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Though red and white wines are the result of the fermentation
of sugar in a wine grape, the processes only parallel each other
to a certain point. As well, a traditional Rose from let's say the
south of France is a blend of red and white wine, a Rose like "white"
Zinfandel is actually a red grape vinted like a white wine. Below
are the basic processes of making both red and white wine.
Red Wines
The juice of most red
grapes is white. Only by coming in contact with the skins can it
change color. Otherwise it will come out like a white wine. In making
red wine, grapes are passed through a "crusher," a machine
that de-stems the grapes and cracks their skins to allow the pulp
to come in contact with the yeast. At this stage, the combination
of skins, seeds, pulp and juice is called "must." The
must is then placed in a temperature-controlled fermenting tank
where either a commercial yeast is added or the natural yeast is
allowed to start fermentation. The pulp of the grape must stay in
contact with the skins in order for it to pick up color.
When fermentation is completed the liquid is now technically
wine. The mass of skins and seeds, called "lees," settles
to the bottom of the fermenter. The clear wine above the lees is
then drawn from the fermenting tank until all of the clear wine
has been removed and only the lees and pulp remain at the bottom.
This is called racking and is how most red wines are clarified before
bottling. The wine may be further clarified by passing it through
a filter or by use of a fining agent such as beaten egg whites.
Egg whites, being positively charged, attract the free-floating
sediment, which is negatively charged, and together fall to the
bottom of the tank or barrel. The wine must still be racked to a
clean barrel or tank in order to keep it clear. This is the gentlest
technique with regard to not removing any of the flavors of the
wine, just the solid particles, which can taste bitter and astringent.
If the intent is to process the wine as little as possible, regardless
of any sediment that may occur in the bottle, it won't be filtered
or fined. It is then aged, either in stainless steel tanks or small
or large oak barrels. Wines such as these are made to age over many
years, even decades. Within even 4 or 5 years of the vintage the
tiny pieces of pulp that were pulverized during fermentation begin
to attract each other to form sediment in the bottle. Unlike tartrate
crystals (explained below), however, sediment tastes awful and must
be removed by decanting or very careful pouring.
At this stage there are hundreds, maybe even thousands of choices
for the winemaker. What kind of barrels do we age in? How
old should they be? For how long? Most red wines go
through another, different type of fermentation called malolactic.
Instead of yeast interacting with sugar and creating alcohol and
carbon dioxide (CO2) by-products, a bacterial culture attacks
the harsh malic acid in the wine (the same acid as on the inside
of green apple skin) and converts it to lactic acid (the same as
lactose in milk). This lowers the wine's overall acidity and makes
it a little smoother and easier to taste. Malolactic (ML) can occur
normally or can be induced with natural cultures.
When ready (a matter of months or years depending on the winemaker's
desire), the wine is bottled and aged further before release. Aging
in the bottle develops what is called "bottle bouquet,"
the final melding of the grape flavors mixed with the processing
techniques.
White Wines
White wines are
made in a similar fashion except, in most cases, without the skin
contact. After crushing, the grapes are pressed quickly to avoid
contact with the skins. The tannic acid in all grape skins (red
or white) adds astringency to the wine. This astringency in reds
helps the aging potential. In whites this astringency may overpower
the delicate flavors so very little, if any, astringency is wanted.
Therefore, the pre-fermented juice is separated from the skins and
seeds in a gentle centrifuge. The remaining liquid is placed in
a fermenting tank or barrel, yeast is added, and fermentation begins.
Unlike red wines, white wines are fermented at very cool temperatures,
sometimes between 50-60 degrees F. This is done to preserve the fresh fruit
qualities and delicacy of white wines. Following fermentation, the
new wine is clarified, then aged in stainless steel or oak barrels
before bottling. Chardonnay and Sauvignon Blanc are occasionally
aged for an additional time in small oak barrels to add extra nuance
and complexity.
Fermentation of white wines is much longer than reds, usually
requiring several weeks versus 7-10 days for reds. During each stage
of winemaking, great care is taken to prevent the wine from coming
in contact with air that can "oxidize" it and cause browning.
Oxidation in wine is best understood by comparing it to the oxidation
that takes place when one cuts an apple. It turns brown within minutes,
as do potatoes when peeled.
Two additional precautions usually are taken with white wines,
but seldom with reds. These precautions are called "heat stable,"
and "cold stable." Heat stability is nothing more than
making certain, before bottling, that no yeast cells remain in the
wine. If the smallest amount of yeast were present and the wine
subjected to even moderate heat, it could begin to ferment even
the smallest amount of sugar. It has been known to happen with sugar
levels as low as .5%. Yeast cells are destroyed by heating the wine
to 120 degrees F for 30 minutes, or passing it through a special membrane
filter capable of straining out even microscopic yeast cells, or
waiting until fermentation has completely stopped and all the yeast
cells have expired. Most wineries choose the latter method since
the heat and filtration process can remove certain nuances, which
would take away from the finished wine.
Cold stabilization is often done in white wines to remove excess
potassium bitartrate, a natural substance found in grapes also known
as "cream of tartar." White grapes contain
fairly large amounts of potassium bitartrate. If most of it is not
removed, the wine will form crystals when placed in the refrigerator.
These tartrate crystals will either cling to the underside of the
cork or fall to the bottom of the bottle where they appear to be
ground glass to the uneducated eye. To remove excess potassium bitartrate
before bottling, the wine is placed in a stainless steel tank and
its temperature dropped to about 30 degrees F. The wine is held at that
temperature for a period of approximately 2-3 weeks. Excess potassium
bitartrate will then crystallize and drop to the bottom of the tank
where it is removed either by filtration or by pumping the wine
out of the tank until the hose is just above the bottom where the
crystals have formed. The "cold stabilized" wine is then
bottled. Some wineries prefer to skip this step, feeling that the
process detracts from the wine's flavor and nuance. Having the tartaric
crystals appear in the wine means nothing more than it was not cold
treated and, thus, may be of higher quality.
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