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.