III.

How Wine Is Made

Wine is the product of the fermentation by yeast of grape juice or grape must, grape juice that still contains the fruit’s skins and seeds. Once the grape sugar has been completely consumed, fermentation is complete, and wine has been produced. The science that deals with wine making is known as enology.

While the basic production elements of wine are simple, manipulation of the grapes, juice or must, and wine to produce the desired combination of flavors and aromas is very difficult, and many recognize this process as an art form. Wine makers try to optimize production of specific aromas and flavors—described with terms like cherry, chocolate, vanilla—and minimize the formation of negative flavors and aromas—described as wet dog, plastic, and rotten egg. It is also important that the wine acids and alcohol are balanced. If the wine is too acidic, the wine may taste sour. If the ethanol level is too high, the wine will have a strong taste of alcohol.

The single most important factor that contributes to a wine’s character is the grapes that are used. Grapes influence the wine’s flavor, alcohol content, acidity, and even its color. White wine, which is actually straw to golden-yellow in color, is produced from white grapes, and red wine is produced from red grapes. Red and white wine production is basically the same except for one primary difference: the presence of the grape skins during fermentation. White grapes are crushed and the juice separated from the skins prior to fermentation. Red wine is fermented with the grape skins. Red pigments called anthocyanins and other compounds in the grape skins are extracted during the fermentation process to impart the characteristic red color of the wine as well as other features. A blush or rosé wine is light pink in color and is produced from red grapes not fermented with the skins. A little pigment is released when the red grapes are crushed, but not to the same extent as during fermentation.

In modern wine production, the grapes are harvested from the vineyards and taken to a winery where they are passed through a machine called a destemmer-crusher that separates the fruit from the stems and cracks the berries open to release the juice. To make white wine, the must is transferred to a press where pressure is applied to separate the juice from the skins. The amount of pressure used influences what flavor compounds are extracted from the skins. After pressing, the white juice without the skins is transported to a fermentation tank. In red wine production, the must from the crusher is transferred directly to a tank for fermentation.

The containers used for fermentation are mostly stainless steel or wood. The type of container used and the temperature of fermentation influence the characters of the wine. Many of the aroma components of wine are volatile—that is, they leave the wine by evaporation. This evaporation occurs faster at higher temperatures, so to retain fruity characters in the wine the temperature of fermentation must be controlled, usually by direct cooling of the fermentation tanks. Stainless steel is much easier to cool than wood and is preferred for temperature-crucial fermentation.

The wine maker may allow fermentation to proceed relying only on the yeast naturally present on the grape skins and in the winery equipment, or the wine maker may add extra yeast in a process known as inoculation. Two yeast species are used in fermentation, Saccharomyces cerevisiae or Saccharomyces bayanus. Yeast is responsible for the presence of positive but also negative aroma characters in wine. For example, when yeast is under stress it produces a compound called hydrogen sulfide, which smells like rotten eggs. To avoid this undesirable quality, a wine maker may add nutrients to the fermentation tank. The duration of fermentation also influences wine character.

Other naturally occurring microorganisms may grow in the must or juice, affecting the flavors and aromas of the finished wine. For example, lactic acid bacteria use the acids in wine as a source of energy, reducing the wine’s acidity. These bacteria also produce other aromas and are responsible for the buttery smells that can be found in wine. Sometimes the wine maker restricts the growth of lactic acid bacteria, especially if the wine is already low in acidity or if the buttery character would clash with other aromas of the wine. Acetobacter, another type of bacteria, can spoil the wine by converting ethanol to acetic acid to make vinegar.

When fermentation is complete, red wine is separated from the stems and grape skins by passing it through a press. Both red and white wines appear cloudy after fermentation, and the wine maker must wait for the yeast and other solids to settle to the bottom of the fermentation tank, forming a sediment called the lees. The clear wine is racked or drawn off the lees and stored in a clean cask. In a process called fining, the wine maker may further clarify the wine by adding ingredients that attract unwanted particles, such as proteins that can cause cloudiness. These added ingredients settle to the bottom and can be easily collected and removed.

After fermentation, the wine maker has to decide how the wine will be aged. Aging of wine significantly affects the flavors and aromas present, and several different techniques are used. For example, wine aged in oak barrels picks up some flavor and aroma characters from the oak wood, a very desirable quality in some wines. A wine may be aged under conditions encouraging the loss of some of the fruity, volatile compounds, producing a wine rich in other characters, such as spicy or toasted flavors. Air exposure during aging can cause the phenolic wine compounds, extracted from grape skins and seeds, to combine with each other, producing large chemical compounds called tannins. Over time the tannins become so large that they form reddish-brown sediment in the bottle. This reduces wine bitterness and astringency. The length of time a wine is aged before it is bottled determines the extent to which these reactions occur. Once the wine has been aged, it is ready to be put into bottles, where it may continue to slowly age for many years.