How to maintain the color and fragrance of green quick-frozen vegetables

Because quick-frozen vegetables have the characteristics of convenient consumption, cleanliness, good preservation performance, and availability of four seasons, they are very popular in the Japanese market. China's annual exports to Japan amount to about 20,000 tons, and more than 10 varieties are available for sale. One of the important signs to measure freshness of fresh-frozen vegetables is color and flavor. In order to maintain the color and flavor of quick-frozen vegetables, the following issues must be noted. 1. Raw material freshness Raw material freshness is the primary factor that determines green vegetable color and aroma. The more fresh raw materials, the more frozen textures, aromas, tastes, and nutrients are stored, the better the quality of frozen products. Therefore, the shorter the distance from the time the raw material is picked to the processing, the better. To achieve proper picking and rapid processing. To prevent acidification of the raw materials, the chlorophyll is destroyed and faded; it prevents the aging of the tissues, the flavor is deteriorated, and even odors are produced. 2. Boil temperature and time The choice of raw material blanching temperature and time is an important part of maintaining the color, smell and taste of vegetables. The blanching is to put the vegetable raw material into boiling hot water for a few seconds to a few minutes to pick up and cool. 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 differ depending on the type of green vegetables such as leafy vegetables and beans. If the blanching time is insufficient, the activity of the enzyme remains in the vegetables, and during the refrigerating process, an enzymatic browning reaction may also occur, so that the frozen vegetables become green and yellow and the flavor of the frozen products is reduced. If the blanching takes too long, the green vegetables will also turn yellow. This is because the green color of vegetables is composed of a mixture of chlorophyll and carotene pigments, and chlorophyll is easily decomposed by acids, alkalis and enzymes. In the case of excessive blanching, organic acids dissolved in water in vegetables react with chlorophyll to produce pheophytin, which causes the vegetables to lose their bright green color and turn yellow-green and yellow-brown. The fragility of the vegetable tissue becomes lower, and the flavor changes. Light and lose some soluble nutrients. Therefore, the scalding water must be frequently renewed or adjusted with an appropriate amount of sodium bicarbonate aqueous solution to control the water quality in the range of pH 7 to 8 in order to achieve the purpose of color retention. The blanching temperature and time should be based on different vegetable varieties. Usually the hot temperature is 95 ~ 98 °C, blanching time from a few seconds to a few minutes, must be tested before it can be put into production. 3. After cooling and hot-burning, the vegetables should be cooled in time, the temperature of the cooling medium should be low, and the speed should be faster. After the blanching, it should be immediately moved to -3°C to -5°C in a cooling pool to cool it thoroughly. It is best to use a sanitary block to cool it with ice water to ensure a bright green color and a good scent. If it is not cooled in time or the temperature of the cooling medium is not low, the vegetables will continue to be affected by the high temperature after blanching. The texture will be softened and the color will be degraded. Even if it is frozen or refrigerated, the frozen vegetables will gradually yellow and produce odors. 4. Rapid freezing is an important guarantee for maintaining the original color, fragrance and taste of green vegetables. Rapid freezing is beneficial to the formation of small and evenly distributed ice crystals in the cell tissue of the vegetables, so that the cell tissue is destroyed and the change of the frozen product reaches the maximum reversibility, so as to maintain the original color, fragrance and taste of the vegetable. Rapid freezing means that the material rapidly passes through -1°C to -5°C ice crystals in 30 minutes or less, and the temperature of the raw material continues to fall below -18°C, allowing the free and combined water energy inside and outside the vegetable tissue. At the same time, a large number of nucleation crystals are precipitated to form a large number of ice crystals with uniform distribution and small particles, so that the pressure inside and outside the cells is kept in balance. The small ice crystals have little damage to the cell wall. After thawing, the ice melted water is reabsorbed by the cells without causing efflux, which is beneficial to the recovery of the cell tissue and preserves the original color, fragrance, taste and nutrients of the vegetables. . If the slow freezing is used, the center temperature of the vegetables should be reduced to -5°C, and the time required will be longer. This will cause most of the water in the vegetable cells to freeze at the intercellular spaces and form larger ice crystals. When water becomes ice, it increases in volume by about 9%, destroys the cell membrane, tendons, and internal structures and breaks the cell tissue. Even when refrigerated at -25°C, frozen products can be discolored and lose flavor. After thawing, a large amount of nutrient juice flows out and the flavor decreases. At present, the flow-type freezing machine is a relatively advanced device for quick freezing of single cells. It has a heat-insulating and insulative box body and installs a mesh-like conveyor, a cooling fan and an evaporator in the box body. The cold source enters the evaporator in the freezer cabinet from the liquid supply pipe in the refrigerator room, and the temperature of freezing is kept at -35°C to -40°C under the agitation of the cooler. The raw materials are placed on a horizontal mesh screen and enter a freezing room. The liquid ammonia exchanges with the heat radiated from the raw material layer. Under the high-speed cold air flow, the raw materials continue to creep forward and freeze, commonly known as IQF (INDIVIDUALLY QUICK FROZEN). Freezing temperature is usually -25 °C ~ -35 °C, the freezing time is a few minutes to ten minutes, depending on the specific vegetable species.

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