Pascale Williams

The mouth-feel properties of polysaccharides and anthocyanins in a wine like medium

Abstract Two fractions containing the major polysaccharides present in wine were isolated, one comprising a mixture of neutral polysaccharides, mannoproteins and arabinogalactan-proteins, and the other containing the acidic polysaccharide, rhamnogalacturonan II. A grape anthocyanin fraction was also prepared. A trained sensory panel, using formal sensory descriptive analysis methods to rate the intensity of mouthfeel attributes while the samples were held in the mouth and after expectoration, individually assessed the fractions, dissolved in a model wine at levels commonly encountered in red wines. Both polysaccharide fractions significantly increased the ‘fullness’ sensation above that of the base wine. The rhamnogalacturonan II fraction significantly decreased the attribute ratings associated with the astringency of the model wine whereas the neutral wine polysaccharide fraction had less affect on reducing the ratings for these attributes. The anthocyanin fraction tended to increase ‘fullness’ although the effect was not great enough to be statistically significant. Unlike the polysaccharides, this fraction also increased perceived astringency but this effect could be due to the presence of some derived tannins in the sample.

Use of an experimental design approach for evaluation of key wine components on mouth-feel perception

Abstract To simultaneously explore the primary and interactive effects of proanthocyanidin (‘tannin’), ethanol, anthocyanin and wine polysaccharide concentrations on the mouth-feel perception of wine like media, a sensory study based on an incomplete factorial design was conducted. Two grape polyphenol fractions, i.e. grape seed tannins and anthocyanins, and two fractions of wine polysaccharides, (mannoproteins+arabinogalactan-proteins and rhamnogalacturonan II) were prepared and analysed. A panel of 15 trained judges generated a series of mouth-feel descriptors and rated their intensities while samples containing various levels and combinations of the components were held in mouth and after expectoration. The sensory perception was primarily determined by tannin concentration. However, the attribute ratings were also strongly influenced by all other factors both directly and through interactions. The intensities of all astringency descriptors increased with tannin concentration and were reduced when rhamnogalacturonan II was added. Bitterness increased with ethanol level and decreased in the presence of proteoglycans. Secondary effects observed included both masking and enhancement of the primary effects but also specific interaction effects. The latter are probably related to differences in the structural organization and properties of molecular assemblies involving polyphenols, polysaccharides, and ethanol.

The polysaccharides of red wine: total fractionation and characterization

Abstract Ethanol-precipitated red wine polysaccharides were fractionated by a combination of anion-exchange, size-exclusion and affinity chromatography steps. This comprehensive fractionation allowed us to prepare a collection of wine polysaccharides in sufficient amount to permit the determination of their intrinsic properties. Glycosyl-residue composition of each polysaccharide fraction was determined by GC–EI–MS of the per- O -trimethylsilylated methyl glycoside derivatives (TMS), a method that has been recently developed and adapted to suit simultaneous determination of neutral and acidic glycosyl-residue compositions of polysaccharides present in plant-derived products. The results showed that mannoproteins released by yeast during fermentation, and grape derived arabinogalactan-proteins, rhamnogalacturonans I and II are the main wine polysaccharides and accounted for 35, 42, 4 and 19%, respectively, of the total polysaccharides. Structural characterization revealed that rhamnogalacturonan I fractions were linked with xyloglucan-like polysaccharides. This finding represents compelling evidence of the existence of cross-linking between pectin and hemicellulose domains in plant primary cell walls.

Polysaccharides from grape berry cell walls. Part I : tissue distribution and structural characterization of the pectic polysaccharides

Abstract Buffer-soluble arabinogalactan-proteins (AGPs) and pectins from grape berry skin and pulp tissues have been isolated and their structure has been partly determined. Pectic polysaccharides from the cell wall material were solubilized by treating pulp and skin cell walls with homogeneous glycosyl hydrolases. Homogalacturonans, rhamnogalacturonans I (RG-I), and rhamnogalacturonan II (RG-II) of each tissue have been fractionated by high resolution size exclusion chromatography and their relative distribution and major structural features have been determined. It has been shown that pulp tissue contains two-fold more buffer-soluble AGPs and pectins than skin tissue and we have determined that 75% of the grape berry walls originates from the skin tissue. There is three-fold more RG-I and RG-II in skin tissue than in pulp tissue and three-fold more RG-I than RG-II in the grape berry cell walls. The results of this study have shown that the grape polysaccharide content of a wine is related to the type of tissue used for wine making and to the solubility of the grape polysaccharides and their resistance to fragmentation by grape and yeast glycanases.

Characterization of five type II arabinogalactan-protein fractions from red wine of increasing uronic acid content

Five arabinogalactan-protein conjugates (AGP) were separated from red wine by two successive anion-exchange chromatography steps and further purified to apparent homogeneity by affinity and size-exclusion chromatography. Together they represent more than 40% of total wine polysaccharides, confirming the abundance of AGPs in red wine. The five purified fractions had a common arabinogalactan core with characteristics typical of wine type II AGPs, but differed mainly in their uronic acid content, as evidenced by differences in the strength of their binding to the anion-exchanger. Their uronic acid content and glycosidic linkage composition revealed that the three less acidic AGPs contained from 3 to 7% glucuronic acid, half in terminal non-reducing positions and half in terminal Rhap-(1-->4)-Glc pA-(1-->sequences. The two more acidic AGP-containing fractions contained both glucuronic (6.1 and 13.3%, respectively) and galacturonic (1.9 and 2.3%, respectively) acid in association with 2- and 2,4-linked rhamnose, indicating the presence of AG-rhamnogalacturonan fragments.

