When a ship is sailing at sea, the supply of vegetables and fruits is directly related to the diet quality of sailors, because vegetables and fruits are rich in vitamins, minerals and dietary fiber. Generally speaking, during the voyage, ship personnel consume leafy vegetables and nightshade vegetables within 1 to 2 weeks, melons and fruits and root vegetables within 3 to 4 weeks, and some root vegetables (such as potatoes, radishes, bamboo shoots, etc.) etc.) can be stored for more than 1 month. However, the long voyage or long voyage time of ships is often more than 1 month, and sometimes even as long as 5 to 6 months. Although canned vegetables and fruits, frozen vegetables, and dried vegetables can make up some of the deficiencies, they cannot compare with fresh fruits and vegetables in terms of nutrition and taste. Therefore, fruit and vegetable storage technology is the main way to effectively prolong the storage period of fruits and vegetables and increase the supply of fresh fruits and vegetables. Modified atmosphere storage is a kind of fruit and vegetable storage technology.

First, Storage methods of vegetables and fruits

The biological reasons for the decline or loss of fresh quality of vegetables and fruits after harvest are mainly aging, physiological diseases (freezing damage, cold damage, gas damage, etc.), water loss, pathogen infection, etc., and appropriate preservation methods must be adopted to prolong their shelf life. There are many ways to store fruits and vegetables, mainly to control the changes in temperature, moisture, gas composition and other aspects during storage, and the most important thing is to control the storage temperature. According to the different temperature control methods, it can be divided into two categories: natural low temperature method and artificial cooling method.

The natural low temperature method is to use the seasonal changes of the natural climate and the temperature difference between day and night to adjust the temperature of the storage place. Insulate when cold and insulate when hot, and try to maintain the required storage temperature. It includes simple storage and ventilation warehouse storage such as stacking, burial, false plant storage, frozen storage and cellar storage. The method is simple, economical and effective. However, because the natural temperature changes are used to adjust the storage temperature, it is often limited by regions and seasons.

The artificial cooling method is to use ice melting to absorb heat or mechanical refrigeration to obtain suitable low temperature, including ice cellar storage, mechanical refrigeration, modified atmosphere storage and decompression storage. These methods are not limited by seasons and regions, and the storage effect is good, but the investment is large and the cost is high.

Controlled atmosphere storage is the abbreviation of gas-regulated storage. It uses the principle of the influence of low oxygen and moderate carbon dioxide on the postharvest, physiological and biochemical processes of vegetables and fruits, and artificially changes the atmospheric composition of the storage environment - reducing the oxygen content of the air and increasing the carbon dioxide content, So as to achieve the storage method of maintaining the quality of vegetables and fruits. Controlled atmosphere storage can inhibit the respiration and ethylene production of vegetables and fruits, delay after-ripening and aging, and inhibit the activity of pathogenic bacteria, delaying the occurrence of spoilage.

There are two ways of modified atmosphere storage, one is real modified atmosphere storage, which requires oxygen and carbon dioxide to be controlled within a small range; the other is limited air storage or simple modified atmosphere storage, also known as MA storage, usually plastic products are used. The tent or bag is sealed, and it is composed of air that is changed by its own breathing. There are no strict indicators for oxygen and carbon dioxide, and relatively large changes are allowed. Modified atmosphere technology combined with refrigeration can get better results.

In addition to the above methods, the silicone rubber window is used as a gas exchange window, which is embedded in a plastic tent or a plastic bag to automatically adjust the gas composition. The principle is that the silicone rubber membrane (a high molecular polymer) in the exchange window has different permeability to carbon dioxide, oxygen and nitrogen, and can automatically adjust and maintain the gas composition in the account.

