A (very brief) introduction to the complex chemistry of

wine flavour, colour and tannins

A bottle of red wine contains over 1000 chemical compoundsQuite amazing when you consider wine is >95% water + alcoholThe chemistry of flavour, colour and astringency in wine is enormously complicated Many chemical and biochemical pathways are not well understoodNew flavour compounds in wine are still being discovered – cutting edge research

Flavour compounds – esters, thiols, terpenes, lactones, acidsColour Compounds – anthocyanins, catechinsAstringent phenolics and tannins – flavanoids, non-flavanoids, tannins (condensed and hydrolysable)Balance in wine is usually ‘defined’ as:

Sweetness + alcohol = acidity + bitterness

Flavour of wine is influenced strongly by volatile compounds that contribute to bouquet/noseThese include alcohols, esters, lactones, terpenes, thiolsNon-volatile compounds that contribute to palate flavour include organic acids and phenolics (acidity and bitterness)

Directly from the grapeFrom non-volatile grape precursors - (many grape aroma compounds occur in a ‘bound’ form – not able to be smelled until they are converted to a ‘free’ form)Yeast and bacterial metabolismOak wood extractionChemical reactions

during wine ageing

Distribution within the grape berry of various phenolic components

Formed by the condensation of an alcohol and an acyl group attached to a coenzyme A molecule (acid + alcohol)

May be derived from the grape, produced by yeast during fermentation, or extracted from oakExist in equilibrium – hydrolysis reaction will split the ester back into the parent alcohol and acid160 different esters have been found in wine

FormatesAcetatesPropanoatesHexanoatesOctanoatesSuccinates

•Usually a combination of ethanol + hexanoic/octanoic/decanoic acids giving ‘fruity’ flavours• Or of higher alcohols (isobutyl, isoamyl) + acetic acid (fruity and vinous flavours)Ester retention requires the wine to be kept cold as these compounds are highly volatile.

A subclass of esters where the esterification is internal, forming a cyclic compoundFrom the grape, synthesised by yeast during fermentation (mostly from succinic and glutamic acids)Extracted from oak – β-methyl-γ-octanoate - oaky/coconut2-vinyl-2methyltetrahydrofuran-5-one – from grape precursors -characteristic of Riesling and Muscat

C13 secondary metabolites found in grape berry, accumulate as non-volatile glycosidesReleased as the free aroma compound through fermentation and ageingContribute tea, honey, violet, tobacco, kerosene flavours1,1,6-trimethyl-1,2-dihydronaphthalene - during ageing -‘kerosene’ note in aged Riesling

Aromatic compounds found in flowers, fruit, seeds, leaves (often floral and herbal aromas)5 carbon isoprene skeletonMay contain functional group such as –OH (terpene alcohols)Primary source is grapes, not fermentationForms a significant part of white wine aroma/flavour profile

Characteristic floral aroma of Muscat, Guwurtztraminer

Thiols contain a terminal sulphur groupDerived from S-containing amino acids broken down by yeast during fermentation May also be sourced from the degradation of S-containing fungicidesMany thiols are considered fault compoundsHowever some are important varietal flavour compoundsSauvignon Blanc aroma profile

Phenolic compounds are any compounds derived from the phenol group Can include acid, ester, glycoside and aglycone formsPhenolics contribute to the colour, structure, astringency and to a lesser extent, flavour of wine‘Tannins’ are large molecular weight compounds resulting from polymerisation reactions of smaller phenolic compounds

Structure of phenol

In general phenolics are the source of astringency in wineLittle or no contribution to aroma (volatile phenols excepted)Provide balance on the palateBitterness and astringency can be seen as the ‘opposite’ of acidThis is because acids promote saliva flow in the mouth ‘mouth-watering’Tannins and phenolics are ‘mouth-drying’ This is due to their property of reacting with and precipitating saliva proteins

Introducing the 'chicken-wire'Introducing the 'chicken-wire'….….3 Subclasses of phenolic compounds in wine

Non-flavonoids – cinnamic and benzoic acid derivatives, volatile phenolsFlavonoids – catechins, flavenols and anthocyaninsPhenolic-protein-polysaccharide complexes

Epicatechin – Flavonoid

Ellagic acid – non-flavonoid

Malvidin - anthocyanidin

Includes over 2000 phenolic compounds in the plant worldMany are brightly colouredIn grapes, source is seeds/skins and tissueMuch of the structure and colour of wine is from this group of compoundsAnthocyanins (red pigments), procyanidins (colour co-factors and tannin precursors)Flavonols (anti-oxidant compounds)Catechins (yellow pigments)

Dominant anthocyanin in all varieties is malvidinTotal anthocyanins in young reds typically 500mg/LHighly reactive compoundsColour hue and density of young red wines result of a complex series of delicately balanced equilibria Particularly sensitive to pH and SO2 levelsOther pigments include hydroxycinnamic esters – non-flavonoid bitter tasting yellow pigments

Benzoic and cinnamic acid derivativesCompounds that oxidise to cause browning in wines (oxidised form is yellow/brown)Generally odourless (but can be precursors to volatile phenol fault compounds)Have a bitter flavourAlso Coumarins – from oakCan exist as the glycoside (bitter) or aglycone (acidic)

Large molecular weight compounds made up of smaller phenolic unitsCharacterised by their precipitation reaction with proteinsBlack tea much more astringent than tea with milk due to precipitation of tannin with milk proteinTannins react with saliva proteinsMolecular weights range from 500 to over 3000Act as ‘oxygen’ soaks – assist in preventing oxidation of red wines

Condensed tannins from grape (skins/seeds)Hydrolysable tannins less from grapes but extracted from oak barrelsHydrolysable tannin structure

Centre of the molecule contains a carbohydrate (often glucose)Hydroxyl groups of the carbohydrate are partially or completely esterified with phenolic units such as gallic or ellagic acidsHydrolysable by weak acids or weak bases in the same way simple ester compounds are, produce the carbohydrate + acid

Linked units of flavonoid phenolics joined by carbon bondsNot hydrolysableUsually soluble but very large MW tannins may precipitateGenerally Chemically stable

Dried tannin products added pre-fermentationAssists colour and flavour extractionInclude ‘fluffy’ or white tannin ex chestnuts, red tannin ex Grape Skins Finishing Tannins – Supra and QuertanninThe latter is a refined oak tannin, very expensiveAdded to refine structure and balanceOnly used in red wine – excess phenolic content or any tannin content in whites is considered inappropriate (bitter)

Condensation reactions (removal of water)Polymerisation of anthocyanidin and procyanidin unitStabilises colourFree anthocyanin levels virtually disappear within 12-18 monthsTannin levels rise slightly, av. MW increasesLarger MW tannins impact on ‘mouthfeel’