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