How Winemaking Works

In the year 2002, 595 million gallons of wine were sold in the United States, totaling about $21.1 billion in consumer spending. This translates to more than 2 gallons (7.6 L) of wine consumed per resident. Many Americans enjoy wine with a meal or have wine at social functions. It has even been reported that a glass of red wine per day can reduce your risk of heart attack and stroke.

Wine is an alcoholic beverage derived from grapes by fermentation, much the way beer is derived from the fermentation of grains. Unlike beer, wines are not carbonated (except champagne and sparkling wines). They also have about twice the alcohol content of beer. In this article, we will examine the steps in the fascinating process of making wine.

Fruit of the Vine

Grapes for winemaking are grown in many areas of the U.S., as well as other countries such as France, Chile and Australia. The major wine-producing area in the U.S. is California, which accounts for 80 to 90 percent of U.S. wine production. Many categories of winemaking grapes are grown throughout the country and include the following:

Vitis Vinifera - European type, such as Chardonnay, Cabernet Sauvignon, Merlot, Sauvignon Blanc, Riesling
French-American Hybrids - Baco Noir, Chambourcin, Seyval Blanc, Vidal Blanc, Villard Noir
Vitis Labrusca - American-type grapes, such as Catawba, Concord, Delaware, Niagara
Vitis Rotundifolia - native to North Carolina, such as Carlos, Magnolia, Scuppernog

**A vineyard**
Photo courtesy Chatham Hill Winery

Typically, the type of grape that is used to make the wine gives the wine its name, such as Chardonnay, Cabernet Sauvignon and Zinfandel. However, some wines are blends of various types of wines, such as a Semillon Chardonnay. The blending of various wines to produce a given flavor is part of the art of the winemaking.

Viticulture

Growing grapes, a process known as viticulture, involves a complex interaction (terroir) between the following factors:

Soil - Soil influences how much water and heat are available. Grapes need a steady, but not excessive, water supply.

Color - Dark soils tend to be warmer than light soils because they are better at absorbing and holding heat.
Geology - Rocky or stony soils allow water to drain better than clay soils; rocks also help to absorb heat in the soil.
Chemicals - The role of chemicals in the soil is not clearly understood (see sidebar).

Topography - This influences the amount of sunlight available (temperature) and the drainage (water supply).

Climate/Microclimate - This influences temperature, sunlight and water (rainfall, fog, mist). Some grapes, such as Vitis Vinifera, tend to grow best in areas where the seasonal temperature varies by about 30 to 35 degrees Fahrenheit (17 to 19 degrees Celsius).

The types of grapes that are planted and grown in any given area depend upon the terroir. In the Northern Hemisphere, grapes begin to bud in late March or early April. The grapes grow, bloom and develop fruit throughout the summer. The grower's goal is to keep the leaf growth small, which allows more sun in and keeps the grape clusters small yet numerous. The growers must also watch for signs of drought, disease and pests. In late September or early October, the grapes are ready to harvest. The actual times vary with the climate, latitude and judgment of individual growers.

Chemicals in the Soil

While the role of chemicals in the soil is not clearly understood, we do know that:

Too much nitrogen yields heavy leaf growth and shade, while too little produces hydrogen sulfide.
Too little potassium makes plants vulnerable to drought and disease and yields grapes that are low in sugar.
pH is not critical, because grapes grow in a variety of soil pH levels.
Too much organic material yields too much nitrogen and water, but too little will not support many soil organisms (earthworms).

Get Your Feet Wet

In the fall, it is crush time. The grapes are harvested. Some vineyards use mechanical harvesting techniques, but most hire workers to pick the grapes by hand. The grapes are then brought to the winery. Many wineries are located on or near the vineyards. If the wineries are far away, the grapes are shipped in refrigerated trucks.

Once the grapes reach the winery, they get crushed. Inside the crusher, there is a perforated, rotating drum. The holes in the drum allow the juice and the skins of the grapes to pass through, but keep the stems inside the drum. The crushed grapes and juice are called must.

What happens next depends on the type of grape. Red-grape must is sent directly to the fermentation tanks. White-grape must is sent first to a wine press, where the juice is separated from the skins, because white wines are fermented from skinless grapes.

