Advancing anthocyanin extraction: Optimising solvent, preservation, and microwave techniques for enhanced recovery from merlot grape marc

Grape marc, a by-product of winemaking, is a rich source of bioactive compounds, yet efficient extraction methods suitable for industrial application remain underexplored. This study presents an integrated, three-stage approach to optimise the extraction of anthocyanins, phenolics, and tannins from Merlot grape marc. In the first stage, 12 solvents were evaluated using conventional solvent extraction, with 50 % ethanol (EtOH) acidified with hydrochloric acid (HCl) achieving the highest anthocyanin recovery after eight extraction cycles (0.66 g/kg of grape marc), followed by formic acid (0.59 g/kg) and citric acid (0.58 g/kg) treatments. The second stage assessed drying methods across eight temperatures combined with a single extraction cycle using 50 % EtOH HCl, identifying 70 °C as the optimal heat-drying condition (1.23 g/kg total anthocyanins, 0.4 g/kg monomeric anthocyanins). Freeze-drying at -105 °C (TN105) with a prewash step (SRT105) further enhanced anthocyanin yields (2.24 g/kg total anthocyanins, 0.69 g/kg monomeric anthocyanins). In the final stage, microwave-assisted extraction significantly increased recovery, with SRT105-MW in 50 % EtOH HCl yielding 8.07 g/100 g total phenolics, 5.76 g/100 g tannins, 3.7 g/kg total anthocyanins, and 2.8 g/kg monomeric anthocyanins. This optimised method preserved anthocyanin composition, including malvidin- and peonidin-3-glucosides (585 and 560 mg/kg, respectively), along with cyanidin-, delphinidin-, and petunidin-3-glucosides (463, 360, and 257 mg/kg, respectively), as well as 66-99 mg/kg of acylated and 37-60 mg/kg of coumaroylated anthocyanins. Citric acid (50 % EtOH CA) demonstrated potential as a sustainable alternative, achieving ∼90 % of the anthocyanin yield of HCl treatments. These findings offer a practical, scalable framework for industrial anthocyanin recovery, advancing sustainable utilisation of grape marc.