Study On The Pathogenic Fungi And Spoilage Characterization Of Chestnut During Refrigeration

Chestnut(Castanea mollissima Blume),as an important woody food,can contribute to food security.However,the high rotting rate of chestnut during postharvest refrigeration(15 ～ 20%)has become a key technology “bottleneck”restricting the development of chestnut industry.The mixed infection of pathogenic fungi is the main reason of chestnut rot during refrigeration.However,the prevention and control of chestnut rot are mostly focused on the optimization of storage methods,and the physio-biochemical changes and the correlation with corruption of chestnut infected by pathogenic fungi have rarely been reported.Therefore,taking Wangmo chestnut as the research object in this study,the main pathogenic fungi were clarified by morphological observation combined with molecular biological identification,and the effects of different conditions on the growth and sporulation of pathogenic fungi were explored.Furthermore,the types and activities of cell wall degrading enzymes produced by pathogenic fungi in different exogenous induction media were determined.In addition,the changes of cell wall degrading enzyme activities,physiobiochemical indexes and ultrastructure of chestnut infected by single and mixed pathogenic fungi were analyzed.The main results are as follows:(1)The main pathogenic fungi of decaying chestnut during refrigeration were Fusarium proliferatum,Penicillium crustosum and Alternaria alternata.The optimal p H for hyphal growth and sporulation of Fusarium proliferatum were 7.0 and 8.0,the optimum carbon source,nitrogen source,temperature and light condition for mycelial growth and sporulation were lactose,glycine,25°C,and 24 h darkness,respectively.The optimal carbon source and p H for mycelial growth of Penicillium crustosum were lactose and 8.0,and the optimum carbon source and p H for sporulation were soybean polysaccharide and 7.0,the optimal temperature,nitrogen source and light condition for mycelial growth and sporulation were 25°C,urea and 24 h darkness,respectively.Sucrose and maltose were regarded as the optimal carbon sources for the mycelial growth and sporulation of Alternaria alternata,the optimal temperature,nitrogen source,p H and light condition for mycelial growth and sporulation were 25°C,peptone,9.0,and 24 h darkness,respectively.(2)Pectin was the best inducer of polygalacturonase(PG)and pectinase(PL)produced by pathogenic fungi,Xylanase and β-galactosidase(β-Gal)were best induced by xylanan medium.The activities of Carboxymethyl cellulase(Cx)and β-glucosidase(β-Glu)were higher in sodium carboxymethyl cellulose medium,and theβ-Glu activity was significantly higher than other media.β-Glu also showed high activity in chestnut pulp medium.The activity of cell wall degrading enzyme increased firstly and then decreased in chestnut during infection process of pathogenic fungi,and the cell wall degrading enzyme activities of chestnut infected by mixed pathogenic fungi were significantly higher than that of chestnut infected by single pathogenic fungi.The β-Glu activity in induction medium and chestnut pulp was obviously higher than other cell wall degrading enzymes(P<0.05).(3)The contents of moisture,fat,protein and starch decreased gradually,but the content of reducing sugar and α-amylase activity increased gradually during infection process of pathogenic fungi.The activities of catalase(CAT),superoxide dismutase(SOD),peroxidase(POD)and polyphenol oxidase(PPO)increased firstly and then decreased.The plasma wall separation and cytoplasm shrinkage were observed in chestnut,and the cell structure disappeared gradually with the time prolongs.In addition,the activity of resistant enzymes(CAT,SOD,POD and PPO)and the damage of cell tissue in chestnut infected by mixed pathogenic fungi were stronger than that infected by single pathogenic fungi.In summary,Fusarium proliferatum,Penicillium crustosum and Alternaria alternata were the main pathogenic fungi of chestnut.During the infected chestnut process of pathogenic fungi,cell wall degrading enzymes destroyed the structural components of chestnut cell wall,and the nutrient contents and the cell resistance decreased by mycelial invasion,the cell tissue was destroyed,which triggered finally the chestnut rot.The results are of great significance for reducing the rot rate of chestnut and prolonging the storage period,and provide a scientific basis for the rotting mechanism of pathogenic fungi.