Effects Of Chitosan/Nanocrystal Cellulose Composite Coating On Storage Quality Of Citrus

Citrus(Citrus sinensis L.Osbeck)are susceptible to pathogen infection,and the loss during storage and transportation is high.At present,the post-harvest preservation measures of citrus fruits in our country are still mainly applying chemical preservatives,and the chemical preservatives have the risk of abuse,and long-term use will lead to the development of drug resistance in the fruit.In this context,the edible coating film preservation technology has attracted more and more attention due to its green,safe,easy-to-operate,and remarkable effects.The edible film coated on the surface of the fruit regulates the gas exchange between the fruit and the outside through its semi-permeability,reduces the respiration rate of the fruit,reduces its transpiration,and delays the ripening and senescence of the fruit,thereby playing a good preservation effect.Our research group preliminary research found that the respiration and transpiration of the fruit will cause micropores in the coating film,which will weaken the preservation effect of the fruit.For this reason,nanocrystal cellulose(NCC)was added to the chitosan coating liquid to adjust the permeability of the chitosan(CS)coating film and enhance its structural stability.Thereby improved its fresh-keeping effect on citrus fruits.At the same time,the permeability of the coating film should match the postharvest respiration rate of the fruit to prevent excessive inhibition.If the barrier property of the coating is too high,it may cause anaerobic respiration of the fruit,which will adversely affect the quality of the fruit.The individual size,peel structure and post-harvest physiological activities of different citrus varieties are quite different,so the requirements for the permeability of the coating film are also different,and the time for the micropores of the coating film to appear on the surface of different fruits is also different.Therefore,this study selected three different varieties of citrus(ponkan,shatangju,tangerine),respectively explored the influence of the NCC content in the coating film on the fresh-keeping effect of the above-mentioned citrus species,and observed the micropores in the coating film.Secondly,the compound coating film with the best NCC content was used to coat the shatang and tangerine respectively at different time points during storage,in order to explored the appearance time of micropores and the fruit of different varieties of citrus fruits.The relationship between storage quality.The main research contents and results are as follows:1.The effect of chitosan/nanocrystal cellulose composite coating on the storage quality of ponkan.The results showed that,compared with the CK group and the CS group,the composite coating with NCC content of 2%and 4%could significantly reduce the weight loss rate and rot rate of fruits in the late storage period.When stored for 16^thday,the fruit rot rate of the 2%NCC composite coating group was 55%lower than that of the CK group,and 43.7%lower than that of the pure chitosan group.This group also maintained the content of nutrients such as titratable acid,ascorbic acid,and soluble protein in the fruit.At the same time,compared with the CK group,all coating groups can significantly inhibit the increase of malondialdehyde content in the peel in the late storage period(after the 10^thday),indicating that the coating can inhibited the membrane lipid peroxidation of the fruit and delay the senescence of the fruit.In general,the composite coating with 2%NCC added to the coating has the best preservation effect on the freshness of the ponkan fruit.2.The effect of chitosan/nanocrystal cellulose composite coating on the fresh-keeping effect of shatangju.The results show that the coating film could not reduced the water loss of the fruit,but it could better inhibited the rot of the fruit.The decay rate of the composite coating group added with NCC was lower than that of the CS group and the FA group in the late storage period.When stored for 24^thday,the decay rate of the 4%NCC composite coating group was 50%and 9%lower than that of the CK group and the FA group,respectively.It shows that the fresh-keeping effect of the composite coating film is better than that of imidamide(FA).Nutrients such as ascorbic acid,titratable acid,soluble solids and total phenols of the fruits of each group fluctuate greatly during storage,and there is no observed change rule.However,the4%NCC composite coating group had the lowest malondialdehyde content in the peel,and could maintain high peroxidase(POD)and catalase(CAT)activities during storage,indicated that the composite coating could significantly delay the peroxidation of the membrane lipid of the fruit and enhance the anti-aging ability of the fruit.From the microscopic morphology of the coating film,it can be seen that due to the high respiration intensity of the fruit(20?48 mg·kg^-1·h^-1),the coating film on the surface of the peel appeared micropores on the first day of storage.However,the addition of 4%NCC to the coating film could help reduce the appearance of micropores and enhance the structural stability of the coating film.In general,the composite coating with 4%NCC added to the coating has the best preservation effect on the fresh-keeping of the fruit.3.The effect of chitosan/nanocrystal cellulose composite secondary coating on the fresh-keeping effect of shatangju.The results showed that the primary coating of chitosan and the composite film could reduced the rot rate of citrus fruit,but after the secondary coating,the rot rate of the fruit increased rapidly in the late storage period,although it was still lower than that of the CK group.But higher than the one-time coating group and FA group.Compared with the primary coating,the secondary coating had no significant effect on maintained the nutrient content in the pulp,and at the same time it was not able to reduced the electrical conductivity and the malondialdehyde content of the peel.The secondary coating group of the composite membrane maintained a high superoxide dismutase(SOD)activity in the late storage period,but the POD activity was lower than that of the primary coating group.The changes in the microscopic morphology of the peel surface indicated that the secondary coating delayed the appearance of micropores on the coating surface,and the secondary coating of the composite film did not observed the appearance of micropores on the 4^thday.However,due to the high respiration intensity of shatangju and very active physiological activities,although the secondary coating improved the stability of the membrane structure,it was not conducive to the physiological metabolism of the fruit.Therefore,its fresh-keeping effect on the shatangju was not as good as the coating primary coating film.4.The effect of chitosan/nanocrystal cellulosecomposite secondary coating on the storage quality of tangerine.The results showed that the composite coating film containing 6%NCC could inhibited the increase in weight loss rate and rot rate of fruits,especially the rot rate of the secondary coating group was much lower than that of the FA group.After 11^thday of storage,the decay rate of CS-6%NCC4 group was40.0%and 28.5%lower than that of CK group and FA group,respectively.The CS-6%NCC4 group could better maintained the content of nutrients such as titratable acid in the fruit.At the same time,the composite coating group could better inhibited the increase of relative conductivity and malondialdehyde,and could maintained higher POD and CAT enzyme activities in the late storage period.The SEM results showed that the chitosan coating on the surface of the red orange had micropores on the third day,and the composite coating with NCC delayed the micropores on the surface of the pure CS coating by 2^thdays.After the second coating,the micropores reappeared in the later storage period,but the CS-6%NCC6 group had fewer micropores.This shows that the secondary coating film could indeed delay the appearance of micropores,maintain the stability of the coating film structure,and help improve the durability of the coating film on the preservation of tangerine.In summary,this study clarified the best chitosan/nanocellulose composite coating formulations for the three citrus fruits,and used the secondary coating to delay the appearance of micropores in the coating.Investigated different varieties citrus fruit relationship between the stability of coating structure and fruit storage quality.The research results showed that the permeability of the edible coating film should match the physiological activity characteristics of the fruit in order to achieve a good preservation effect,which provides theoretical guidance for the rational application of the edible coating film in fruit preservation.