Analysis of the factors affecting the storage qual

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Analysis of factors affecting the storage quality of walnuts

Abstract: This paper introduces several storage methods of walnuts, systematically summarizes several main factors affecting the storage quality of walnuts at home and abroad, and puts forward some countermeasures for the above influencing factors

key words: walnut; Storage; Main factors

walnut (Juglans regia L), also known as walnut and Qiang peach, is a walnut plant in the walnut family. Due to the high content of fat in walnut kernel, it is prone to oxidative rancidity during storage, resulting in quality decline. In order to solve this storage problem, the author referred to a large number of relevant research materials at home and abroad, and summarized them in order to provide a theoretical basis for the research and development of stored walnuts

1 storage method

now it is a popular material

walnuts for storage must be harvested after they are fully mature. Zhang Hongchao et al. (1981) studied the relationship between different harvest dates and walnut yield and quality, and believed that before fruit ripening, with the delay of harvest time, walnut kernel yield and fat content increased; The study also pointed out that if walnuts in Beijing were harvested 15 days in advance, their yield and quality would suffer economic losses to varying degrees [3]

1.1 indoor storage and low-temperature refrigeration

newly harvested walnuts have high water content and are prone to decay. They must be peeled off, dried in the air and fully dried before storage. Generally, the dried walnuts are put into cloth bags or gunny bags, or stored indoors, and the bottom is padded with wood or masonry, so that the walnuts are 40 ~ 50cm away from the ground. The environment in the storage room must be cold, dry, ventilated and backlit, and attention should be paid to prevent rodents. The above method is only suitable for short-term storage of walnuts, which can be stored at room temperature until summer, and the quality of walnuts remains basically unchanged. Walnut should have low temperature conditions for long-term storage. When storing in large quantities, it is best to pack them in gunny bags or refrigerated boxes and store them in a constant temperature cold storage at 0 ~ 5 ℃. The shelf life of walnut kernel can be as long as 2 years [3]

1.2 sealed storage with film curtain

when walnut storage is large and cold storage conditions are not available, plastic film curtain can be used for sealed storage. Walnuts will be recorded after being fully dried in autumn, and will be sealed before the temperature rises in February of the next year. When sealing, it should be kept at a low temperature. CO2 is introduced into the account, which can inhibit walnut respiration, reduce consumption, inhibit mold activity and prevent mildew. When the CO2 concentration reaches more than 50%, it can prevent oil oxidation from causing deterioration and insect damage. Nitrogen filling in the account can also prevent walnut senescence to a certain extent. After 4 weeks of nitrogen storage, its color and flavor were significantly better than indoor storage; After 25 weeks, it still maintained good quality, and at this time, the walnut stored indoors was prone to rancidity, and the walnut skin was dark

2 factors affecting the stability of walnut storage quality

2.1 effect of hard shell structure on Walnut storage quality

hard shell, as an important part of walnut nuts, plays an important role in the growth, development, maturation, rinsing, transportation and storage of nuts. David (1985) divided the tightness of walnut suture into five levels from extremely tight to extremely easy to crack

radicati (a professionally designed automatic control and data collection system 1990) divides nuts into good, medium and poor. The national standard of Walnut High Yield in hard fruit quality issued by China in 1987 stipulates that nuts with suture cracking exceeding 10% of the number of sampling samples cannot be rated as excellent or first-class [4]

fruit cracking refers to the cracking at the suture of walnut nuts. Zhang Zhihua et al. (2004) found that after 60 days of storage, the difference of fruit cracking rate among different types and varieties reached a significant level, and the fruit cracking rate of iron walnut varieties was significantly lower than that of ordinary walnut varieties. After one year storage of walnut nuts, the insect fruit rate of different varieties was also very different. The fruit rate of Shisheng No. 8 and Luguo No. 1 stored at room temperature was 55.7% and 57.80%, respectively. The difference of insect fruit rate among different types and varieties reached a significant level, and the insect fruit rate of iron walnut varieties was significantly lower than that of ordinary walnut varieties. The thinner the hard shell, the flatter the suture, the higher the fruit cracking rate, the higher the probability of seed kernel contamination, and the higher the rate of insect fruit during storage. Through analysis, it was found that the tight density of suture and the thickness of hard shell were the most important factors affecting the fruit cracking rate and insect fruit rate of walnut

2.2 effect of water content on the storage quality of walnuts

the water content of walnuts that need to be stored in China is controlled at 6%~8%[1,3], which is close to the requirements of the United States; In France, the water content of walnut kernel is required to be less than 12% [5]. When the water content of walnut kernel is less than 8%, its water activity (AW) is generally less than 0.64. Under this condition, it can inhibit the growth and reproduction of most microorganisms. Zhang Zhihua et al. (1994) showed that when the water content of walnut nuts is less than 8%, its respiratory rate is low and stable, which is consistent with the nut water content standard introduced by David (1985). Yang Jianting et al. (2001) also believed that water is an important factor affecting the rancidity of walnut oil. Under the condition of 3 ~ 4 ℃, walnut with high water content (12.77%) is more serious than that with low water content (5.89%). However, drying can not completely eliminate the occurrence of Walnut Rancidity. If the water content is too low, it will increase the possibility of rancidity. Rockland (1978) believed that the water content of walnut kernel should not be less than 3.5%

