Processing and Export of Dried Vegetable Products

The moisture content of fresh vegetables is high, most of which is more than 90%, of which free water accounts for most of them, so it is easy for microbial infection, and because of the large volume and crispness of fresh vegetables, it brings certain difficulties for storage and transportation. Dehydrated processed vegetables have low water content, which can effectively inhibit the activities of microorganisms and the activities of enzymes in the tissues. Therefore, the storage period is extended, the volume is reduced, the weight is reduced, and storage and transportation are convenient. The dehydration process is a water evaporation process. The first is the evaporation of moisture from the surface of the raw material, which is called "outward diffusion." The larger the surface, the faster the air flows; the higher the temperature, the lower the relative humidity of the air, and the faster the out-diffusion of water. As the surface moisture evaporates, more water inside the raw material moves toward the surface with less moisture. This transfer phenomenon is called "inner diffusion" of water. The speed of outdiffusion and inward diffusion must be coordinated. If the rate of out-diffusion of water is much greater than that of internal diffusion, that is, the internal water does not have time to transfer to the surface, the surface of the raw material will form a hard shell due to excessive drying. This phenomenon is called "encrustation. ". Encrustation will impede the evaporation of surface moisture, affect dehydration, and because the internal moisture is high, the steam pressure is high, thus crushing the structure of the softer part, cracking the surface of the raw material, spilling internal soluble substances, affecting the appearance and quality of the product. There are many methods for drying, which can be divided into artificial drying and natural drying depending on the source of heat energy used and the method of treatment. 1. Artificial Drying (1) Drying of dryers uses heating of fuels (such as coal, charcoal, wood, oils, etc.) to achieve drying. It is the most commonly used dry method in China. The equipment used for ordinary drying is relatively simple, including baking ovens and baking rooms, and larger drying machines. There are many types of dryers, and tunnel dryers are commonly used in production. The drying chamber of the tunnel type drying machine is one or two narrow-shaped tunnels. The tunnels are provided with rails, and the raw materials are loaded on the drying screen of the vehicle, and are fed from one end to perform relative movement with the hot air and complete dewatering from the other. Send it out at one end. A part of the exhaust gas is discharged through the exhaust hole, and the other part is returned to the heating room. According to the direction of hot air flow and the direction in which the vehicle moves forward, it can be divided into forward-flow dryer, counter-flow dryer, and hybrid dryer. The direction in which the raw material of the co-current drying machine moves forward is the same as the direction of hot air flow. That is, when the drying begins, the raw materials are in a high temperature and dry bath environment, the water evaporates quickly, and the car moves forward. The lower the temperature, the higher the humidity. The general starting temperature is 80-85°C and the final temperature is 55-60°C. The use of higher temperatures in the early stages of drying is to better inhibit enzyme activity. The forward direction of the raw material carrier of the countercurrent dryer is opposite to the direction of hot air flow. That is, when the drying begins, the raw materials are in a low-temperature and high-humidity environment, and finally the evaporation is completed in a high-temperature, low-humidity environment. The general inlet temperature is 40-50°C and finally 65-85°C. This method is suitable for fruits with high sugar content and thick juice. The dry process of the hybrid dryer is divided into two stages, one is the downstream stage and the other is the countercurrent stage. That is, the hot air is blown toward the middle from both ends, and the exhaust port is located at the center. The advantage of this method is that the raw materials flow first, so that at a higher temperature, the water evaporates quickly; in the intermediate stage, the temperature is low, the humidity is high, the evaporation is slow, the raw materials are not easy to form a hard shell, and finally it enters high temperature and low humidity to ensure The raw material reaches the required degree of drying. There are also tumble dryers and belt dryers. The former consists of 1-2 steel drums, which are both the heating part and the dry part, and the raw material is dried on the drum. The latter is to lay the raw material on a conveyor belt and dry it in contact with the drying medium while rotating forward. (2) Freeze-drying Freeze-drying is also called sublimation drying or vacuum freezing sublimation drying. That is, the raw material is frozen to a temperature below the freezing point, and the water becomes ice, and then the ice is converted into steam at a higher vacuum to be removed and the material is dried. Freeze-drying can maintain the original flavor, less heat denaturation, but the high cost, only applies to products with particularly high quality requirements (high-grade food, medicine, etc.). (3) Microwave drying Microwave drying is a high-frequency alternating current with a microwave frequency of 300 MHz to 300 kMHz and a wavelength of 1 m to 1 mm. Microwave drying has the advantages of fast drying speed, short drying time, uniform heating, and high thermal efficiency. (4) Far-infrared infrared rays with a wavelength of 5.6-1000 μm are far-infrared rays. The far-infrared rays are absorbed by the heated object and directly converted into heat energy to reach the heat and dryness. When dry, each layer in the object is subjected to a uniform thermal effect. It has the advantages of fast drying speed, high production efficiency, energy saving, small equipment scale, low construction cost and good drying quality. (5) Drying under reduced pressure Drying under reduced pressure is based on the principle of using boiling water for self-depression at the time of decompression, and a part of the water in the vegetable tissue is discharged mechanically to achieve the purpose of drying. 