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Basic technology of storage and preservation of fruits and vegetables
At present, the widely used storage methods at home and abroad can be classified into two categories: one is low temperature storage, that is, the use of natural low temperature or artificial cooling (mechanical refrigeration or ice) method to store the storage environment at low temperatures; Is to control the storage of gas components (referred to as modified atmosphere storage). Most of the storage methods are to adjust the gas components in the storage place under low temperature conditions so as to achieve gas indicators suitable for storage of fruits and vegetables, so as to obtain better storage effect. With the continuous reform and innovation of storage technology and some processing methods for fruits and vegetables, in addition to the above methods for storage, research on radiation treatment, electromagnetic field treatment, and decompression storage at home and abroad has also paid more attention to avoiding the storage of fruits and vegetables. New research approach. (1) Simple Storage The simple storage consists of three basic forms of storage, burial (buried) and cellaring, and the resulting pseudo-storage and storage. These are the use of natural low temperatures to maintain the required storage temperature, structural equipment is simple, and have a certain degree of spontaneous preservation. 1. Stacking is a temporary storage method for directly stacking fruits and vegetables on the fields or on the ground or in open fields. Stacking can also be used as a pre-storage method. During the accumulation, the fruits and vegetables are generally piled directly on the ground or in the shallow ditch (pit). According to the change of temperature, the thickening is covered in several layers to provide shade or cold insulation. The coverings used were made in the field, and the covering materials for the resident use were mats, straw curtains, crop straws, and soil. Because the heap is stored on the ground, the fruit is less affected by the ground temperature after it is stored, but is affected by the temperature, especially at the early stage of storage, because the temperature is high, the heap temperature is difficult to reduce. Therefore, the heap should not be used in areas with high temperatures, and it is generally only used during short-term storage in autumn and winter. The width and height of the storage pile should be determined according to the local climate characteristics and the types of fruits and vegetables. 2. Gutter is a relatively simple method for storing fruits and vegetables. Root vegetables, chestnuts, walnuts, and hawthorn are commonly preserved by this method; apples and other fruits are also preserved by this method. The trenches should be trenched or pitted on the ground. The location where the groundwater should be collected should be high and dry, with a relatively heavy soil, good drainage, and a low groundwater level. The direction of the ditch is relatively long in the cold regions, and the north and south are appropriate. In the warmer regions, the east-west direction is mostly adopted. The depth of the ditch is generally determined according to the thickness of the local permafrost, and is stored below the permafrost. The effect of burial is not only affected by the soil temperature, but also related to its width. The methods of stacking fruits and vegetables in the ditch are generally the following: First, the accumulation method, that is, the fruits and vegetables are piled up in the ditch, and then covered with soil (sand); the second is the stratification method, that is, each layer of fruits and vegetables, and a layer of sand After stratification to a certain height, cover with soil (sand); third, mixed sand burying method, mix the fruits and vegetables with sand, stack them in the ditch, and then cover; and fourthly, put the fruits and vegetables into the ditch after loading the basket. 3. There are many types of cellar storage pits, of which cellar pits are the most common. In addition, there are cave dwellings in Shanxi, Shaanxi, Henan and other places, and the storage of citrus in Sichuan Nanchong, etc., in the form of well pits. These cellars are built according to the characteristics of the local natural and geographical conditions. It can use both stable soil temperature and simple ventilation to regulate and control the temperature in the pit. Fruits and vegetables can be pitted into the cellar at any time, and storage conditions can be checked in time. However, the cellar was reduced by the general ventilation of other types of cellars in Karachi. (II) Ventilation storerooms The storage and ventilation storehouses are the development of open cellars, and their form and performance are similar to those of open cellars. The cellar pit is a temporary storage place. The ventilated storage is a permanent construction. Brick, wood and cement structures are used. The cost is higher than that of the cellar pit, but the storage volume is large and can be used for a long time. At present, the output of fruits and vegetables has increased substantially year by year, and the storage of fruits and vegetables cannot fully rely on cold storage. Therefore, the ventilation storage has great practical value for a considerable period of time. Ventilation library is also the use of air convection principle, the introduction of external cold air and cooling effect. Mainly in the warehouse with good thermal insulation performance, set