How to keep frozen vegetables fresh?

Quick-frozen vegetables are popular in the Japanese market because of their convenience, cleanliness, good fresh-keeping performance and seasonal supply. China's annual export to Japan is about 20,000 tons, with more than 10 marketable varieties. One of the important indexes to measure the freshness of quick-frozen vegetables is color and flavor. In order to keep the color and flavor of quick-frozen vegetables, we must pay attention to the following problems.

1, raw material freshness. The freshness of raw materials is the primary factor to determine the color and flavor of green vegetables. The fresher the raw materials, the better the color, fragrance, taste and nutritional components of frozen products are preserved, and the better the quality of frozen products. Therefore, the shorter the time distance from picking to processing, the better. It is necessary to select at the right time and deal with it quickly. It is necessary to prevent raw materials from acidification under heap pressure and chlorophyll from being damaged and faded; Prevent tissue aging, flavor deterioration and even odor.

2, the temperature and time of the water. The choice of blanching temperature and time of raw materials is an important link to maintain the color, fragrance and taste of vegetables. Blanching is to put vegetable raw materials in boiling hot water for a few seconds to a few minutes, and then take them out and cool them. Its main function is to destroy the oxidase activity in vegetables, prevent ester oxidase from destroying chlorophyll and vitamin A in green vegetables, and avoid oxidative discoloration and odor of quick-frozen vegetables. The suitable temperature and time for blanching vary with the types of green vegetables, such as leafy vegetables and beans. If the blanching time is not enough, the activity of enzymes will remain in vegetables, and enzymatic browning will occur during refrigeration, which will turn frozen vegetables green and yellow and reduce the flavor of frozen products. If the blanching time is too long, the green vegetables will turn yellow. This is because the green color of vegetables is composed of mixed pigments of chlorophyll and carotene, and chlorophyll is easily decomposed by acid, alkali and enzyme. Under the condition of excessive blanching, the organic acids dissolved in water in vegetables react with chlorophyll to produce pheophytin, which makes vegetables lose their bright green color and turn into yellow-green or yellow-brown, and the brittleness of vegetable tissues becomes lower, the flavor becomes weaker and some soluble nutrients are lost. Therefore, it is necessary to update the water frequently or adjust it with proper amount of sodium bicarbonate solution to control the water quality within the range of pH 7 ~ 8 in order to achieve the purpose of color protection.

The temperature and time of blanching should be determined according to different vegetable varieties. Usually, the blanching temperature is 95 ~ 98℃, and the blanching time varies from a few seconds to several minutes. It must be tested before it can be put into production.

3. cool down. Vegetables should be cooled in time after blanching, and the cooling medium temperature should be low and the speed should be fast. Immediately after blanching, it should be placed in a cooling pool at -3℃ ~-5℃ for thorough cooling, and it is best to use sanitary block ice water for cooling to ensure bright green and good fragrance. If it is not cooled in time or the temperature of the cooling medium is too low, the blanched vegetables will continue to be affected by high temperature, the texture will become soft and the color will become bad. Even if they are frozen and refrigerated, frozen vegetables will gradually turn yellow and produce odor.

4. Quick freezing. Quick freezing is an important guarantee to maintain the original color, fragrance and taste of green vegetables. Rapid freezing is beneficial to the formation of small and evenly distributed ice crystals in vegetable cell tissues, which slightly damages the cell tissues, and the changes of frozen products are reversible to the maximum extent, thus maintaining the original color, fragrance and taste of vegetables to the maximum extent.

Quick-freezing means that in 30 minutes or less, the material quickly passes through the ice crystal formation zone at-1℃ ~-5℃, and the central temperature of the raw material continues to drop below-18℃, so that free water and bound water inside and outside vegetable cells precipitate a large number of nuclei at the same time, forming a large number of ice crystals with uniform distribution and fine particles, and maintaining the pressure balance inside and outside the cells. Small ice crystals do little damage to the cell wall, and the water melted by the ice crystals after thawing is absorbed by the cells again without juice flowing out, which is beneficial to the restoration of the original shape of the cell tissue and the maintenance of the original color, fragrance, taste and nutritional components of vegetables. If slow freezing is used, it will take a long time to reduce the central temperature of vegetables to -5℃, which will freeze most of the water in vegetable cells at the intercellular space and form large ice crystals. When water turns into ice, its volume will increase by about 9%, which will destroy the cell membrane, cell wall and internal structure, and rupture the cell tissue. Even if refrigerated at -25℃, the frozen products will change color and taste. After thawing, a large amount of nutrient juice flows out and the flavor is reduced. [Page]

At present, the fluid freezing machine is an advanced equipment for rapid freezing of monomer. It has a heat insulation box, which is equipped with a screen conveyor, a cold fan and an evaporator. The cold source enters the evaporator in the freezer from the liquid supply pipe in the freezer, and the freezing temperature is kept at -35℃ ~-40℃ under the excitation of the cold fan. Raw materials are placed on a horizontal screen and then enter the freezer. Liquid ammonia exchanges heat with the raw material layer, which makes the raw material creep forward and freeze under the drive of high-speed cold air flow, which is usually called iqf (separate quick freezing).

The freezing temperature is usually -25℃ ~-35℃, and the freezing time is several minutes to ten minutes, depending on the variety of vegetables.