Exogenous cysteine oxidation induced secondary browning of light-colored glycine-xylose Maillard reaction products through promoted formation of glyoxal and methylglyoxal

The browning degree in light-colored Maillard reaction products (L-MRPs), derived from the glycine-xylose system with cysteine (Cys) addition increased during storage. This secondary browning reaction might compromise their long-term stability. To elucidate the underlying mechanism, accelerated storage and model systems (glycine-xylose Amadori compound (GX-ARP) and GX-ARP/cystine) were employed. The findings demonstrated that cysteine undergoes oxidative conversion to cystine, triggering adverse changes including promoted degradation of GX-ARP and retro-aldolization of deoxyosones (DXs) into glyoxal (GO) and methylglyoxal (MGO), thereby accelerating secondary browning. Storage tests with different Cys levels indicated low concentrations insufficiently inhibited browning, while high levels exacerbated darkening from cystine accumulation, and 2 mmol/L Cys achieved optimal balance between browning inhibition and minimized side reactions. In conclusion, the conversion of Cys to cystine and its interference in the GX-ARP degradation pathway cause the instability of L-MRPs. This finding provides a novel theoretical basis for improving the color stability of L-MRPs.