Aude A. Watrelot

Interactions between pectic compounds and procyanidins are influenced by methylation degree and chain length.

The interactions between procyanidins and pectic compounds are of importance in food chemistry. Procyanidins with low (9) and high (30) average degrees of polymerization (DP9 and DP30) were extracted from two cider apple varieties. Commercial apple and citrus pectins, as well as three pectin subfractions (homogalacturonans, partially methylated homogalacturonans with degree of methylation 30 and 70) at 30 mM galacturonic acid equivalent, were titrated with the two procyanidin fractions (at 30 mM (-)-epicatechin equivalent) by isothermal titration calorimetry and UV-vis spectrophotometry. Slightly stronger affinities were recorded between commercial apple or citrus pectins and procyanidins of DP30 (Ka = 1460 and 1225 M(-1) respectively, expressed per monomer units) compared to procyanidins of DP9 (Ka = 1240 and 1085 M(-1), respectively), but stoichiometry and absorbance maxima differed between apple and citrus pectins. It was proposed that methylated homogalacturonans interacted with procyanidins DP30 mainly through hydrophobic interactions. The stronger association was obtained with the longer procyanidin molecules interacting with highly methylated pectins.

Neutral sugar side chains of pectins limit interactions with procyanidins.

Interactions between seven hairy regions of pectins, rhamnogalacturonans II and arabinogalactan-proteins and procyanidins with different average degrees of polymerization, low (DP9) and high (DP30), were investigated by isothermal titration calorimetry and absorption analysis to study the impact of neutral sugar side chains of pectins on these associations. Associations between pectic fractions and procyanidins involved hydrophobic interactions and hydrogen bonds. No difference in association constants between various hairy regions and procyanidins DP9 was found. Nevertheless, arabinan chains showed lower association constants, and hairy regions of pectins with only monomeric side chains showed higher association with procyanidin DP30. Only very low affinities were obtained with rhamnogalacturonans II and arabinogalactan-proteins. Aggregation could be observed only with the procyanidins of DP30 and the protein-rich arabinogalactan-protein. Associations were obtained at both degrees of polymerization of the procyanidins, but differed depending on neutral sugar composition and the structure of pectic fractions.

Red Wine Tannin Structure-Activity Relationships during Fermentation and Maceration.

The correlation between tannin structure and corresponding activity was investigated by measuring the thermodynamics of interaction between tannins isolated from commercial red wine fermentations and a polystyrene divinylbenzene HPLC column. Must and/or wine samples were collected throughout fermentation/maceration from five Napa Valley wineries. By varying winery, fruit source, maceration time, and cap management practice, it was considered that a reasonably large variation in commercially relevant tannin structure would result. Tannins were isolated from samples collected using low pressure chromatography and were then characterized by gel permeation chromatography and acid-catalyzed cleavage in the presence of excess phloroglucinol (phloroglucinolysis). Corresponding tannin activity was determined using HPLC by measuring the thermodynamics of interaction between isolated tannin and a polystyrene divinylbenzene HPLC column. This measurement approach was designed to determine the ability of tannins to hydrophobically interact with a hydrophobic surface. The results of this study indicate that tannin activity is primarily driven by molecular size. Compositionally, tannin activity was positively associated with seed tannins and negatively associated with skin and pigmented tannins. Although measured indirectly, the extent of tannin oxidation as determined by phloroglucinolysis conversion yield suggests that tannin oxidation at this stage of production reduces tannin activity. Based upon maceration time, this study indicates that observed increases in perceived astringency quality, if related to tannin chemistry, are driven by tannin molecular mass as opposed to pigmented tannin formation or oxidation. Overall, the results of this study give new insight into tannin structure-activity relationships which dominate during extraction.

Yield and composition of pectin extracted from Tunisian pomegranate peel.

A central composite design was employed to determine the influence of extraction conditions on production yield and chemical composition of pectin from pomegranate peels. Response surface methodology (RSM) was used to quantify the integral effect of the three processing parameters (extraction duration, temperature and pH) on yield. A second-order polynomial model was developed for predicting the yield of pomegranate peels pectin based on the composite design. Yields ranged from 6.4 to 11.0±0.2%. Optimal temperature, duration and pH value of the extraction were 86°C, 80min and 1.7, respectively. The uronic acid and the total neutral sugar content of the extracted pectins ranged from 377 to 755mg/g and from 161 to 326mg/g, respectively. Moreover, the degree of methylation varied with the extraction conditions and the extracted pectins were low methylated. On high pressure size exclusion chromatography (HPSEC), the elution pattern of the acid-extracted pectins showed that severe conditions were associated with lower hydrodynamic volume.

