Study And Analysis On Storage Characteristics And Metabolism Differences Related To Husk Browning In Different Cultivars Of Pomegranate

Shannxi province is one of the main productions of pomegranate in China,which cultivates and breeds various varieties.Different cultivars exhibit different characteristics during low temperature storage.Browning is an important physiological change which affects the quality of pomegranate during cold storage and seriously affects its merchantability and storage resistance.At the present study,‘Tunisia soft seed’,‘Jingpitian’and‘Lishanhong’cultivars from Shaanxi,China was used as raw materials.They were stored at 4±0.5℃and 85-90%relative humidity for 90 d which aimed to elucidate storability differences,the changes and differences of physiological active substances,microstructure changes and waxy layer structure changes of pomegranate husk as well as the mechanism of metabolism related to peel browning.By comparing the storage characteristics among the three cultivars,the study could provide theoretical basis for long-term storage of pomegranate.The results are as follows:1.During the cold storage,the changes of husk of Tunisia soft seed pomegranate were as follows:the browning index of reached 1.25 on the 90^th day of storage,the husk became thinner by 1.51 mm and the value L*andΔE*changed insignificantly.Under electron microscope and fluorescence microscope,it was observed that the husk maintained a relatively complete microstructure and waxy layer with few cracks,and no notch was found in the longitudinal section.The changes of husk of Jingpitian pomegranate were as follows:the busk was severely browned reached to 3.94,the thickness reduced by 1.10 mm,value L*decreased by 10.9%and valueΔE*increased by 38.1%.Under electron microscope,the cracks on the skin surface were wide and long,and gradually widened and deepened.Large pieces of waxy layer disintegrated and fell off.Longitudinal slices of husk showed that the husk was gradually fractured under fluorescence microscope.The changes of husk of Lishanhong pomegranate were as follows:the browning index was higher than that of Tunisia soft seed reached to 1.88 at 90^th of storage,the thickness of husk decreased by 1.02 mm but the value L*andΔE*changed insignificantly.Under microscope,the cracks in pomegranate husk were elongated and increasing,the waxy layer partially disintegrated,and notch was found in the longitudinal slices.2.Tunisia suffered from high moisture loss during the cold storage,reaching to 3.77%.The hardness of pomegranate seed of Jingpitian increased by 12.87 N,and the hardness of Jingpitian increased by 37.44 N.The contents of soluble solid,titrable acid and ascorbic acid of three kinds of pomegranate fruit decreased significantly in the middle and later stages of storage,total phenolic content decreased significantly,total flavone content increased first and then decreased,and anthocyanin content fluctuated.The contents of above substances in Tunisian soft seed pomegranate juice were higher than those in Jingpitian and Lishanhong pomegranate juice.There are 37 kinds of volatile compounds in Tunisian soft seed pomegranate juice,but 11 kinds of volatile compounds are not detected at late storage,while only 2 and 4 kinds of volatile compounds of Jingpitian and Lishanhong were not detected at late storage.3.The results active oxygen species and membrane lipid metabolism of husk showed that with the increase of browning index,the O₂·^-and OH free radical scavenging rate,superoxide dismutase（SOD）activity,glutathione,carotenoids and chlorophyll decreased significantly,while reactive oxygen species increased.Besides,the tissue electrolyte leakage and malondialdehyde content also increased,while the soluble protein content did not change significantly.Among them,the Tunisia soft seed pomegranate showed a lower degree of browning,and the changes of other indexes were clearly distinguished from Jingpitian and Lishanhong.The results of correlation analysis showed that the browning of pomegranate husk was negatively correlated with O₂·^-and OH free radical scavenging rate,hydrogen peroxide content,SOD activity,glutathione content and chlorophyll content of active oxygen metabolism and positively correlated with the O₂·^-production rate;positively correlated with tissue electrolyte leakage and malondialdehyde content of membrane lipid metabolism.Additionally,the results of principal component analysis further confirmed that the browning of pomegranate husk was closely related to the metabolism of reactive oxygen species and membrane lipid,and the storage quality of Tunisia soft seed pomegranate was obviously different from those of Jingpitian and Lishanhong.4.The results of phenolic and energy metabolism showed that:with the extension of storage time,the browning index increased,the DPPH radical scavenging ability decreased in the later stage,the ABTS⁺radical scavenging ability,total phenolic and total flavonoids contents fluctuated slightly,and the anthocyanin content increased first and then decreased.The PPO activity of Tunisia soft seed remained lowest,and PAL activity retained highest,while the POD activity of Jingpitian was the highest.In general,those phenolic metabolism indexes of Tunisia soft seed pomegranate husk performed better than those of Jingpitian and Lishanhong.In addition,the Tunisia soft seed pomegranate husk maintained higher ATP,ATP contents,energy charge,and higher Mg²⁺-ATPase and H⁺-ATPase activities during storage,while the change of energy state of Jingpitian and Lishanhong was similar and performed worse than Tunisia soft seed.The results of correlation analysis showed that pomegranate husk browning was positively correlated with the contents of total phenolic,total flavonoids,POD and PAL activities of phenolic metabolism,negatively correlated with PPO activity but not significant.It was also positively correlated with the contents of ADP,AMP and Mg²⁺-ATPase activities in energy metabolism.The results of principal component analysis further confirmed the correlation between pomegranate husk browning and phenolic metabolism and energy metabolism indexes.In conclusion,Tunisia showed better husk structure and fruit quality under long-term cold storage.In contrast,the husk of Jingpitian pomegranate was less tolerant to storage under long-term refrigeration,but the contents of bioactive substances changed insignificantly during storage.The storage quality changes of Lishanhong pomegranate were similar to those of Jingpitian.Active oxygen metabolism and phenol metabolism were the main metabolisms that caused the difference of pomegranate husk browning.