| Grapes are one kind of fruits which have high nutritional and economic value, but theyare so easily mechanical damaged and pathogen infected because of their soft pulp, high sugarand moisture content that the logistics and anniversary supply are seriously affected. Nowthey are often combated by application of sulfur dioxide (SO2) to maintain quality. However,SO2is dangerous to people who are allergic to sulfites and its applications have beenrestricted in many countries.Chitosan is non-toxic, safe, antibacterial, film, and degradable, so people have becomestudy the mechanism of its induction of disease resistance in fruit and vegetable. Toinvestigate the effect of chitosan preharvest sprays on quality, texture, ultrastructure andpostharvest physiology of Red globe grape, the fruits were treated with0.1%,0.5%and1.0%chitosan solution before harvest and then stored at (0±1)℃. On the basis of preharvesttreatment, the role of chitosan in grape fresh-keeping was explored; the mechanism ofchitosan induced disease resistance of grape was studied. In the end, the feasibility wasdiscussed if it is possible to control postharvest diseases of grapes by preharvest chitosansprays. At the same time, the effect of moderate dehydration treatment on grape was studiedin order to explore a new method no sulfur dioxide of fresh-keeping. The objective oftreatment was to study the effect of dehydration on quality, dielectrical parameters andphysiology of grape. Through these studies, it is possible to provide theoretical and practicalbasis for green method. All the results were showed below:1. The fruits were treated with0.1%,0.5%and1.0%chitosan solution before harvest andthen stored at (0±1)℃. The results indicated that preharvest sprays with chitosan couldsignificantly keep the soluble solids and titratable acid content, the chitosan treatment washelpful on keeping the quality of grapes. The chitosan sprays could inhibit the decline ofhardness, chewiness and cohesiveness, keep the springiness of grape. In the late storage, thechitosan sprays were helpful of keeping resilience and inhibition adhesiveness rise of grape fruit.2. The ABA content of pulp in chitosan treated group was lower than control, but thecontent of IAA, GA3, ZR, and the value of ZR/ABA and GA3/ABA were higher than control.Preharvest sprays with chitosan could also maintain the value of IAA/ABA and ZR/ABA,inhibit the ABA content in pericarp at the same time.3. The content of total phenolics, flavonoids and soluble protein in pulp in treated groupwas higher than control, the same as the activity of POD, PPO, SOD, PAL and GLU, all ofthese indicated that chitosan sprays induced the disease resistance of grape.4. Furthermore, transmission electron microscope was used to observe the ultrastructureof the grape. The results showed that in the first day of storage, there was no significantdifference between chitosan treated grape and control except for macromolecular sedimentand thicker cell wall in chitosan treated grape cell. There were some pits in cell wall in grapesprayed with0.5%chitosan. After seventy-five days, the middle lamella of preharvest treatedgrape was not completely dissolved; leaving a lot of small microfibrils, and the primary cellwall was integrated. Chloroplast and mitochondria of preharvest-treated grape were integrated;there were pits in cell wall, and macromolecular sediment in the cell. But the cell wallstructure of control grape swell and flab, the primary cell became crooked and thicker; mostof the middle lamella was dissolved and disappear. Chloroplast gradually becamedisorganization, the brink of membrane and grana lamella was unclear. Mitochondriaremained numerous, mitochondrial crista disappeared. There had little pits and nomacromolecular sediment in the cell.5. In the paper, the effect of spray-shock was found and put forward for the first time inthe process of preharvet treatment, the possible mechanism was also been studied about howthe spray-shock led grape injury in the storage. The grape damaged in cold storage wouldappear such phenomenon as peel cracking, pulp collapse and juice outflow from20d to30d,all of these made fruit quality decrease rapidly and grape became rot quickly. Fruitspray-shocked with1.0%chitosan had higher soluble solid content than control fruit. Solublesolid and titratable acid content fluctuated and appeared a pear in45d. The spray-shockedfruit had higher POD activity than un-shocked fruit. Compared with the un-shocked grape, theshocked grape had lower SOD, CAT and PPO activity. The spray-shock led to the injury andfast decay of grape in the storage.6. Dielectric parameters of Red globe grape during cold storage at0℃werenon-destructively measured over frequency range from0.1kHz to3980kHz. The resultsshowed that there are significant exponential relationship between impedance, resistance,inductance reactance, admittance, susceptance and frequency, but no exponential relationship for other dielectric parameters. Fruit impedance, resistance, inductance and reactancedecreased gradually by linear form with the increase of frequency, admittance andsusceptance increased linearly. At the optimum test frequency, impedance, resistance,inductance and reactance decreased linearly with the extension of storage time.7. Firmness, chewiness, springiness and adhesiveness of grape were measured as textureattributes. In the storage of45d, fruit firmness and chewiness decreased gradually with thetime went on, springiness and the absolute value of adhesiveness went up contrarily. At0.1kHz frequency, the high correlation was found between dielectric and texture parameters,such as reactance with texture parameters (P<0.05). At last, the regression equation was builtfor dielectric and texture parameters, it was possible for the realization of grapenondestructive testing.8. Postharvest dehydration treatment was a safe fresh-keeping method without sulfur. Allthe test results showed that postharvest moderate dehydration was one more effective way inkeeping the quality of grape during cold storage. Grape dehydrated3%has significant higherfirmness, PPO and POD activity than control. Compared with the un-dehydrated grape, thedehydrated one has higher impedance, reactance, inductance, resistance and lower dielectricconstant, impendence phase angle during45days storage. |
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