Freezing before packaging is adopted, and fluidized bed is mostly used to realize quick freezing of fruit monomers, and the obtained products are loose. When the temperature is -26 ~-30℃ and the product thickness is 30 ~ 40 mm, the freezing time of blueberry, raspberry, strawberry and cherry is only 4 ~ 15 min. Liquid nitrogen freezing method has fast freezing speed and small crystallization of ice, which is especially suitable for berries with soft tissue. During production, the fruit is sent to the quick-freezing tunnel by the conveyor belt, sprayed with liquid nitrogen and immediately frozen. For example, the freezing time of strawberries from 20℃ to-18℃ only takes 2 minutes, and the tissue state of strawberries does not change much. After thawing for 90min, the juice loss is 50% lower than that of air-blown monomer quick-frozen products, but the cost is higher. The method of freezing after packaging is suitable for: ① bulk freezing of candied fruits; (2) freezing soft fruits such as raspberries and overripe fruits such as strawberries; ③ Retail packaging is frozen with syrup or sugar. The freezing system mostly adopts horizontal aluminum plate freezer (if forced circulation is adopted, the surface temperature of the plate is -4℃ when the packaging thickness is 25mm, and the freezing time is about 30min) and fixed air blower freezer (the temperature is usually -38 ~-40℃ and the wind speed is 4 ~ 7m/s).
The general processing flow of quick-frozen fruit is: raw material selection, cleaning, grading and shaping (such as peeling, coring and cutting). ), adding protective agents such as sugar and vitamin C, packaging (or bulk), quick freezing and frozen storage.
The characteristics of fruit raw materials play a decisive role in the quality of finished products. Fruits used for freezing should be suitable for freezing preservation. After thawing, they should be able to basically maintain the original microstructure and brittleness, moderate maturity, neat appearance, and not easy to be oxidized and discolored. Commonly used fruits are peaches, apricots, pears, apples, plums, strawberries, raspberries, melons, cherries, mangoes, pineapples and kiwis. Adding sugar is an important step to maintain the quality of fruit, in order to prevent a lot of water from freezing and damaging the tissue of fruit, prevent air oxidation, weaken the activity of oxidase and avoid the discoloration of pulp. Soak the fruit in 2 ~ 4% vitamin C solution for a few minutes before freezing, which can make the color of quick-frozen fruit close to fresh fruit. Before packaging or monomer quick freezing, it should be dripped dry with vibration dehydration device. The pH value of fruit is 2.5 ~ 5.0, so the pretreatment equipment is required to be made of stainless steel.