Factors affecting wine texture, taste, clarity, stability and production efficiency
Wine texture is considered a major product differentiator both for wine style and value in the marketplace. In addition, clarity and colour stability (absence of haze development and the retention of colour) are generally considered essential for market success. Achieving desired textural qualities, clarity and stability can involve processing steps with significant costs. The ability to modulate these characteristics of wine while maintaining profitability is a significant challenge for the wine industry. This project aims to elucidate key compositional drivers of texture, bitterness, clarity, stability (protein and colour) and wine filterability, and seeks to develop strategies to modulate them in a production- based environment. This research will provide knowledge of grape and wine composition for polyphenols, polysaccharides and proteins and a clearer understanding of the impact of winemaking processes on macromolecule concentrations and colloidal profiles.
The following specific aspects are being investigated in this project:
- the compositional drivers of texture, hotness and bitterness
- the role of macromolecules such as tannins, polysaccharides, proteins and their aggregate colloids in the expression of texture, stability, clarity and filterability
- the impact of other wine matrix components on macromolecule function and expression
- the source of these molecules or their precursors in grapes and yeast and the impact of winemaking processes such as clarification, flotation, vinification and filtration on their retention and/or transformation
- the impact of filtration on macromolecules
- strategies for modulation of specific compositional drivers through the use of grape-based fining agents
- alternative strategies for achieving protein stability
- practical methods for wineries to determine likely extractability of macromolecules during winemaking
- strategies for the stabilisation of colour independent of vintage effects.
The knowledge generated by the project will provide a framework for the development of winemaking strategies, practical tools and recommendations for managing colour (and colour stability), astringency, viscosity, hotness, bitterness, filtration processes and protein hazes.
Fundamental methods for understanding texture, taste, clarity, stability and filterability
Typical methods for investigating the formation and properties of colloids in wine (i.e. macromolecular aggregates with particle sizes in the nanometre range) are quite complicated and have mainly been developed for applications in areas other than wine science. They are, however, fundamental to determining how colloids influence the results from many processes in wine production including extraction, settling, clarification, filtration and stability in the bottle. To improve existing methods and tailor them to applications in the wine industry, collaborations in this project have recently been strengthened through Dr Agnieszka Mierczynska-Vasilev’s adjunct position at UniSA’s Future Industries Institute and Dr Paul Smith’s position at Flinders University, gaining access to a wide range of cutting-edge physical chemistry characterisation equipment.
In exploratory studies plasma polymerisation has been used to create surfaces of varied properties (e.g. polarity or charge) to explore how wine molecules and colloids interact with them. Quartz crystal microbalances (QCM) allowed study of the surface-binding characteristics of different types of colloids (e.g. polar or hydrophobic) in wine without disturbing them through complex isolation and handling. X-ray photoelectron spectroscopy (XPS) analysis has been used to determine the surface composition of polymer surfaces with and without wine deposited on them. Atomic force microscopy (AFM) has been used to provide topographical images of those surfaces. In essence, all these methods support the characterisation of the wine components that bind to a particular surface. This is critical to understanding processes such as filter fouling, binding to tanks and fittings and interactions with processing aids (e.g. bentonite).
Figure 1 shows an example of the spectra for bare and wine-bound surface samples obtained by XPS. This method gives information about the types of bonds that are present in the molecules bound to a surface and allows better understanding of what types of molecules they are (e.g. proteins, polysaccharides, polyphenols or a mixture). One outcome of this work was the recognition that tannin bound up with polysaccharide in ‘complexes’ may account for approximately 20% of total tannin in red wine and that these complexes are not affected by filtration, and would therefore be expected to remain in finished wine after filtration.
Figure 2 shows example of an AFM image of red wine adsorbed onto two different surfaces. These AFM images of the adsorbed layers allow an estimation of the amount of wine adsorbed on the surface which allows an assessment of, for example, how resistant a surface is to fouling. This gives insight into the types of surface coatings that may be useful in a wine industry setting.
Surface Plasmon Resonance (SPR) has been used to characterise surface
layers that form between tannins and proteins. Isothermal Titration Calorimetry (ITC) has been used to assess the binding strength of macromolecules, and showed that the binding strength of tannins with proteins decreases as wine tannins age. This may account for the softening of red wine tannins with age.
