Study On The Mechanism Of Postharvest Pomegranate Fruit Quality Deterioration And Its Prevention And Control Mechanism

Pomegranate(Punica granatum L.),which was originally produced in Iran,has been cultivated for more than two thousand years in China.Because of the fine implication of pomegranate and its medicinal value,the pomegranate industry has developed rapidly since the reform and development.Up to now,the planting area of pomegranate in China is nearly 1.8 million mu,and the output exceeds.2 million tons.However,due to the high moisture content of pomegranate fruit,peel browning,grain discoloration,softening and decay are easy to occur after harvest,which seriously affects the commercial value of the fruit.Chinese traditional indoor and well cellar storage technology is backward,and it is difficult to solve the problem of quality deterioration.In recent years,most of the research reports on pomegranate storage at home and abroad have focused on the conventional preservation methods,such as low-temperature refrigeration,plastic bag packaging and so on,to maintain fruit quality by adjusting the external environment of a single factor,not to mention a systematic study from the theoretical level.Modified atmosphere preservation and coating preservation are high efficiency and harmless new storage methods,because of its high stability,good effect,safety and non-toxic advantages,has been widely used in fruit and vegetable storage.In this paper,the mechanism of pomegranate peel browning was studied from the characteristics of enzymatic reaction and the relationship between enzymatic reaction and active oxygen metabolism.The pathogen of pomegranate brown rot during storage was isolated and identified by molecular biological methods.The special preservative for pomegranate storage was prepared and its antibacterial mechanism was studied.The effect of gas composition on active oxygen metabolism of pomegranate peel was studied.The prevention and control mechanism of pomegranate quality deterioration by combined measures of low temperature,controlled atmosphere,preservative and intermittent warming was explored.The scientific and practical operation rules of storage technology were put forward in order to provide scientific basis for the development of pomegranate storage industry in China.The main research contents and results are as follows:1.Pomegranate peel browning is enzymatic browning.The optimum temperature and pH of polyphenol oxidase(PPO)in peel were 50℃ and 4.4,while the optimum temperature and pH of peroxidase(POD)in peel were 27℃ and 6.4 respectively.The content of total phenol in peel was negatively correlated with browning index,PPO and POD activities were positively correlated with browning index,while the content of superoxide anion(O2-),malondialdehyde(MDA)and relative conductivity of tissues were positively correlated with browning index,while the activities of superoxide dismutase(SOD),catalase(CAT)and ascorbic acid peroxidase(APX)were negatively correlated with browning index.With the prolongation of storage time,the activities of PPO and POD in pomegranate peel increased.At the same time,the metabolic rate of reactive oxygen species increased,the activities of SOD,CAT and APX decreased,the harmful components such as O2-,H2O2 and MDA increased,lipid peroxidation intensified,cell membrane and intracellular structure damaged,resulting in phenolic substances leakage,combined with PPO and POD under aerobic conditions.The formation of quinones was further polymerized into melanin,resulting in the aggravation of pomegranate peel browning.2.The pathogens causing pomegranate brown rot during storage were isolated and purified,then identified by fungal taxonomy,ITS sequence amplification and phylogenetic analysis.It was found that the pathogens causing pomegranate brown rot were Botryotinia fuckeliana and Penicillium spinulosum.Low temperature had no lethal effect on the two pathogens.They could adapt to a wide pH range,grew well under neutral,weak acid and weak alkaline conditions,and their spore reproduction were inhibited under strong acid and strong alkaline conditions.Ultra-low or ultra-high O2 concentration is not conducive to the growth of the two pathogens.Botryotinia fuckeliana and Penicillium spinulosum are both specific aerobic bacteria,and the latter has a higher degree of oxygen demand,so the growth characteristics of the two pathogens can be changed by adjusting the composition of ambient gases.3.The optimum formula of special preservative for pomegranate storage is chitosan-oligosaccharide 0.30 g,natamycin 0.02 g,gluconic acid-delta-lactone 0.06 g,citric acid 2.50 g,ascorbic acid 2.50 g,sodium hexametaphosphate 0.10 g,sodium caseinate 0.60 g,distilled water 1000 g.The preservative has the advantages of low cost,simple operation,convenient operation,safety and innocuity.Compound preservative coating treatment can effectively maintain the nutritional components of pomegranate arils,maintain fruit hardness,slow respiratory metabolism,prevent peel browning,and reduce decay loss.The fruit had good appearance,quality and high commodity value after storage.Compound preservatives formed a tight permeable closed-loop environment on the surface of pomegranate,which isolated the fruit from external 02 and inhibited the activities of PPO and POD.Phenolic substances in the peel were difficult to oxidize and were not prone to browning.In addition,the treatment was helpful to keep the activities of CAT and SOD at a higher level,enhance the scavenging of H2O2 and O2-,and retard tissue senescence.4.The appropriate concentration of modified atmosphere has a good effect on regulating active oxygen metabolism of pomegranate peel after harvest.Firstly,the activities of SOD,CAT,APX in pomegranate peel could be maintained by controlled atmosphere treatment,and the free radicals of reactive oxygen species(O2-)could be scavenged effectively.The production of harmful substance H2O2 could be inhibited,then its toxicity could be reduced.The ability of anti-oxidation and anti-aging of pomegranate peel could be enhanced.Secondly,proper controlled atmosphere treatment can also inhibit the rapid increase of MDA content in the peel,delay lipid peroxidation,slow down the increase rate of relative conductivity of tissues,and maintain good cell permeability.Thirdly,controlled atmosphere treatment protected the ascorbic acid-glutathione cycle composed of reduced ascorbic acid(ASA)and reduced glutathione(GSH)to a certain extent,and helped to improve the endogenous antioxidant capacity of fruit.5.The optimum composite storage technology of pomegranate was as follows:pomegranate was pre-cooled,coated by a special preservative solution with the concentration of 160 mg·L-1,dried and stored at the temperature of 2.0-3.0℃C,relative humidity of 90%-95%,gas composition of O2 = 8.0%,CO2 = 4.0%,ventilated every 10 days,and intermittent warmed at 10.0℃ for 24 hours every 15 days,then stored back at 2.0-3.0℃,recycled the above measures until the end of storage.By using this technique,pomegranate can be stored for 150 days,the total sugar content in arils is 12.6%,the titratable acid content is 0.56,the commodity fruit rate is 94.0%,and the peel browning index is 0.12.After storage,the shelf life of pomegranate was 19.1 days when it was stored at 3.0-5.0℃.The fruit had bright color,good sensory quality and ideal fresh-keeping effect.