Combined effects of sunlight and tempering treatment on the oligomeric procyanidin formation in dried ume (Prunus mume Sieb. et Zucc.)

In this study, the combined effect of sunlight irradiation and tempering treatment on the promotion of oligomeric procyanidins (OPCs) formation in the dried ume (Prunus mume Sieb. et Zucc.) was investigated and firstly reported. Water content, water activity, and chemical structure of the OPCs were analyzed during the drying process. Two nights of tempering treatment reduced the water content and water activity significantly. Two major OPCs, procyanidin B2 and procyanidin C1 were characterized in sun-dried ume and the (-)-epicatechin model by both the positive and negative ionization modes using an electrospray ionization source (ESI). The concentration of procyanidin B2 and procyanidin C1 increased by 1.76 and 1.24 folds, respectively in the intermittently sun-dried ume. The developed photocatalytic closed-type dryer was used in this experiment confirmed successfully that tempering treatment enhanced the moisture redistribution and precursor accumulation leading to the subsequent photooxidation of (-)-epicatechin for OPCs. To minimize the drawback of outer drying under climatic change this is a possible protocol design with effective drying technology to improve the quality and mass production of the functional umes.