Speciation analysis for biomolecular complexes of lead in wine by size-exclusion high-performance liquid chromatography-inductively coupled plasma mass spectrometry

Size-exclusion chromatography with ICP-MS detection was developed for speciation of lead in wine. The method required minimal sample preparation and allowed routine analysis of wine samples, showing the distribution of lead among biomolecular compounds within 30 min. The quantification of lead bound to biomolecules was done by comparing the chromatographic peak area with that of a signal obtained by flow-injection ICP-MS analysis. A study carried out on a population of 20 wines of different origin showed that mineral lead on which the toxicity conclusions were based may not exist in wine. Lead was found to be associated with one major biomolecular (ca. 10 kDa) species present in all wine samples, and one to three minor compounds whose number depended on the wine sample. The dominant species which accounted for 40–95% of lead was identified as the complex formed with the dimer of a pectic polysaccharide, rhamnogalacturonan II. Other minor species with apparent molecular masses in the range 500–3000 Da were not identified.

Rhamnogalacturonan II, a dominant polysaccharide in juices produced by enzymic liquefaction of fruits and vegetables

Rhamnogalacturonan II (RG-II), a small complex pectic polysaccharide, is released from apple (Malus domestica), carrot (Daucus carota), and tomato (Solanum lycopersicum) by treatment with two commercial liquefying enzyme preparations. RG-II was isolated by size-exclusion chromatography from apple, tomato, and carrot juices obtained by enzymic liquefaction. All the RG-IIs contained the diagnostic sugars, apiose, 2-O-methyl-L-fucose, 2-O-methyl-D-xylose, aceric acid, Kdo and Dha. Glycosyl-linkage compositions of the neutral and acidic sugars, including aceric acid, were consistent with the hypothetical model described for sycamore RG-II confirming the conservation of RG-II in plants. Thus, when pectinolytic enzyme preparations are used to process fruits and vegetables, RG-II is released as a main soluble polysaccharide fraction while other pectic polysaccharides are heavily degraded.

Polyphenolic, polysaccharide and oligosaccharide composition of Tempranillo red wines and their relationship with the perceived astringency

The influence of the proanthocyanidic, polysaccharide and oligosaccharide composition on astringency perception of Tempranillo wines has been evaluated. Statistical analyses revealed the existence of relationships between chemical composition and perceived astringency. Proanthocyanidic subunit distribution had the strongest contribution to the multiple linear regression (MLR) model. Polysaccharide families showed clear opposition to astringency perception according to principal component analysis (PCA) results, being stronger for mannoproteins and rhamnogalacturonan-II (RG-II), but only Polysaccharides Rich in Arabinose and Galactose (PRAGs) were considered in the final fitted MLR model, which explained 96.8% of the variability observed in the data. Oligosaccharides did not show a clear opposition, revealing that structure and size of carbohydrates are important for astringency perception. Mannose and galactose residues in the oligosaccharide fraction are positively related to astringency perception, probably because its presence is consequence of the degradation of polysaccharides.

Changes in Polysaccharide and Protein Composition of Cell Walls in Grape Berry Skin (Cv. Shiraz) during Ripening and Over-Ripening

Polysaccharide modification is the most fundamental factor that affects firmness of fruit during ripening. In grape, because of the lack of information on the modifications occurring in cell wall polysaccharides in skins, but also because this tissue contains large amounts of organoleptic compounds for winemaking, a study was performed on the evolution and extractability of polysaccharides from grape skins of Shiraz cultivar throughout ripening. A HEPES/phenol extraction technique was used to analyze Shiraz grape cell wall material isolated from skins of berries harvested from one to ten weeks after veraison. Total amounts in cell wall polysaccharides remained constant during ripening (4.2 mg/berry). A slight decrease in galactose content of insoluble polysaccharides was observed, as well as a significant de-esterification of methoxylated uronic acids, indicating that some modifications occur in cell wall polysaccharides. The water-soluble fraction represented a very small fraction of the whole polysaccharides, but its amounts increased more than 2-fold between the first and the last sample. Isolated cell walls were also analyzed for their protein composition. Last, hydroalcoholic extractions in model-wine solution were also performed on fresh skins. This extracted fraction was very similar to the water-soluble one, and increased during the entire period. By comparison with polysaccharide modifications described in flesh cell wall in previous works, it can be assumed that the moderate skin polysaccharide degradation highlights the protective role of that tissue.

Speciation of metal-carbohydrate complexes in fruit and vegetable samples by size-exclusion HPLC-ICP-MS

Kinetically inert and thermodynamically stable metal complexes with polysaccharides were detected in aqueous leachates and enzymatic digests of apple and carrot samples by size-exclusion chromatography with parallel refractometric and ICP-MS detection. The method developed allowed detection in the water-soluble fraction and the identification of a high molar mass polysaccharide fraction (>50 kDa) containing Pb, Ba, Sr, Ce and B, whereas other metals (Zn, Cu, Mg) eluted as complexes with low molar mass non-carbohydrate compounds. The majority of the metal-carbohydrate complexes were located in the solid water-insoluble fraction of the analysed samples. An extraction procedure with a mixture of pectinolytic enzymes was developed to release these species into the aqueous phase. The metal-binding carbohydrate component was identified as the dimer of rhamnogalacturonan-II, a pectic polysaccharide present in plant cell walls. The unidentified residual metal species contained less than 5% of the metals present.