The use of many products is simply impossible without their heat treatment in one way or another. However, when preparing, the properties of the products, the content of various useful and harmful substances, can change very substantially.
Proteins
Coagulation or clotting of proteins occurs at a temperature of 70 ° C.Proteins lose their ability to retain water, and from hydrophilic ones turn into hydrophobic ones, as a result of which the mass of meat and fish decreases. The secondary and tertiary structure of protein molecules undergo partial destruction, some of the proteins are converted into polypeptide chains, due to which they are better cleaved by proteases of the gastrointestinal tract.
Proteins contained in products in the form of a solution, during cooking are folded flakes forming a foam on the surface of the broth, and the collagen and elastin contained in proteins turn into glutin( gelatin).During the heat treatment, the total loss of protein products can reach up to 7%.
Exceeding the temperature and duration of the heat treatment worsens the consistency of the products and leads to a tightening of the muscle fibers. This especially applies to the liver, heart and seafood. In the process of strong heating, starch destruction begins on the product surface, sugar and amino acids enter into reactions during which melanoids are formed, giving the crust a dark color, a specific taste and aroma.
In the process of frying and cooking meat products lose about 30-40% of their mass. This is due to the compaction of proteins, melting fat and evaporation of moisture and soluble substances. The least loss is the transfer of breaded products from the cutlet mass, in which the moisture extracted by whites is retained by the filler( bread), and thanks to the breading layer it does not evaporate from the frying surface.
Fats
During cooking, fat from products is heated. As a result of the breakdown of fatty acids, its nutritional value decreases. For example, the losses of arachidonic and linoleic acids can reach 20-40%.During cooking, about 40% of the fat passes into the broth, where it is partially emulsified and oxidized. Under the influence of acids and salts contained in the broth, the emulsified fat decomposes into glycerol and fatty acids, because of what the broth becomes cloudy and gets an unpleasant smell and taste. That's why it should be cooked on moderate heat, and accumulate fat on the surface to remove from time to time.
When frying, a deeper change in fat occurs. If the temperature of the frying surface is above 180 ° C, the fat begins to disintegrate with the appearance of smoke, and the taste of the product may deteriorate sharply. Therefore, the products should be fried at a temperature just below the smoke temperature( 170-175 ° C).
During frying, most of the fat is lost due to splashing. The reason for this is the rapid evaporation of water when the fat is heated above 100 ° C.Such fat loss is called carbon haze, and they are most significant in fats, which contain a lot of water( margarine), as well as in the case of frying moistened foods( meat, raw potatoes, etc.).Breaded products lose less fat.
If we talk about chemical changes in fats, then they are most pronounced during deep-frying. During the hydrolysis, polymerization and oxidation, accumulation of harmful compounds takes place, giving the fat an unpleasant odor and a rancid taste. On the surface of the frying products toxic products of thermal oxidation of fats( ketones and aldehydes) are adsorbed. Also, fats contaminate the particles of products falling into it.
To prevent unwanted changes in fat, fryers are used, in the lower part of which there is a cold zone, the temperature of the fat is much lower, so that the food particles that get there do not burn.
To protect the fryer from spoilage, several techniques are used: it is periodically filtered, hands and equipment are lubricated with vegetable oil, and the products intended for frying are not breaded in breadcrumbs.
Carbohydrates
In the process of heating starch, mixed with a small amount of water, at 55-60 ° C the process of its gelatinization begins, which is markedly accelerated as the temperature rises to 100 ° C.In the process of potato heat treatment, the gelatinization of the starch contained in it is due to moisture contained directly in the potato.
During the baking of dough products, starch is gelatinized due to the moisture that gluten-free gluten secures. Similarly occurs with cooking pre-soaked beans. Starch, present in dry foods( cereals and pasta), is gelatinized by moisture absorbed by them from the environment, and this process does not affect the increase in the mass of products.
The human body is not able to assimilate starch in its pure form, therefore all products containing it are consumed only after cooking. In the case of heating starch to a temperature exceeding 110 ° C, without adding water to it, it will split to dextrin dissolved by water. In the process of heat treatment, dextrinization takes place on the surface of the processed products( when passing the flour, roasting the cereals, baking the pasta).
When cooking fruits and berries, the sucrose in them is split under the action of acids into glucose and fructose. If sucrose heats up to a temperature above 140-160? C, it caramelizes - dissolves, in the process of which dark-colored substances are formed - the ginger. Zhzhenka used for tinting confectionery, soups and sauces.
In the process of heat treatment protopectin, through which plant cells are connected to each other, is converted into pectin. At the same time, the consistency of the products becomes more gentle and they are better absorbed. The rate of conversion of protopectin to pectin depends on:
- product properties - some products are more resistant( beets, cereals, legumes), and in some products it is less stable( fruits, potatoes);
- cooking temperatures - the higher the cooking temperature, the faster protopectin is converted to pectin;
- the reaction of the medium - the process of transformation slows down in an acidic environment, so when cooking soups with sauerkraut, the potatoes must be laid before it, and in case of soaking the legumes it is necessary to prevent their souring.
During the heat treatment, the cellulose, the main structural component of the walls of plant cells, undergoes minor changes, swelling and becoming porous.
Vitamins
During the heat treatment, fat-soluble vitamins( A, D, E, K) are well preserved. So, in the process of carrot secretion, its vitamin value does not decrease, but on the contrary - it dissolves in fats, carotene is converted to vitamin A. Such stability of carotene allows long-term storage of passaged vegetables in fats, although with prolonged storage due to air, vitamins will partially break down.
Water-soluble B vitamins are resistant to heating in acidic medium, and in neutral or alkaline environment they are destroyed by 20-30%, partially turning into a decoction. Most of all thiamine and pyridoxine are lost when combined heat treatment( quenching, etc.).They are best preserved with short-term heat treatment, accompanied by a small amount of flowing juice.
Vitamin PP is the best heat resistance.
Vitamin C suffers the worst heat treatment. It breaks down during oxidation with oxygen, which is facilitated by factors such as:
- cooking with the lid open;
- laying products in cold water;
- long cooking and long storage of food in hot condition;
- increase the contact surface of the product with oxygen during grinding and rubbing.
In an acidic environment, vitamin C is better preserved. During cooking, it partly turns into a decoction. When frying potatoes in deep fat, vitamin C is preserved better than when it is fried in a standard way.
Mineral substances
Most of all( by 25-60%) the products lose the mineral substances contained in them during cooking in a large volume of water - they become a decoction. This is why broths cooked from organic vegetables are used as a basis for first courses and sauces.
Colorants
During the cooking of green vegetables, the chlorophyll contained in them is destroyed under the influence of acids, converting to brown substances. Resistance to heat treatment is the anthocyanins of plums, cherries and black currants, as well as carotene carrots and tomatoes. To preserve the bright color of the beet, a concentrated broth with acidic medium is necessary, otherwise the beets acquire a brown tint. The bright pink color of meat changes as a result of hemoglobin change to gray.
Summary of the
Most of all nutrients are lost when cooking in the usual way - they turn into a decoction. Also, increasing the loss of nutrients contributes to the complexity of the technology of cooking foods( grinding, quenching and wiping both raw and boiled products), too high temperature and cooking time.