Metabolomics analysis reveals the non-enzymatic browning mechanism of green peppers (Piper nigrum L.) during the hot-air drying process

Non-enzymatic browning (NEB) reduced the colour quality of hot-air dried peppers, but the specific mechanism remains unclear. This may be related to the degradation of chlorophyll, ascorbic acid, and polyphenols. The findings revealed that the surface of pepper gradually browned during hot-air drying, with the ΔE^⁎ value and browning degree (BD) significantly increasing by 119.4 % and 62.9 %, respectively. However, the total phenol content (TPC) and ascorbic acid content decreased by 74.2 % and 84.3 %, respectively. TPC was negatively correlated with BD (R = -0.86), and its value (31.7 %) was 300 times higher than that of other colour-related components (ascorbic acid, chlorophyll). UPLC-MS/MS analysis further identified 345 polyphenols, among which 1, 3-dicaffeoylquinic acid and 5,7-dihydroxy-3', 4', 5'-trimethoxyflavone were the two key monophenols influencing NEB (R = -0.87). The results suggested that NEB pathways involve chlorophyll and ascorbic acid degradation, the Maillard reaction, and polyphenol degradation, with the latter playing a major role.