Stability of Anthocyanins in Grape Pomace Extracts under Different pH and Temperature Conditions

Anthocyanins are water-soluble phenolic pigments widely used as natural colorants and recognized for their functional properties. Among the main factors affecting their stability, pH and temperature stand out. This study investigated the influence of these parameters on an aqueous extract of grape pomace obtained by probe ultrasound with recirculation, using 50% ethanol as solvent (solid:liquid ratio 1:20) for 90 minutes. The extracts were concentrated in a rotary evaporator to remove ethanol and subsequently adjusted to pH 2.5, 4.5, and 6.5 using HCl (1N) and NaOH (1M). Samples were subjected to temperatures of 5, 35, 55, and 80 °C for up to 240 minutes, with aliquots collected at different times for analysis. Determinations included monomeric anthocyanins (differential pH method), total phenolic content (Folin–Ciocalteu), antioxidant capacity (DPPH and ABTS), and color (L, a, b*). At pH 2.5, anthocyanin content remained relatively stable, with approximately 90% preservation after 240 minutes at 5 and 35 °C. However, preservation decreased to 72% and 51% at 55 and 80 °C, respectively. At pH 4.5, a similar trend was observed, although retention dropped to below 60% at highest temperatures. At pH 6.5, degradation was more pronounced, with only 57% preservation at 30 °C and further reductions to 40% and 30% at 55 and 80 °C, respectively. Overall, pH 2.5 conferred the greatest stability, attributed to acylation promoted by HCl, which provides additional protection compared to non-acylated anthocyanins. A similar pattern was observed for total phenolics and antioxidant activity value. In regard to color characterization, extracts at pH 4.5 and 6.5 showed ΔE values of 25.05 ± 0.36 and 29.06 ± 0.16, respectively, compared to pH 2.5, indicating visually perceptible changes. The luminosity parameter (L*) also demonstrated progressive darkening with increasing pH, decreasing from 10.30 ± 0.36 (pH 2.5) to 2.73 ± 0.08 and 2.18 ± 0.04 (pH 4.5 and 6.5). This darkening can be attributed to the formation of pigments derived from anthocyanin oxidation and polymerization. It can be concluded that high temperatures compromise anthocyanin stability at all pH conditions, while values closer to neutrality intensify degradation, reducing the preservation of bioactive compounds and color stability.