Condensed Tannin Reacts with SO2 during Wine Aging, Yielding Flavan-3-ol Sulfonates

Numerous monomeric and oligomeric flavanol sulfonation products were observed in 10 wines. Levels of 0.85-20.06 and 0-14.72 mg/L were quantified for two monomeric sulfonated flavan-3-ols and, surprisingly, were generally higher than the well-known native flavan-3-ol monomers. Increasing SO2 levels during wine aging increased the sulfonate-modified flavan-3-ol monomers and dimers along with higher concentrations of native monomers. The results indicate that >10% of SO2 is reacting with the C-4 carbocation, formed from acid cleavage of the interflavan bond, perhaps by a bimolecular SN2-type reaction, and as a reducing agent. In addition, the high SO2 wine had the lowest protein-binding tannin levels, tannin activity, and mean degree of polymerization (mDP), and acidic SO2 treatment of condensed tannin abolishes protein binding. Thus, SO2 changes tannin composition during wine aging, and the substantial formation of sulfonate-modified flavan-3-ols may provide an additional explanation for the reduction in astringency of aged red wines.

Pear ripeness and tissue type impact procyanidin-cell wall interactions

Procyanidins and cell walls were extracted from pear at ripe and overripe stages in order to investigate the impact of ripening stage on their association. Procyanidin composition and structure remained stable at the overripe stage. Mid Infrared Spectroscopy (MIR) discriminated cell wall-procyanidin complex from initial purified cell wall material (CWM). Interactions between procyanidins and CWM isolated from the whole flesh (FL), parenchyma cells (PC), stone cells (ST) and skin (SK) at ripe and overripe stage were characterized using UV-Vis spectrometry using Langmuir isotherm formulation and Isothermal Titration Microcalorimetry (ITC). The affinity between procyanidins and CWM decreased as follows: PC > FL > ST > SK. The proportion of bound procyanidins increased at the overripe stage for all CWM and the maximal saturation level was obtained for overripe FL and ST. ITC indicated that associations between pear cell walls and procyanidins involved hydrogen bonds and mainly hydrophobic interactions for overripe PC.

Friction forces of saliva and red wine on hydrophobic and hydrophilic surfaces

The physical aspect of human oral astringency perception - the mouthfeel - of red wine has not been quantitatively studied in depth. In this study, the interfacial friction/lubrication properties of saliva (mucin from bovine submaxillary glands or human saliva) with red wines (cv. Cabernet sauvignon and Pinot noir) were measured with a surface force apparatus (SFA). In SFA measurements sliding occurs between smooth, undamaged surfaces with a well-defined contact area and film thickness. The surfaces were either hard, hydrophilic mica or soft, hydrophobic PDMS-coated mica which mimic in-mouth conditions. Saliva was a better lubricant than mucin with the soft, hydrophobic surfaces. In addition, salivas lubricity was 2.5 times better on the soft hydrophobic surfaces than hard hydrophilic surfaces. The addition of red wine with saliva further decreased friction and improved lubrication. Surprisingly, the coefficient of friction measured for red wine with saliva as the lubricant was higher for Pinot noir than Cabernet sauvignon wine. The aggregation and precipitation of salivary proteins by tannins is well known. The lower friction of high tannin Cabernet sauvignon compared to lower tannin Pinot noir was attributed to exclusion of these aggregates and depletion of more polymeric and protein material from the interfacial sliding region.

Understanding microoxygenation: Effect of viable yeasts and sulfur dioxide levels on the sensory properties of a Merlot red wine

Abstract Microoxygenation (Mox) is widely used in winemaking to improve color, in-mouth properties and aroma, but its use is not always predictable. Here we investigate the effect of Mox, (while monitoring viable yeasts and SO2 levels), on color, anthocyanin-derived pigments, tannins, aroma and in-mouth sensory properties as well as on hedonic rating by wine experts. Results on this Merlot wine show that the re-appearance of viable Saccharomyces cerevisiae yeasts modulates oxygen consumption rates, and dramatically increases acetaldehyde levels. This led to significant sensory changes, particularly for aroma. Mox reduced green-vegetable and reduction-related aromas, but also astringent mouth-feel properties related to tannins, and lower astringency was correlated with lower tannin activity. The Mox wines that exhibited yeast growth had higher hedonic scores from one group of expert tasters based on increased jammy/dried fruit flavors, while another group of tasters rated the non-Mox wines higher due to the green vegetable and spicy aromas. These results show that the chemical and sensory impact of a Mox treatment is highly dependent on the absence or presence of yeast growth, so it is important to monitor for yeast populations during Mox treatment.