| Soften and flavor changes are the main quality deterioration of table grape after harvest. Water loss, degradation of pectin and volatile component transformation are main reasons for its quality changes. This study began with the textures soften of grape fruit and volatile flavor compounds changes caused by water evolution and pectic macromolecules degradation. Using T2obtained by low-field nuclear magnetic resonance (LF-NMR) and magnetic resonance imaging (MRI), atomic force micro technology (AFM), solid phase micro extraction/GC-MS technique (SPME/GC-MS) and other technique to study fruit moisture status, combination in the cell, distribution in tissues, moisture migration during storage, and effects of environmental temperature and humidity on the moisture migration systematically. Then establishing correspondence between changes in texture (TPA) of grape fruit with the main parameters; and the processes of original pectin degradation to different solubility of pectin, such as, water soluble pectin (WSP), hydrochloric acid soluble pectin (HSP), sodium hydroxide soluble pectin (SSP), ammonium oxalate soluble pectin (ASP), pectic acid and monosaccharides induced by exo-PG, endo-PG, EGase, PE and other pectinase were also studied in this project. The relationship between pectin degradation and changes in the texture of fruit was dynamic analyzed. The main volatile flavor compounds changes in four typical kinds of grape were studied before and after storage. The revealed mechanism of quality deterioration of grape fruit after harvest will provide a new theoretical basis for the storage and preservation of berries. The main findings are as follows:(1) Water content had great influence on grape softening and springiness. When stored at15℃, the correlation of grape moisture content and hardness, cohesiveness, springiness, gumminess and adhesion was significant; at30℃, moisture content and hardness, elasticity were significantly correlated positively and correlated with cohesion negatively. Humidity also plays an important role in change of fruit moisture status. When stored at20℃, higher humidity was beneficial to maintaining the stability of bound and free water and the hardness of the grapes, have a, The grape fruit bound water content tracking and storage quality can be better characterized and predicted due to the strong correlation of bound water and a number of indicators when storage in high humidity; the total water content detection should be more reasonable indicator at low humidity storage;, the use of free water and total water content to the common characterization of the quality of the grape fruit is more reasonable under varying ambient humidity conditions.(2) Grape moisture was divided into four parts based on transverse relaxation time T21, T22, T23, T24, and was characterized as cell wall water, intercellular substance water, cytoplasmic water and vacuoles water respectively. Based on cell moisture migration, cell wall and intercellular water migrated slowly at4℃than0℃, but cytoplasmic water and vacuoles water were more stable at0℃than4℃. The different combined status of water in the fruit revealed that the overall effect is better at0℃, but resulting more migration of vacuoles water because of the vacuole membrane damage in late stage of storage. When stored at low temperature the hardness and viscosity showed closely relation with vacuole and cytoplasm water. Large vacuole water and key textural parameters showed a significantly positive correlation, while the cytoplasm water and texture parameters showed a significantly negative correlation. MRI analysis indicated that with the development of storage period, free water is bound to the cell and become bound water dueto the shrinking of fruit structure. The rate of moisture migration, degree of structural shrinking and deterioration rate of grape fruit without stems was significantly faster than the fruit with stems.(3) PE activity of grape fruit at0℃and4℃storage showed similar trend which increased at early storage time and decreased at late period. Two peaks were determined in EGase activity which was emerging earlier in the fruit storage at4℃than the fruit storage at0℃. Endo-PG activity in grape fruit had significant difference at0℃and at4℃in28d storage, then the difference reduced in the following storage period. Exo-PG activity decreased continuously when stored at low temperature. Glucose and fructose content are the highest monosaccharide in Kyoho grapes and glucose levels slightly higher than fructose content. Both glucose and fructose contents increased at the early storage period, decreased in the middle storage time and increased again in the following period. Highly correlated relation was found between total pectin and soluble pectin content. More glucose and fructose were produced in the transformation process of protopectin to soluble pectin. When stored at 0℃, higher PE activity induced faster degradation rate of soluble pectin. Endo-PG plays an important role in the degradation process from protopectin to soluble pectin. Correlation analysis indicated the similar results during storage at4℃and0℃. More fructose and glucose were produced with the increase of EGase activity.(4) The content of HSP in Kyoho grape was higher than other soluble pectin, ASP and SSP levels were close and WSP content was the lowest. High and low humidity showed greater impact on ASP and HSP content, respectively. High humidity was beneficial tomaintain the stability of the total sugar and reducing sugar contents, but has little effect on the changes of soluble sugars. The total sugar content significantly increased due to WSP degradation under high humidity; however, soluble sugar content increased with HSP degradation at low humidity., SSP content is an important indicator during ambient humidity storage, total sugar and reducing sugar content showed a significantly positive correlation. AFM was used to study the changes of the fine structure of pectin, fresh grapes SSP existed as long chain with less branching, and the molecular chains is smaller. Break point of SSP strand increased after storage and the SSP chains became shorter that pectin chain molecules belong to the basic junction overlap. Molecular weight of WSP was smaller than the SSP, chain width and height values of WSP significantly changed after storage. WSP content in fresh grape fruit showed high degree of cross-linking and aggregation. Hydrolysis degree of WSP intensified, the molecules dispersed more, molecular weight became smaller, but the height of the chain significantly increasedafter storage.(5)65μm PDMS/DVB fiber was selected for extraction of volatile compounds from grape fruit by SPME at50℃for30min. Volatile compounds in Kyoho, Longan, Cow’s Teat and Emerald grape changed greatly during storage and indicated different trends. Species of volatile compounds in late storage were much less than the initial storage. Trans-2-hexene-l-alcohol in most varieties of grapes is a major alcohol in early storage period, and significantly reduces in late storage period. Aldehydes, especially2-hexenal was found higher in two kinds of green varieties include Cow’s Teat grape and Emerald grapes in the initial storage. But little differences was found in the content of2-hexenal in the four varieties of grape fruit at the end of storage. Content of ketones in four kinds of grapes were remarkably reduced at late storage. The relative content of esters in Emerald grape obviously increased with the development of storage time. Total sugar and total acid contents of four grape cultivars showed a stable increase with the extension of storage time. Total sugar and total acid contents of Cow’s Teat grape were higher than the other three cultivars of grapes. Total sugar and total acid in Kyoho grape showed opposite trend that compared to the other three cultivars of grape. |
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