Studying the Effect of Coating and Storage Duration on some Quantitative and Qualitative Characteristics of Oranges and Lemons in Above-zero Cold Storage

Fruits and vegetables play an important role in healthy nutrition of the society and have an important place in the consumer's food pyramid and in meeting the food needs of the society. However, the short shelf life and high perishability of agricultural products, especially citrus fruits and vegetables, have caused undesirable changes, which makes the buyer consume them fresh. The use of healthy preservatives and proper storage, along with maintaining the health and quality of the product, can greatly help in increasing the storage time and reducing spoilage in the products. In the present study, the effect of coating and storage time on post-harvest quality and some quantitative and qualitative characteristics of oranges and lemons in a subzero cold store was investigated.

Introduction
Citrus fruits are one of the most important fruits produced in tropical and horticultural regions of the world. According to estimates, the waste from citrus fruits is approximately between 28 and 31 percent. Therefore, it is of great importance to investigate various methods that will result in reducing the amount of citrus waste. The use of synthetic chemicals in fresh produce is one of the fundamental and important problems in relation to human health, because the residual chemicals, especially in the diet of children and sensitive groups, endanger the health of the people in society, and in view of this, we must look for alternative materials. Today, new technologies have wide applications in the stages of production, processing, storage, packaging and transportation of agricultural products.

Materials and Methods
In this experiment, local blood oranges and lemons were used. These samples were picked from citrus orchards in Sari city on the morning of the experiment. They were brought to the experiment site by observing all the necessary points both during picking and during transportation. For each sample of the coverings, 6 healthy fruits without scratches, fungus, and Mediterranean fly attacks were selected and then wiped with a completely clean cloth and prepared for placement in packaging materials and conducting the necessary tests. It is necessary to explain that the same number of treatments were considered as control treatments. In the treatments that used covering materials, after the treatments were completely covered using a predetermined recipe, they were placed in baskets and transferred to the cold store. For example, for the chitosan treatment, after weighing the number of lemons and oranges with a digital scale and measuring the firmness, soluble solids and pH, the fruit was completely covered with a chitosan solution at a dose of 1 gram (1%) in 1 liter of water and transferred to a cold storage for 60 days at a temperature of 2 to 4 degrees Celsius and a relative humidity of 85%. The rest of the treatments were also treated in the same way and transferred to a cold storage. The weight of all packages was weighed and recorded every 15 days with a digital scale with an accuracy of 0.01. After a 60-day storage period, the percentage of weight loss of the fruits was calculated. The firmness, wheight reduction, soluble solids, titratable acidity, pH and vitamin C of the fruit juice was determined by suggested methods by researchers. In this study, the data were analyzed using a completely randomized design with three replications and a factorial test. Statistical analysis was performed using SPSS software and graphs were drawn using Excel.

Results and Discussion
The results showed that the lowest weight loss, pH change, and vitamin C retention were achieved in the propolis and chitosan coating among the coatings. The firmness of the samples' texture and soluble solids also showed the least changes in the hot water steam, propolis, and chitosan treatments. In general, coating fruits with edible and non-chemical materials improves the quality and shelf life of the product during storage. The use of propolis creates a favorable atmosphere and better preserves the characteristics of citrus fruits. In terms of weight changes, propolis and chitosan coating is the best option for coating compared to other options, which is due to the antibacterial and antifungal properties and the resinous nature of propolis. The use of coating during storage had a significant effect on maintaining the pH of the fruit. In the first 30 days of testing, no change in the pH of the propolis and chitosan samples, water vapor, and chitosan fungicide samples was observed, but in the rest of the cases, changes were observed in the first 30 days. In the second and final 30 days of the test, we had an increase in pH, such that the propolis and chitosan coating had the least change and the ethanol and imizaline coating and the control sample had the most change. Among the coating treatments in orange fruit, edible hot water vapor coatings, propolis, chitosan, and thiabendazole chemical coatings had the least change, which indicates that the use of these coatings, especially propolis and chitosan, has a great effect on preserving soluble solids. The use of coating has a significant effect on maintaining titratable acidity, but the storage period and the interaction effect of coating and storage period on maintaining titratable acidity are not significant. Propolis has a significant function in maintaining titratable acidity in lemon. Propolis controls ethylene synthesis, which causes premature aging and increases respiration rate. A decrease in fruit firmness was observed in all coatings until the end of fruit storage. This phenomenon is due to a decrease in the hydrolysis of cell wall pectins, and this effect is caused by a decrease in the activity and also a decrease in the level of cell wall degrading enzymes or a decrease in the rate of ethylene production due to a decrease in the activity of enzymes involved in ethylene production. Propolis, chitosan, and hot water treatment had the best effect on preserving fruit texture, respectively. Vitamin C decreased during storage, and only chitosan and propolis coatings were able to preserve these values to some extent, but as observed in other parameters, the coating effect on oranges was more favorable than on lemons.

Conclusion
In this study, the physical and chemical properties of two types of local lemon and sanguine orange fruits were investigated using the coating method during the storage period. The results showed that propolis coating performed better than other coatings in preserving the quantitative and qualitative indicators of the fruit and was more effective compared to chemical coatings. Propolis was better than other coatings in all measured traits and had a great impact on preserving the physical and chemical properties of the fruits by controlling respiration, oxidation, delaying the aging process, creating a suitable atmosphere inside the fruit, and better exchange with the cold storage atmosphere. In the next stage, chitosan coating and water vapor for 120 seconds performed better than other coatings. These results clearly state that the use of propolis coating, in addition to preserving the properties of the fruit, is the best option to replace conventional coatings on the market because it is edible, non-toxic, economical, and has the least waste. On the other hand, because chemical fungicides are completely imported and a high cost must be paid to purchase them, and also by saving money, maintenance costs can be reduced and the cost of the product can be reduced, thus bringing health to the consumer community.