Ethanol and citric acid: an efficient alternative for extracting anthocyanins and proanthocyanidins, and co-pigments associated with thermostability from Syrah grape pomace

Grape pomace, a by-product from the wine industry with a high content of anthocyanins and proanthocyanidins, is considered a potential therapeutic resource. However, conventional extraction of these compounds often involves high concentrations of acidified methanol and acetone. This study aims to identify conditions that may enhance anthocyanins and proanthocyanidins contents by reducing the use of methanol and acetone and improving their stability, and identifying co-pigments related to this stability of these compounds. Syrah grape pomace was obtained, and different proportions of methanol or ethanol: water, acidified water and acetone: water were evaluated in the extraction process. Anthocyanins and proanthocyanidins concentrations were similar between methanol and ethanol. As the solvent concentration increases, the extraction of proanthocyanidins also enhance. In the next stage of extraction, only ethanol was used due to its lower toxicity compared with methanol. Additionally, the proportion of acetone and water was modified, and the results showed that samples acidified with citric acid/acetone: water (EW_C/AW 5:5/5:5) and (EW_C /AW 5:5/7:5) improved total phenolic compounds content by 5–19%, anthocyanins by 56–87%, and proanthocyanidins by 8–17% compared to extraction using methanol (MW_HCl/AW 5:5/5:5). Additionally, with the HPLC results, the PCA grouped in these extracts was characterized by 3-(4-dihydroxyphenyl) propionic acid, gallate epicatechin, ellagic acid, and malvidin 3-O-glucoside. Both extracts showed high antioxidant capacity, low browning index and high concentration at 60 °C, 80 °C, and 100 °C. Malvidin 3-O-glucoside, 3-(4-hydroxyphenyl) propionic acid, and ellagic acid were identified as tentative co-pigments. Citric acid-acidified ethanol is an alternative for extracting EPPs and it shows greater stability than the conventional methodology.