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Ageing in Barrels
From the wine-making
occurring in vats, the wine is endowed with a wide aromatic
pallet, nevertheless the oak wood adds some other numerous
tinges that come to enrich the aromatic complexity of
the wine. During the ageing in oak barrels, naturally
aromatic, the dissolution of the extractive elements
of the wood, made possible by the alcoholic composition
of the wine allows the latter to get a woody taste so
much appreciated by its consumers.
Most of the elements coming from the wood are identified
and for each of them an aromatic and gustatory well-known
conformity has been granted, thus we can find many spicy
charactaristics such as clove, pepper…or floral
ones like carnation, rose or empyreumatic elements :
smoke, toasted bread….
The gathering of those various fragrance makes up the
basis of the woody taste.
At the time of its making, the barrel goes through an
essential stage for the toasting quality. This stage
permits the coming of various volatile components deriving
from the “thermodegradation” of certain
elements.
Those components can be studied, measuring them out
by chromatography in liquid stage and in fizzy stage.
I. Introduction to the chromatography
The chromatography
is a method of chemical analysis consisting in separating
the constituents of a blend. That separation goes off
thanks to an exchange between a movable period and a
fixed period.
The movable period is made of a sample to be analysed
and of solvent that will permit the migration of the
different elements of the sample all along a column
whose capillary wall (grapes or gel) set up the stationary
period.
The oak wood has got various very important phenolic
components in the quality of the ageing of “Grand
Cru” in barrels. The liquid chromatography is
also going to help us to put into relief the set of
the phenolic components of the heart of the wood and
of the toasted wood, and to realize the differences
as far as the origin of wood and the barrel manufacturing
techniques are concerned.
Nevertheless, we will not be able to detect the volatile
phenol.
II. The non-volatils components
The high performance
liquid chromatography allows to put into relief the
non-volatile components of the oak transferred to the
wine.
We set apart the gallic, the ellagic, the vanillic,
the ferulic and the syringic acids from the vanillic,
the syringic, the coniferilic and the sinapic aldehyde
and from the aescutine coumarin, scopolitine.
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Those
are polyphenolic components i.e they gather one or several
polyphenol functions. We notice phenol acids, phenol
aldehydes and coumarins. |
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Woods from France or America have few phenol aldehydes
(vanillin and syringaldehyde). However the toasted wood
is full of them.
The lignine (on the right) is a neutral complex phenolic
polymeric, that impregnates the “polysaccharid”
mould of the cellular wall of the duramen (heart of
the wood). |
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The
phenol aldehydes derive from the lignin which is part
of the oak.
In the condition of aerobic pyrolysis (case of the
heating), the terminal units of the lignin would be
likely to breaking and to rearranging themselves at
the level of T link making up aldehydic structure
: vanillin, syringaldehyde.
This rearrangement functions like the following
sketch : |
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Those
components specially enrich the aromas of the wine because
they bring about vanilla flavour (vanillin). Their quality
depend on the duration of the toasting but if the latter
is too long, they are going to degrade into phenol acids
and into volatile phenols that can grant tinge of “board,
sawdust”. |
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The
phenol acids have little interest because their organoleptic
role is of little importance. Indeed, the ellagitannins
(ellagic and gallic acids) play a part only in the fizzy
exchanges between the wood and the wine at the time
of the malolactic fermentation which consumes a part
of the oxygen.
The coumarins, among which the scopoletine, are polyphenolic
components present in the wood that become integrated
to the wine as the time goes by. Thus we can prove that
the concentration in scopoletine is all the more important
than the duration of the ageing in barrel.
It proves to be a very good marker of the ageing of
a wine. It has on the other hand no gustatory influence.
There are also various other types of phenols; the volatile
phenols, impossible to analyse by CLHP. Here is another
method of chromatography : the chromatography in fizzy
period. This analysis is going to determine the volatile
rich acids that the wood offers to the wine and the
very volatile scented components, of pyrolitic origin.
III.The Volatile Components
The scented components
found in the staves of a barrel are on the one hand,
natural components part of the brut wood, particularly
the polyphenols studied previously, and on the other
hand, substance made up from certain constituents of
the wood under the effect of the heating. Those are
of pyrolitic origin deriving from a thermodegradation
of elements already part of the wood.
