The Process of Winemaking
By: Heather
Barr
Some
Background Information
Enology,
a word that comes from the Greek words for wine and study, is the term that is
used to describe the science of winemaking.
This science has its roots in prehistoric times, as the effects of
alcohol were probably discovered when rotten fruit was consumed and found to
have an intoxicating effect, which was viewed as pleasant. Fermented liquid could be stored for a long
time without fear of decomposition, and a reliable drinking source was obtained
(Wade, 1999).
The
process of fermentation is described by the reaction:
C6H12O6 yeast 2C2H5OH + 2CO2
glucose (sugar)
ethanol carbon dioxide
Yeast is added to a solution containing glucose, and
the yeast cells convert the simple sugars to ethanol and carbon dioxide. The alcoholic solution that results from
fermentation contains about 12-15% ethanol, as yeast cells cannot survive in
higher concentrations of ethanol (Wade, 1999).
High
sugar content is necessary for fermentation to proceed in a desirable
manner. Because of this, the sugar-rich
species of grape Vitis vinifera is
used for the majority of the wines produced around the world.
The Perfect Bunch of Vitis Vinifera*
As
many as 4000 varieties of Vitis Vinefera
have been developed and are used in the production of wines. Diversity and quality of wine results not
only from the type of grape used, but also from the distinctive qualities of
soil, topography, and climate. Although
the specifics of winemaking can vary from location to location and the
individual fermenting techniques, the basic steps involved in winemaking are
similar for most wineries (Napa Valley Vintners Association, 10/12/00).
There
are three main categories of wine: table wines, sparkling wines, and fortified
wines. Table wines can be red, white,
or rose, are allowed to ferment naturally, and account for the bulk of the
world’s wine production. Sparkling and
fortified wines are made by processes similar to the table wines, but extra
steps are added to give the carbonation or extra alcohol desired (Wine Making,
10/12/00).
Initial Stages (For All Wines)
Once
the grapes have been picked and transported to the winery, certain preparatory
steps must be taken before the actual winemaking can begin. Cleanliness and sanitation are essential for
good winemaking, as troublesome bacteria can cause disastrous results. Equipment must be sanitized with an O2 based
caustic solution, rinsed with water, and finally treated with an anti-bacterial
sulfite solution. To rid the equipment
of excess sulfite, everything is rinsed with water one more time (The Grape
Escape, 10/12/00). It is estimated that
approximately ten gallons of good quality water are needed for every one gallon
of wine produced.
Upon
arriving at the winery, grapes are treated with 50-75 ppm of free sulfur
dioxide. This process is called
sulfating, and inhibits the unwanted microorganisms and wild yeast species on
the grapes (Vine, 1981).
White Wine
Pressing the grapes
Grapes that are meant for
the production of white wine are picked and immediately
processed in the winepress.
A Winepress*
The grapes are gently squeezed for about 2 hours and
the juice is pumped (or fed by gravity) into holding tanks (Edgefield,
10/12/00). In the tanks, the juice is
chilled, and sediment from the fruit drops to the bottom. The sediments are
removed and wine is ready to be fermented with yeast (Edgefield, 10/12/00).
Fermentation
The
juice is transferred to fermenting vats, and the yeast is added.
Fermentation Vats*
With the addition of yeast, the term wine can now be
used to describe the grape juice. The species of yeast that is used to ferment
grape juice into wine is Saccharomyces
cerevisiae. This yeast is a
domesticated species that has been acclimatized to the effect of free sulfur
dioxide. In case some wild yeast still
exists in the juice, the domestic species can dominate and the fermentation
will be able to proceed in a predictable fashion (Vine, 1981).
The
juice is put in large vats from which air is excluded. In this way, oxidation is prevented and the
growth of bacteria is discouraged (Napa Valley Vintners, 10/12/00). The most problematic bacterium is of the
genus Acetobecter. This organism has the potential to convert
wine into vinegar overnight.
Fortunately, Acetobecter is
sensitive to free sulfur dioxide and preventative measure against the bacteria
can be taken (Vine, 1981).
Fermentation
is a process that takes place slowly over a period of ten to thirty days. The temperature of the liquid is maintained
at approximately 25oC, as severe changes in temperature can kill the necessary
yeast cells (Wine Making, 10/12/00).
