It has often been said that wine quality is made in the vineyard, and few experienced winemakers disagree with this statement. The soil, climate, the viticulture and all other aspects of the vineyard environment contribute to the quality of the wine. Even if the winemaker does a perfect job, the quality of the starting grapes always determines the potential quality of the wine. Grape quality is extremely important. Many winemakers feel that when a grape growing problem develops, the difficulty must be recognized and promptly resolved to assure fruit quality. Consequently, both professional and amateur winemakers prefer to grow their own grapes. Then they have complete control over the vineyards.


Two different fermentations occur in most red wines. The same two fermentations are also encouraged in some heavier styled white wines like Chardonnay or Sauvignon Blanc. A variety of yeast and bacteria can grow in wine, and many of these microorganisms cause other fermentations that reduce the quality of the wine.

Primary Fermentation

Conversion of the two major grape sugars (glucose and fructose) into ethyl alcohol is called “primary” fermentation. Yeast in the wine produce enzymes, and the enzymes convert the sugars into alcohol. Converting grape sugars into alcohol is not a simple process. Many steps are involved in this transformation, and the yeast must produce several different enzymes.

Malolactic Fermentation

Malic acid in the grapes is converted into lactic acid during the “secondary” fermentation. The necessary enzymes are produced by bacteria rather than by yeast. Several different types of bacteria can produce malolactic (ML) fermentation, and these bacteria are called “lactic bacteria.” Lactic acid is weaker than malic acid, so malolactic fermentation reduces the overall acidity of the wine. In addition, some byproducts produced during the ML fermentation can make a positive contribution to the complexity of the wine.

Other Fermentations

Depending upon the winemaking conditions, several other fermentations can and often do occur in wine. Some bacteria can ferment the glycerol in the wine into lactic and acetic acids. The natural grape sugars can be transformed into lactic and acetic acid by other types of bacteria. A few species of bacteria can ferment the tartaric acid in the wine into lactic acid, acetic acid and carbon dioxide gas. Vinegar bacteria can convert the alcohol into acetic acid. Then the same bacteria convert the acetic acid into water and carbon dioxide gas. These other transformations can produce several materials that detract from wine quality. Byproducts of these undesirable fermentations can be devastating, and when these fermentations occur, wine is often called “diseased” or “sick.”

During the fermentation phase, the primary function of the winemaker is to make sure the primary and secondary fermentations take place in a controlled and judicious way. Making sure the unwanted fermentations do not occur is also very important, so the wine is measured, smelled and tasted often.


At the end of the primary fermentation, the new wine contains many spent yeast cells, several different types of bacteria, tartrate crystals, small fragments of grape tissue, bits of dirt, etc. All these particles interact with light that passes through the new wine. The particles absorb or scatter the light, and they give the wine an opaque, turbid appearance.

Gravity will slowly pull most of these particles down to the bottom of the wine container. Then the winemaker can decant the clear wine off the sediment. The larger sized particles may settle out in a day or two, but smaller particles may take several weeks to fall. Some suspended material may be so small it never completely settles out of the wine. After gravity has removed most of the impurities from the wine, the winemaker may add a “fining” material to help the settling process. Alternatively, most commercial winemakers would choose to filter the wine and mechanically remove the remaining particles.

At this stage of its evolution, the wine may be clear and bright, but the wine probably is not completely stable. In other words, the wine may not remain in a clear condition over an extendedtime. Most wines contain excessive amounts of protein and potassium hydrogen tartrate. When wine is stored under certain conditions, the protein and the tartrate can precipitate out of the wine and produce a haze or a sediment. Any white or blush wine will probably be a total loss if either of these materials precipitates after the wine has been bottled. Wine stability is very important to the winemaker because of the protein and tartrate problems. Several techniques have been developed to remove excessive amounts of protein and tartrate from wine, and these procedures are part of the normal winemaking process. After the excess protein and tartrate materials have been removed, the wine will be chemically stable. Then the winemaker can continue the winemaking process with reasonable assurance that the wine will remain clear and bright after it has been bottled.


Odors in the wine that came directly from the grapes are called wine “aroma.” “Bouquet” is the term used for the odors in the wine produced by the winemaking process, and winemakers use the term “nose” when referring to both the aroma and the bouquet components.


Wine aromas come from the grapes. Aromas do not result from the winemaking process. Cabernet Sauvignon wine smells like Cabernet Sauvignon because of specific aromatic materials in that particular variety of grape. The “grassy” aroma, so characteristic of Sauvignon Blanc wine, is a consequence of the grape variety, not the winemaking process.


The formation of wine bouquet is a more complicated process. Wine bouquet is a result of the winemaking process. Wine bouquet is produced by the yeast, bacteria, barrels, winemaking procedures, etc. Some bouquet components are prevalent soon after the completion of fermentation, but these components decrease in intensity with time. Other bouquet components may require several years to develop fully. Byproducts produced by the yeast contribute to the fresh, fruity nose so typical of white table wines such as Gewurztraminer, Riesling and Chenin Blanc. However, these odor components are short-lived. They often disappear in less than a year or so. Consequently, these types of wines are best consumed when they are young, and the nose is still fresh and fruity.

Bouquet components decrease, remain constant or increase in intensity as the wine ages. By products produced by lactic bacteria can give wines a lasting buttery attribute. Wines stored in oak barrels slowly accumulate vanillin and other substances from the wood. Wine acids react with alcohols to produce volatile esters, and during bulk storage, oxidation slowly changes many wine ingredients. All these different materials contribute to the bouquet of the wine. After the wine is bottled, oxygen is no longer available, and a different type of aging begins to take place. Winemakers call these transformations reduction reactions because they take place without oxygen. Reduction aging is responsible for the changes that produce “bottle bouquet.” This is the bouquet that develops after a wine has been in the bottle for some time. As a wine ages, the aroma gradually decreases, and the wine becomes less and less varietal in character. Wine becomes more vinous as the aroma decreases, and the bouquet increases. When wines are blind tasted, wine experts sometimes have trouble distinguishing old Zinfandel wines from old Cabernet Sauvignon wines.


Winemaking can be divided into four major steps. First, grapes are harvested in optimum condition. Second, the grapes are fermented. In the third step, the new wine is clarified and stabilized. In the last step, the wine is aged to enhance its sensory qualities. Each of the four steps contributes to the quality of the finished wine. However, basic wine quality is determined in the first step.

The potential quality of any wine is established when the grapes are selected and harvested. Once the fruit is harvested, the winemaker attempts to realize the potential quality by carefully guiding the wine through the other three winemaking steps. Making high quality wine from poor quality grapes is impossible, but making poor quality wine from high quality grapes is very easy.

The winemaking process may take a few months, or it can extend for several years. During this time many procedures and operations are preformed, so both professional winemakers and home winemakers keep accurate records of the procedures used to make each wine. This record documents all the winemaking details starting from several weeks before the grapes were harvested until the wine is bottled and aged to perfection.

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