In controlled atmosphere storage, the product must be sealed with a controlled atmosphere warehouse or plastic film to avoid interference from outside air. The structure of the air-conditioned warehouse is similar to the general refrigerator, but it requires higher air tightness. It can be lined with metal sheets on the inside of the four walls, ceiling and floor, or airtight plastic plates, or sprayed with polyurethane foam. The door and various pipes passing through the wall must also have an airtight structure. In ordinary cold storage, the use of plastic film tents (bags) to seal vegetables and fruit products is also a method of modified atmosphere. The key to controlled atmosphere storage is to control the gas composition at an appropriate level. The optimal gas composition varies with vegetables, fruit species, storage temperature, and storage stage. In order to facilitate operation and management, the gas index should be allowed to have a certain range of change. There is a process of reducing oxygen and increasing carbon dioxide from the adjustment of normal air composition to the required composition. Different vegetables and fruits have different requirements for the appropriate storage concentration of oxygen and carbon dioxide.

Modified atmosphere storage technology is a modern preservation technology developed on the basis of traditional refrigeration and preservation, and is considered to be a good storage method for storing vegetables and fruits today.

Second, Current status of controlled atmosphere storage of fruits and vegetables at home and abroad

Commercial air-conditioned storage has a history of more than 60 years abroad. The long-term storage of various fruits in developed countries, such as apples, pears, and kiwifruit, mainly adopts air-conditioned storage. Controlled atmosphere storage in my country began in the 1970s. Over the past 30 years, through the introduction, digestion and absorption of foreign advanced technology and equipment, coupled with the continuous research and exploration of Chinese researchers, the controlled atmosphere storage technology has developed rapidly. device capability.

Due to the large investment and high cost of modified atmosphere storage facilities and equipment, its storage capacity is far less than that of cryogenic storage. The low-temperature storage capacity of fruits and vegetables in foreign developed countries accounts for about 60% to 70% of the total output of fruits and vegetables, of which the controlled atmosphere storage capacity only accounts for less than 10% of the low-temperature storage capacity. my country's existing low-temperature storage capacity only accounts for 1% to 2% of the total output of fruits and vegetables, about 6 million tons, of which controlled atmosphere storage is less than 50,000 tons.

The foreign controlled atmosphere storage technology of fruits and vegetables is mainly used for commercial purposes, and there is no report on the application on ships. The application of controlled atmosphere storage technology for fruits and vegetables in China is mainly concentrated in the vegetable and fruit production bases that provide supplies for large supermarkets and manufacturers. The research and application of controlled atmosphere storage technology for fruits and vegetables has not been carried out on ships.

The controlled atmosphere storage technology mainly includes the design and construction of the controlled atmosphere warehouse, the selection, installation and commissioning of the controlled atmosphere equipment, the determination of the gas composition, and the management of the storage period.

Third, Characteristics, design and construction of air-conditioned warehouses

3.1.1 Features

(1) Air tightness. Air tightness is a major feature that differentiates the air-conditioned storage from the cold storage in terms of architectural requirements. In addition to the requirement of thermal insulation, moisture-proof, and reduction of cold and heat exchange with the outside, the air-conditioned warehouse also requires strict sealing around the walls of the warehouse, the doors of the warehouse and the connections of all incoming and outgoing pipelines to minimize the gas exchange inside and outside the warehouse. Only in this way can the gas composition in the library be adjusted artificially, and the interference of the external gas on the gas composition in the library can be reduced or avoided.

(2) Security. Since the air-conditioning storage has strict requirements on the airtightness of the envelope structure, in the process of dehumidification and air regulation, with the change of temperature and pressure in the storage, a pressure difference will be generated on both sides of the envelope structure, which will damage the The air tightness can even cause the building envelope to burst or collapse. In order to balance and reduce the pressure difference inside and outside the warehouse, the air conditioning warehouse must design a safety valve and an air regulating valve.

(3) Fast forward and complete. The shorter the time that the fruit enters the modified atmosphere state, the better, otherwise the storage effect will be affected. Therefore, the storage of controlled atmosphere warehouses requires fast storage of vegetables and fruits, so that they can be filled, sealed and air adjusted as soon as possible. Minimize the number of door openings during storage. Frequent opening of doors will not only affect the storage effect, increase operating costs, but also reduce the air tightness of the door. When leaving the warehouse, in order to ensure the shelf life of vegetables and fruits, it is best to sell them out at one time or in batches in a short period of time.