2. Natural dry natural drying is the use of natural conditions (such as the sun, hot air, etc.) to make vegetables dry. The raw materials are directly subjected to sun exposure, called sun drying or sun drying; dried in a well-ventilated room or under a shade, dried or dried. 3. The main processes of dry process artificial drying are: raw material cleaning, peeling and slicing, rinsing, blanching and cooling, color protection, dehydration, wet rejuvenation, selection and packaging. (1) Raw material selection The quality of raw materials is related to the product qualification rate and economic benefits. As a raw material for dehydrated vegetables, it requires fresh, bright color, thick flesh, dense tissue, less crude fiber and waste, suitable shape, size, length, maturity, no rot and serious damage. (2) Before cleaning the fresh vegetables, the inedible and unqualified parts such as skin, shell, root, leaf, etc. need to be removed to remove the deposited sediment, impurities, pesticides, and microbial contaminated tissues so that the raw materials can basically reach the requirements for dehydration processing. . (3) Peeling and removing the skin of the raw vegetable can improve the food quality of the product and facilitate the evaporation of the moisture of the material to facilitate dehydration and drying. Peeling methods include hand peeling, mechanical peeling, thermal peeling and chemical peeling. (4) Dividing and rinsing The raw material is cut into a certain size shape so that the water evaporates. Generally cut into pieces, strips, grains and filaments and so on. Its shape, size and thickness should be based on different types and export specifications, using mechanical or manual operations. For some vegetables, such as onions, garlic, etc. in the slicing process also need to use the water to continuously wash out the gum juice, until the glial fluid rinsed to facilitate drying and dehydration to make the product more beautiful color. (5) Bleaching and cooling The so-called blanching involves short-term heat treatment of raw materials. Blanching inhibits the activity of enzymes, prevents browning of raw materials, reduces microbial contamination, and softens tissue to facilitate dehydration. Usually steam or boiling water is used for treatment. The blanching time differs depending on the type of raw materials, shape and size, tissue tenderness, etc., usually 2 to 5 minutes, and some only a few seconds. In order to maintain the original bright colors of vegetables, some vegetables such as green pea pods, broccoli and carrots also need to add a small amount of edible sodium bicarbonate or citric acid in boiling water. After the blanching, it should be quickly rinsed and cooled to prevent the material from softening and deforming, losing its elasticity and luster. (6) Color protection Most vegetables are forbidden to use color-protecting agents in the production process, but for some browning vegetables, non-toxic color-protecting agents such as sodium bicarbonate and citric acid are allowed. After the color protection, centrifugal dryer must be used to drain the surface water until the raw material is almost water-free. (7) Dehydrated exports Dehydrated vegetables are usually dehydrated and dried using a tunnel hot air dryer. The specific operation is as follows: Spreading The sieved material is laid on bamboo sieves or non-toxic baking screens. Baking sieve is mostly rectangular, the general size is 1.0m1.0m0.48m, the sieve hole is 6mm6mm square, and each baking sieve is 2.0-5.0kg, which varies according to different vegetable types. The loading screens of the paved material are loaded on the loading frame. Each car has 18-20 layers, can be placed 36-40 baking screen. Into the baking: filled with the car can be along the ground track, into the drying room dehydration drying. At a certain time, a loader truck is incremented from the entrance of the drying room, and a dehydration-laden baking truck is discharged from the exit, so that the dewatering operation is continuously performed. A drying room can generally accommodate 8— 9 loaders. Drying room temperature is usually controlled at about 60 °C, generally does not exceed 65 °C, after 6-8 hours to complete, depending on the moisture content of baked products may be. Excessive temperatures can cause rapid swelling of the material in the tissue due to sudden high temperatures, causing loss of contents, crusting, and coking. Vegetables rich in sugars and volatile substances should also be dehydrated at lower temperatures. (8) Balanced moisture Due to the different types of vegetables, different shapes and sizes, the uneven thickness of the shop material often causes a slight difference in the water content of the product. Therefore, after the product is cooled slightly, it should be immediately put into a closed tinplate barrel or a plastic bag, and kept for 1-2 days so that the moisture of the dry product can be transferred to each other. After the balance is reached, the next operation is performed. (9) Select the sieve to remove the crumbs, chips and impurities in the product, then pour it on the picking station to pick up the unqualified products. Select operations should be rapid to prevent moisture absorption and moisture recovery of the product. After the selection of finished products still need to conduct quality and moisture inspection, unqualified persons need re-bake. (10) The compacted vegetables are fluffy after dehydration and have a large volume, which is not conducive to packaging and transportation. Therefore, some of them need to be compressed. Briquetting conditions are generally 60-65°C, humidity is properly controlled, and a pressure of 20-80 kg/cm2 is used. (11) Packages are generally packaged in corrugated boxes. The boxes are lined with moisture-proof aluminum foil bags and plastic bags. For products that are easily oxidized and browned, they must be packed in composite plastic bags and aluminum foil bags. The net weight of each box is 20kg or 25kg. After the product is packaged, it must be stored in a low-temperature library at about 10°C. Storage must be dry, cool, odor free, and free of pests. Periodically check the moisture content of the finished product and the pest situation during storage. The method of natural drying is generally raw material selection, washing, slicing, drying (or dried).

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