Do enzymatic or non-enzymatic pathways drive the postharvest darkening phenomenon in carioca bean tegument?

• Darkening is mostly determined by an increase in a* value, while b* often masks reddish of tegument.
• Total phenol content is not a sufficient parameter to identify genotypes most prone to dark.
• Final tegument color is affected by the phenolic profile and not by its amount.
• Principal response curves indicate different pathways for the darkening process.
• Darkening phenomenon may be enzymatic or non-enzymatic, and even a combination of both process.

Four genotypes of carioca bean were accelerate aged (40 °C/75% relative humidity) and evaluated to determine if differences in postharvest darkening trait was an enzymatic or non-enzymatic process. Chromaticity a* was the colour parameter with major alterations, and increase in chromaticity b* often masked the reddish of tegument. Peroxidase activity was not detected. Polyphenoloxidase remained active along the storage time, but its activity was higher in the lighter genotypes. Flavonoid, proanthocyanidin and total phenol content were much higher in darker genotypes, although just this last component presented significant alterations. Genotypes showed different susceptibilities to the darkening independent of phenolic content. Results of principal response curves analyse suggested different pathways for the darkening process: darkening in lighter genotypes seem to be mostly due to polyphenoloxidase activity while in dark ones there are the combination of enzymatic and non-enzymatic oxidation.