Transcript Phenolic Compounds and Tannins in Wine

A (very brief) introduction
to the complex chemistry of
wine flavour, colour and tannins
A bottle of red wine contains over 1000
chemical compounds
Quite amazing when you consider wine is >95%
water + alcohol
The chemistry of flavour, colour and astringency
in wine is enormously complicated
Many chemical and biochemical pathways are
not well understood
New flavour compounds in wine are still being
discovered – cutting edge research
Flavour compounds – esters, thiols, terpenes,
lactones, acids
Colour Compounds – anthocyanins, catechins
Astringent 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/nose
These include alcohols, esters, lactones, terpenes,
thiols
Non-volatile compounds that contribute to palate
flavour include organic acids and phenolics (acidity
and bitterness)
Directly from the grape
From 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 metabolism
Oak wood extraction
Chemical 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 oak
Exist in equilibrium – hydrolysis reaction will split the
ester back into the parent alcohol and acid
160 different esters have been found in wine
Formates
Acetates
Propanoates
Hexanoates
Octanoates
Succinates
•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 compound
From the grape, synthesised by
yeast during fermentation
(mostly from succinic and
glutamic acids)
Extracted from oak – β-methylγ-octanoate - oaky/coconut
2-vinyl-2methyltetrahydrofuran5-one – from grape precursors characteristic of Riesling and
Muscat
C13 secondary metabolites found in grape berry,
accumulate as non-volatile glycosides
Released as the free aroma compound through
fermentation and ageing
Contribute tea, honey, violet, tobacco, kerosene
flavours
1,1,6-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 skeleton
May contain functional group such as –
OH (terpene alcohols)
Primary source is grapes, not
fermentation
Forms a significant part of white wine
aroma/flavour profile
Characteristic floral
aroma of Muscat,
Guwurtztraminer
Thiols contain a terminal
sulphur group
Derived from S-containing
amino acids broken down by
yeast during fermentation
May also be sourced from the
degradation of S-containing
fungicides
Many thiols are considered
fault compounds
However some are important
varietal flavour compounds
Sauvignon Blanc aroma profile
Phenolic compounds are any compounds derived from
the phenol group
Can include acid, ester, glycoside and aglycone forms
Phenolics 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 wine
Little or no contribution to aroma (volatile phenols
excepted)
Provide balance on the palate
Bitterness and astringency can be seen as the ‘opposite’ of
acid
This 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'….
3 Subclasses of phenolic compounds in wine
Non-flavonoids – cinnamic and benzoic acid
derivatives, volatile phenols
Flavonoids – catechins, flavenols and anthocyanins
Phenolic-protein-polysaccharide complexes
Epicatechin –
Flavonoid
Ellagic acid –
non-flavonoid
Malvidin - anthocyanidin
Includes over 2000 phenolic compounds in the
plant world
Many are brightly coloured
In grapes, source is seeds/skins and tissue
Much of the structure and colour of wine is
from this group of compounds
Anthocyanins (red pigments), procyanidins
(colour co-factors and tannin precursors)
Flavonols (anti-oxidant compounds)
Catechins (yellow pigments)
Dominant anthocyanin in all varieties is malvidin
Total anthocyanins in young reds typically 500mg/L
Highly reactive compounds
Colour hue and density of young red wines result of a
complex series of delicately balanced equilibria
Particularly sensitive to pH and SO2 levels
Other pigments include hydroxycinnamic esters – nonflavonoid bitter tasting yellow pigments
Benzoic and cinnamic acid derivatives
Compounds 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 flavour
Also Coumarins – from oak
Can exist as the glycoside (bitter) or aglycone
(acidic)
Large molecular weight compounds made up of
smaller phenolic units
Characterised by their precipitation reaction
with proteins
Black tea much more astringent than tea with
milk due to precipitation of tannin with milk
protein
Tannins react with saliva proteins
Molecular weights range from 500 to over 3000
Act 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 barrels
Hydrolysable 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 acids
Hydrolysable 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 bonds
Not hydrolysable
Usually soluble but very large MW tannins may
precipitate
Generally Chemically stable
Dried tannin products added pre-fermentation
Assists colour and flavour extraction
Include ‘fluffy’ or white tannin ex chestnuts, red
tannin ex Grape Skins
Finishing Tannins – Supra and Quertannin
The latter is a refined oak tannin, very expensive
Added to refine structure and balance
Only 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 unit
Stabilises colour
Free anthocyanin levels
virtually disappear
within 12-18 months
Tannin levels rise
slightly, av. MW
increases
Larger MW tannins