up a relatively complete and flexible ventilation system, using the temperature difference of the liquid, through the air guide equipment, the low temperature air outside the library into the library, and then the library of hot air, ethylene, etc. The bad gas is discharged outside the warehouse through the exhaust equipment, so as to maintain a more suitable storage environment for fruits and vegetables. However, due to the fact that the ventilation warehouse is cooled and stored by natural temperature, it is subject to a large temperature limit, especially at the early and late stages of storage, and the warehouse temperature is high and difficult to control, affecting the storage effect. In order to make up for this deficiency, electric fans, blowers, ice, or mechanical refrigeration can be used to accelerate the reduction of the temperature in order to further improve the storage effect and prolong the storage period. Ventilation storage should be built in a place with high dryness, good ventilation, no air pollution, and convenient traffic. The direction of the ventilation warehouse depends on the local minimum temperature and direction. It is advisable to keep the north and south of the north long, which can reduce the direct attack of the cold wind in winter and avoid the low temperature of the reservoir. In the south, the length of the east and west is used to reduce the sun exposure and the irradiation surface in the west and increase the windward surface. Ventilation storage generally has three types: ground floor, semi-underground, and underground. The body of the above-ground ventilation storage is all built above the ground, so it is greatly affected by the temperature; in the semi-underground ventilation storage, a part of the warehouse is below the ground and part of it is above the ground. The temperature of the reservoir is affected by the temperature, The influence of the soil temperature; the underground body of the ventilated storage is constructed below the ground and is most affected by the ground temperature and is the least affected by the temperature. As to which type of ventilating storage is adopted, it is decided according to the conditions of the local climate (mainly air temperature, ground temperature) and geography. In cold winter regions, the underground type is mostly used, which is conducive to cold insulation; in warm areas, above-ground type should be adopted to facilitate ventilation and cooling; in warmer winter regions, above-ground type should be adopted, which is conducive to cooling and cooling in the winter. In warm areas, semi-underground type should be used; in low-lying areas where groundwater level is higher, aboveground type should be adopted. (C) Cold Storage Storage Cold storage refers to mechanical refrigeration storage. Therefore, cold storage first requires a permanent building warehouse with good insulation and insulation equipment, as well as mechanical refrigeration equipment. Such ancillary equipment can be cooled and stored using mechanical cooling devices. According to the different types and varieties of fruits and vegetables stored, temperature adjustment and control are performed to achieve long-term storage. Mechanical refrigeration can meet the needs of different fruits and vegetables for different temperatures, so it can be stored throughout the year. The common cold storage can be divided into three categories according to the nature of their use: productive cold storage, distributed cold storage and retail cold storage. The production cold storage is generally built in the production area where the supply is relatively concentrated for the frozen processing and storage after the product is concentrated. This cold storage requires a large cooling capacity and a certain turnover capacity. Distribution cold storage is generally built in large and medium-sized cities or transportation hubs and industrial and mining areas where population is more concentrated, and is used as the market for transit and storage of goods. This kind of cold storage also requires a large amount of cooling capacity, and is suitable for storage of multiple varieties. Therefore, it is usually compartmentalized into several storage rooms, which can maintain different storage temperatures. The transportation in the warehouse should be smooth and the throughput should be rapid. Retail cold storage is generally a type of cold storage used by the retail sector. Its library capacity is small, the storage period is short, and the storage temperature can be changed as needed. At present, most of the cold storage of fruit and vegetables in our country belongs to the production library as an allocation library, and often both. The cold storage is a permanent building, and the thermal insulation of the warehouse is extremely important. There are two main types of heat-insulating materials: one is a rigid material processed into plates and other fixed shapes, such as cork boards, polystyrene foam boards, and the like. The other is bulk loose bulk materials such as wood chips, expanded perlite, rice husks, and the like. The fixed shape material maintains its original shape and is durable. The loose granular material is suitable as a filler material for filling it between two layers of brick walls. Filling should pay attention to fill in, fill in the real, it is best to layer settings, so as not to sink, otherwise it will cause the upper part of the insulation layer empty, the formation of heat leakage channels. Since the hot air must meet the cool air before