2.3 effects of oxygen, relative humidity and temperature on the storage quality of walnuts

low temperature environment is the preferred condition for long-term storage of walnuts. Hadorn et al. (1980) believed that the environment with temperature of 12~14 ℃ and relative humidity of 50%~60% was the best condition for walnut storage. The storage temperature recommended by other researchers is 0~2 ℃; Kader and labavitch suggest that the temperature of walnut storage is 10 ℃, and the relative humidity is 70% [5]. A. Lopez et al. (1995) studied the quality changes of walnut kernel under low temperature storage. The results showed that the shelf life of walnut kernel could reach 1 year at 10 ℃ and 60% relative humidity, and its physical, chemical, sensory and other quality indexes were within the specified range; During one year of low-temperature storage, the stability of oil in walnut kernel showed a downward trend in the first four months of storage, which may be related to the reduction of antioxidant factor (VE) content in walnut kernel, but in the next few months, the stability of oil did not change significantly, and the fatty acid composition of oil remained the same

oxygen concentration is one of the main environmental factors affecting oil oxidation. When the oxygen content in the environment is high, the oxidation rate has nothing to do with the oxygen concentration; On the contrary, the oxidation rate is positively correlated with oxygen concentration [6]. Yang Jianting et al. (2001) believed that when the storage environment and time are the same, and the water content of walnut kernel is less than 7%, due to its direct exposure to the air, the water content decreases, and the acid value, saponification value and peroxide value are higher than those of shelled walnut kernel, while the iodine value is very low, indicating that shelled walnut kernel is easy to deteriorate without special oxygen isolation packaging. Other scholars believe that oxygen barrier packaging can prolong the shelf life of walnuts. Mate et al. (1996) found in the study on the effect of oxygen concentration and relative humidity on Walnut Rancidity, Walnut has high oxygen content (> 21%) and low oxygen content respectively (<2.5% = after 28 days of storage in the environment, there is a significant difference. The increase or decrease of resistance indicates the direction of displacement; by the 6th week of storage, the peroxide value and hexanal content of walnuts stored in the environment with high oxygen content are significantly higher than those stored in the environment with low oxygen content; walnuts stored in the environment with relative humidity of 53% are more prone to oil oxidation than walnuts stored in the environment with relative humidity of 21%. Roasted walnuts are separated by Oxygen packaging or nitrogen filled packaging can extend its shelf life

2.4 effect of irradiation on storage quality of walnuts

insect damage is also one of the main factors affecting storage quality of walnuts. Traditional methods mainly use fumigation to control pests in the storage and transportation of walnuts. The fumigants used include ethylene dibromide, methyl bromide, ethylene oxide, etc. However, fumigants have adverse effects on human body and the environment, and have now been banned or are being eliminated. Therefore, it is particularly important to develop a non chemical pest control method with strong pertinence. Many researchers have found that radiation treatment can effectively replace chemical methods- Bachir and Abdus Sattar (2004) believe that the radiation dose used to control insect pests is low, generally 1 kGy, which will not cause significant changes in nut composition, has no negative impact on sensory quality, and can also kill all breeding insects. Wang et al. (2002) used radiation treatment to control the third and fourth instar Apple bark moth of walnut with shell. This treatment raised the central temperature of walnut to 53 ℃ within 3 minutes and kept the controller chirping for 5 minutes, and the insect mortality reached 100%, but it would not cause the decline of peroxide value and fatty acid index of walnut

jan et al. (1988) reported that the walnut kernel treated with 0.5 kGy and 1.0 kGy radiation had no effect on its oil oxidation and sensory quality- Bachir (2004) stored irradiated and non irradiated walnuts in an environment of 15~18 ℃ and 50% - 70% relative humidity. The results showed that, γ X-ray increased the total acid value of walnut, reduced the iodine value and volatile alkali nitrogen, and there was no significant difference in flavor between the test group and the control group; After 12 months storage, γ The acid value and peroxide value of irradiated walnuts were relatively low, while the iodine value and volatile alkali nitrogen were relatively high. 1.5 kGy and 2.0kGy high dose γ X-ray treatment had adverse effects on sensory characteristics of walnut. Other research results show that fluorescence and γ X-ray irradiation is easy to cause oxidation of walnuts. Packaging with opaque materials can protect walnuts during storage

3 discussion

at present, scholars at home and abroad have made fruitful research on the storage methods of walnuts and the key factors affecting their storage quality, but there are still many problems to be further investigated, such as the factors affecting the antioxidant rancidity of walnuts, such as variety, fatty acid composition, vitamin E, tannin in walnut seed coat The effects of changes in the activities of related enzymes (catalase, SOD, lipase, lipoxygenase, etc.) in walnut kernel on the quality of walnut kernel, and the effects of different storage times on its nutritional efficacy

Zhang ye1, Wang Kejian 2, Hao Yanbin 2, Zhao Liqin 1

(1. School of food science and engineering, Inner Mongolia Agricultural University; 2. Institute of forestry and fruit trees, Beijing Academy of agricultural and Forestry Sciences

source: China preservation

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