Certain types of wines are fermented in ways that give them their
characteristic flavors. For example,
Chardonnay is placed in oak barrels to ferment, and an oaky flavor in the final
wine product results (Edgefield, 10/12/00).
Malo-lactic Fermentation
Oak
fermented wines may go through secondary fermentation, called malo-lactic
fermentation. This is a reaction in
which malic acid is converted into lactic acid, and results in the texture of
the wine changing from crisp and light to creamier buttery (Edgefield,
10/12/00). Malo-lactic fermentation can
either be introduced, or may naturally occur.
It is not an easily predicted reaction: it may begin immediately, or it
may take months for the process to begin.
The progress of malo-lactic fermentation is monitored with paper
chromatography. The benefits of
secondary fermentation are that it reduces the amount of total acidity and
causes a mellowing of the tartness in the wine (Vine, 1981).
Aging
The
amount of time that a wine is aged is equal to the time that elapses between
fermentation and drinking. White wine
tends not to be aged for long, though some ‘complex’ white wines can be aged
for 3-7 years. This process is much
more common in red wines. It is
described in more detail in the red wine section (UC Davis, 10/29/00).
Separation, Chilling, and
Bottling
After
fermentation, the wine is drawn off to separate it from the dead yeast cells
and other sediments that have precipitated from the juice. The wine is chilled to create more
clarification, and then bottled (Edgefield, 10/12/00).
Red Wine
Making the Must
Grapes intended for red wine are initially processed
in the crusher-destemmer.
Grapes entering the crusher-destemmer*
The grapes are separated from the stems and gently crushed
into a pulpy material called ‘must’.
The must is transferred into tanks or fermenting bins where it will
‘cold soak’ for a few days. Cold
soaking allows the juice to gain color and fruit flavor (Edgefield,
10/12/00). The addition of more sulfur dioxide
is usually necessary at this point to suppress the growth of wild yeast and
bacteria that had not been killed by the first sulfation (Napa Valley Vinters,
10/12/00).
Fermentation
After
a couple days of cold soaking, the must is inoculated with yeast and
fermentation begins. The carbon dioxide
that is produced by the fermentation pushes skins to the top of the tank or
bin, making a ‘cap’ on top of the liquid.
This cap is kept in contact with liquid as much as possible because
gives the juice color and tannins.
Tannins are the group of chemicals that naturally exist in the skin and
seeds of the grapes (and many other plants).
They give finished wine varying degrees of astringency (Napa Valley
Vintners, 10/12/00).
There
are two ways that the cap is kept in contact with the wine: ‘punching down’ and
‘pumping over’. In the process of
punching down, the caps are manually pushed down into the juice, usually with a
pole device. Historically, people have
used their feet for punching down the cap. For larger quantities of wine, pumping over is more common: a pump that attached to the bottom of the
tank, and the juice is pumped over to the top of the cap and circulated this
way for 15-20 min. Either process may
be used, but both must be repeated two to four times a day, until fermentation
is complete (Edgefield, 10/12/00).
Pumping over the wine*
Pressing and Settling
The
amount of time that a wine ferments varies on the type of grape and the method
of the winemaker. In average musts,
yeasts convert sugar into alcohol and carbon dioxide in 10 to 14 days. In wines that have a higher concentration of
sugar, fermentation can take up to months.
Although there is no definitive test to tell when fermentation is done,
the loss of cloudiness in the must indicates that fermentation in complete
(Deutsches Wieninstitut, 12/8/00).
After fermentation, the juice (now wine) is pressed away from skins into
a holding tank, where it sits for a few days to allow sediments and dead yeast
cells to settle out (Edgefield, 10/12/00).
The wine is separated from the skins*
Aging
The
wine is put into oak or redwood barrels for aging.
Wine is aged in barrels*
Aging allows oxygen to enter, and water and alcohol to
escape. The acidity decreases, clarification takes place, and components of
wine form compounds to enhance flavor and aroma. The wood also contributes to the flavor. Malo-lactic fermentation commonly occurs
during the aging of red wines, and contributes to the mellowing of the
wine.
