Oxidation occurs when a chemical compound loses electrons. Oxidation is considered beneficial in some winemaking circumstances, but excessive exposure to oxygen under the wrong circumstances can cause a wine to spoil. Oxidation sets in as soon as grape crushing takes place. This is one of the reasons grapes are handled so carefully in their transit from the vineyard to the winery (placement in stackable shallow plastic boxes, as one technique). Oxygen reacts with chemical compounds in the newly liberated grape juice. A grape enzyme called polyphenol oxidase starts to work on the grape’s phenolic compounds and starts to turn the juice brown, not a good thing for a beverage so dependent on color for its visual appeal. Natural molds that reside on the grape skins get into the act, accelerating the browning. The necessary solution to this problem is the addition of small amounts of sulfur dioxide to the juice to deactivate the enzymes. Later in the winemaking process, additional SO2 will be added to act as a preservative.
Acetaldehyde is a substance that occurs naturally in grapes. When acetaldehyde gets out of hand, unpleasant odors result. A key part of the process of alcoholic fermentation involves the opposite of oxygenation: the reduction of acetaldehyde to alcohol. Acetobacter bacteria sometimes can react with oxygen to reverse the conversion, converting the alcohol to acetaldehyde. Things can go downhill from this point. Fresh fruity flavors and aromas in the wine become casualties in this chemical battle. Acetaldehyde reacting further with oxygen converts into acetic acid. The result is vinegar instead of wine, activating every winemaker’s worst nightmare.
The various steps involved in producing red wines often occur in the presence of some oxygen, in open top fermentation vessels, as one example. Punching down, pumping over, racking and other procedures provide measured oxygen contact. Red wines have high levels of phenolic compounds like anthocyanins and pigmented tannins, which not only protect against oxygenation, they also promote flavor and texture complexity. White wines differ profoundly on this issue. The phenolic compounds in white wine are incidental to the intended end result, but like acetobacter, they can react with oxygen to convert alcohol into even more acetaldehyde, seriously compromising color, aromas, flavor and freshness. For this reason, most white wines are fermented in closed fermentation vessels, keeping oxygen contact to a minimum.
Acids in Wine
Acidity in wine is one of its core characteristics. A wine that pleases has a good balance of acidity to other components like sweetness and tannin, and also has the right kind of acidity.
We measure acidity in two ways: the amount and the strength. The total acidity, or “titratable acidity,” is measured in grams per liter of wine. We measure the strength in pH, the lower, the more acidic. On the pH scale, 7 is neutral, 0 is extremely acidic, 14 the highest base reading. Most wine has a pH somewhere in the middle of the 2.9 to 3.9 range.
Two organic acids play important roles in wine grapes and hence in the wine it produces: malic acid and tartaric acid (citric acid, also from the grape, plays only a minor role). Malic acid is present in most berries and fruits, in green apples above all. The term “malic” is derived from the Latin malum, meaning apple. In the growing grape, malic acid enhances enzymatic reactions that are essential to the growth of the vine. Malic acid, however, is less desirable in the ultimate wine. It lends a harsh taste, like green apples. Tartaric acid is found primarily in grapes, but its positive effects are found primarily in the wine. It helps maintain the chemical balance and stability of the wine, helps the wine keep its color, and, most important, has a stimulating taste.
Both malic and tartaric acids inhibit bacterial growth, with two important exceptions. The acetobacter that can turn wine into vinegar is one of them. Lactic bacteria is another. This lactic bacteria, existing in the wine or added at a certain point in the winemaking process, drives the process called malolactic fermentation (or malolactic conversion) which converts harsh-tasting malic acid into softer lactic acid, rounding out mouthfeel, and in the case of some white wines like Chardonnay, adding that buttery taste. Malolactic fermentation is promoted for nearly all red wines, but for only some whites.
Acetic acid is not resident in the grapes. Fermentation creates small amounts of this harsh-tasting unwelcome guest. If conditions and oxygen exposure are not carefully monitored and controlled, acetobacter can react with oxygen to turn alcohol into larger quantities of acetic acid, as we have already discussed in the section on oxygenation. Wine begins to taste bad, and rapidly turns to vinegar (and not the balanced pleasant kind we use for our salad dressings). The term “volatile acidity” refers to acetic acid as it constitutes a wine fault.
Finally, during the primary (alcoholic) fermentation, yeast metabolizes nitrogen to create small amounts of succinic acid, which creates a taste that is a combination of saltiness, bitterness and acidity, the basic taste of fermentation, also present in beer.
Esters are aromatic compounds that form during winemaking when an acid combines with an alcohol. The two components, acid and alcohol, have no aroma. When they combine during the esterification process, an often wonderful array of aromas results.
We classify esters into two groups. Biochemical esters are created through yeast action. Chemical esters do not depend on yeast action, but occur as a result of wine maturation.
When acids meet alcohol and esterize, the perception of acidity decreases even if the total acidity remain constant. This is a flavor and aroma phenomenon. Esters may also be quite delicate, breaking down when wine is moved around. Chemical esters may eventually reconstitute, but biochemical esters can be permanently lost (as the yeast that creates them is long dead). This is why it is best not to move or shake a complex wine.
Grapes are among the sweetest of fruits, containing between 15 and 28 percent sugar. Glucose and fructose are the fermentable sugars. Arbinose, rhamnose, and xylose are unfermentable sugars that find their way into the ultimate wine in small amounts. Some of the fermentable sugar may also inhabit the wine, either by winemaking choice or by failure of yeast action. With global warming, depending on grape type, sugar content of grape juice is often so high that the yeast dies because of too much alcohol in its environment before it gets a chance to metabolize all the sugar into more alcohol. In high acid wines like Riesling, leaving in some natural grape sugar is a legitimate means of reducing the perception of acidity. In poor quality wines, leaving in residual sugar can be an attempt to mask winemaking defects.
Winemakers have three basic ways to arrest fermentation in order to leave residual sugar. One means is to fortify the wine by adding grape or grain spirits. This, of course, increases the alcohol level, which may not be desirable. The winemaker may also arrest fermentation by using a centrifuge to remove all yeast from the wine. A third way is to chill the wine to deactivate the yeasts and then filter them out.
The other option is to add sweetness to a wine that is already fermented by using unfermented grape juice. This option actually adds less sweetness than does arresting fermentation because of the difference in sweetness between glucose and fructose. Fructose is sweeter. In fermentation, yeast first ferments the glucose, so if fermentation is arrested the sugar left in the wine is more likely to be sweeter fructose. If unfermented grape juice is added, fructose and glucose will be about equal.
One of the major differences between red and white wine resides in the fact that red grapes have a higher level of phenolic compounds. Phenolics (also called polyphenols) include tannins, flavor precursors, pigments, and substances like resveratrol (which, some claim, has health benefits) and vanillin (which brings the characteristic flavor and aroma of vanilla to some wines). Phenolics come to the wine from the skins, stems and seed of the grape. The term “phenolic ripening” refers to the concept that exposure to sunlight over the ripening period increases phenolics (as opposed to “physiological ripening,” which refers to grape sugar creation). Once the juice ferments into wine, a process we call polymerization occurs. Pigments, tannins and other phenolics form long molecule chains that ultimately fall to the bottom as sediment and result in a lighter, friendlier, less astringent wine.
Wine color comes from the grape skin alone. It is a function of the level of anthocyanins in the skins. More acidic wines (meaning wines with lower pH values) are redder than wines with higher pH value (less acidic), which tend to have bluer tones. White wines have different types of pigments called flavones. Low pH acidic wines are paler while high pH less acidic wines congregate around the golden edge of the color spectrum.