entering the warehouse through the wall of the refrigerator, the moisture content in the hot air is greater than the moisture content of the cold air, and thus the condensed water is generated when the hot air is cooled. If the insulating material absorbs condensed water, its insulation effect will be significantly reduced for most insulating materials, and the accumulation of condensed water at low temperatures will pose a danger to the structure of the library. Therefore, in order to prevent water vapor from penetrating into the heat insulating material to form condensed water, it is necessary to provide a heat insulating layer on the hot side where the hot air penetrates the heat insulating layer. Commonly used thermal insulation materials include asphalt, linoleum, and emulsified asphalt. There are three oils and two felts, that is, three layers of asphalt oil brush on the inner and outer sides of two layers of linoleum. In the case where the temperature difference between the inside and the outside is small and the relative humidity outside the library is low, one felt, two oils, and aluminum foil can also be used. Cold storage refrigeration equipment includes a compressor unit, a condenser, and an evaporator. The refrigeration compressors can be divided into ammonia compressors, fluorine-12 compressors, and fluorine-22 compressors, depending on the refrigerant used. In terms of the source and price of the refrigerant, the ammonia refrigerant source is convenient and the price is relatively cheap. However, the structure of the ammonia refrigeration system is more complicated. Due to the suffocability of ammonia, irritation, and explosiveness of air, its safety precautions are required in use. Fluorine-22 refrigerant is non-toxic, non-irritating, volatile and non-explosive, so it is safe to use. The condenser is a heat exchange production device using a cooling medium and a high-temperature and high-pressure gaseous refrigerant coming out of the compressor. Water is usually used as an exchange medium for condensers, and air is also used as a cooling medium. The role of the evaporator is to ensure that the refrigerant evaporates at low pressure and at low temperatures and absorbs the heat of the outside air to achieve cooling to the outside world. (4) Air-conditioned storage and plastic film packaging are air-conditioned, fresh-keeping, air-conditioned, and stored in a relatively tight storage environment. At the same time, the proportion of gas components such as oxygen, carbon dioxide and nitrogen in the storage environment is changed and adjusted. A method of stabilizing them within a certain concentration range. The controlled atmosphere storage is carried out while maintaining a low temperature. Therefore, in addition to the storage facilities, there must be high airtightness in the storage area, and the warehouse should be able to withstand a certain pressure. Atmosphere storage can be divided into controlled atmosphere storage and spontaneous atmosphere storage. 1. In addition to the features of an ordinary cold storage, the modified atmosphere cold storage air-conditioned cold storage should have high airtightness to maintain the gas concentration required for the air-conditioned storage. The structure and equipment of the air-conditioning refrigerator in terms of heat insulation, cooling, and maintenance are the same as those of the conventional refrigerator, except that the former requires a certain degree of airtightness, and the library can bear a certain amount of pressure when the air pressure changes. For this purpose, an airtight layer is usually added inside the insulation. The materials and structures used for the inner liner are various and were first welded and sealed with galvanized iron sheets or steel sheets. Later, high-density plywood (ie, plastic-impregnated plywood) and aluminum foil sandwich panels (with moisture-proof paper on both sides of the aluminum foil or Polymeric film) etc. All of these materials can achieve better airtightness, and can also be easily converted from an ordinary cold storage to an air-conditioned storage. With the development of the plastics industry, there have been new breakthroughs in the airtight structure of the air-conditioning store: First, prefabricated sandwich panels (usually 10cm thick polystyrene foam) have been used, and polyurethane foams have been used. Styrofoam is used as a sandwich panel, which can resist the temperature difference of 40°C and play a good role in heat insulation. The outer metal plate has good air and air tightness. The use of polyurethane foam not only has excellent thermal insulation properties, but also the fine bubbles in the foam do not communicate with each other independently, ie, the closed-cell ratio of the material is extremely high, and therefore, it has good gas barrier and airtight properties. Therefore, a layer of polyurethane foam structure can simultaneously play the role of airtight, gas barrier, heat insulation and other three aspects. This material can also be prefabricated in the field laying, can also be sprayed directly on the scene, and can be added as an appropriate amount of asbestos, silica, fiberglass and expanded perlite as filler. The air-conditioning library should have a certain degree of airtightness, but it does not require absolute sealing, which is also difficult to achieve in actual production. Technically speaking, air tightness is considered good when objects stored in the library consume more oxygen than leaked oxygen. The general rule of thumb is that the positive