action of a jet of hot air coming from heaters; moisture is removed through special ventilation systems.
Spray drying, used for dehydration of liquid food products (milk, eggs, tomato juice), is carried out in a special chamber at t ° 90-150 ° by spraying the liquid product through a nozzle into a fine suspension, which dries quickly under the action of hot air and in the form of a powder sinks to the bottom of the chamber. During spray drying, the product is subjected to short-term exposure to high temperatures, and therefore it changes little and retains all its natural properties. The dry products (powders) obtained in this way, when water is added, are easily restored to the original product suitable for consumption.
Preservation of liquid products can also be carried out by film drying by applying a liquid product to the heated surface of a rotating drum. Products obtained by film drying are significantly inferior to products made by spray drying. Thus, the solubility of milk powder from spray drying reaches 97-99%, while film-dried milk powder dissolves only 80-85%.
Vacuum drying, usually carried out at a low temperature, ensures the complete preservation of food products. One type of vacuum drying is lyophilization. The main principle of lyophilization as a preservation method is the dehydration of the product under vacuum conditions and the removal of moisture directly from the ice crystals, bypassing the liquid phase. There are three periods in the lyophilization process. In the first period, the products loaded into the sublimator are subjected to the action of a deep vacuum, in which the product self-freezes and moisture evaporates directly from the ice crystals. In dried products, the temperature reaches -17°. This period lasts 15-25 minutes, during which about 18% of moisture is removed. In the second period, at t° -10-20°, about 80% of moisture is removed, then the plates on which the dried products are located are heated. In this case, the products are not defrosted, and the removal of moisture continues directly from the ice crystals. Drying in the second period lasts 10-20 hours depending on the humidity and weight of the product. In the third period, thermal vacuum drying is performed at t° 45-50° for 3-4 hours.
Salting and canning with sugar
Salting and canning with sugar are produced on the basis of an increase in osmotic pressure. This preservation method is based on the ability of microorganisms to remain viable only under the condition of a certain difference in osmotic pressure inside the bacterial cell and the environment (the osmotic pressure in the bacterial cell is somewhat higher than in the environment). An increase in osmotic pressure in a food product leads to a disruption in the exchange between the microbial cell and the external environment, to dehydration of the cell, a decrease in the volume of protoplasm and death of the microbial cell. Salt and sugar solutions are characterized by high osmotic pressure. So, the osmotic pressure of a 1% solution of sodium chloride or sugar is 6.1 atmospheres.
When preserving with salt, 8-12% common salt solutions are used, which corresponds to 50-73 atmospheres of osmotic pressure, which provides a reliable preservative effect. However, there are microorganisms (Serratia salinaria) that can withstand high concentrations of salt (up to 15-20%). Dry, wet, warm and cold salting are used in practice. With dry salting, salted products are processed with dry salt, without brine. Wet, or brine, salting is made by immersing the product in a pre-prepared saturated saline solution. The ambassador of frozen products is called cold, and the ambassador at ambient temperature is called warm. K. salting is accompanied by some loss of nutrients.
When preserving with sugar, its concentration is usually created at about 60%, which corresponds to 350 atmospheres of osmotic pressure. Such a high pressure provides a reliable preservative effect - storage for a long period at any ambient temperature.
Marinating and pickling
Pickling and pickling are based on the ability of microorganisms to grow within narrow pH ranges. A change in the pH value disrupts the dispersity of the protoplasm of a microbial cell and stops its vital activity. So, at a pH below 4.5, the vital activity of putrefactive bacteria stops (a change in the concentration of hydrogen ions is in practice carried out by the pickling method). When pickling, food acids are used, including acetic acid, which at a concentration of 4-6% causes the death of microorganisms, and at a concentration of 1-1.8% weakens the vital activity of microorganisms and puts them into a state of suspended animation. To increase the effectiveness of canning, pickling is combined with pasteurization and salting. Pickled products should be stored at a temperature not exceeding 6 °.
During fermentation, a change in the concentration of hydrogen ions is combined with the specific action of lactic acid - sugar is fermented into lactic acid. Under the influence of fermentation, the vital activity of pathogenic non-spore-bearing microflora is completely suppressed and inactivation of helminth eggs occurs.
