This preservation method can not only achieve good anti-corrosion effect, but also no toxicity and no pesticide residue. The post-harvest rot of fruits has brought great losses to the world's fruit production. According to incomplete statistics, the post-harvest rot of fruits generally amounts to 20% to 30%, and China is even more severe. The post-harvest rot rate reaches 30% to 40%. Because of the environmental factors in the tropics, it is more conducive to post-harvest diseases. The post-harvest rot rate of fruits is as high as 50%. Heat treatment is one of the popular methods of fruit post-harvest preservation in recent years. In particular, with the increasing seriousness of pesticide residues, scholars have begun to pay attention to this method of non-toxic, non-pesticide fruit preservation. Research in related fields has become Quite active. The relevant research on the application of post-harvest heat treatment in fruit preservation is briefly described below. First, the basic meaning of post-harvest heat treatment fruit Post-harvest heat treatment, the post-harvest fruit is placed at an appropriate high temperature and maintain its freshness. Second, the application of heat treatment in the post-harvest preservation of fruits The application of heat treatment in fruits was first started from the control of fruit rot. In 1922, Fawcett first reported the use of heat treatment to control the decay of oranges caused by anthrax. Later, due to the advent of various chemical agents, heat treatment was abandoned for almost 30 years due to the difficulty of implementation and the effect was not significant enough. With many chemical preservation methods banned or restricted in various countries one after another, heat treatment has once again attracted people's attention. In 1953 Akamine et al. reported that papaya was treated with hot water at 44-49Â°C for 20 minutes to control papaya anthrax. After 1980, China began to conduct post-harvest heat treatment of fruits and fruits such as mangoes and bananas. Liu Xiujuan et al. used post-harvest treatment of different varieties of mango with heat treatment methods. Generally soaked in warm water at 52-55Â°C for 5-10 minutes, both obtained effective anti-corrosion and preservation effects, making the post-harvest preservation period of mango more than 15 days, and the color No significant change in flavor. In 1955 Huang Chaohao et al. reported using hot water to control the decay of mangoes caused by anthracnose. The specific methods were: 15Â°C warm water soaking fruit for 15 minutes, or 54Â°C soaking fruit for 5 minutes. 50010-6 benzene could also be used. The special 52Â°C hot liquid of the special, enemy Kesong, 100010-6 carbendazim was soaked for 5-10 minutes. In the past half a century of research, scientists at home and abroad have used the same method to achieve post-harvest preservation and preservation of citrus fruits, apples, peaches, melons, strawberries, bananas and many other fruits. In 1999, Liu Xiujuan and others used heat treatment to determine the pathogenicity of the anthracnose pathogens on bananas and mangoes. The results showed that when the heat treatment temperature was 55Â°C and the time was more than 30 minutes, the mycelia of anthracnose had a significant killing effect. The heat treatment temperature of 60Â°C and a time of 30 minutes or more significantly kills the anthrax spores. Researches on heat treatment and preservation of fruits are gradually expanding to the deeper pathological and physiological mechanisms of heat treatment. Third, the mechanism of fruit heat treatment preservation For a long time, the fruit heat treatment temperature is mainly used 35-55 Â°C, designed to promote the healing of wounds on the surface of the fruit, as well as to kill or inhibit the growth of pathogenic microorganisms in the peel or pulp. 1. The physiological effect of heat-treating and fresh-keeping of fruits reduces the respiration intensity of postharvest fruits. The post-harvest rot of many fruits is due to the arrival of the respiratory peak, which causes the fruit to enter the physiological aging period. Accumulation of a large number of toxic metabolites gradually weakens the disease resistance of the fruit, and the tissue disintegrates and rots. Since tropical fruits such as mangoes, bananas, etc., mostly belong to the type of respiration, how to delay the arrival of the respiratory peak in preservation techniques is a crucial issue. At high temperatures, many fruits such as tomatoes show reduced respiratory intensity. Based on this, Japanese scholar Ogura Masao and others proposed the method of high-temperature storage of tomatoes as early as 20 years ago. Han Tao et al. found that treatment of fruit at 35Â°C for 2 days could significantly reduce the respiratory intensity of pre-storage fruits. Effect on endogenous ethylene synthesis in fruit. Fruit ripening occurs almost simultaneously with the peak of ethylene release. Before the arrival of the respiratory peak, the main factor affecting the normal release of ethylene was the temperature, except for the wounds formed by harvesting. When the temperature is higher than 35Â°C, the activity of ACC (aminocyclopropanecarboxylic acid) oxidase in the ethylene synthesis pathway is significantly inhibited. Han Tao reported that the inhibition of ethylene production by high temperatures is mainly through the inhibition of ethylene