Helen Holt

Interaction between grape-derived proanthocyanidins and cell wall material. 2. Implications for vinification.

Proanthocyanidins (PAs) were isolated from the skins, seeds and flesh of commercially ripe grapes, and from wine and marc produced from the same source. In the grape berry, skin PAs accounted for 54% of the total extractable PA, while seed and flesh-derived PA accounted for 30% and 15% of the total, respectively. Following fermentation, 25% of the fruit PA was found in the wine, while 27% was found in the pericarp isolated from marc, and 48% was unaccounted for (either remaining in the seed or adsorbed to lees). To investigate the role that cell wall material (CWM) has on PA extraction during fermentation, CWM isolated from skin and flesh were combined with PA in model suspensions. In general, the affinity of flesh CWM for PA increased with increasing PA molecular mass (MM); however, this relationship was not observed for the interaction of skin CWM with skin PA. Subsequent experiments suggest that the differences in the interaction of flesh and skin CWM with PA of higher MM (>15000 g/mol) may be limited by the structure of the CWM. Observed variations in the composition between skin and flesh CWM may explain the differences in PA interaction at high MM. Among wine-derived PA, no higher MM material was detected, suggesting that, during vinification, higher MM PA are nonextractable and/or are removed from the wine by interaction with CWM.

Relationships between harvest time and wine composition in Vitis vinifera L. cv. Cabernet Sauvignon 1. Grape and wine chemistry

The study aimed to quantify the effects of grape maturity on wine alcohol, phenolics, flavour compounds and polysaccharides in Vitis vinifera L. cv Cabernet Sauvignon. Grapes were harvested at juice soluble solids from 20 to 26 °Brix which corresponded to a range of wine ethanol concentrations between 12% and 15.5%. Grape anthocyanin and skin tannin concentration increased as ripening progressed, while seed tannin declined. In the corresponding wines, monomeric anthocyanin and wine tannin concentration increased with harvest date, consistent with an enhanced extraction of skin-derived phenolics. In wines, there was an observed increase in yeast-derived metabolites, including volatile esters, dimethyl sulfide, glycerol and mannoproteins with harvest date. Wine volatiles which were significantly influenced by harvest date were isobutyl methoxypyrazine, C(6) alcohols and hexyl acetate, all of which decreased as ripening progressed. The implications of harvest date for wine composition is discussed in terms of both grape composition and yeast metabolism.

Relationships between harvest time and wine composition in Vitis vinifera L. cv. Cabernet Sauvignon 2. Wine sensory properties and consumer preference

A series of five Vitis vinifera L. cv Cabernet Sauvignon wines were produced from sequentially-harvested grape parcels, with alcohol concentrations between 12% v/v and 15.5% v/v. A multidisciplinary approach, combining sensory analysis, consumer testing and detailed chemical analysis was used to better define the relationship between grape maturity, wine composition and sensory quality. The sensory attribute ratings for dark fruit, hotness and viscosity increased in wines produced from riper grapes, while the ratings for the attributes red fruit and fresh green decreased. Consumer testing of the wines revealed that the lowest-alcohol wines (12% v/v) were the least preferred and wines with ethanol concentration between 13% v/v and 15.5% v/v were equally liked by consumers. Partial least squares regression identified that many sensory attributes were strongly associated with the compositional data, providing evidence of wine chemical components which are important to wine sensory properties and consumer preferences, and which change as the grapes used for winemaking ripen.

Sensory Properties of Wine Tannin Fractions: Implications for In-Mouth Sensory Properties

Different molecular structures of grape tannins have been shown to influence astringency, however, the in-mouth sensory effects of different molecular structures in red wine tannins remains to be established. The objective of this research was to assess the impact of wine tannin structure on in-mouth sensory properties. Wine tannin was isolated from Cabernet Sauvignon wines of two vintages (3 and 7 years old) and separated into two structurally distinct subfractions with liquid-liquid fractionation using butanol and water. The aqueous subfractions had greater mean degree of polymerization (mDp) and contained a higher proportion of epigallocatechin subunits than the butanol-soluble subfractions, while the older wine tannin fractions showed fewer epicatechin gallate subunits than the younger tannin fractions. The red wine had approximately 3:1 mass ratio of the aqueous and butanol tannin subfractions which approximated an equimolar ratio of tannin in each subfraction. Descriptive sensory analysis of the tannin subfractions in model wine at equimolar concentrations revealed that the larger, more water-soluble wine tannin subfractions from both wines were perceived as more astringent than the smaller, more hydrophobic and more highly pigmented butanol-soluble subfractions, which were perceived as hotter and more bitter. Partial least squares analysis indicated that the greater hydrophobicity and color incorporation in the butanol fractions was negatively associated with astringency, and these characteristics are also associated with aged wine tannins. As the larger, water-soluble tannins had a greater impact on the overall wine astringency, winemaking processes that modulate concentrations of these are likely to most significantly influence astringency.

Degradation of white wine haze proteins by Aspergillopepsin I and II during juice flash pasteurization

Bentonite is commonly used to remove grape proteins responsible for haze formation in white wines. Proteases potentially represent an alternative to bentonite, but so far none has shown satisfactory activity under winemaking conditions. A promising candidate is AGP, a mixture of Aspergillopepsins I and II.; a food grade, well characterized and inexpensive protease, active at wine pH and at high temperatures (60-80°C). AGP was added to two clarified grape juices with and without heat treatments (75°C, 1min) prior to fermentation. AGP showed some activity at fermentation temperatures (≈20% total protein reduction compared to control wine) and excellent activity when combined with juice heating (≈90% total protein reduction). The more heat stable grape proteins, i.e. those not contributing to wine hazing, were not affected by the treatments and therefore accounted for the remaining 10% of protein still in solution after the treatments. The main physicochemical parameters and sensorial characteristics of wines produced with AGP were not different from controls.

White wine taste and mouthfeel as affected by juice extraction and processing.

The juice used to make white wine can be extracted using various physical processes that affect the amount and timing of contact of juice with skins. The influence of juice extraction processes on the mouthfeel and taste of white wine and their relationship to wine composition were determined. The amount and type of interaction of juice with skins affected both wine total phenolic concentration and phenolic composition. Wine pH strongly influenced perceived viscosity, astringency/drying, and acidity. Despite a 5-fold variation in total phenolics among wines, differences in bitter taste were small. Perceived viscosity was associated with higher phenolics but was not associated with either glycerol or polysaccharide concentration. Bitterness may be reduced by using juice extraction and handling processes that minimize phenolic concentration, but lowering phenolic concentration may also result in wines of lower perceived viscosity.

Addition of carrageenan at different stages of winemaking for white wine protein stabilization.

Carrageenan added at different stages of winemaking was assessed for its protein removal and impact on wine heat stability and on the chemical and sensorial profile of the wines. Carrageenan was added to a Semillon during fermentation and after fermentation and to finished wines, and the effect of each addition was compared to that of bentonite fining at the same time point. Data on protein concentration, heat stability, and bentonite requirement indicate that when added at the correct dosage carrageenan was very effective in stabilizing wines at dosages at least three times lower than those of bentonite. In addition, carrageenan treatment did not cause an increase in lees volume relative to bentonite and resulted in very similar chemical parameters to the unfined and bentonite-treated wine. Sensorially, although carrageenan-treated wine was significantly different from the unfined wine, the magnitude of difference did not vary significantly when compared to bentonite treatment. The feasibility of carrageenan use in a winery production setting will need to be determined by individual wineries, as technical issues including frothing, slower filterability, and risk of overfining will need to be considered relative to the benefits, particularly when carrageenan is used before or during fermentation.