Retention of Proanthocyanidin in Wine-like Solution Is Conferred by a Dynamic Interaction between Soluble and Insoluble Grape Cell Wall Components

Abstract

For better understanding of the factors that impact proanthocyanidin (PA) adsorption by insoluble cell walls or interaction with soluble cell wall-derived components, application of a commercial polygalacturonase enzyme preparation was investigated to modify grape cell wall structure. Soluble and insoluble cell wall material was isolated from the skin and mesocarp components of Vitis vinifera Shiraz grapes. It was observed that significant depolymerization of the insoluble grape cell wall occurred following enzyme application to both grape cell wall fractions, with increased solubilization of rhamnogalacturonan-enriched, low molecular weight polysaccharides. However, in the case of grape mesocarp, the solubilization of protein from cell walls (in buffer) was significant and increased only slightly by the enzyme treatment. Enzyme treatment significantly reduced the adsorption of PA by insoluble cell walls, but this effect was observed only when material solubilized from grape cell walls had been removed. The loss of PA through interaction with the soluble cell wall fraction was observed to be greater for mesocarp than skin cell walls. Subsequent experiments on the soluble mesocarp cell wall fraction confirmed a role for protein in the precipitation of PA. This identified a potential mechanism by which extracted grape PA may be lost from wine during vinification, as a precipitate with solubilized grape mesocarp proteins. Although protein was a minor component in terms of total concentration, losses of PA via precipitation with proteins were in the order of 50% of available PA. PA-induced precipitation could proceed until all protein was removed from solution and may account for the very low levels of residual protein observed in red wines. The results point to a dynamic interaction of grape insoluble and soluble components in modulating PA retention in wine.

The Role of Soluble Polysaccharides in Tannin-Cell Wall Interactions in Model Solutions and in Wines

“…Our results showed that this material did not behave in the same way as the commercial polysaccharides, since it caused a substantial decrease in the concentration of skin tannins in solution (46.5%) and the formation of a precipitate. Similar findings were observed by Bindon et al [2], who noted that, in the interaction of the mesocarp or skin cell wall solubilized in the supernatant with tannins, the tannin concentration was reduced and a visible precipitate was formed. The cause of this reduction in tannin concentration and precipitate formation was mainly attributed by these authors to the formation of insoluble protein-tannin complexes and their consequent precipitation, especially in the case of mesocarp cell walls.…”

“…The final concentration of phenolic compounds in the must/wines is usually lower than what might be expected given the concentration measured in grapes. The reason for this is that, during the first stages of vinification, the large amount of vegetal material/cell walls in suspension arising from the degradation of the grape skin and flesh interacts and binds the extracted tannins [1,2]. In previous studies, our research group found that removing some of this suspended vegetal material before the skin maceration stage improved the final wine chromatic characteristics due to the increase of the phenolic content, especially in the concentration of tannins [3].…”

“…Only when the soluble polysaccharides are present do all the added tannins interact with them and the probability of precipitation of this soluble material increases. Bindon et al [2] stated that although pectic polysaccharides have the capacity to interact with tannins in solution to form a stable, soluble complex, coprecipitation of soluble material and tannins has also been observed probably resulting from complexes formed with the polysaccharides. Riou et al [50] reported that with Rhamnogalacturonan (II) dimers a co-aggregation with tannins was observed and that could lead to precipitation.…”

“…They are used with the objective of breaking down the structure of the cell walls to allow the extraction of the phenolic compounds located inside the cells but they could also contribute to deconstructing the polysaccharide network of the cell walls that remain in suspension during the maceration period [10,11], thus reducing their capacity to absorb tannins. Several studies have been carried out with the objective of increasing our knowledge of the role of maceration enzymes in tannin-cell wall interactions [2,5,[12][13][14], and it has been commonly observed that the effect of enzymes in increasing the tannin concentration in the medium is limited. Bindon et al [2], in a study to explain this low effect, observed that, as expected, the cell walls previously treated with a commercial pectolytic enzyme (polygalacturonase) decreased their ability to interact with tannins in model solutions.…”

“…Several studies have been carried out with the objective of increasing our knowledge of the role of maceration enzymes in tannin-cell wall interactions [2,5,[12][13][14], and it has been commonly observed that the effect of enzymes in increasing the tannin concentration in the medium is limited. Bindon et al [2], in a study to explain this low effect, observed that, as expected, the cell walls previously treated with a commercial pectolytic enzyme (polygalacturonase) decreased their ability to interact with tannins in model solutions. However, the problem was that, after the enzymatic treatment, if the solubilized part of the cell walls was left in contact with tannins, precipitates were observed.…”