Impact of juice clarification on macromolecules and phenolics in white wine
A broad laboratory-scale scoping study compared the effects on wine polysaccharides and phenolics of different levels of juice solids obtained by gravity settling using enzymes, bentonite or without the aid of a clarifying agent. Wines made from higher solids juices resulted in wines with higher polysaccharide concentrations. Subsequent analysis of the molecular weight distribution and monosaccharide composition of the polysaccharides indicated that yeast-derived mannoproteins were the main contributors to the elevated polysaccharide levels. The relative abundance of arabinogalactans and rhamnogalacturonans (polysaccharides derived from grape solids), which are thought to influence fullness and hotness in wine, was influenced in a complex manner by solids content and the clarification method used to achieve it.
The study also showed that the total phenolic content of wines made from juices clarified either naturally or with the assistance of enzymes were similar to those made from full solids. This result was consistent with results obtained using juices sourced from small commercial wineries made during the 2015 vintage. However, the laboratory-scale study also found that an intermediate solids level resulted in lower total phenolics than either full or low solids. The results suggest that the final concentration of grape-derived polysaccharides and phenolics in wine is the net result of the ease of their extraction from grape cell walls, settling time, and losses due to fining effects from other grape components.
Molecular drivers of texture and taste
Contemporary interpretations of wine complexity and overall quality generally include the contribution of mouth-feel/texture which includes the stylistic attributes of viscosity, oiliness, creaminess and astringency/dryness, and the more negatively perceived attributes of bitterness and hotness. Previously it has been shown that white wine polysaccharides, particularly those in the medium molecular weight range (15-90 kDa), can suppress alcohol hotness and increase viscosity in model and white wine. Trials were conducted to assess if red wine polysaccharides could have a similar positive effect on red wine mouth-feel. Whole polysaccharides were extracted from a red wine and fractionated into three fractions based on molecular weight using preparative size exclusion chromatography. Formal sensory assessment of the fractions in model wines of varying pH and alcohol levels showed that the alcohol hotness was reduced by medium molecular weight red wine polysaccharides. These results were consistent with the previous work on white wine. Work to establish the monosaccharide composition of the red wine polysaccharide fractions is underway.
Improved understanding of white wine protein haze
A set of unstable wines (vintage 2015) has been collected for use in the development of a predictive haze model, and for exploring alternatives to the industry-standard heat test that are faster and more accurate. These wines were analysed for a range of wine components and the results compared to the haze potential determined by the heat test. As part of this project a new rapid HPLC method for profiling proteins in wines has been developed and MS analysis has confirmed the identity of the previously uncertain peaks, validating the rapid analytical method for separating wine proteins.
Evaluation of bentonite alternatives continued, with experiments comparing two bentonites and three novel resins. The most viable of the three resins was found to be sulfonated silica, although the active concentration required was five times that of bentonite. Larger quantities of the novel protease BcAP8 (described previously) have been isolated and purified for use in fermentation trials and will be benchmarked against bentonite and two known proteases, bromelain and aspergillopepsin. Experiments have begun using colloid characterisation techniques to measure the impact of different polysaccharides on protein stability. An investigation of the impact of the matrix components phenolics, ionic strength and sulfate on the stability of a chitinase protein is also underway.
Impacts of filtration on wine macromolecules
A range of red wines were filtered using cross-flow filtration, followed by lenticular filters and then 0.65 µm and 0.45 µm membrane filters. Cross-flow filtration was shown to remove some larger particles while all other levels of filtration (lenticular filtration, 0.65 µm and 0.45µm membranes) had negligible effect on macromolecules, as shown in Figure 3. Analysis at the next time point will show if macromolecules re-form complexes post-filtration and with wine ageing.
Macromolecule and colour extraction, stability and retention – influence on wine style and production practice
A collaboration was started with Dr Cassandra Collins and PhD student Dylan Grigg at the University of Adelaide looking at vine age impacts on wine colour during ageing. Another activity with Drs Anna Carew and Fiona Kerslake (TIA) was initiated to look at the interactive effect of leaf removal in the vineyard and microwave maceration on wine colour development. Two papers have been published on the changes in polymeric pigment structure during ageing (Bindon et al. 2014a, 2014b) and this methodology underpins current research on wine colour development during ageing.
