The preservation function of critical point low temperature and high humidity storage is embodied in two aspects: first, under the premise of no chilling injury, adopting as low a temperature as possible during preservation can effectively control the respiratory intensity of fruits and vegetables and make some perishable fruit varieties dormant; Secondly, adopting a relatively high humidity environment can effectively reduce the evaporation of fruit moisture and reduce weight loss.
In principle, the critical point of low temperature and high humidity storage can not only prevent the fruit from rotting and deteriorating during the preservation period, but also inhibit the fruit from aging, which is an ideal preservation method.
(2) Structured modified atmosphere preservation of interstitial water
Structured water technology can make the water between fruit and vegetable cells participate in the formation of structured water, which will increase the viscosity of the solution in the whole system, thus producing the following two effects: first, the enzymatic reaction rate is slow, which is expected to control the physiological activities of organisms; Second, the process of fruit water evaporation was inhibited. This provides a new principle and method for the short-term preservation and storage of plants.
Some researchers used xenon gas to prepare structured water for Chinese cabbage and flowers, and explored its preservation technology, and achieved satisfactory preservation results. However, the cost of using high-purity xenon is too high, so researchers often reduce the cost by mixing and pressurizing inert gases, so as to find another fresh-keeping method.
(3) Ozone modified atmosphere preservation
Ozone is not only a strong oxidant, but also a good disinfectant and bactericide, which can not only kill and eliminate microorganisms and toxins secreted by fruits, but also inhibit and delay the hydrolysis of organic substances in fruits, thus prolonging the storage period of fruits. Since the discovery of 1785, ozone has been widely used in food, transportation, storage, tap water production and other fields as a gas bactericide.
Ozone modified atmosphere preservation is a new preservation technology developed in China in recent years, and researchers have made some achievements in using this technology to preserve perishable litchi. Its preservation function is reflected in three aspects: first, it eliminates and inhibits the production of ethylene, thus inhibiting the ripening speed of fruits; Second, it has a certain bactericidal effect, which can prevent the fruit from mildew; Thirdly, inducing stomatal contraction of fruit epidermis can reduce water evaporation and fruit weight loss.
(4) Low dose radiation pretreatment and ultraviolet preservation.
Radiation preservation is mainly based on the principle that γ-rays emitted by Co -60 and Cs-137, as well as electrons and X-rays accelerate the ionization of water and other substances in organisms to generate free radicals or ions. It has insecticidal, bactericidal, mildew-proof and physiological and biochemical effects on bulk or prepackaged fruits, and can replace chemical reagents such as ethylene, dibromo, bromomethane and ethylene oxide.
The radiation treatment of fresh fruit should use a relatively low dose, generally less than 3 kGy, otherwise it will soften the fruit and lose a lot of nutrients.
Low-dose radiation pretreatment can be combined with other technologies, such as freezing and blanching, to reduce the radiation dose required for radiation preservation. Soaking in hot water or steam heating (temperature 50 ~ 55℃) for 5 minutes can produce good fresh-keeping effect. This technology is widely used in the preservation of oranges, peaches and cherries.
The ultraviolet preservation technology has the characteristics of safety, environmental protection and high efficiency. The ultraviolet wavelength is 2600 A, and the sterilization effect is the greatest.
(5) coating to keep fresh
The method covers the heterogeneous interface on the food surface or inside the food by wrapping, dipping, coating, etc., and provides multiple functions such as selective gas barrier, moisture barrier, content loss barrier and harmful influence of external environment, respiratory inhibition, delaying maturity and aging, inhibiting the growth of microorganisms on the surface, improving storage quality, etc., so as to achieve the purposes of food preservation and prolonging shelf life.
At present, the coating materials widely used in fruit preservation include sugar, protein, polysaccharide sucrose ester, polyvinyl alcohol, monoglyceride, and the composite film composed of polysaccharide, protein and lipid.
(6) high-pressure storage
The principle of high-pressure preservation is to apply a small pressure from outside to inside above the container, so that the external atmospheric pressure of the container is higher than its internal vapor pressure, forming a full positive pressure difference from outside to inside, and the general pressure is 2500-4000 atmospheric pressure. Such positive pressure can prevent water and nutrients from spreading outward, slow down respiration rate and ripening rate, so it can effectively prolong the storage period of fruits.
In addition, the combination of high-pressure preservation technology and cold storage technology has a better effect. Grapes are preserved at 5℃ for 5 months and strawberries at 8℃ for 30 days.
(7) preserved by genetic engineering technology.
The purpose of preservation is mainly to reduce the production of endogenous ethylene during the physiological ripening of fruits and delay the softening of fruits in the later ripening process. Apples, peaches, bananas, tomatoes and other fruits with respiratory peaks will automatically promote the release of ethylene during the ripening process, and control the production of ethylene in plants through different ways.
At present, scientists have found the gene that produces ethylene. If this gene is turned off, it can slow down the release rate of ethylene, thus delaying the ripening of fruits and prolonging the shelf life of fruits at room temperature.
By inhibiting polygalacturonase, pectinase and other enzyme genes that degrade tissue and cell integrity, the purpose of delaying fruit softening can be achieved. Therefore, using DNA recombination and manipulation technology to modify genetic information, or using antisense RNA technology to inhibit mature genes, can delay the senescence of fruits and prolong the preservation period.
(8) Regulation and preservation of cell expansion pressure
Through the control technology of temperature, relative humidity, surface control degree, ventilation speed and other related thermodynamic characteristics and the corresponding test technology of tissue swelling pressure change, the fruit cell swelling pressure is kept intact, and its texture is controlled and preserved.
Some scholars have carried out tissue expansion and pressure control on apples and pears, and achieved good long-term preservation effect.
(9) Modified atmosphere preservation
By adjusting the composition of environmental gas, the storage period and shelf life of food can be prolonged. Since the invention of modified atmosphere preservation of apples, modified atmosphere preservation has been widely promoted in the world, which has become an important means of fruit preservation in industrialized countries and an important trend of fruit preservation in the future.
According to whether the established ambient gas can be readjusted, modified atmosphere preservation can be divided into two forms: CA (Modified Atmosphere Storage) and MA (Modified Atmosphere Storage). CA means that the concentration of the selected conditioning gas remains constant during controlled atmosphere storage; MA is a predetermined adjusted gas concentration initially established in the modified atmosphere system, and it is not adjusted manually in the subsequent storage process. MA technology originates from the respiratory mechanism of fruit decay, and achieves the purpose of preservation by inhibiting the rapid respiration and endogenous ethylene production.
MA can prolong the shelf life of food, which has been recognized by the world for many years. As a pollution-free preservation method, modified atmosphere preservation has attracted much attention all over the world. In foreign countries, low oxygen modified atmosphere technology or ultra-low oxygen storage is a new breakthrough in the application of modified atmosphere technology in postharvest fruits and vegetables.
Modern consumers' demands for convenience, freshness and health will further broaden the application scope of MA.
(10) comprehensive protection
Single preservation methods all have their own weaknesses, the preservation effect is not ideal, or the problem can not be completely solved. Therefore, the idea of comprehensive preservation is more and more accepted by people, and people gradually realize that preservation is not only carried out after harvest, but only by taking enough measures before harvest and during picking can the purpose of preservation be achieved.