Isolation And Identification Of Main Spoilage Fungi On The Post-harvest Lotus Roots And Antimicrobial Mechanisms Of Air Discharge

Air discharge is employed to keep fruits and vegetables fresh and reduce the spoilage rate by inhibiting the growth and reproduction of microorganisms.However,the antimicrobial mechanism of air discharge is not clear yet.In this study,the spoilage fungi from lotus roots stored in simulated cold chain environment were isolated,purified and identified,and Fusarium oxysporum,one of the major spoilage fungi on post-harvest lotus roots,was further studied.The antimicrobial mechanism of air discharge on Fusarium oxysporum was studied through the changes of the growth and ultrastructure of mycelial and spores,the changes occur in cell surface charges,chitin and chitosan,fatty acids and cell wall degrading enzymes activity.The main results are as follows:(1)By the combination of morphological,molecular and biological methods,Fusarium oxysporum and Fusarium solani were identified as the main spoilage fungi in post-harvest lotus roots.(2)Air discharge had significant(P<0.05)inhibitory effects on spore germination and mycelial growth of Fusarium oxysporum.The effect of antimicrobial was positively correlated with treatment time.The longer the treatment time,the slower the mycelial growth rate and lower the spore germination rate.After air discharge treatments,the mycelium was dry and wrinkled,the surface was rough,outer structure was incomplete,while the mycelia of the control group was smooth with uniform thickness,no wrinkles were observed.In air discharge groups,cells of mycelial and spores were irregular in shape,cell wall had invagination or protrusions with uneven thickness.And in cytoplasm,mitochondria decreased significantly and large vacuoles appeared,cytoplasm lost compact structure.In the control group,the cells were oval and normal,and cell wall was uniform in thickness while cytoplasm had compact structure.The apoptotic rate of the spores treated by air discharge was significantly(P< 0.05)higher than the control group.(3)After air discharge treatments,zeta potential increased;negative charges on spore cell surface were the least after ozone+negative air ions(ozone+NAI)treatment,followed by the ozone treatment group and negative air ions(NAI)treatment group.Negative charges on spore cell surface were marked by CdSe/ZnS core-shell quantum dots,the fluorescence intensity of spore cells in the control group was stronger and surface negative charges distribution was even,but the distribution of the quantum dots in air discharge treatment groups was not uniform,the fluorescence intensity was weak as a whole,and the localized distribution was obvious.(4)After air discharge treatments,cell wall permeability and chitinase activity increased significantly(P<0.05).Combined scanning electron microscope,highly dispersed structure with small particles and low dense was clearly seen in chitin and chitosan,while large block granular and high dense structure was clearly visible in the untreated chitin and chitosan.In chitin and chitosan,there was no significant(P<0.05)difference in wave numbers of functional groups between the control and the air discharge groups except the transmittance by infrared spectra.Crystallinity of chitin increased,while crystallinity of chitosan decreased(P<0.05);the degree of deacetylation of chitosan increased,the C/N ratio of both chitin and chitosan decreased(P<0.05).(5)After air discharge treatments,the cell membrane permeability increased(P<0.05),ergosterol content decreased(P<0.05);the content of saturated fatty acids such as myristic acid,pentadecanoic acid,arachidic acid,behenic acid,tricosanoic acid and lignoceric acid increased.While that of palmitic acid and heneicosanoic acid decreased.The content of pearl acid increased in the ozone treatment group while in NAI treatment group and ozone+NAI treatment group decreased,but there was no significant change(P<0.05)was observed in the content of stearic acid.The content of unsaturated fatty acids such as palmitic acid,arachidonic acid and CIS erucic acid increased,linoleic acid decreased,while there was no significant changes in the content of oleic acid.The ratio of unsaturated fatty acids to saturated fatty acids decreased significantly(P<0.05)after air discharge treatments.(6)After air discharge treatments,xylanase activity decreased significantly(P<0.05).But during the first 18 h,the activity of cellulase(Cx ? ?-glucosidase ? FPA)and pectinase(PMG?PG?PMTE?PGTE)significantly(P<0.05)decreased,then the activity of enzymes increased gradually.And the activity of the above enzymes was lower than thecontrol group.