or negative pressure of 10 mm water column caused by inflating or evacuating the warehouse will not be restored to zero within 30 minutes. The air-conditioning equipment of the air-conditioning warehouse mainly includes a nitrogen generator capable of reducing oxygen and a carbon dioxide remover. At present, there are four types of nitrogen generators used: combustion nitrogen generators, nitrogen generators for carbon molecular sieves, nitrogen generators for hollow fiber membranes, and nitrogen generators for cracked ammonia. The removal of carbon dioxide used to absorb hydrated lime in the past, and it can be used for small-scale storage products, but it cannot be used in large-scale air-conditioning storage. Removal of carbon dioxide by activated carbon adsorption is a common method at home and abroad. In addition, carbon dioxide can be removed with water and sodium hydroxide solution. It is also very important to remove ethylene gas, usually using activated carbon, potassium permanganate solution or potassium permanganate clay particles and high-temperature catalytic removal of ethylene. 2. The plastic film is packaged into a gas-tight bag and the plastic film is pressed into a bag. The bag is put into the bag and the bag is tightly closed, which means that it becomes a sealed storage place. Plastic bags can be stacked directly on shelves in a freezer or ventilated storage, or they can be placed in baskets (boxes). The fruit baskets (boxes) are stacked in stacks and stored. Some of the fruit baskets are packed in plastic bags, which are then tied tightly and then stored in the storage room. According to their different management methods, can be divided into the following three: (1) regular adjustment or release: Sealed bags are mostly made of 0.05 to 0.07 mm thick polyethylene plastic film bag, bag length 100 cm, width 80 cm. 15 to 20 kilograms of fruits and vegetables in each bag. Because of the respiration of fruits and vegetables inside the bag, the oxygen in the bag is reduced and the carbon dioxide is gradually increased. When the gas composition exceeds the required specifications, the bag opening is released, fresh air is replaced, and the bag is closed again. (2) Do not adjust the air: The thickness of the plastic film bag is 0.025-0.03mm. This film is thin and transmissive, and it can maintain a proper level of low oxygen and high carbon dioxide in a relatively short period of time without reaching a harmful level. Therefore, it is not necessary to perform air conditioning or air release. This method is suitable for short-term storage, long-distance transportation or retail sales. (3) Silicon window bag storage method: The silicone rubber film is embedded in a plastic film bag. Using the special permeability of silicone rubber, the high carbon dioxide inside the bag penetrates through the silicone rubber window and the external oxygen penetrates inside. Its penetration ratio is 6:1, so that it plays a regulatory role. (e) Other methods of storage and preservation 1. Electromagnetic treatment (1) High-frequency magnetic field treatment: The product is placed in or passed through the magnetic field of the electromagnetic coil and is directly affected by the magnetic field lines. (2) High-voltage electric field treatment: The product is placed on or through a high-voltage electric field consisting of metal plates. There may be such a role: 1 the direct effect of the electric field; 2 the formation of ion air effect of high-voltage discharge; 3 the role of discharge to create odor, and so on. (3) Ion air and ozone treatment: The product is not directly in the electric field, but ionized air and ozone treatment products formed by high-voltage discharge. It has been reported that negative ion air has an effect of inhibiting physiological activity on some fruits and vegetables. Positive ion air often plays a promoting role. Ozone is a strong oxidant, and there may be other physiological effects besides the disinfection and preservation. Electromagnetic treatment is used to store fruits and vegetables and is still in the experimental stage. Some devices are still indeterminate, and the mechanism of action needs to be further explored. 2. Decompression storage of fruits and vegetables under reduced pressure is one of the modern methods of storage. The technical point of decompression storage is that the product is placed in a closed room, some air is drawn from the sealed room, the internal pressure is reduced to a certain degree, and a constant low pressure is maintained during storage. Decompression storage can also be said to be a special atmosphere storage method. In decompression storage, the control of the amount of oxygen and relative humidity is more accurate than ordinary controlled atmosphere storage. 3. Radiation treatment Radiation storage technology mainly uses gamma (γ) rays generated by 60-cobalt (60Co) or 137铯 (137CS), or electron flow generated by an electron accelerator having an energy of 10 MeV (million electron volts) or less. Gamma ray is a kind of ray with strong penetrating power. When it passes through biological organisms, it will ionize water and other substances, generate free radicals or ions, thus affecting the body's metabolism, and in severe cases it will kill the cells. . The penetrating power of the electron flow is weak, but it can also play an ionizing role. From the perspective of food preservation, radiation treatment is the use of ionizing radiation to cause insecticidal, bactericidal, fungicidal, and physiological and biochemical effects.