(4) Full warehouse storage. In addition to leaving the necessary ventilation and inspection channels, the fruits in the warehouse should be stacked as high as possible. This is because: on the one hand, the more fruits and vegetables in the warehouse, the fewer pores in the warehouse, and the less air. In addition to the respiration of fruits and vegetables, the time for fruits and vegetables to enter the state of modified atmosphere is shorter; on the other hand, the storage volume utilization coefficient large, the operating costs of refrigeration and air conditioning can be reduced accordingly.

3.1.2 Design and build

(1) Type and structure. According to the air conditioning method, it can be divided into inflatable type and circulating type; according to the cooling method, it can be divided into internal cooling type and external cooling type; according to the building structure, it can be divided into masonry type and color-plated sandwich panel assembly type. The inflatable type refers to the use of nitrogen generators to continuously inflate the nitrogen generated into the controlled atmosphere warehouse, supplemented by other adjustment methods, so that the oxygen and carbon dioxide in the warehouse can reach the predetermined indicators. The circulating type refers to introducing the air in the controlled atmosphere warehouse into a combustion device, and burning the oxygen in the air. The gas composition after combustion includes nitrogen, carbon dioxide and water vapor, etc. These gases are cooled and then charged into the warehouse; When the carbon dioxide exceeds the predetermined index, the carbon dioxide removal system is turned on. In this way, the two cooperate with each other to make the oxygen and carbon dioxide in the warehouse reach the predetermined target. The internal cooling type is to place the cooling system (evaporator) inside the air-conditioned warehouse, generally above one end of the warehouse. This cooling method cools down quickly, the temperature in the warehouse is uniform, and the effect is good. Most of the controlled atmosphere warehouses in my country are of this type. External cooling, that is, jacketed air-conditioned storage, generally installs a layer of metal airtight structure with good thermal conductivity in the original refrigerator, and still uses the original cooling facilities, and the blown cold air is directly blown onto the metal airtight structure layer. , use the sealed wall to cool the inside of the storehouse, and the air conditioning is carried out in this airtight structure, but the cooling effect of this method is poor. The building structure of the masonry air-conditioned warehouse is basically the same as that of the ordinary cold storage. It is built with traditional building insulation materials, or the cold storage is transformed. A layer of airtight layer is added on the surface of the library body, which is directly laid on the enclosure structure. This type of masonry controlled atmosphere storage is reasonable in investment, but has a long construction period. Because the color-plated sandwich insulation board is a factory-produced warehouse material, it only needs to be simply assembled at the construction site, the construction period is short, and the investment is slightly higher than that of the masonry type, and the construction of the airtight layer is more convenient and reliable than the masonry type.

(2) Design and construction of airtight layer. The prefabricated combined air-conditioning library adopts color-plated sandwich panels for its enclosure structure. Both sides of the insulation layer are covered with color-plated steel plates. The air tightness and moisture resistance and air barrier of color plated steel plate are very good. As long as the joints between the panels are sealed on the built enclosure walls, the entire enclosure structure can have good air tightness.

(3) Air-conditioned warehouse door. The warehouse door is the place where gas leakage is likely to occur in the controlled atmosphere warehouse. In order to ensure the sealing between the door and the body, the airtight strip of the door can be made into an inflatable type. This method has good sealing performance, but it is troublesome to use. Usually, a fastening device is added on the basis of the cold storage door. When the door is sealed, the device is used to tightly fasten the door to the door frame, and the door seam is sealed by the sealing strip. During the process of the door being dropped and fastened, the sealing strip at the lower end of the door is pressed against the ground to seal. There are observation windows on the doors of the air-conditioned warehouse, which can be opened, so that when there is a problem in the warehouse, it is convenient for the staff to enter the warehouse for inspection. Its outer frame is a metal component, and the middle is inlaid with double-layer glass or hollow double-layer glass. If double-layer glass is used, desiccant should be placed in the interlayer or evacuated to prevent condensation.