Earlier harvesting of grapes is one option to achieve lower alcohol wines but the practice is limited by losses in colour and texture. Two collaborations with PhD students on projects based at the University of Adelaide were initiated to investigate ways to retain colour and texture in wines made from earlier harvested grapes. PhD student Sijing Li investigated the role of enzymes, mannoprotein and tannin additions on wine colour and texture. Through enzyme application, she was able to produce a lower alcohol Shiraz wine from earlier harvested grapes with the same tannin concentration as a wine made from later harvested grapes. However, since enzyme application also significantly modifies polysaccharide concentration and composition, she is further investigating the sensory implications. Another PhD student, Olaf Schelezki has begun a project using earlier-harvested grapes as blending options in the production of lower alcohol wines, and the AWRI is assisting with the analysis of grape and wine tannins.
Further studies have focused on evaluating yeast strains and maceration processes during winemaking as tools to alter wine macromolecule concentration and composition. In wine made in the 2014 vintage it was found that the choice of yeast strain resulted in highly variable polysaccharide and tannin concentrations. At the end of primary fermentation, the two yeasts which yielded highest wine tannin concentrations (1.5 g/L) resulted in wine with the lowest (0.45 g/L) and highest (0.66 g/L) polysaccharide concentrations respectively. It was found that high wine polysaccharide concentration in the case of one yeast was due to the release of pectic polysaccharides rich in galacturonic acid and arabinose from the grapes. This has implications for wine sensory properties, since larger tannins may exert reduced astringency when they are associated with polysaccharides. Based on leads from this trial, in 2015 an experiment was performed to investigate the interactive effect of maceration time (7 vs 30 days), macerating enzyme and yeast strains (‘high-tannin’ vs ‘low-tannin’ yeast) on wine macromolecules in 50 kg Shiraz ferments.
1 Bindon, K.A., McCarthy, M.G., Smith, P.A. 2014a. Development of wine colour and non-bleachable pigments during the fermentation and ageing of (Vitis vinifera L. cv.) Cabernet Sauvignon wines differing in anthocyanin and tannin concentration. LWT-Food Sci. Technol. 59(2): 923-932.
2 Bindon, K., Kassara, S., Hayasaka, Y., Schulkin, A., Smith, P. 2014b. Properties of wine polymeric pigments formed from anthocyanin and tannins differing in size distribution and subunit composition. J. Agric. Food Chem. 62(47): 11582-11593.
AWRI publication #1444. Marangon, M., Van Sluyter, S.C., Robinson, E.M.C., Muhlack, R.A., Holt, H.E., Haynes, P.A., Godden, P.W., Smith, P.A., Waters, E.J. Degradation of white wine haze proteins by Aspergillopepsin I and II during juice flash pasteurization. Food Chem. 135(3): 1157–1165; 2012.
AWRI publication #1518. Gawel, R., Day, M., Schulkin, A., Smith, P., Herderich, M., Johnson, D. Thescienceoftexture. WineVitic.J. 28(2): 30–34; 2013.
AWRI publication #1552. Gawel, R., Van Sluyter, S.C., Smith, P.A. and Waters, E.J. Effect of pH and alcohol on perception of phenolic character in white wine. Am. J. Enol. Vitic. 64: 425–429; 2013.
AWRI publication #1563. Van Sluyter, S.C., Warnock, N.I., Schmidt, S., Anderson, P., van Kan, J.A.L., Bacic, A., Waters, E.J. Aspartic acid protease from Botrytis cinerea removes hazeformation proteins during white winemaking. J. Agric. Food Chem. 61(40): 9705–9711; 2013.
AWRI publication #1587. Bindon, K.A., Madani, S.H., Pendleton, P., Smith, P.A., Kennedy, J.A. Factors affecting skin tannin extractability in ripening grapes. J. Agric. Food Chem. 62(5), 1130–1141; 2014.
AWRI publication #1596. Gawel, R., Schulkin, A., Smith, P.A., Waters, E.J. Taste and textural characters of mixtures of caftaric acid and Grape Reaction Product in model wine. Aust. J. Grape Wine Res. 20(1): 25–30; 2014.
AWRI publication #1617. Bindon, K.A., Kassara, S., Cynkar, W., Robinson, E.M.C., Scrimgeour, N., Smith, P.A. Comparison of Extraction Protocols to Determine Differences in Wine-Extractable Tannin and Anthocyanin in Vitis vinifera L. cv. Shiraz and Cabernet Sauvignon Grapes. J. Agric. Food Chem. 62(20): 4558–4576; 2014.
AWRI publication #1644. Ruiz-Garcia, Y., Smith, P.A., Bindon, K.A. Selective extraction of polysaccharide affects the adsorption of proanthocyanidin by grape cell walls. Carbohyd. Polym. 114: 102–114; 2014.