Analysis of the potential fading mechanism of sweet cherry after freezing and thawing using untargeted metabolomics

Freezing can prolong the shelf life of sweet cherries, while cherries will fade after thawing, which limits their further application. After thawing, the redness and chroma of cherries decreased due to anthocyanin degradation. The increase of hue angle and the decrease of total phenols indicated that browning occurred in sweet cherries after thawing. The micromorphology of sweet cherry skin deformed after freezing and thawing, leading to the destruction of cell structure. Metabolomics analysis showed that 37 differential metabolites (DMs) were up-regulated and 43 DMs were down-regulated after freezing, and 75 DMs were up-regulated and 104 DMs were down-regulated after thawing. Anthocyanins, phenols and flavonoids were the key DMs related to color change of cherries after freezing and thawing. Cyanidin-3-rutinoside might degrade to protocatechuic acid in β-glucosidase catalysis pathway. The main enrichment pathways involved in the fading after freezing and thawing of cherries were anthocyanin biosynthesis, biosynthesis of secondary metabolites and flavonoid biosynthesis. Therefore, the fading of sweet cherry after freezing and thawing was mainly attributed to anthocyanin degradation catalyzed by β-glucosidase and enzymatic browning of phenols catalyzed by polyphenol oxidase.