Characterization and stability analysis of anthocyanins from Pachyrhizus ahipa (Wedd) Parodi roots

Abstract

Pachyrhizus ahipa is an Andean leguminous plant which has purple-pigmented edible roots. The content and distribution of the phenolic compounds in the skins and flesh of P. ahipa roots as well as their antioxidant capacity were studied. Total phenols, flavonoids, tannins and anthocyanins were quantified and the effect of the extraction solvent on the maximum absorbance wavelength (λvis-max) was measured. The ATR-FTIR spectrum and stability of the purified anthocyanin extracts were analyzed. Individual anthocyanins were partially characterized using high-performance liquid chromatography-mass spectrometry and absorbance spectra. Ahipa skins had much more phenols (3.25 mg gallic acid equivalents/g), flavonoids (3.0 mg catechin equivalents/g) and tannins (4.6 mg tannic acid equivalents/g) than the flesh, which was reflected in a higher antioxidant capacity. Changing the extraction solvent from acidified methanol to acidified water shifted the λvis-max 16 nm lower, suggesting copigmentation reactions. Anthocyanin in the skins was >8.7 content in the flesh, and the chromatographic analysis showed at least 6 different anthocyanins with identical retention times for skins and flesh, including delphinidin, pelargonidin and cyanidin as aglycons. The absorbance spectra showed that two of them are acylated structures. Pigments were stable for at least 180 min at pH 1 and 25 °C, but they were affected by increased pH and temperature. These anthocyanins may be a potential source of natural pigments.