Canning with antiseptics and antibiotics
Chemical preservatives are used to a limited extent in food preservation in our country; only certain chemicals are allowed in quantities that are not harmful to the health of consumers. Food antiseptics (benzoic, sulfurous and sorbic acids), antibiotics and antioxidants are used as chemical preservatives. Benzoic acid in the amounts used for preservation is harmless, but its preservative properties are small. The unconditionally permissible daily dose of benzoic acid is up to 5 mg / kg and the conditionally permissible dose is 5-10 mg / kg of body weight. In the USSR, benzoic acid is allowed in marmalade, marshmallow, marmalade and melange in the amount of 700 mg/kg; in conserves (sprat) and fruit juices - 1000 mg / kg. Sulfurous acid, sulfurous anhydride, sodium bisulfate and sodium pyrosulfate are used for sulfitation of fruits and vegetables. Under the influence of sulfitation, a better preservation of products and a higher content of ascorbic acid in them are ensured. Sulphated vegetables and fruits undergo partial desulphurisation during heat treatment. The content of sulfurous acid in fruit juices and dry fruits is allowed up to 100 mg/kg, in tomato puree - up to 1500 mg/kg. Sorbic acid is most suitable for food preservation. It is characterized by a high antimicrobial effect and the least manifestation of any negative effects on the body. The transformation of sorbic acid in the body occurs according to the type of transformation of unsaturated fatty acids. The unconditionally permissible daily dose of sorbic acid is up to 12.5 mg/kg, the conditionally permissible dose is 12.5-25 mg/kg of body weight. Sorbic acid is allowed in soft drinks in the amount of 300-500 mg / kg, in fruit and berry juices and condensed milk - 1000 mg / kg, when processing the surface of cheeses - 2000 mg / kg, and semi-smoked sausages - 5000 mg / kg. Antibiotics for conservation purposes are used in an extremely limited range and volume. In the food industry, only those antibiotics are allowed that are not used in medicine for medicinal purposes and which, along with a high antimicrobial effect, are unstable in the environment and are inactivated during heat treatment. As an exception, biomycin is used in the food industry - only in the form of biomycin ice (5 g of tetracycline per 1 ton of ice). Biomycin ice is used in the transportation of cod fish and meat over long distances. The use of chlortetracycline hydrochloride for food preservation is not allowed in our country. In the USSR, the use of two antibiotics - nystatin and chlortetracycline hydrochloride for the treatment of meat carcasses by irrigating them with solutions (chlortetracycline hydrochloride - 100 mg and nystatin - 200 mg per 1 liter of water) is temporarily allowed. Nisin is used to process some vegetable and fruit products, to which staphylococci are especially sensitive. Nisin has the ability to reduce the resistance of spores to heat, which contributes to their more effective inactivation. Antioxidants are used to prevent spoilage of fats. Butylated hydroxyanisole, butylated hydroxytoluene and dodecyl gallate are allowed as fat antioxidants. Ascorbic acid and ascorbyl palmitate can be used as fat antioxidants.
Smoking and preservation
Smoking and preservation are combined methods of preservation. Smoking is based on the action of smoke, which has preservative properties, on the product. Smoke contains a complex of sublimation products, which exhibits an antiseptic effect. Among the products of smoke smoke there are resins and some substances classified as carcinogenic. In connection with the carcinogenic danger in modern conditions, smoke smoking has been replaced by the use of smoke liquid, free from substances that have carcinogenic properties. Smoking includes a complex of effects on the food product - salting, drying, heating. There are hot (t° 80-140° for several hours) and cold (at t° not higher than 40°) smoking. These types of smoking are mainly used for canning fish. Hot smoked fish is a perishable product, its shelf life should not exceed 72 hours. Pre-salted fish is subjected to cold smoking.
Reservation is a set of actions that ensure the safety of products in hermetic containers without signs of spoilage for several months. Pickling, salting and pasteurization are used as preservative factors. Preserves should be stored at t° 6-8°.
Conservation by ionizing radiation has some promise. This method is comprehensively studied in many countries of the world.
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