Study on the browning mechanism of apple juice based on untargeted metabolomics

Browning of apple juice is a major quality defect that occurs during storage, yet the molecular basis of the browning process remains unclear. This study utilised an untargeted metabolomics approach to investigate the untargeted metabolomic differences in Xinjiang Aksu sugarheart apple juice before and after 100 days of storage. Employing high-resolution liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF/MS), we identified 6 264 metabolites, with 1 588 significantly upregulated and 1 158 downregulated. Multivariate statistical analysis, including principal component analysis (PCA) and orthogonal partial least squares-discriminant analysis (OPLS-DA), revealed that storage time was the primary factor affecting metabolic differences (PCA1 : 55.54%; OPLS-DA Q2Y = 0.971). Key findings suggest that browning is triggered by enzyme activation through tyrosine metabolism activation (substrate supply) and dopaquinone accumulation. Changes in transmembrane transport by ABC transporters also contribute to this process. Non-enzymatic browning is exacerbated by Maillard intermediate products and lipid peroxidation products. Simultaneously, disrupted glutathione metabolism and antioxidant system failure lead to redox imbalance. KEGG enrichment analysis indicated coordinated changes in phenylpropanoid biosynthesis (secondary metabolic polymerisation), alkaloid metabolism, and the pentose phosphate pathways. These results suggest that oxidative stress, cell membrane damage, and polyphenol metabolism disturbances are key drivers of apple juice browning, offering a molecular foundation for quality control in apple juice production.