Changes Of Pectin Structure And Metabolism Upon Texture Deterioration In Citrus Fruit During Postharvest Storage

Citrus is one of the most widely grown and output fruit species in China,which gains consumers’appreciation due to its numerous nutrients and pleasant flavor.The development of the citrus industry is of great significance for ensuring a stable supply of fresh fruits and vegetables,promoting the development of agriculture and rural areas,and helping the country fully implement the"village revitalization"strategy.However,deterioration of citrus nutrients and flavor often occurs during postharvest storage,which is largely due to the fruit senescence,and they are sometimes unbearable for consumption.This is in stark contradiction with the requirement to transform the consumption structure of fresh fruits in my country from"enough"to"delicious"in the new era,restricting the high-quality development of the citrus industry.Therefore,studying the quality deterioration of citrus during postharvest storage is an urgent demand for industrial development.Fruit texture is one of the important indicators to measure the quality of citrus fruit,and it is closely related to changes in cell wall material content and fruit moisture content.The deterioration of pulp texture represented by granulation and the deterioration of the cortex during storage are common post-harvest citrus fruit texture deterioration,which seriously affects the quality of post-harvest citrus.Controlling fruit texture,improving fruit quality and postharvest storage are long-term focus in the field of postharvest biology.More than 30 years’research has revealed that pectin was closely related to fruit texture.Therefore,this study was designed to investigate the mechanism underlying the texture deterioration of the pulp and peel of citrus fruit during postharvest senescence from the perspective of pectin.This paper has carried out the following research work.First,the research was designed to employ the pomelo fruit as material to systematically analyze dynamic changes in fruit quality,pectin content,enzyme activities relating to pectin metabolism,calcium content,pectin nanostructures,with the aim to elucidate the relationship between the pectin and texture deterioration of granulated juice sacs.Second,based on the results of the first part of the study that calcium ions may positively regulate granulation,we explored and established an in vitro culture system for pomelo juice sacs,then treated juice sacs with exogenous calcium,further elucidate the effect of calcium on the inducing process of juice sacs granulation,and the influence on the content of pectin substances and the activity of pectin metabolizing enzymes were further analyzed.Third,using Satsuma Mandarin as materials,systematically compared the effect of stalk on fruit texture during postharvest storage,and clarified the relationship between pectin and the deterioration of fruit peel.The main results were as follows:（1）Granulated juice sacs were not at the same physiological state,while they were highly heterogeneous.For example,there were at least three types of juice sacs in granulated pomelo,which were expanded,gelated and lignified,respectively.These different types of juice sacs may represent the dynamic granulation progress of citrus fruit.（2）In this study,the juice sacs in different states during the granulation process were used as experimental materials.First,the nutritional and sensory qualities of the juice sacs were analyzed,and the results found that:TSS,TA,As A,and sensory qualities of juice sacs showed a sharp decrease since gelation stage and then through the whole granulation process.Among them,the chewiness of granulated juice sacs increased,and the taste became dry and rough.Although dry mouthfeel was a typical feature of the citrus granulation,the relative water content was relatively constant in the juice sacs at different granulation stages,and the juice yield of the juice sacs gradually decreased as the granulation progresses.Contents of WSP,CSP,SSP increased during granulation process,especially since the gelation stage.Taken together,it could be inferred that during the granulation process,the dry mouthfeel and the decreased of juice yield were caused by the formation of gel in juice sacs and the trap of free water.（3）To further analyze the activityies of pectin metabolism related enzymes,the results showed that the PME activity increased and the PG activity decreased during granulation process,indicating that the high methylesterised pectin was demethyl-esterised by PME,the resultant low methylesterised pectin did not enter the degradation pathway as the decreasing of PG activity.The results of the atomic absorption experiment showed that the calcium content increased significantly during the granulation process,especially since the gelation stage,indicating that the de-methylesterised pectin molecules cross-linked with calcium to form"egg box"structures,evidenced by the increase of CSP content.The AFM analysis suggested that,both length and width of three types of pectin increased upon granulation process,linear structure of pectin decreased and pectin polymers increased.It is suggested that the depolymerization and degradation of pectin were inhibited during citrus granulation,and the network structure formed by pectin in the cell wall was remodeled.The remodeling of the pectin network may occur in the early stage of the granulation process,and it participates in trapping the free water of the juice sac and forming a gel,resulting in deterioration of texture.At the same time,the gelation of juice sacs may result in a physiological drought stress to the cells of juices sacs,which may further stimulate the lignification process of juices sacs.Taken together,remodeling of the pectin network was likely an important driving factor for the occurrence of granulation,which regulated the deterioration of pulp texture during the process of juice sacs granulation.（4）The low methylesterised pectin could be cross-linked with the calcium to form an"egg box"structure upon granulation,and then the pectin structure network was remodeled.It suggested that Ca²⁺was a positive regulator of granulation,which induced the deterioration of pulp texture during the granulation process..In order to verify this analysis,this study established an in vitro juice sac culture system and analyzed the effect of exogenous calcium ion treatment on juice sacs.The results showed that exogenous calcium,including 25 m M,50 m M,75 m M and 100 m M,could be able to induce the granulation process of juice sacs.Since calcium treatment,the degradation of WSP and CSP in juice sacs was inhibited,while the SSP content increased.The increase in PME enzyme activity indicated that calcium treatment may activate PME enzyme activity,then reduce the degree of pectin methylesterification modification,and facilitate the cross-linking of pectin with calcium ions to form an"egg box"structure,and induced the gelation of juice sac,and the texture became harder.（5）During the storage period of citrus fruit,the firmness of normal fruit showed a significant decreasing trend,while the firmness of the no-stalk fruit was significantly lower than that of normal fruit since the early storage period.The relative water content and weight loss rate were measured and found,the relative water content of the peel decreased significantly,and the weight loss rate increased significantly during the storage of citrus fruit.However,removal of stalk increased the water loss of the citrus fruit peel and increased the weight loss rate.The contents of WSP and CSP in the peel of citrus fruit gradually increased during storage,and the content of WSP and CSP of no-stalk fruit peel increased much more dramatically;the SSP content in the peel of normal fruit peel gradually increased,but the SSP content of the no-stalk fruit peel showed a decreasing trend since the 5th day of storage.This may be the reason for the significant decreased in firmness of the no-stalk fruit in the storage period;the content of soluble pectin increased during the storage stage,and the increase in no-stalk fruit peel was more significant,while the propectin content gradually decreased during fruit storage.We further analyzed the activity of pectin metabolism related enzymes of citrus fruit peel during the postharvest storage.The results showed that both the PME activity and the PG activity of the citrus fruit peel increased significantly,and the PME activity and PG activity of the peel of non-stalk fruit peel exhibited a greater increase trend.Therefore,as the key tissue for maintaining the quality of postharvest citrus,the fruit stalk was able to effectively alleviate the deterioration of the peel texture of the postharvest citrus mainly via both preventing water loss and inhibiting the degradation of pectin,finally maintain the fruit quality during postharvest storage.In summary,the substantial texture deterioration of postharvest citrus was closely related to the pectin metabolism,the present study unraveled the mechanism underlying texture deterioration of both pulp and peel of citrus fruit,mainly via the pectin content,structure and metabolism.These results provide invaluable information for the citrus industry to improve the strategy of postharvest storage of citrus fruit.