(4) Sealing standards, experimental methods, safety devices and precautions. In order to ensure that the gas composition in the controlled atmosphere warehouse is adjusted quickly, the fluctuation range is small, the storage quality is improved, and the cost is reduced, it is necessary to ensure that the warehouse body has good air tightness. In GB 50274-98 "Code for Construction and Inspection of Installation Engineering of Nitrogen Generators and Air Separation Equipment", the section about combined (air-conditioned) cold storage stipulates: "After the air-conditioned cold storage is installed in the storage body, the air-tightness of the storage body should be checked. Test. The test should meet the following requirements: start the blower, stop when the pressure in the warehouse reaches 100 Pa (10 mm H2O), and start timing, when the test reaches 10 minutes, the pressure in the warehouse should be greater than 50 Pa”, that is, the half pressure reduction time for 10 minutes. In order to prevent the warehouse body from being crushed or burst due to the imbalance of internal and external pressure, the warehouse should be equipped with air pressure balance bags (referred to as modified atmosphere bags). When the internal temperature decreases, the storage gas contracts, and the storage gas will flow into the storage room from the modified atmosphere bag. In addition, a pressure balance safety valve (balance valve for short) should be installed in the warehouse. When the pressure difference between the inside and outside of the warehouse is greater than 200 Pa, the gas inside and outside the warehouse will be exchanged through the balance valve to prevent the structure of the warehouse from being damaged.

(5) Refrigeration equipment. In the design of the entire refrigeration system of the air-conditioned warehouse, according to different situations, the centralized cooling of a large unit or the independent cooling of a small unit in a single warehouse can be used, and the latter has greater flexibility. The use of glycol chillers not only improves the temperature control accuracy, but also greatly simplifies the refrigeration pipeline structure. In order to make the refrigerator run economically during the air-conditioning process, the thermal insulation layer of the envelope structure of the air-conditioning warehouse should be thicker than that of the general high-temperature warehouse.

(6) Humidification equipment. Due to the long storage period of fruits and vegetables in the air-conditioned warehouse, and the nitrogen charged is very dry, the moisture of the stored products evaporates more. In order to reduce the partial pressure difference of water vapor between the storage environment and the stored products, suppress the evaporation of water, maintain a relatively high relative humidity (90%-95%) in the air-conditioned warehouse, and reduce dry consumption, a humidifier should be installed in general air-conditioned warehouses. . There are two main types of humidifiers: ultrasonic humidifiers and centrifugal humidifiers.

3.2 Selection, installation and commissioning of air-conditioning equipment

3.2.1 Nitrogen production and oxygen reduction equipment. The 95%-98% pure nitrogen produced by the nitrogen generator can be used to replace (or dilute) the gas in the controlled atmosphere warehouse and reduce the oxygen concentration in the warehouse; in small controlled atmosphere warehouses, it can also be used to remove excess carbon dioxide and ethylene. or other gases.

3.2.2 Carbon dioxide remover. During the controlled atmosphere process of fruits and vegetables, the concentration of carbon dioxide is generally required to be controlled within the range of 1% to 5%. The respiration of fruits and vegetables will increase the concentration of carbon dioxide in the warehouse. Excessive carbon dioxide concentration can lead to poisoning of fruits and vegetables, and produce a series of adverse symptoms, and eventually rot and deteriorate. Therefore, a carbon dioxide remover (also known as a scrubber) must be installed and used to remove excess carbon dioxide in the warehouse to meet the requirements of the controlled atmosphere parameters.

3.2.3 Ethylene removal device. Ethylene is a product of the metabolism of fruits and vegetables, and it is a plant hormone that can promote the respiration of fruits and vegetables and accelerate the aging and aging of fruits and vegetables. When storing fruits and vegetables that are very sensitive to ethylene, the ethylene concentration in the storage environment must be reduced to below the threshold, generally to a level below 2×10-8 (volume ratio). The method of removing ethylene: Potassium permanganate is often used as a strong oxidant, and porous materials such as alumina and molecular sieve are used as carriers to make a one-time-use composite material. Ethylene removal. With the development of air-conditioning technology, a high-efficiency ethylene removal device based on the principle of high-temperature catalysis has been developed in recent years.