forming enzyme (EFE) to prevent the conversion of ACC to C2H4. The scientists confirmed that although different forms of EFE have different thermal resistances, they are all susceptible to heat inactivation. However, the effect of heat treatment is largely to delay the fruit ripening process by reducing the ethylene release level of the fruit. The fruit storage period has a positive effect. The effect of various enzyme activities such as PG (polygalacturonase). Appropriate tropical treatment can inhibit some of the fruit surface physiological disorders, so that the fruit maintains a higher hardness during storage. First, the level of enzyme activity of pectin-degrading enzymes and PG is directly related to the post-harvest softening process of the fruit. After proper heat treatment, the PG enzyme activity of the fruit is very low and even not observed. Although the PG enzyme activity of the fruit was restored after normal temperature was restored, its activity level was rather low. A review reported that semi-mature papaya was treated in hot water at 46Â°C for 90 minutes, then at 24Â°C for 3 days, the PG enzyme activity was reduced by 48%, and after 6 days, it was still increased by 25%, but it was not heat-treated. 10% of fruit activity. In the same way, the apple's softening was effectively delayed and the hardness was kept high. Secondly, heat treatment can inhibit the activity of various cell wall degrading enzymes, such as pectin-degrading enzymes, and reduce the damage caused by stress caused by subsequent treatments (such as low temperature, radiation, etc.). If heat treatment of grapefruit can reduce or even eliminate the cold-induced peel spots; adding TBZ (thiazolyl) in hot water can effectively reduce the fruit's low-temperature damage and reduce the occurrence of low-dose radiation treatment of fruit depression. Finally, heat treatment can significantly reduce the activity of acid phosphatase. According to Wang Dianjiu, when bananas were heat-treated at 40Â°C, their acid phosphatase activity gradually decreased. If they were treated for 3 days, the activity could be restored when they were returned to 20Â°C, and they could not be recovered if they were treated for more than 5 days. Effects on protein synthesis. Some experiments showed that during the heat treatment of fruit, the synthesis of its protein changed. Picton et al. found that the bands of labeled proteins were different from those observed at 25Â°C during processing in tomato at 35Â°C. Although the proteins were continuously synthesized, they lost normal mature proteins below 25Â°C. In this regard, he believes that changes in protein synthesis during fruit heat treatment inhibit or accelerate certain processes that require the synthesis of proteins to proceed. For example, the synthesis of PG enzyme and ethylene depends on the synthesis of proteins, and they are all inhibited by heat treatment. The process of fruit ripening after heat treatment is inhibited, heat treatment can also induce mango and other fruits produce heat shock protein, which may be related to antibacterial effects. 2. The pathological effects of heat-treating and fresh-keeping of fruits reduce the physiological diseases of postharvest fruits. Tiger skin disease is a serious physiological disease that occurs during the storage period of apples. According to reports, apple was heat-treated at 38Â°C for 4 days before conventional atmosphere storage at 0Â°C, and tiger skin disease was suppressed. The main mechanism is to inhibit the accumulation of Î±-farnesene and conjugated triene in the epidermis of apple during storage. In addition, the preservation of fresh tropical and subtropical fruits by cold storage methods is prone to physiological chilling. For example, most mangoes suffer cold injury when their temperature is below 10Â°C. Experiments have shown that heat treatment at 27Â°C for 7 days before cold treatment can minimize post-harvest chilling injury of the lemon. Sterilization. A large number of studies have shown that heat treatment can not only kill the pathogens attached to the surface of the fruit, but also has obvious killing and inhibiting effects on the latent infection bacteria. Liu Xiujuan et al. (1999) reported the effect of heat treatment on the growth and pathogenicity of two species of latent infecting anthrax. The results showed that the pathogenic bacteria belonging to different species belonged to different reactions to heat treatment: When the heat treatment temperature was 55Â° C. and the time was more than 30 minutes, the mycelia of the anthrax were significantly killed; when the heat treatment temperature reached 60Â° C. and the time was more than 30 minutes, The spores of Anthracnose had an extremely significant killing effect; treatment at 55Â°C for 10-20 minutes significantly reduced the pathogenicity of these two species of latent bacteria. Therefore, the use of heat treatment to control postharvest diseases should be based on different fruits and pathogens using different temperature ranges and processing times. According to reports by Barkai-Golan et al., the dry spores of the fungi are heat-resistant and the dormant structures are more heat-resistant than the active structures. For example, 90% of wet spores of Penicillium digitatum can be killed after 1 minute at 70Â°C, but the same treatment only killed 10% of dry spores.
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