In view of the physical and chemical changes of cooking materials during heating and the influence on food after heating, according to my years of cooking experience. The methods of baking, roasting, steaming and water heating in food processing are discussed.
Keywords: physical and chemical changes of cooking materials during heating
With the continuous improvement of people's living standards, the requirements for cooking and processing of diet are getting higher and higher, and more attention is paid to the nutrition and safety of various edible raw materials, seasonings and accessories in the cooking process. Because the cooking materials are all living animals and plants, and they all contain various nutrients. Physically, they exist in solid form, but they contain liquid aqueous solution, and some also contain gas. However, the amount and proportion of nutrients contained in various edible raw materials, seasonings and auxiliary materials are also different, so their physical and chemical properties are also different.
In the process of food heating, due to the different conditions such as temperature, heat transfer medium and PH value, specific physical and chemical changes occur first, and some are carried out at the same time. Cooking heating is carried out by a variety of raw materials, and the changes between them are also different. Generally speaking, there are chemical changes in physical changes, which creates conditions for chemical changes. In the process of chemical changes, a series of changes have taken place between various chemical elements and compounds, resulting in new compounds and new physical properties of new dishes. Therefore, the physical changes and chemical changes of cooking materials during heating are interrelated and mutually conditional.
First, the general changes of food during heating.
As we know, after the raw materials of dishes are heated, various changes will occur during the heating process due to their different properties, shapes and cooking methods.
(a) Physical dispersion of food after heating
Physical dispersion of food after heating, including water absorption, expansion, disintegration and dissolution. Fresh vegetables and fruits: Their cells are full of water, and there is a kind of vegetable gum between cells, which adheres to each cell, so most of them are hard and full before heating. When the raw materials are heated to a certain temperature, gelatin softens, which is mixed with water to form glue solution. At the same time, the cell membrane ruptures, and some substances contained in it, such as minerals and vitamins, dissolve in water, making the whole tissue soft. Therefore, after the vegetables are heated, soup will appear in the pot, which is rich in minerals and vitamins. Example ② Rice flour and flour: Generally, the water content of rice flour and flour is about 65,438+02%-65,438+04%, but their maximum water absorption can reach about 35%, so they can continue to absorb water and expand in warm water and boiling water. As a result, the layers that make up the starch granules are separated, finally broken and gelatinized. The gelatinization temperature varies with the type of starch granules. Sweet potato and taro have low gelatinization temperature and are easy to cook, while rice and wheat have high gelatinization temperature and long cooking time. Starch paste contains three parts, one part is real solution, the other part is starch sugar, and the other part is starch glue. The more starch colloidal particles contained in starch paste, the greater the viscosity. Starch grown in rhizomes, such as lotus root, sweet potato and potato, contains more starch colloidal particles than grains (such as rice and wheat). Therefore, their starch paste is sticky, which can be used as soup, hanging paste, sizing and thickening.
(2) heating and hydrolyzing food
Usually, when food is heated in water, many of its components will be hydrolyzed. For example, starch is hydrolyzed into paste and sugar, which is sweet after maturity; Protein hydrolyzes to produce various amino acids, which have a delicate flavor after maturity; Raw rubber in meat will be hydrolyzed into animal glue, which can be frozen after cooling. When starch is heated in water, part of it will be hydrolyzed into paste, which will further produce maltose and glucose. It is sticky and sweet when it is ripe. Protein is further hydrolyzed after heating to produce amino acids with delicate flavor. After meat is heated, connective tissue is gradually hydrolyzed. Connective tissue is mainly composed of ligaments and gums. Ligament is not easy to hydrolyze, but gum is easy to hydrolyze into animal glue. Animal glue has strong hydrophilicity and can absorb water to form gel. It can be dissolved into colloidal solution when heated and coagulated into colloid after cooling. Therefore, when the connective tissue (raw rubber) of meat is destroyed by hydrolysis after stewing, protein fiber bundles will be separated, making the meat soft and brittle. At the same time, the soup contains a lot of hydrolysate-animal glue, which turns into aspic after cooling.
(3) curing
When food is heated, some water-soluble protein will gradually solidify. If there is electrolyte in the solution, it is easy to coagulate quickly. We know that there are many kinds of protein, and many protein are water soluble. Most of the water-soluble protein gradually solidifies after being heated. For example, the protein of an egg condenses into a lump after heating. Hemoglobin is also a water-soluble protein. When heated to about 85℃, it will solidify into a block, and the degree of solidification will increase with the extension of heating time. Therefore, the heating time of boiled eggs, chicken and duck blood soup, pig blood soup, etc. It should not be too long, otherwise the food will harden, which will not only reduce the umami taste, but also be detrimental to digestion. In the presence of electrolyte, protein colloidal solution coagulates faster. For example, adding electrolytes such as gypsum calcium sulfate or brine magnesium chloride to soybean milk can condense into tofu. Because salt and sodium chloride are electrolytes, it is not allowed to add salt too early when cooking beans, barbecue or vegetables that need thick soup, because adding salt too early will make protein in beans, meat or raw materials for making thick white soup coagulate too early, so water will not easily penetrate into beans and meat, and it will not be made. Of course, the effect of this electrolyte on various raw materials is different from that of protein, so salt should be put early and late according to the specific situation of dishes.
(4) esterification
When the fat is heated by adding water, part of it is hydrolyzed into fatty acids and glycerol. If spices such as wine and vinegar are added, they can be combined with fatty acids to form aromatic vinegar. This action is called esterification. Ester is easier to play than fat, and it has an aromatic smell. Therefore, the raw materials of fish and meat are fragrant when cooked with wine, which is the reason.
(5) oxidation
Many vitamins are easily destroyed by oxidation when heated or in contact with air, and are more easily oxidized quickly in the presence of alkaline solution or a small amount of copper salt. The biggest loss in food cooking is vitamins. Many vitamins are easily destroyed by oxidation after contact with air, and lose their nutritional value. They are relatively stable in heated solution, but they are more easily oxidized in alkaline solution. A very small amount of copper salt can be a catalyst for vitamin C oxidation. Therefore, vegetables containing more vitamin C should try to avoid contact with air and heating for too long when cooking. It is not suitable to put alkali and soda, and it is not easy to use copper pots and shovels.
(vi) Other functions
In addition to the above major changes, various other changes will occur when food is heated. For example, starch and sugar may be partially carbonized in different media, different temperatures and heating times, resulting in yellow or burnt black. Another example is the change in the heating of green vegetables. Blanching vegetables in 100℃ water makes them greener, which is formed by excluding water and air in vegetables. If you fry vegetables in a pan, the vegetables will decompose various organic acids and sulfides when heated. When these substances meet magnesium atoms in chlorophyll, chemical reactions will occur, and the color of vegetables will change from green to yellowish brown. If these acids are dispersed or neutralized by adding alkaline substances and acids, vegetables will remain green.
Second, the effects of different temperatures, materials and heating methods on food.
Some changes in the process of food heating are good, we need to use them, and some are bad. We need to prevent them as much as possible. In the cooking process, different temperatures, different materials and different heating methods are used, that is to say, in order to achieve this goal, we must first master the following measures in terms of temperature and heating time.
Generally speaking, large pieces of tough raw materials should be heated with medium fire or small fire for a long time to soften the tissue and make the meat brittle and rotten. Small pieces of tender raw materials are usually heated for a short time with a big fire. Otherwise it is easy to corrode or become tough. As for the use of different temperatures, raw materials and heating methods, it will have a certain impact on the changes of food. Introduce the following methods respectively:
(1) Use oil as heating body.
Oil can produce high temperature and is suitable for cooking dishes in a short time. With the cooperation of other cooking techniques, it can not only prevent the water in the raw materials of dishes from evaporating and dehydrating due to high temperature heating, but also keep their fresh and tender texture, and make them loose, crisp and crisp by removing more water. Heating at high temperature for a short time can also keep the original flavor of the raw materials of the dishes and prevent the loss of nutrients. Because the specific gravity of oil is less than that of water, oil quickly diffuses in water to form an oil film with uniform thickness, which makes it difficult to disperse the temperature and fragrance in soup. Heating with a small fire can not only shorten the heating time, but also obtain a good flavor and preserve the nutrients in the raw materials of the dishes.
When making shortbread, the oil is mixed and stirred, and the starch particles are quickly surrounded by the formed oil film, so that the starch particles are not easy to expand and coagulate, and a crisp texture is produced after maturity. Similarly, when oil is mixed in the cooking process of mashed vegetables, oleic acid is dissolved by heating, and its fluidity is also increased, and it is quickly and evenly distributed in all parts of mashed mud to prevent water loss in mashed mud. So the texture is very soft. The spices contained in food are easy to vaporize at high temperature, emitting fragrance and becoming a dry and delicious dish. At the same time, using the high temperature of oil, the tough and brittle raw materials treated by a certain tool are made into various neat and beautiful shapes.
(2) Drying and baking methods
Firepower must be uniform, and its characteristics can make the dishes dry outside and fragrant inside, and tender inside. The method of baking and baking is to heat the food raw materials in dry and hot air, so the water on the surface of the raw materials is easy to evaporate. After the sauce overflows, it is dry and hot on the surface of raw materials and immediately condenses into a film. This film can prevent the moisture inside the raw materials from evaporating outside, so the dishes are dry and fragrant outside and tender inside. However, if it is a closed oven, the water evaporates slowly, and the spilled sauce is not easy to solidify on the surface of the raw materials, and will fall into the oven drop by drop. Therefore, the loss of nutrients is more than that of open-air baking. As for mud baking, it is an indirect baking method. Because the raw materials are sealed by layers of mud, they are not in direct contact with the flame, but are slowly baked outside and braised inside to make the raw materials mature. Of course, the moisture of raw materials is not easy to evaporate, and more nutrients can be maintained, so the taste is particularly fresh and tender, and it has a certain flavor.
(3) using steam as a heating body
The characteristics of its dishes are: soft and tender, keeping the shape of raw materials intact and beautiful. For example, before heating, various raw materials are molded according to certain requirements, and scattering and deformation will not occur during heating. This is because the raw materials did not turn over when steaming. In order to keep the steamed vegetables soft and original, there must be enough steam in the steamer to make the humidity basically reach the saturation point, so that the moisture of raw materials is not easy to evaporate and the loss of nutrients is less. However, steaming also has a disadvantage, that is, it is not easy to taste. Because the humidity in the steamer is saturated, the moisture in the raw material is not easy to evaporate, so the molecules of seasoning are not easy to penetrate into the raw material. So steamed vegetables are often seasoned before or after heating.
(4) using water as a heating body
Judging from the taste, quality, nutrition, fragrance, color and shape of dishes, the required temperature and heating time are different, so using water as a heating body has produced many different cooking methods, such as high-temperature (100℃) blanching and short-term high-temperature heating, and so on. The water temperature requirements of raw materials are also different, some are boiled in cold water, and some are boiled in boiling water. For example, the smaller, tender and colored raw materials in vegetables (especially green leafy vegetables) must be boiled and put into the pot for heating. Because the cell membrane of vegetables is destroyed after heating, it will produce an oxidase, which has a strong destructive effect on vitamins, but the oxidase itself is not resistant to high temperature. It is very active at 65℃, but it is destroyed when the temperature reaches 85℃. For example, when vegetables are heated with cold water, when the water temperature rises to about 65℃, the activity of oxidase increases, and two important nutrients in vegetables are seriously destroyed. But when the water temperature reaches 100℃, its activity will be inhibited. At this time, heating vegetables with boiling water pot can greatly reduce the loss of vitamin C.
To sum up, it is the physical and chemical changes of food during heating, and these changes are determined according to the performance, characteristics and texture of various dishes. China has a large number of cooking materials. It includes animal raw materials, plant raw materials, mineral raw materials and chemical synthetic raw materials, so the knowledge of cooking raw materials covers a very wide range, which requires our cooking workers to study, practice and explore for a long time.