Red wines are aged for
several months to several years, depending on the type and quality of the wine
desired (Napa Valley Vinters, 10/12/00). Some red wines are aged up to forty
years. It is important to note, however,
that it is a common misconception that wines must be aged. While some wines improve with age, others
can and should be drunk immediately. It
is possible that if wines are aged for too long, the tannins that give the wine
its flavor will precipitate out, and the wine will go ‘over the hill’ (UC
Davis, 10/29/00).
Sparkling
Wines
General Production
Sparkling
wines, including champagne, are produced in the same way as white wines until
fermentation is complete.
Addition of Carbonation
The
wine is put into bottles, and then sugar and more yeast is added to spur
fermentation. The bottles are capped
with an airtight seal so that the carbon dioxide that is created by the
additional yeast and sugar will be trapped.
The yeast will die and precipitate out of the solution, creating the
need to get this excess sediment out of the wine (Edgefield, 10/12/00)
Methode Champenoise
How
do you get yeast and sediment out of bottle without losing carbonation? The
‘Methode Champenoise’ is used. The bottles are placed in a specially designed
rack that keeps the bottle tilted at an angle.
Twice a day the wine is ‘riddled’ (the bottles are lifted out of rack
slightly, turned a quarter of a turn, then slammed back down again, eventually
settling the sediment into the neck of the bottle). Once the sediment has
condensed, the bottles are turned upside down and sent through a ‘disgorging
line’. In the disgorging line, the
necks of the bottles are frozen and small ice plugs form in the very end. The caps are removed, and the pressure
created by the carbon dioxide shoots the ice plug out. The sediment is frozen in the ice plug, so
the sediment leaves the bottle at the same time. A little wine, called a dosage, is put back in the bottle to
replace any wine lost. Finally, bottles
are corked quickly to prevent the carbon dioxide from escaping (Edgefield,
10/12/00).
Fortified
Wines
Fortification
Fortified
wines differ from table wines in that they have alcohol added to them. Port and Sherry are examples of two common
fortified wines. Port is a type of wine
that has alcohol added to it during the fermentation process. The grapes are picked when they are
extremely ripe, and processed in the same manner as red wine. When the fermentation is approximately
halfway complete, neutral spirits of grape base are added. Neutral spirits are made by distilling an
alcoholic beverage until it contains 95% or more alcohol by volume (BC Liquor
Stores 12/8/00). The natural
fermentation will cease at this point because the alcohol content is too high
for the yeast to survive. Port wines
are generally in the range of 17-24% alcohol.
Sherries are typified as wines that have alcohol added to them after
fermentation, and the percentage of alcohol in the wine varies (Edgefield,
10/12/00).
Helpful Links
Information
gathered from books and the internet were used to create this website. The websites that were found to be the most
useful included:
Edgefield Winery, The Winemaking Process, http://www.mcmenamins.com/Edge/Winery/process.html.
Napa Valley Vintners Association, The Wine Making
Process,
http://www.napavintners.com/nvvaproc.
University of California Davis, Wine and Grape
Information,
http://wineserver.ucdavis.edu/winegrape/index.htm.
Wine Making, http://fs.broward.cc.fl.us/~pelleng/2210pro.htm.
References
BC Liquor Stores,
http://www.bcliquorstores.com/glossary.cfm?term=neutral%20spirits, viewed 12/8/00.
Deutches Wieninstitut,
http://www.germanwines.de/english/wissen/wiss1_1.htm, viewed 12/8/00.
Edgefield Wine Making Process, http://www.mcmenamins.com/Edge/Winery/process.html,
viewed 10/12/00.
The Grape Escape: Detail of the Winemaking Process, http://tarpontale.com/grapescape/chemistry.htm,
viewed 10/12/00.
Napa Valley Vintners Association, The Wine Making
Process, http://www.napavintners.com/nvvaproc.html,
viewed 10/12/00.
UC Davis, Aging Wine, http://www.sbwines.com/usenet_winefaq.agingwine.html,
viewed 10/29/00.
Vine, Richard P., 1981, Commercial Winemaking:
Processing and Controls, Westport, Connecticut: AUI Publishing Company,
Inc.
Wade, L. G., 1995, Organic Chemistry, 3rd ed.,
Princeton, New Jersey: Prentice Hall, Inc.
Wine Making, http://fs.broward.cc.fl.us/~pellingh/2210pro.htm,
viewed 10/12/00.
* Pictures courtesy of Seven Hills Winery