Stability of Hydroxycinnamic Acid Derivatives, Flavonol Glycosides, and Anthocyanins in Black Currant Juice

The stability of phenolic compounds was followed in black currant juice at ambient temperatures (in light and in dark conditions) and at +4 °C for a year. Analyses were based on high-performance liquid chromatography-diode-array detection-electrospray ionization-mass spectrometry (or tandem mass spectrometry) and high-performance liquid chromatography-diode-array detection-electrospray ionization-quadrupole time-of-flight mass spectrometry methods supported by nuclear magnetic resonance after selective high-performance liquid chromatography isolation. Altogether, 43 metabolites were identified, of which 2-(Z)-p-coumaroyloxymethylene-4-β-d-glucopyranosyloxy-2-(Z)-butenenitrile, 2-(E)-caffeoyloxymethylene-4-β-d-glucopyranosyloxy-2-(Z)-butenenitrile, 1-O-(Z)-p-coumaroyl-β-d-glucopyranose, (Z)-p-coumaric acid 4-O-β-d-glucopyranoside, and (Z)-p-coumaric acid were novel findings in black currant juice. Hydroxycinnamic acid derivatives degraded 20-40% at room temperature during one year of storage, releasing free hydroxycinnamic acids. O-Glucosides of hydroxycinnamic acid compounds were the most stable, followed by O-acylquinic acids, acyloxymethyleneglucosyloxybutenenitriles, and O-acylglucoses. Light induced the isomerization of (E)-coumaric acid compounds into corresponding Z-isomers. Flavonol glycosides stayed fairly stable. Flavonol aglycones were derived mainly from malonylglucosides. Over 90% of anthocyanins were lost at room temperature in a year, practically independent of light. Storage at low temperatures, preferably excluding light, is necessary to retain the original composition of phenolic compounds.