Polyphenolic profile and dietary fiber content of skins and seeds from unfermented and fermented grape pomace

Abstract

The valorization of winemaking byproducts is subordinated to the knowledge of their chemical characteristics. This work concerned the determination of the polyphenolic profile and the dietary fiber content of skins and seeds from unfermented and fermented pomace of different cultivars (Moscato bianco, Cortese, Arneis, Pinot Noir, Barbera, Grignolino, Nebbiolo), sampled from some wineries in the Piedmont area (Italy) during the 2020 harvest.
The pomace was dried under controlled conditions (pre-drying for 48 h at room temperature in a ventilated and dehumidified room, followed by drying at 40 °C for 48 hours, up to constant weight). Before drying, the unfermented pomace was washed with water (3 successive washes) to remove residual sugars. Once dried, the skins were separated from the seeds; the 2 fractions were then separately milled to obtain skins and seeds flours.
The determination of the polyphenolic composition of the flours was carried out on the extracts obtained with a double extraction: first with methanol/water 50:50 %v/v and then with acetone/water 70:30 %v/v. The content of total polyphenols (GAE), total flavonoids and condensed tannins was determined in the extracted liquid solution (EPP, extractable polyphenols), while the fraction of non-extractable condensed tannins was determined in the residue after extraction (NEPP, non-extractable polyphenols), with the method proposed by Saura Calixto et al., 1991 (modified). The percentage content of dietary fiber (DF) of the flours was also determined with the official method AOAC 985-29. Limited to the skins samples, the analysis was extended to the distinction between soluble and insoluble dietary fiber (SDF and IDF).
Significant differences in the EPP content were observed between skins (GAE values ranging between 21.5 and 35.6 mg/g of flour, dry weight) and seeds (GAE values between 28.7 and 94.3 mg/g d.w.). The skins had lower GAE values, (from 30.0 % to 75.6 %) than the respective seeds. The seeds with the highest polyphenolic content were, in decreasing order, unfermented Pinot Noir, Moscato bianco, fermented Pinot Noir and Cortese. The tannins concentration was overall higher for unfermented seeds, with important differences between cultivars.
Conversely, the NEPP content was higher in the skins, 2.6 to 4.4 times higher than seeds. The content in polyphenols linked to the dietary fiber was more homogeneous among the different seed samples (3.41 to 4.93 mg/g d.w. compared to the skins (between 11.6 to 18.4 mg/g d.w).
Finally, significant differences between skins and seeds were found in the total DF content. The seeds had total DF contents (52.0 -65.6 % d.w.) 1.1 to 1.3 times higher than the skins. The winemaking practices have influenced the quantity and characteristics of the DF in the skins (SDF and IDF fractions): the total and insoluble DF content was higher in the flours from unfermented pomace than in the fermented ones.