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Fresh Grifola frondosa contains 85% to 90% of water, and microorganisms and enzymes easily work to degrade it. Therefore, drying and drying methods are generally used to extend the preservation time of mushroom products, but the loss of nutrients is greater. For example, the chlorophyll and vitamin content of fresh vegetables is 100%, only 3% after drying, 17% dried and dried, and the remaining 40% fast drying; while the remaining 95% freeze-dried, basically no loss of nutrients. In addition, freeze-dried products do not need to be stored in cold storage, and can be stored for more than 5 years after being sealed at room temperature (room temperature). Freeze-dried food can be eaten after rehydration, which is a convenient food for high-efficiency and fast-paced modern life. At present, this new generation of convenience foods has been rapidly popularized in some developed countries. The freeze-dried foods on the Japanese market have reached 22 categories, including meat and its products, various fish and shellfish, various vegetables, dairy products, and eggs. , cereals and fruits, and has formed a scale of 100 billion yen in the industry. Freeze-dried vegetables, meat and aquatic products, freeze-dried tea, and instant coffee have been developed in large quantities in Europe, the United States, and Japan.
The foods currently produced in China using freeze-dried equipment mainly include: soup, shrimp, semi-finished products (such as egg powder), health products (such as freeze-dried ginseng) and food ingredients (such as dehydrated vegetables in instant noodles), the annual output is less than 0.3 million Ton. Experts predict that by 2010 China's only high-grade instant noodles accessories, about 40,000 tons of freeze-dried food, plus a variety of food and fast food ingredients, soup, drinks, the annual freeze-dried food consumption will exceed 100,000 tons. The tremendous demand for frozen foods at home and abroad has provided great opportunities for the development of China's freeze-dried food industry.
The freeze-dried food in China started late. In 1992, it established four freeze-drying plants in Shanghai, Shandong, and Zhejiang in joint ventures with foreign countries, and later introduced freeze-dried production lines from abroad. The JDG series of food vacuum freeze-drying equipment pioneered and developed by the Institute of Modern Physics of the Chinese Academy of Sciences passed the expert appraisal organized by the Science and Technology Committee of Gansu Province at the end of 1995. In 1996, 270 square meters and 120 square meters were put into operation in the cities of Yuncheng and Laiyang, Shandong Province. The two freeze-dried production lines had good equipment and the freeze-dried food produced had excellent quality, which promoted the development of freeze-dried food industry in China.
The principle of freeze-drying is to first freeze the water in the raw material into ice crystals, and then remove the ice by direct sublimation and gasification under a high degree of vacuum, thereby realizing the dehydration of the mushroom body. Vacuum freeze sublimation drying is the use of this principle, first frozen in the water of fresh mushrooms into ice crystals, when the pressure is reduced to a certain value, and then supply the sublimation heat, the ice in the raw materials will continue to sublimate, to achieve the purpose of drying. Due to the relatively brittle texture of the dried product of this method, care should be taken in selecting suitable packaging materials; in order to achieve long-term preservation, vacuum packing is preferable and nitrogen is filled in the packaging bag.
The main part of the vacuum freeze-drying device is a drying chamber equipped with freezing, air extraction, heating and control measurement systems. After the raw materials are frozen, they are sent to a drying room where they are evacuated, sublimated and dried, and then filled with dry air and nitrogen to restore atmospheric pressure. Freeze-drying technology can process any food, including various vegetables and mushrooms. The process of freeze-dried foods includes:
1. Pretreatment refers to the selection, cleaning, slicing, blanching, and sterilization of processed foods; removal of attached eggs and silt; removal of metallic debris by magnetic fields; removal of foreign matter such as hairs and scales by electrostatic fields.
2. Quick-freeze Place the cleaned mushrooms in a closed container and freeze at -20°C. The quicker the freezing is performed, the smaller the crystals in the article are, and the smaller the damage to the material structure, the less the cells will undergo qualitative change.
3. Sublimation Dry-frozen foods are quickly vacuum-sublimed and dried, typically using radiant heating. Then, under a higher vacuum, the temperature is gradually increased. After about 10-12 hours, the mushroom body is dehydrated and dried due to sublimation. After the above-mentioned treatment, the water loss rate accounts for about 90% of the total weight of the mushroom, and the product still contains 7% moisture. If the water content is less than 7%, odor may be generated due to the start of oxidation of fat.
