A first approach towards the relationship between grape skin cell-wall composition and anthocyanin extractability

Abstract

The composition of skin cell-walls from four different grape varieties (Vitis vinifera L., cv. Cabernet Sauvignon, Merlot, Syrah and Monastrell, the later harvested in three different locations) has been studied, trying to correlate those differences found in cell-wall composition among varieties with their degree of ripeness and with the easiness of anthocyanin extractability. The results of an anthocyanin extractability assay showed that the anthocyanins from Monastrell grape skins might be more difficult to extract than those from the other varieties. The study of the cell-wall composition showed some differences in uronic acids, cellulosic glucose, proteins and polyphenol content of cell-walls among varieties, and also differences between the Monastrell grapes cultivated in different locations were detected. Little differences could be found among the different samples regarding the non-cellulosic neutral sugars. In trying to look if differences in the extractability index were related to cell-wall composition and to help to clarify some of the mechanism involved in the anthocyanin extractability, a multiple regression analysis was conducted. The results showed that a model could be built up explaining a high percentage of the variability in the extractability index, using the content of the different components of the skin cell-wall as independent variables. Following this approach, the differences on the easiness of anthocyanin extractability in grape samples could be based in differences in pectin and cellulose content, but also, differences occurring on arabinoxylan, arabinogalactan and xyloglucan could be of importance.

Introduction

Anthocyanins are the compounds responsible for wine color. They are located in the skin. Inside the cells, they are located in the vacuoles, in a free, non-complex form [1], [2], [3], [4] and they diffuse into the must and wine during the maceration step in winemaking. Evidently, the barrier for these compounds is the skin cell-wall. These cell-walls are highly complex and dynamic, composed of polysaccharides, phenolic compounds and proteins, and stabilized by ionic and covalent linkages. Hemicellulose, pectins and structural proteins are inter-knotted with the network of cellulose microfibrils, the skeleton of cell-walls [5]. In grape cell-walls, cellulose and pectins accounts 30–40% of the polysaccharide components of the cell-wall [6].

Extraction of anthocyanins during the maceration process requires that the pectin rich middle lamella to be degraded to release the cells, and the cell-walls to be broken to allow their vacuole contents to be extracted, or to diffuse into the wine [4]. The extraction of anthocyanins from the grape into the wine is, therefore, essentially a diffusion process, and the rate and extent of extraction is influenced by the grape anthocyanin concentration, the composition of berry cell-walls and processing methods. However, experience has shown that highly colored grapes do not necessarily produce high colored wines, being any difference probably related with the easiness of anthocyanins to be extracted from grape skins into musts [4].

The structural properties of the cell-walls of the different varieties may determine the mechanical resistance, the texture and the ease of processing berries [7]. The loosening of cell-walls is realized by the breakdown of chemical bounds among the structural components [5], therefore, changes in the cell-wall polysaccharide structure could affect the solubility and the tissue disassembling mechanisms [8].

In this study, the composition of skin cell-walls from four different grape varieties (Vitis vinifera L., cv. Cabernet Sauvignon, Merlot, Syrah and Monastrell, the later harvested in three different locations) has been studied, trying to correlate those differences found in cell-wall composition among varieties with their degree of ripeness and with the easiness of anthocyanin extractability, calculated with an extractability assay developed by Saint-Cricq de Gaulejac et al. [9].