3.3 Automatic control equipment

Controlled atmosphere storage must accurately measure and control the gas composition in the storage throughout the storage period. If it is detected that the oxygen in the warehouse exceeds the standard, the nitrogen generator should be turned on, and nitrogen will be charged to reduce oxygen until the oxygen content reaches the set value of the process parameters. If the carbon dioxide exceeds the standard, the outlet and return valve of the warehouse should be opened, and the carbon dioxide removal device should be opened to remove carbon dioxide until the carbon dioxide in the warehouse reaches the set value. The automation of the above work can be realized by using automatic control equipment and computer-specific software.

3.4 Determination of gas composition

3.4.1 Determination of oxygen and carbon dioxide. The measuring instruments mainly include austenitic gas analyzers, oxygen electrode oxygen testers, infrared carbon dioxide testers, oxygen and carbon dioxide sensors, etc.

3.4.2 Determination of ethylene. Detection by a professional using a gas chromatograph.

3.4.3 Storage management of controlled atmosphere warehouse. In order to maintain the stability of storage parameters in the warehouse, it is necessary to manage refrigeration equipment, humidification equipment, air conditioning equipment, pipeline valves and various control equipment. The airtightness of the air-conditioning warehouse, the air-conditioning bag, and the balance safety valve should also be kept in good condition. From the storage of fruits and vegetables to the delivery of the storage, the quality monitoring of the stored products throughout the storage period should always be done. In addition, the air-tightness damaged parts of the air-conditioning warehouse should be re-tested after being repaired, and all kinds of machinery and equipment, pipeline valves, control instruments, and electrical components should be subject to annual inspection and overhaul as required.

3.4.4 Technical indicators of controlled atmosphere storage. The main technical indicators of controlled atmosphere storage of vegetables are shown in Table 1. The main fruit controlled atmosphere storage indicators are shown in Table 2. Table 1 Technical indicators of air-conditioned storage of main vegetables (omitted) Table 2 Technical indicators of air-conditioned storage of main fruits (omitted)

Fourth, Application conditions of controlled atmosphere storage technology for fruits and vegetables on ships

4.1 Time to go to sea

For ships that have been out to sea for more than one month, it is only necessary to design and build an air-conditioned storage warehouse for fruits and vegetables. Because the fresh vegetables and fruits carried by most ships, plus canned fruits and vegetables, quick-frozen vegetables, dried vegetables, etc., can basically meet the food needs of ship personnel within one month.

4.2 Number of fruit and vegetable storages

For ships with 4 to 5 fruit and vegetable storage warehouses with similar volumes, it is possible to consider setting up one controlled atmosphere storage warehouse, and it is recommended to set the ratio at 1:4 to 1:5. Because when the ship goes to sea, ordinary low-temperature refrigeration is the main way of storing fruits and vegetables, and its storage capacity should not be less than 80% of the total storage capacity of fruits and vegetables.

4.3 Professional and technical personnel

Controlled atmosphere storage of fruits and vegetables is a relatively professional technology, involving the safety indicators of storage body sealing performance, pressure control, gas composition detection, etc. for human and fruit and vegetable storage products. Therefore, it is necessary to cultivate technologies with good technical quality and strong sense of responsibility. Only personnel can ensure the smooth progress of the controlled atmosphere storage of fruits and vegetables on the ship.

4.4 Construction plan and storage method suitable for naval applications

Since the research and application of controlled atmosphere storage of fruits and vegetables on ships has not yet been carried out, relevant scientific researchers and design and construction personnel should, on the basis of in-depth and extensive research, according to the needs of ships for controlled atmosphere storage of fruits and vegetables, combined with ship design status and According to the actual environmental conditions, the design parameters of the controlled atmosphere storage warehouse for fruits and vegetables suitable for ships are studied, the suitable varieties of fruits and vegetables are screened, and the relevant storage methods and operating procedures are formulated.