to wine: all about oak
Let’s talk biology. When we discuss wine, we focus almost
exclusively on the grape vine, Vitis vinifera, in its many
varieties. But there are two other organisms crucial to wine
production that are often forgotten about. The first is the yeast,
Saccharomyces, without which our favourite tipple would
just be grape juice. The second – and the subject of this piece
– is the oak tree, Quercus.
An odd choice, you might think. However, oak’s accidental
association with wine has been a critical one. The majority of
fine red wines are dependent on oak barrels for a vital component
of their flavour, as are a good number of whites.
The reason oak barrels were initially chosen for storing wine
in had nothing to do with the flavouring effects they have: it’s
simply that in the past barrels were used as all-purpose
containers, and oak is a tight-grained wood capable of making
leak-proof barrels ideal for storing liquids in. The shape of the
barrel makes it extremely strong and once on its side it can be
moved by rolling, even when full.
The barrel-manufacturing process involves heating the staves
over a brazier so that they can be bent into shape. Somewhat
fortuitously, this slight charring – referred to as
‘toasting’ – coupled with the chemical properties of the
wood, means that the interaction of the wine with the inside of a
new barrel imparts pronounced flavour characteristics to the wine.
When used appropriately, new barrels can have a significant
beneficial impact on the wine that is aged in them.
Another equally important, but less talked-about, effect of
ageing wine in barrels is that this allows a very slight and
controlled exposure to oxygen. Normally, winemakers do all they
can to avoid exposing their wines to air, but in this case the
low-level oxidation that barrels permit is beneficial to the
structure and character of the wine.
So how does oak affect the flavour of the finished wine? For
red wines, barrels often add a little spice, enhance the
structure, and may add some sweet vanillin characters. A white
wine that has been fermented and aged in barrels will often have a
noticeable nutty, buttery character along with the spice and
vanilla characteristics that reds often pick up. Barrel aged wines
are generally a little more complex and have a more interesting
texture than those aged in tank, although some white grape
varieties, such as Riesling and Sauvignon Blanc rarely benefit
from being oaked.
A key difference in the effect of oak barrels is whether they
are made of French or American oak. If you ever get a chance to
try side-by-side cask samples of the same wine aged in French and
American oak, take it: the differences are marked. The relatively
wide-grained American oak imparts a much stronger flavour, with
more obvious sweet vanilla flavours and spicy notes. French oak
has a more subtle, slightly more savoury effect. Producers have to
decide which suits their wine better, although cost can be a
factor here: French barrels are much more expensive than American
ones. The degree of ‘toast’ and even the manufacturer of the
barrel are also important factors in the effect barrels have on
crucial variable here is the age of the barrel. New barrels impart
the most flavour, and this effect is subsequently diminished with
each re-use of the barrel, such that third-use barrels don’t add
much flavour at all. Many producers juggle their barrels
carefully, ageing their wine in a mix of new and used barrels to
avoid over-oaking it. Great care must be taken with the use of
older barrels, since they can harbour bacteria and yeasts that
might contaminate the wine.
Because new barrels are expensive, their use is usually
reserved for premium wines. But winemakers are only human: they
want the beneficial effects of oak for their cheaper wines as
well, without the high cost. As a result, barrel substitutes have
become increasingly popular. These can range from small oak chips
in teabag-like nets to barrel staves bolted into the inside of the
tank. Results can be variable, and are generally not as good as
those achieved by barrels. If you see the words ‘oaked’ on the
label of an inexpensive wine without mention of barrels, the
chances are one of these alternative techniques has been used.
A hi-tech twist on this theme is a technique called
micro-oxygenation. This process aims to simulate the gradual,
low-level exposure to oxygen that occurs in barrel. A specialized
device is set up inside the tank that releases a slow stream of
tiny oxygen bubbles in a controlled manner. The exact scientific
basis isn’t clear, but it’s a technique that’s becoming
widely adopted. Converts claim that it enhances the structure,
stabilizes the colour, and removes unwanted vegetal notes from red
wines treated in this fashion. Increasingly, micro-oxygenation is
being used in tandem with oak chips in a sophisticated emulation
of the process of barrel ageing, but at a much-reduced cost.
However, this is a technique still in its infancy, and it’s hard
to gauge its likely take-up with winemakers currently using the
real thing. Whatever happens, it looks likely that the historical
association between wine and oak is likely to be an enduring one.
also: How oak barrels are made:
an illustrated guide; and Learn about wine: