During the freezing process of fruits and vegetables, their organizational structure and internal components will still undergo some physical and chemical changes, affecting the quality of the product. The degree of influence depends on the type, maturity, processing technology and freezing method of fruits and vegetables.
1. Effect of freezing on fruit and vegetable structure
Generally speaking, the cell tissue of a plant can cause changes in the cell membrane during the freezing process, increase permeability and reduce swelling pressure, that is to say, cold treatment increases the permeability of the cell membrane or cell wall to moisture and ions, which may cause tissue damage . In the freezing process, the supercooling temperature of fruits and vegetables is limited to a few degrees below freezing, and the time is short, mostly within a few seconds, under special circumstances, there are also longer supercooling time and lower supercooling temperature . During freezing, the water in the intercellular space freezes before the water in the cell protoplasts, and even at a freezing temperature as low as -15°C, the protoplasts can still maintain their supercooled state. The supercooled water inside the cell has a higher vapor pressure and free energy than the ice crystal outside the cell. Therefore, the water inside the cell flows out of the cell through the cell wall, causing the ice crystal outside the cell to continue to grow, and the concentration of the solution inside the cell continues to increase. This Until the intracellular water freezes. The freezing point and freezing speed of fruit and vegetable tissues are controlled by the concentration of soluble solids inside them, such as salts, sugars and acids.
In the case of slow freezing, ice crystals are mainly formed in the cell gap, the intracellular water continues to flow out, and the concentration of inorganic salts in the protoplasts continues to rise, enough to precipitate protein, denature or irreversible solidification, and cause cell death. The tissue disintegrated and the texture softened. It is different in the case of quick freezing. Such as quick-frozen tomatoes, the thin-walled cell tissue observed under a microscope revealed that the ice crystals present in the cell and in the cell wall are very small, the cell gap did not expand by 3v, and the protoplasm adhered to the cell wall to prevent the water from moving out. , This tiny ice crystal has little effect on the tissue structure. In the faster thawing, the damage to the protoplast was also extremely small, the plastid was preserved intact, and the vacuole membrane was sometimes not damaged. Maintaining the structural integrity of the cell membrane is very equal to maintaining the intracellular static pressure, which can prevent browning.
Importantly, it can prevent juice flow and tissue softening. The purpose of frozen storage of fruits and vegetables is to maintain the characteristics of fresh fruits and vegetables as much as possible, but during freezing and thawing, the texture and appearance of the product are different from those of fresh fruits and vegetables. The degree of tissue disintegration, softening, juice flow, etc. varies according to the type and condition of the product. For example, although the cells in the succulent tissue of edible rhubarb have hard cell walls, the ice crystals formed in the tissue during freezing make the Cell wall separation occurs, and many cells close to the frozen crystal are distorted and broken, causing the cell content to flow into the gap between human cells, and the juice is lost after thawing. Shi Diaobo freezes at different temperatures, but it is difficult to restore the original freshness after thawing.
It is generally believed that the destruction of fruit and vegetable tissues caused by freezing, and the softening and juice flow caused are not due to the direct effect of low temperature, but the mechanical damage caused by the expansion of crystals. At the same time, the freezing of the intercellular space causes the dehydration of the cells, the concentration of the salt solution increases, the colloidal properties of the protoplasts are destroyed, the cells die, and the ability to control the fresh characteristics is lost.
2. Chemical changes caused during freezing and freezing The temperature, freezing, freezing storage and thawing of fruit and vegetable raw materials may change their flavor, texture, color, and nutritional ingredients, thus affecting the quality of the product.
(1) Taste change During freezing and storage, carbonyl compounds and ethanol will accumulate in fruit and vegetable tissues, producing volatile odors. If the raw material contains many lipids, it will also produce odor due to oxidation. It has been reported that pea, string bean and sweet corn have changed in lipid compounds during frozen storage, and the free fatty acids in their lipid compounds have increased significantly.
(2) The texture becomes soft, frozen and thawed. The vegetable tissue softens. One of the reasons is that the presence of pectinase hydrolyzes the original pectin into soluble pectin. In addition, when frozen, the water inside the cells permeates, and after thawing, it cannot be completely absorbed and restored by the protoplasts, and Jdz tends to soften the fruits and vegetables.
(3) Discoloration During freezing, the color of fruits and vegetables also changes to varying degrees, mainly from green to grayish green. This is due to the conversion of chlorophyll to pheophytin, which affects the appearance and reduces the value of the product. In terms of color changes, fruits and vegetables often brown during freezing and storage, especially after thawing. This is due to the oxidation of phenolic substances under the action of enzymes, such as chlorogenic acid and catechin in apples and pears are the main components of polyphenol oxidase. This enzyme browns quickly and changes color quickly , Affecting quality.
For the enzyme browning, some preventive measures can be taken, such as blanching the raw materials and adding inhibitors (so and ascorbic acid).
(4) Changes in nutrients The freezing itself has a protective effect on the nutrients. The lower the temperature, the stronger the protective effect. Because the chemical reaction speed of organic matter is directly related to temperature. However, due to a series of treatments of raw materials before freezing, such as washing, peeling, and cutting, the raw materials are exposed to the air, and vitamin c is reduced due to oxidation. These chemical changes continue in the frozen storage, but much slower. Vitamin B is heat sensitive, but there is little loss in refrigeration. Vitamin B is reduced in the treatment before freezing, but there is not much loss in frozen storage. Many changes in color and flavor of frozen products are carried out with the participation of enzymes. The activity of the enzyme is greatly influenced by the temperature, but also by the pH substrate. The activity of the enzyme is destroyed at around 93.3°C, and when the temperature drops to -73.3°C, there is still some activity, but the rate of the enzymatic reaction is greatly reduced. Food freezing only has an inhibitory effect on enzyme activity and reduces its activity, but the enzyme activity has not disappeared. On the contrary, the activity of enzymes is often stimulated under supercooled conditions. Therefore, in order to maintain the excellent quality of frozen fruits and vegetables, it is generally required that the temperature of frozen storage is not higher than -18°C, and some countries adopt a lower temperature of frozen storage.
3. The effect of freezing on microorganisms The growth, reproduction and activity of microorganisms have their proper temperature. Above or below the optimal temperature, the growth and activity of microorganisms will gradually weaken until they stop or be killed. Most microorganisms will be inhibited from growing at temperatures below 0°C. The raw materials of fruits and vegetables are easily infected by bacteria before freezing. And the longer the delay, the greater the infection. Sometimes the raw materials are packaged and frozen immediately after blanching. Because the packaging material hinders the heat conduction, the cooling is slow, especially the temperature of the center of the package drops very slowly, and the corruption of microorganisms still occurs during freezing. Therefore, it is best to cool the raw materials to near freezing temperature before packaging, and then it is safer to freeze the packaging. The survival rate of pathogenic bacteria decreases rapidly after the food is frozen, and it has a strong inhibitory effect and a slow killing effect in frozen storage. The test proves that Bacillus can grow under the condition of -2℃. Aspergillus sp. and yeast can grow at -4℃, and some psychrophilic bacteria can survive at -10℃～20℃. Therefore, the storage temperature of frozen fruits and vegetables is generally -18 ℃ or lower. Freezing can kill many bacteria, but not all bacteria. Some molds, yeasts and bacteria can survive for years in frozen foods. Once the frozen fruits and vegetables are thawed, the temperature and humidity are appropriate, and the remaining microbial activities are intensified, causing decay and deterioration. Therefore, the food should be consumed as soon as possible after thawing.
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