4. Packaging freeze-dried foods are porous, the tissue surface is 100-150 times larger than the original, so the chance of contact with oxygen increases. When recovering normal pressure from low temperature and low pressure, it should be carried out in a nitrogen atmosphere. Crushing and packaging should be performed at a relative humidity of 30%-40% and a temperature of 25°C or less. In order to keep the moisture content of dry foods below 5%, desiccant should be placed in the package to absorb trace moisture. Packaging materials should be selected for poor air permeability, high strength, and deep color.
Taking the JDG lyophilizer manufactured by the Institute of Modern Physics of the Chinese Academy of Sciences as an example, its technical performance and the quality of dried products produced are well received by foreign companies. After many years of production practice tests, it can fully satisfy the quality and efficiency of the Huishuhua products. Deep processing requirements. Freeze-drying production includes pre-processing, quick freezing, sublimation, drying and packaging. In order to create working conditions for quick-freezing warehouses and sublimation drying warehouses, it is necessary to provide refrigeration systems, heating systems, equipment monitoring systems and material transport systems, as well as steam boilers and circulating water. The most critical equipment for a freeze-drying line is a sublimation drying bin, which is a large horizontal cylinder with a multi-layer heating frame inside the cylinder and a tray inserted between multi-layer heating frames. The skips go along the sky rails from the pre-processing room (loading), through the quick-freezing warehouse (quick freezing) and the drying warehouse (sublimation drying dehydration) to the packing room (unloading), and then return to the pre-processing room along the sky rail. This kind of environmental transportation makes it very clean, hygienic and safe. It takes only 5 minutes to transport dry materials into a drying warehouse from a quick-freezing warehouse to ensure that the frozen materials do not melt. The vapor trap is installed on both sides of the heating rack (or is independent of the outside of the warehouse). The heating plate on the heating rack undergoes special surface treatment to transfer thermal energy (sublimation heat) to the material in an auxiliary manner. The vapor trap can be quickly captured. Water vapour that sublimates from the material.
Freeze-dried food production line can be large or small, can provide freeze-dried area of ​​60-1200 square meters production line. It is advisable to produce medicinal materials in small sizes, and medium-sized ones are appropriate for processing vegetables. For processing freeze-dried mushrooms or Grifola frondosa, with a freeze-drying area of ​​120 square meters, 2.4 tons of fresh mushroom material can be loaded every 24 hours, and 418 kg of freeze-dried products can be processed. The power consumption per kilogram of dry matter is 9 kilowatts. Time. If the whole slicing is processed, the yield can be increased, the electricity consumption can be reduced, and the benefit can be increased. Freeze-dried ash tree mushroom strips have good resilience, so long as they are soaked in hot water for a few minutes, they can restore their original shape, and the rehydration rate can reach 80%. Besides the hardness is slightly inferior to fresh mushrooms, the flavor and Compared with fresh mushrooms, there is almost no difference. The use of vacuum freeze-drying, because of the need to maintain a high vacuum during the drying process, so the greater energy consumption, higher product costs, thus limiting the promotion of this technology. If combined with hot air drying and freeze drying, production costs can be reduced.
Also known as vacuum freeze-drying or sublimation drying, the advantage is that the product can better maintain the original color, aroma, taste, shape and nutritional value, commercial quality is much better than the general heating and drying method. Therefore, the vacuum freeze-drying technology is currently the best processing technology in the world. With the advancement of society and the improvement of people's living standards, freeze-dried foods are no longer the foods enjoyed by astronauts. It will be an important convenience food in the 21st century. Enter the food consumption of ordinary people. More than ten years ago instant noodles seasoning mostly powder, now after the high-grade instant noodles seasoning added boiling water, that is, verdant jujube, Hongyan dill 11, fragrant beef Ding, like fresh in general, these are freeze-dried food. It is fundamentally different from air-dried, sun-dried, and frozen foods, and is the latest result of high-tech solutions to the problem of food preservation.