Identification of the major drivers of 'phenolic' taste in ... - GWRDC

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AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines 1 General Introduction From the moment of grape harvesting, the juice that is expressed from the pulp of the white grape that ultimately makes up the majority of the final wine is subjected to processes that affect its final phenolic composition. Harvesting techniques, juicing methods, pre-ferment skin contact and the creation and use of press fractions influence both the total amount and type of phenolics that are found in the finished wine. The phenolic content can be further modified both quantitatively and qualitatively either by fining juice or wine with proteinaceous fining agents, subjecting the juice to oxygen, or less commonly by the addition of commercial preparations of phenolics. The use of pre-fermentation skin maceration and the judicious use of press fractions have been traditionally used by winemakers to achieve good expression of varietal flavour. These strategies are based on the knowledge that many of the volatile compounds responsible for flavour in these cultivars are mostly found in the fleshy cells immediately below the skin. However, varietal expression (or typicity as it is called in a broader sense) involves more than just aroma and flavour characteristics. The typicity of most varieties also depends on in-mouth textural characteristics. The three varieties used in this study – Viognier, Riesling and Chardonnay provide good examples. High quality examples of Viognier are as much defined by their oily, rich in-mouth texture as by their distinctive peach and apricot like aromas and flavours. Conversely a typical high quality example of an Australian or Germanic dry Riesling wine is typified by a palate structure that is light bodied and generally ‘lean’ in character. Finally, winemakers worldwide invest in high cost methods in an attempt to create the creamy and viscous texture that defines great examples of fuller bodied oaked Chardonnay styles. While some textures in white wine such as ‘hotness’ can be confidently attributed to major features of the wine matrix such as alcohol (Gawel et al. 2008), others such as bitterness and astringency have been associated with the presence of phenolics (Singleton et al. 1975). The cause of other mouth-feel characters such as viscosity, oiliness, metallic or pungency/burning are less clear, but are often found in wines that were made using processes that encouraged phenolic pick-up during winemaking. While understanding that the term ‘phenolic taste’ is ill-defined, we initially include the textural attributes of astringency, bitterness, viscosity, oiliness, metallic and pungency/burning as sub-attributes of ‘phenolic taste’ in white wine. 1.1 Classes of Phenolics in White Wine White wine phenolics can be broadly categorized into the non-flavonoids and the flavonoids. Non- flavonoids include the hydroxybenzoic and hydroxycinnamic acids. The flavonoids include flavonols and flavanols. Page | 10
AWRI: Identification Of The Major Drivers Of ‘Phenolic’ Taste In White Wines 1.1.1 Hydroxycinnamates The major non-flavonoids in white wine are the hydroxybenzoic and hydroxycinnamic acids. Hydroxycinnamic acids (HCA’s) and their derivatives, collectively referred to as hydroxycinnamates, are the most abundant phenolic compounds in white wines. The hydroxycinnamic acids present in wine are mainly derived from caffeic, p-coumaric, ferulic, and sinapic acids. While they can be present in both cis- and trans forms, the trans form is more prevalent. The concentrations of HCA in their free form are low in comparison to their esters of l-(+)-tartaric acid – the most important being the ester of caffeic acid known as caftaric acid. (Ong and Nagel 1978; Singleton et al. 1978; Somers et al. 1987). Caftaric acid is the most abundant hydroxycinnamate in both juice and wine, but the tartaric esters of p-coumaric acid and ferulic acid are also present (Somers and Ziemelis 1985; Vérette and Somers 1988; Crothers 2005). An important derivative of caftaric acid that has significant winemaking implications is 2-S- glutathionyl caftaric acid, better known as Grape Reaction Product (GRP). It forms when polyphenol oxidase (PPO) enzymes oxidise caftaric acid to the quinone which then chemically (i.e. non- enzymatically) react with the grape peptide glutathione. Similar GRP-like conjugates involving other hydroxycinnamic acids and peptides including cysteine and glutamine are also formed by the same mechanism. However, these are far less abundant than glutathionyl caftaric acid. Numerous other derivatives of HCAs have been found in white wine. These are the ethyl esters of caffeic acid and coumaric acid, the ethyl esters and diethyl esters of caftaric acid and the glucosides of all the major free hydroxycinnamic acids (Baderschneider and Winterhalter 2001; Somers et al. 1987; Monagas et al. 2005). The most prevalent benzoic acids in white wine are gallic acid, gentisic acid, p-hydroxybenzoic acid, protocatechuic acid, syringic acid and salicylic acid. Unlike the hydroxycinnnamic acids, they are usually found in their free form, and in comparatively low concentrations (Baderschneider and Winterhalter 2001). Of these gallic acid is the most abundant. While it is found in the grape itself, it also is released following the hydrolysis of the gallic acid esters of flavan-3-ols. Ethyl and methyl esters of vanillic and protocatechuic acids, and the glucose ester of vanillic acid have also been isolated from white wines. Hydroxycinnamic acids, benzoic acids and their derivatives are mostly found in the pulp of white grapes with up to 200 mg/L being reported in free run juice (Ong and Nagel 1978). White skins have also been reported to contain up to 45 mg/kg of hydroxycinnamates (Rodriguez Montealegre et al. 2006). Given that white wines are primarily made from the pulp of grapes, it is not surprising that the combined level of hydroxycinnamates can be substantial relative to other phenolic compounds found in white wine. As an example, a survey of 26 Greek white wines found an average hydroxycinnamate level six times higher than the combined concentration of the other phenolics present (Makris et al. 2003). Similarly, the combined concentration of caffeic and coumaric acids in dry Muscat wines was found by (Karagiannis et al. 2000) to be between 6 and 16 times higher than the combined concentrations of the important flavan-3-ol monomers, catechin and epicatechin. Page | 11
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