Ultrasound-Assisted Recovery of Pectic Fractions from Grape Pomace: Combined Effects of Acid Type, pH, and Temperature on Physicochemical and Antioxidant Properties

Grape pomace represents an underexploited source of pectic polysaccharides whose properties are strongly influenced by extraction conditions. This study investigated the combined effects of acid type (citric acid or HCl), pH (1.5, 2.0 and 3.0), and temperature (55, 65 and 85 °C) on pectic fractions recovered from grape pomace using sequential ultrasound-assisted extraction followed by dynamic maceration. A 2 × 3 × 3 full-factorial design was applied, combining citric acid or hydrochloric acid, three pH values (1.5, 2.0 and 3.0) and three temperatures (55, 65 and 85 °C), resulting in 18 extraction conditions. The recovered fractions were characterized in terms of extraction yield, degree of esterification (DE), methoxyl content (MeO), anhydrouronic acid content (AUA), molecular weight (Mw), apparent viscosity, carbohydrate and protein contents, color, FTIR profile, total phenolic content (TPC) and antioxidant activity. Extraction yield ranged from 1.12 to 7.88%, with the highest recovery obtained using citric acid at pH 3.0 and 55 °C. DE varied markedly from 14.89 to 63.63%, with citric acid consistently yielding low-methoxyl fractions under all conditions, whereas HCl at pH 1.5 yielded high-methoxyl fractions. MeO varied accordingly from 1.66 to 7.65%, while AUA remained within a comparatively narrow range of 63.27–69.24%, close to that measured for commercial citrus pectin (67.21%). Molecular weight ranged from 22.7 to 607 kDa, and increasing extraction temperature generally reduced both Mw and apparent viscosity, indicating greater structural modification under harsher conditions. FTIR spectra confirmed the preservation of the characteristic pectic polysaccharide framework while revealing differences in the carboxyl and fingerprint regions among extraction conditions. The recovered fractions contained 180.22–310.78 mg glucose eq./g total carbohydrate and 8.56–32.83 mg BSA eq./g protein. Phenolic compounds were also retained, with TPC values of 9.50–14.29 mg GAE/g and ABTS IC50 values ranging from 4.26 to 10.38 mg/mL. The factorial analysis demonstrated significant interactions among acid type, pH, and temperature for the main physicochemical and functional responses. Overall, the results demonstrate that these extraction variables can be adjusted to recover grape pomace pectic fractions with distinct esterification, molecular and antioxidant profiles.