Structure and function of phenolic compounds in grapes and wine
Background
One of the keys to driving applied phenolics research forward is to increase the fundamental knowledge of how the chemical ‘structures’ of grape and wine phenolics affect their ‘function’ in wine. Whether the function of interest is mouth-feel, colour or perhaps bitterness, it is crucial to understand the molecular structures of the phenolics responsible. Currently we have placed particular focus on characterisation of tannin structure, reactivity and function. As tannins in wine constitute a hugely diverse mixture, and reference substances are largely unavailable from commercial sources, separation, isolation and characterisation of compounds are a significant challenge. Progress in characterisation of red wine tannins requires new technologies for separating these complex polymers and the project has progressed significantly in this area in the last year.
Progress
A variety of new methods for characterisation of the physico-chemical properties of tannins has been developed. New analytical HPLC techniques using novel solid phases now allow characterisation of several tannin sub-fractions. A solid phase extraction (SPE) method, using a novel solid phase, has been finalised in our laboratory for rapidly isolating pure wine tannins with minimal handling. This allows total wine tannin to be isolated; which can be further separated into two wine tannin sub-fractions of significantly different polarity. Gaining access to these two tannin sub-fractions is a major step forward as the relationship of each of the two sub-fractions is now being investigated with respect to wine age, variety, sensory properties and other parameters. Furthermore, a series of experiments is being undertaken to assess the influence of oxidation on wine tannins.
A survey of flavonols in Australian wines was also completed. As the first comprehensive study of this type it provided information on levels of flavonols in Australian wines, but also highlighted several important aspects of flavonol analysis, including the fate of rutin in wine. Rutin is rapidly degraded to the aglycone quercetin and does not exist to any measurable extent in bottled wine. Hence rutin is unlikely to contribute significantly to the mouth-feel properties of wine.
Mass spectrometry has proven to be valuable for the characterisation of red wine tannins and, together with the new SPE fractionation and partition methods, these methods now form essential parts of the ‘tannin toolkit’ available to the AWRI.
Highlights
- New analytical HPLC techniques using novel solid phases now allow characterisation of several tannin sub-fractions.
- A solid phase extraction (SPE) method, using a novel solid phase, has been finalised in our laboratory for rapidly isolating pure wine tannins with minimal handling
- The first comprehensive study of Australian wines provided information on levels of flavonols.
Project leader: Dr Paul Smith
Project team members:
- Dr David Jeffery
- Eric Dennis (PhD student, Flinders University)
- Dr Yoji Hayasaka
Publications:
1058 Jeffery, D.W., Mercurio, M.D., Herderich, M.J., Hayasaka, Y., Smith, P.A. Rapid isolation of red wine polymeric polyphenols by solid-phase extraction. J. Agric. Food Chem. 56(8), 2571–2580; 2008 (click here to order).
971 Smith, P.; Mercurio, M.; Dambergs, R.; Francis, L.; Herderich, M. Red grape and wine quality – the roles and relevance of tannin. Aust. N.Z. Wine Ind. J. 22(3): 47–52; 2007 (click here to order).
962 Gawel, R.; Francis, L.; Waters, E.J. Statistical correlations between the in-mouth textural characteristics and the chemical composition of Shiraz wines. J. Agric. Food Chem. 55: 2683–2687; 2007. (click here to order).
857 Herderich, M.J.; Smith, P.A. Analysis of grape and wine tannins: methods, applications and challenges. Aust. J. Grape Wine Res. 11: 205–214; 2005 (click here to order).
763 Vidal, S.; Courcoux, P.; Francis, L.; Kwiatkowski, M.; Gawel, R.; Williams, P.; Waters, E.; Cheynier, V. Use of an experimental design approach for evaluation of key wine components on mouth-feel perception. Food Qual. Pref. 15: 209–217; 2004 (click here to order).
768 Vidal, S.; Francis, L.; Noble, A.; Kwiatkowski, M.; Cheynier, V.; Waters, E. Taste and mouth-feel properties of different types of tannin-like polyphenolic compounds and anthocyanins in wine. Anal. Chim. Acta 513: 57–65; 2004 (click here to order).