Fractionation of Winemaking Grape Stalks by Subcritical Water Extraction to Obtain Added-Value Products

Abstract

Grape stalks (GSs) from winemaking were submitted to a green process to valorise its lignocellulosic biomass that applied subcritical water extraction (SWE) at 170 °C and 180 °C to obtain active extracts and cellulose-enriched fractions. The sum of the total phenolic content of the soluble extract and the solid residue fractions from the SWE exceeded that of the GS, which suggests the generation of compounds with antioxidant properties through SWE. All SWE fractions showed high antioxidant power. The increased temperature promoted the extraction of polyphenolic compounds, enhancing the antioxidant power of both extracts and solid residues. These solid residue fractions were bleached with alkaline hydrogen peroxide solutions (4 and 8% v/v) to purify cellulose. After two bleaching cycles, no notable delignification progress was observed, as the bleaching yield or whiteness index did not significantly change in the further cycles. The first bleaching cycle led to a significant reduction in the lignin content at both SWE temperatures. The cellulose purity was higher in the samples obtained at 170 °C and bleached with 4% alkaline hydrogen peroxide. SWE at 180 °C led to greater cellulose oxidation during the bleaching step regardless of the hydrogen peroxide concentration.

Keywords: antioxidant; cellulose fibres; hydrogen peroxide bleaching; hydrothermal treatment; integral valorisation; lignocellulosic waste; phenolic compounds.

Figure 1

Flow chart of the process applied to valorise grape stalks. The Y-values represent the mass yield (dry out-flow mass with respect to the dry in-flow mass) of the different fractions obtained in the subcritical water extraction process at 170 °C and 180 °C. Mean values ± standard deviation. Insoluble residues (R-170, R-180) and extracts (E-170, E-180).

Figure 2

(a) Bleaching yield (BY) and whiteness index (WI) obtained with the different one-hour treatment cycles with alkaline hydrogen peroxide at 4 and 8% v/v. The LSD intervals are 2.22 for BY and 0.03 for WI. (b) Images of the raw material, subcritical water extraction (SWE) residues at 170 °C (R-170) and 180 °C (R-180), and bleached residues (BR) at the different steps of the bleaching process. Bleached residues are coded as the BR temperature of SWE concentration (% v/v) of alkaline hydrogen peroxide number of 1 h bleaching cycles.

Figure 3

Cellulose, lignin, and ash content of residues after subcritical water extraction (SWE) at 170 °C (R-170) and 180 °C (R-180) and after two bleaching cycles (C1 and C2) with hydrogen peroxide solutions at 4 and 8% v/v. (a) Results for SWE at 170 °C (LSD intervals: 13.04 for cellulose, 8.10 for lignin, and 2.49 for ash). (b) Results for SWE at 180 °C (LSD intervals: 23.95 for cellulose, 11.63 for lignin, and 0.88 for ash).

Figure 4

FESEM images of cellulose fibres after two bleaching cycles with 4% or 8% (v/v) alkaline hydrogen peroxide solutions (BR-170-4, BR-170-8, BR-18-4, and BR-180-8), compared with non-bleached fibres from subcritical water extraction at 180 °C (R-180) and grape stalk (GS) samples.

Figure 5

FTIR spectra of grape stalk (GS), non-bleached samples (R-170 and R-180) and samples bleached with 4 or 8% v/v alkaline hydrogen peroxide solutions (BR-170-4, BR-170-8, BR-180-4, and BR-180-8) after one and two bleaching cycles (1C–2C).

Figure 6

XRD spectra and crystallinity index (CI) of the grape stalk (GS), non-bleached samples (R-170 and R-180) and the two-cycles-bleached samples (BR) under different conditions (4 or 8% v/v alkaline hydrogen peroxide solutions).

Figure 7

Thermogravimetric analysis (TGA) and DGTA curves of the grape stalk (GS), non-bleached subcritical water extraction residues (R-170 and R-180) and the bleached residues (BR) after one and two bleaching cycles with 4% or 8% (v/v) alkaline hydrogen peroxide solutions. Bleached residues are coded as BR temperature of subcritical water extraction concentration (%) of hydrogen peroxide-number of 1 h bleaching cycles.