**A wine press separates the juice from the skins.**

The wine press consists of a stainless steel cylinder with an inflatable rubber bladder inside. The must is poured inside the cylinder and the bladder is inflated with air. The bladder squeezes the skins against the side of the cylinder and forces the juices out. The juices are collected and sent to the fermentation tanks. At some wineries, the skins are recycled to local nurseries for fertilizer.

Sell No Wine Before Its Time

The must, whether from red grapes or pressed white grapes, is ultimately sent to the fermentation tanks. The fermentation tanks are airtight, made of stainless steel and can hold 1,500 or 3,000 gallons (5,678 or 11,356 liters). The tanks are cooled with glycol to maintain a temperature in the 40-F range (4-C range). The winemaker adds sugar and yeast to start the process of fermentation. The type of yeast and the amount of sugar added depends on the type of grape.

Fermentation

When the yeast first hits the must, concentrations of glucose sugar (C6H12O6) are very high, so it is through diffusion that glucose enters the yeast. In fact, it keeps entering the yeast as long as there is glucose in the solution. As each glucose molecule enters the yeast, it is broken down in a 10-step process called glycolysis. The product of glycolysis is two three-carbon sugars, called pyruvates, and some ATP (adenosine triphosphate). ATP supplies energy to the yeast and allows it to multiply. The two pyruvates are then converted by the yeast into carbon dioxide (CO2) and ethanol (CH3CH2OH), which is the alcohol in wine. The overall reaction is:

The fermentation process takes about two to four weeks. During this time, the winemaker samples the fermenting must and measures the pH or acid levels to determine that the fermentation process is proceeding as it should.

Storage

Once the fermentation process is completed, red wines are sent to the press to separate the skins from the wine. The red wines are then filtered to remove the yeast. White wines are allowed to settle and are filtered to remove the yeast. Once the yeasts are removed, the wines are stored in either stainless steel storage tanks or oak barrels (oak gives many wines a characteristic flavor) depending on the type of wine. In some red wines, a second type of fermentation, called malolactic fermentation, is undertaken while in storage. In malolactic fermentation, the winemaker adds a bacteria to the wine that breaks down malic acid, a byproduct of aerobic (oxygen-requiring) metabolism, into lactic acid, a byproduct of anaerobic (no oxygen) metabolism. Lactic acid is a milder acid than malic acid. The aging process can be anywhere from three months to three years.

Cork in the Bottle

After the wine has aged sufficiently, as determined by the winemaker, it is time to bottle and package it for sale. The operator pumps the wine from the storage tank to the bottling machine. There, bottles are loaded by hand and a pre-measured amount of wine flows into each bottle. After each bottle is filled, the operator removes it and places it in the corking machine. The machine draws a vacuum inside the bottle that sucks the pre-loaded cork into the neck of the bottle.

After the bottle is corked, the operator places the neck into the foil machine, which seals an aluminum foil wrapper over the cork. Next, the operator moves the bottle to the labelling machine, where the winery's self-adhesive label is placed on the bottle. Finally, the operator loads the bottle into a case for shipping and distribution.

Many wineries offer tours so you can see the winemaking process. They may also have tasting rooms where you can sample and purchase their products.

Champagne

Champagnes and sparkling wines are treated somewhat differently than other wines:

The grapes are grown and fermented the same as with any other wine.
After fermentation, the wines are aged for about five months.
The wine is bottled with extra yeast and sugar. The bottles are capped to allow for a second round of fermentation, which lasts for about a year.
The wine is aged for one or more years after the second fermentation.
The yeast is removed through riddling, whereby the bottle is placed upside-down and rotated one-eighth of a turn every day. The dead yeast cells settle into the neck of the bottle.
The neck of the bottle is frozen in an ice/salt water bath and the cork is removed. The pressure forces the frozen plug of dead yeast cells out of the bottle. This process is called disgorging.
A mixture of white-wine brandy and sugar (dosage) is added to top off the bottle.
The bottle is corked and wired to secure the high pressure inside.

Home Winemaking

The steps taken by commercial wineries can be carried out at home in a cool basement. Many people enjoy making their own wines from grapes that they have grown themselves, grapes that they have purchased or from other fruits, such as blackberries, strawberries, plums or peaches. Several home-winemaking kits are available, and there are stores that cater to the home winemaker by selling equipment and giving advice. Equipment and information is readily available over the Internet.

For more information on winemaking and related topics, check out the links on the next page.