Aromatic Higher Alcohols in Wine: Implication on Aroma and Palate Attributes during Chardonnay Aging.

The higher alcohols 2-phenylethanol, tryptophol, and tyrosol are a group of yeast-derived compounds that have been shown to affect the aroma and flavour of fermented beverages. Five variants of the industrial wine strain AWRI796, previously isolated due to their elevated production of the 'rose-like aroma' compound 2-phenylethanol, were characterised during pilot-scale fermentation of a Chardonnay juice. We show that these variants not only increase the concentration of 2-phenylethanol but also modulate the formation of the higher alcohols tryptophol, tyrosol, and methionol, as well as other volatile sulfur compounds derived from methionine, highlighting the connections between yeast nitrogen and sulfur metabolism during fermentation. We also investigate the development of these compounds during wine storage, focusing on the sulfonation of tryptophol. Finally, the sensory properties of wines produced using these strains were quantified at two time points, unravelling differences produced by biologically modulating higher alcohols and the dynamic changes in wine flavour over aging.

Novel wine yeast with ARO4 and TYR1 mutations that overproduce 'floral' aroma compounds 2-phenylethanol and 2-phenylethyl acetate.

It is well established that the choice of yeast used to perform wine fermentation significantly influences the sensory attributes of wines; different yeast species and strains impart different profiles of esters, volatile fatty acids, higher alcohols, and volatile sulphur compounds. Indeed, choice of yeast remains one of the simplest means by which winemakers can modulate the sensory characteristics of wine. Consequently, there are more than 100 commercially available Saccharomyces cerevisiae wine yeast strains available, mostly derived by isolation from vineyards and successful fermentations. Nevertheless, some desirable characteristics such as 'rose' and 'floral' aromas in wine are not present amongst existing strains. Such aromas can be conferred from the higher alcohol 2-phenylethanol (2-PE) and its acetate ester, 2-phenylethyl acetate (2-PEA). These metabolites of the aromatic amino acid phenylalanine are present at concentrations below their aroma detection thresholds in many wines, so their contribution to wine style is often minimal. To increase the concentration of phenylalanine metabolites, natural and chemically mutagenised populations of a S. cerevisiae wine strain, AWRI796, were exposed to toxic analogues of phenylalanine. Resistant colonies were found to overproduce 2-PE and 2-PEA by up to 20-fold, which resulted in a significant increase in 'floral' aroma in pilot-scale white wines. Genome sequencing of these newly developed strains revealed mutations in two genes of the biosynthetic pathway of aromatic amino acids, ARO4 and TYR1, which were demonstrated to be responsible for the 2-PE overproduction phenotype.

Oxygen exposure during red wine fermentation modifies tannin reactivity with poly-l-proline.

Red wines injected with nitrogen or oxygen during fermentation were used to identify the effect of gas exposure on tannin structure and reactivity with poly-l-proline. Tannin was purified from wine after fermentation and after three years of bottle storage. Tannin from nitrogen-treated wine had a lower percentage of galloylation and were less pigmented than tannin from oxygen-exposed wine. Self-aggregation of tannin was measured by nanoparticle tracking analysis and a larger particle size was observed for the oxidized treatment. The interaction of tannin and poly-l-proline was measured by isothermal titration calorimetry, and involved more hydrogen bonding than hydrophobic interactions in the case of nitrogen-treated wine tannin. Conversely, oxidized tannin was more hydrophobic and the association with poly-l-proline was entropy-driven due to a change of solvation. The results show meaningful changes in the structure and reactivity of tannin as a result of oxygen exposure during fermentation, which may impact astringency perception.

Properties of wine polymeric pigments formed from anthocyanin and tannins differing in size distribution and subunit composition.

To explore the effect of tannin composition on pigment formation, model ferments of purified 3-O-monoglucoside anthocyanins (ACN) were conducted either alone or in the presence of two different tannins. Tannins were isolated from grape seeds (Sd) or skins (Sk) following exhaustive extraction in 70% v/v acetone. The Sd and Sk tannin fractions had a mean degree of polymerization of 5.2 and 25.6, respectively. The Sd fraction was highly galloylated, at 22%, but galloylation was <2% in the Sk fraction. The Sk fraction was distinguished by a high proportion of prodelphinidin, at 58%. After a 6 month aging period, polymeric pigments were quantified and their color properties determined following isolation by solid-phase extraction. Wine color and polymeric pigment were highest in the treatment containing ACN+Sd and similar in the ACN+Sk and ACN treatments. The same trend between treatments was observed for total and polymeric nonbleachable pigments. Only minor changes in tannin subunit composition were found following ACN incorporation, but the size distribution of polymeric pigments determined by gel permeation chromatography decreased, in particular for the ACN+Sk treatment. Color incorporation in the higher molecular mass range was lower for ACN+Sk wines than for ACN+Sd wines. Compositional differences between the two tannin fractions may therefore limit the incorporation of ACNs in the colored form. The results suggest that in the ACN+Sk and ACN treatments, the formation of lower molecular mass oligomeric pigments was favored. In polymeric pigments derived from ACNs, the presence of ethyl- and vinyl-linked ACNs to the level of trimers was identified using mass spectrometry.

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.

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.