Taste and mouth-feel properties of different types of tannin-like polyphenolic compounds and anthocyanins in wine

Abstract

The taste and mouth-feel properties of three different types of tannin-like polyphenolic compounds, representative of some of the tannin-like polyphenolic compounds found in red wines, were determined using descriptive sensory analysis. Ethyl-bridged flavanols were produced by reaction of (+)-catechin with acetaldehyde under acidic conditions. Red coloured tannin-like polyphenolic compounds from wine and from wine pomace were isolated by multi layer coil counter-current chromatography (MLCCC). Mouth-feel attributes and bitterness of the fractions dissolved in a model wine (MW) medium were rated while the fractions were held in the mouth and after expectoration. The sensory properties of the fractions described above were compared to those of apple procyanidins (tannins) with degree of polymerisation (dp) 3 and 9. Both wine and pomace-derived coloured tannin-like polyphenolic compounds were rated as significantly less astringent when compared to both the apple tannin fractions but were similar to the MW. The ethyl-bridged flavanols (with mean dp of 5) were rated significantly more bitter than both the apple procyanidin fractions. Astringency of the ethyl-bridged flavanols was lesser than the apple procyanidin fraction with dp9 and more than that with dp3. Highly purified grape anthocyanidin monoglucosides and monoglucoside coumarates, prepared either by solid-phase extraction or by multi layer coil counter-current chromatography were rated similarly to the MW indicating that free anthocyanins, like the coloured tannin-like polyphenolic compounds from wine and pomace, do not contribute astringency nor bitterness to wine.

Introduction

Bitterness and astringency are two sensory terms of crucial importance for describing the sensory properties of wine. Whereas bitterness is a taste mediated by sensory receptors, astringency is considered to be a tactile sensation resulting from the precipitation of salivary proteins and leading to a loss of mouth lubrication. Wine astringency is mainly attributed to the presence of the phenolic compounds called tannins.

Wine tannins include a variety of phenolic compounds as described elsewhere [1]. Proanthocyanidins or condensed tannins originate from grapes and can be extracted from seeds, skins and stems during wine-making. Proanthocyanidin structures vary in the nature of constitutive sub-units, degree of polymerisation (dp) or chain-length and linkage-position. Grape seed proanthocyanidins (procyanidins) are composed of (+)-catechin, (−)-epicatechin and (−)-epicatechin-3-O-gallate (∼20%) resulting from acylation of epicatechin by gallic acid [2]. Skin proanthocyanidins are composed of the same units with only 5% (−)-epicatechin-3-O-gallate and additionally include (−)-epigallocatechin (∼20%) [3] and are consequently referred to as prodelphinidins. Stem proanthocyanidins have a composition between that of skin and seed proanthocyanidins [4] but only account for 5 to 10% of total grape proanthocyanidins.

Mouth-feel properties of grape seed and skin proanthocyanidins have been examined through many studies [5], [6]. A recent study showed that astringency increased with the dp and that an increasing degree of galloylation was responsible for an increasing coarse perception of the proanthocyanidins [7]. Increasing the degree of B-ring trihydroxylation (given by epigallocatechin content) seemed to decrease astringency.

Once extracted from grapes, phenolic compounds, including the seed and skin proanthocyanidins undergo chemical reactions during vinification to form tannin-like polyphenolic compounds in wine, following two main reaction pathways. The first involves acetaldehyde-mediated condensation between flavanols [8] leading to molecules referred to as ethyl-bridged flavanols. This type of condensation can also occur between anthocyanins [9] or between flavanols and anthocyanins. In the latter case, both ethyl-bridged flavanol–anthocyanin [10] and pyranoanthocyanin–flavanol adducts have been reported [11]. The second type of wine tannin-like polyphenolic compound is derived from direct reactions between flavanols and anthocyanins. Because these compounds can act as electrophiles or nucleophiles at wine pH, both anthocyanin–flavanol and flavanol–anthocyanin adducts can be formed. The decrease of astringency which occurs during wine-ageing may be the result of the formation of these tannin-like polyphenolic compounds, which have been suggested to be less bitter and astringent than the parent compounds [12], [13]. Because of the difficulty in isolating and characterising these types of molecules, only limited studies of their mouth-feel properties have been made to date.

The aim of the present study was to improve our understanding of the mouth-feel properties of wine tannin-like polyphenolic compounds. Descriptive analysis of the taste and mouth-feel properties was conducted on isolated and characterised wine tannin-like polyphenolic compound fractions, native proanthocyanidins and native anthocyanins in a wine-like model solution.