Effect of polysaccharide extracts from grape pomace on the oxidative evolution of hydroxycinnamic acids

Abstract

Phenolic acids are especially sensitive to oxidation, so they can greatly impact wine sensory characteristics and stability [1]. Furthermore, extracts derived from grape pomace have been previously postulated as possible oenological adjuvants for wine protection [2].

Thus, in this work, the oxidative evolution of the main three hydroxycinnamic acids (HA) present in wine (caffeic, p-coumaric and ferulic acids) was studied for two months by means of HPLC-DAD-MS [3], identifying and quantifying their degradation products. Ethanol-free wine like media (pH 3.4; 5 g/L tartaric acid; 4 mg/L FeCl₃; 0.4 mg/L CuSO₄) containing the HA (67 mg/L each) were oxygen-saturated with air. The absence of ethanol ensured oxygen availability for the oxidation of HA and oxygen consumption was monitored. Additionally, the effect of soluble polysaccharidic extracts (PS), obtained from white (WPS) or red grape pomace (RPS), on the evolution of the media was studied. These PS were fully characterized, and their antioxidant activity was assayed by the ABTS and FRAP methods [4]. The antioxidant activity of HA in the presence of PS was also studied and ITC experiments were performed to elucidate the nature of the interactions between HA and PS.

All HA decreased their concentration with time and oxygen consumption, especially in the presence of PS, which was contrary to the expected protective effect. Several degradation products of HA were identified, with a notable increase and subsequent degradation over time of the hydroxybenzoic acids derived from each respective HA. These degradation products could, therefore, serve as oxidation markers in wines rich in phenolic acids, such as white or rosé wines, which are especially susceptible to oxidation. Ferulic acid, followed by p-coumaric acid, greatly decreased in concentration, achieving a reduction of around 90% with PS addition (WPS and RPS). RPS showed greater antioxidant activity than WPS, and its presence in the HA evolution assays resulted in a greater degradation of HA.

PS acid hydrolysis and HPLS-DAD-MS analysis revealed the presence of polyphenolic residues that cannot be removed and that are only released after PS hydrolysis, including cyanidin and delphinidin derived from their proanthocyanidin precursors. In RPS, malvidin was also found, so the anthocyanin content could explain the difference in antioxidant activity.

In the presence of transition metals (like Fe₃₊ and Cu₂₊), some polyphenols can act as prooxidants [4], favouring the oxidative degradation of phenolic acids. In this experiment, the absence of other antioxidants in the matrix (such as free anthocyanins or tannins) can explain the faster degradation of HA, stimulated by the polyphenols in the PS, contrary to what it is believed to be the overall protective effect of polysaccharides seen in wine. The findings in HA degradation and their relationship with PS provides a deeper insight in wine oxidation and the molecular interactions in its matrix.

References

[1] Wirth, J.; Caillé, S.; Souquet, J. M.; Samson, A.; Dieval, J. B.; Vidal, S.; Fulcrand, H.; Cheynier, V. (2012). Food Chem., 132 (4), 1861–1871.

[2] Manjón, E.; Li, S.; Dueñas, M.; García-Estévez, I.; Escribano-Bailón, M. T. (2023) Food Chem., 400, 134110.

[3] García-Estévez, I.; Alcalde-Eon, C.; Escribano-Bailón, M. T. (2017). J. Agric. Food Chem., 65 (31), 6359–6368.

[4] Maurya, D. K.; Devasagayam, T. P. A. (2010) FCT., 48 (12), 3369–3373.