We will thus find by-products of the thermodegradation
of sugars, of volatile phenols and of by-products of
lipids (the lactones).
Experiments are carried out on wines whose maturing
is different and above all the level of heating of the
barrel varies.
Results testify that the wood grants
volatile components (derived from lipids and volatile
phenols) such as the eugenol. It endows the wine with
those components.
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The eugenol (on the left) is a volatile phenol coming
from the lignin thermodegradation (as seen previously).
It can be thus found in the toasted wood. We can notice
that its concentration goes up with the level of the
heating. It is responsible for scents of clove, carnation,
pepper, leather, crushed herbs. It enriches the woody
character of the wine. Other volatile phenols can bring
unpleasant scented tinges such as the sweat, the horse…if
the heating is too long.
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The
wood enriches also the wine in “cis-3 methylane-4-lactone”
better known under the name of whiskylactone.
Indeed the heating tends to increase the number of
rich acids already existing in the wood. The complex
rich acids are scentless whereas the volatile rich acids
take part in the global aroma. The thermal degradation
of the lipids “hydroxyled” rich acids create
lactones by the natural loss of a molecule of water.
The lactones are cyclic esters of 5 or 6 stable unrestrained
atoms, widely spread in the plants. In the case of the
oak tree, the only lactones to be found are the two
isomers “A-methylactone-B-lactone”.
The isomer “cis” (sketch 13) has much more
fragrance than the “trans”(sketch 14). Those
substances disappear if the heating is too strong (spreadsheet)
but are necessary for the woody aroma of the wine because
they offer tinges of coconut or freshly chopped wood.
As for the rich acids, the wood like almost most of
the organic elements contains “polysaccharides”
that are going to be dissolved into the wine; they take
part in the sensation of some fatness in the wine and
they decrease the astringency that we will study later
on.
Finally, we can notice a high rise of the polyoxydes
by-products (sugar) thanks to the heating and the toasting.
Indeed, the heating of sugars in the wood can provide
several components by direct pyrolysis. The furanic
aldehyde with a toasted almond scent are their main
by-products.
Others very scented molecules coming from the thermal
degradation of the glucids can be recognised : those
are the oxygenated heterocycles responsible for the
toasted and burnt scents.
We also find nitrogenous heterocycles like pyrazine
which are well-known for giving at a small concentration,
some intensive scents of cocoa, praline, fresh bread.
Those dosage allow to emphasise the main aromatic components
of the wine which are brought about by the oak wood.
Those reinforce the typicity of the wine that acquires
its woody character all along its maturing.
We also find the pleaseant flavours of coconut, freshly
chopped wood, vanilla, clove, toasted, pepper or caramel.
However if ever the manufacturing of barrel is not harsh
enough, one can see appear unpleasant components smelling
the horse, the sweat.
The maturing in barrel offers also the wine the possibility
to alter its colour. There are some coloured polyphenols
in the wine containing 3 groups of pigments : free anthocyans,
tannins and the tannins-anthocyans combined.
Réactions:
- 2C2H5OH + O2 ---->
2CH3CHO + 2H20
- A + CH3CHO ----> A-O-CH-CH3
- 2SO2 + O2 + 2H2O
----> 2SO42- + 4H+
The free anthocyans
come from the grape, they have a red colour due to their
concentration, the PH and the content in free SO2 (preservative
of the wine with anti-oxidising put at the very beginning
of the fermentation).
The tannins derive from the procyanide and from the
polysaccharides their colours vary from colourless to
orange regarding the oxydation of the milieu.
The combinaison of the two in the presence of oxygen
leads to molecules more or less coloured in red. In
fact, the barrel has the distinctive feature of being
porous and thus allows the infiltration of oxygen into
the wine. It allows the combinaison tannins-anthocyans
and therefore a fixing agent of the wine colour as the
time goes by. We can observe little by little a disappearance
of free anthocyans and an oxidation of tannins that
causes a yellowing of the wine characterised therefore
of “brick-red”.
Moreover, tannins found in the wood have the distinctive
feature to bring out an unpleasant bitterness to the
wine, called astringency. The astringency is characterised
by the components of some bodies that produce a closing
of the mucous membrane of the oral wall (feeling of
pungency). It is necessary to limit at the most the
supply of tannins by the barrel. The major interest
of the drying process of the stave-wood in the open
air before the manufacturing happens in here.
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