ANTHOCYANINS EXTRACTION FROM GRAPE POMACE USING EUTECTIC SOLVENTS

Abstract

Grape pomace is one of the main by-products generated after pressing in winemaking.Emerging methods, such as ultrasound-assisted extraction with eutectic mixtures, have great potential due to their low toxicity, and high biodegradability. Choline chloride (ChCl) was used as a hydrogen bond acceptor and its corresponding hydrogen bond donor (malic acid, citric acid, and glycerol: urea). Components were heated at 80 °C and stirred until a clear liquid was obtained. Distilled water was added (30 % v/v). A solid-liquid ratio of 1 g pomace per 10 ml of eutectic solvent was used. Total anthocyanins were determined. Malvidin-3-O-coumaroylglucoside was quantified by HPLC. Metabisulfite bleaching and the viscosity of the extracts were also determined. The highest extraction was obtained for the methanol/water system.The eutectic system that showed the highest extraction was the mixture of choline chloride, urea, and glycerol in a molar ratio of 1:1:1. Glycerol is classified as a polyol. It can modify the polarity of water so it can be used as a co-solvent in the extraction of polyphenols. In addition, it is considered a highly flexible molecule, capable of forming intra- and intermolecular hydrogen bonds [1]. The higher extraction of choline chloride glycerol and urea (molar ratio 1:1:1) could be due to the influence of the lower polarity of glycerol presenting a higher affinity, probably with malvidin-3-O-coumaroylglucoside, which is less polar than Mv-3-O-glc. In HPLC analysis, malvidin-3-O-coumaroylglucoside was the main anthocyanin identified in all extracts.In eutectic mixtures, viscosity is the property that limits the extraction process compared to conventional solvent extractions. The extract obtained with the choline chloride: urea: glycerol (1:2:2) system had the lowest viscosity values, while the rest of the extracts presented higher viscosities. Viscosity reflects how compact a molecular structure is. Therefore, it can be inferred that the systems with malic acid and citric acid with choline chloride in molar ratios 1:2 present a compact molecular structure with a minimum of holes, which results in less diffusion during the extraction process. The choline chloride: malic acid (1:1) system presented significant resistance to sulfite bleaching at pH 3.5, losing approximately 34 % of color. The choline chloride: urea: glycerol (1:1:1) system lost approximately 50 % of the color, presenting a lower resistance to discoloration.

1. A. P. Abbott, R. C. Harris, K. S. Ryder, C. D’Agostino, L. F. Gladden, and M. D. Mantle, “Glycerol eutectics as sustainable solvent systems,” Green Chem., vol. 13, no. 1, pp. 82–90, 2011