Copyright 1999
Chapter 6
pH AND SULFUR DIOXIDE
Winemakers are always concerned with titratable acid (TA) and pH because both parameters influence wine characteristics. As discussed in the previous chapter, titratable acid is primarily responsible for the tart taste of table wines, but pH has little relationship on the tart taste. However, pH strongly affects several other important wine properties including color, oxidation, biological and chemical stability, etc. Although pH depends on the total acid content, other factors like potassium content influence pH, and because of these other factors, pH is not directly related to titratable acid. Nevertheless, wine pH is a fundamental parameter. pH has a profound influence on the biological and chemical effectiveness of sulfur dioxide in wine.
pH Chemists use the pH scale to describe the number of hydrogen ions present in a solution. pH uses an upside-down, logarithmic scale, and because of the upside-down scale, a smaller pH value represents more hydrogen ions. For example, a wine with a pH value of 3.0 contains ten times more hydrogen ions than in a wine with a pH of 4.0. Consequently, the pH value of a solution becomes smaller as the acid content of the solution becomes larger. Sometimes novice winemakers are confused by the upside-down scale. pH can be measured by several different methods, but a pH meter with three-digit accuracy is the most practical way of measuring wine pH.
Factors Affecting Wine pH
pH is a measure of the number of hydrogen ions present in a solution. Consequently, the pH value reflects the quantity of acids present, the strength of the acids and the effects of minerals and other materials in the wine. Many different factors are involved, but wine pH depends upon three major factors: (1) the total amount of acid present, (2) the ratio of malic acid to tartaric acid, and (3) the quantity of potassium present. These three factors are discussed below.
(1) Wine acids produce hydrogen ions, and pH is a measure of the number of hydrogen ions present in a solution. Overall, wine pH will be lower when the titratable acid is higher. However, high titratable acid does not always produce low pH values. The presence of potassium and several other factors alter wine pH. Malic acid is weaker than tartaric acid, so wines unusually high in malic acid can have a high TA and a high pH value. High acid, high pH wines require special treatment using an ion exchange technique. However, ion exchange equipment is very expensive, so most small producers have difficulties handling high acid, high pH wines.
(2) Tartaric acid produces almost three times more hydrogen ions than malic acid, so gram for gram, tartaric acid produces a much lower pH than malic acid. Therefore, when the total acid content is fixed, pH depends upon the relative amounts of tartaric and malic acid in the juice or wine. For example, a wine containing an unusually large amount of malic acid might have a titratable acid of 0.65 percent and a pH of 3.9. A second wine containing more tartaric and less malic acid might have a titratable acid of 0.65 percent, but the pH might be 3.4. Wine pH increases as the relative amount of malic acid increases.
(3) Potassium (K) is essential for vine growth and fruit production. Potassium is a mineral, and vines obtain potassium through their roots. The roots remove potassium from the soil, and the potassium is distributed to all parts of the vine. Early in the season, when the growth rate is high, much of the potassium accumulates in the leaves. Then the potassium ions are moved from the leaves into the berries later in the season when the fruit starts to ripen.
Potassium ions carry a positive electrical charge just like hydrogen ions. Under certain conditions, potassium ions can change places with the hydrogen ions at the extreme ends of the tartaric acid molecules. These are the hydrogens that ionize easily in water solutions, and these are the hydrogens shown in bold type in Figure 2. Potassium bitartrate is formed when potassium is exchanged for hydrogen, and the hydrogen then becomes a free ion in the solution.
Tartaric acid has two hydrogen atoms that can ionize. One hydrogen atom ionizes easily, and tartaric acid is the strongest of the primary wine acids. Potassium bitartrate only has one ionizable hydrogen atom, and it does not ionize so easily. Therefore, potassium bitartrate produces fewer hydrogen ions than tartaric acid.
Grapes contain from one-half to three grams of potassium per liter of juice. Grape skins contain about nine grams of potassium per liter, so grape skins contain four or five times more potassium than the juice. When grape juice and skins remain in contact for extended periods, potassium leaches out of the skins into the juice. The additional potassium from the skins reacts with tartaric acid in the juice and forms potassium bitartrate. When alcohol accumulates during fermentation, the juice cannot hold all the additional potassium bitartrate, and some tartrates precipitate out of the liquid. Red wines usually have a lower titratable acid content and higher pH values than white or blush wines because of the extended skin contact time.
Significant amounts of potassium bitartrate can also precipitate as the wine is bulk aged. When potassium bitartrate precipitates, the titratable acid of wine decreases, but wine pH may increase, decrease or stay the same. If the starting pH of the wine is 3.6 or less, the pH will become smaller as the bitartrate precipitates out of the wine. If the starting pH is 3.8 or greater, the pH will become larger as the bitartrate precipitates. Little change will occur when the starting pH falls between about 3.6 and 3.8.
Advantages of Low pH
Titratable acid is primarily responsible for the tart taste of table wine. Over the range of 3.0 to 4.0, pH has little influence on wine taste. Nevertheless, pH strongly influences other important wine characteristics. pH values range from about 2.9 to 4.2 in wine. This may seem like a small range, but the pH scale is logarithmic, and a pH change of 0.3 represents a change in hydrogen ion content of about 20 times.
The chemical stability and the biological stability are both very sensitive to the pH value of the wine, and that winemakers prefer to have wine pH values between 3.0 and 3.5. Chemical and biological stability are improved so much at these lower pH values, most winemakers believe pH is the more important wine acidity parameter.
Wine yeasts are quite tolerant of pH. Yeast growth does not change significantly over the normal range of wine pH values, and overall fermentation characteristics are little affected by pH. On the other hand, wine bacteria do not tolerate low pH values, and wine pH strongly influences both bacterial growth rate and bacterial fermentation characteristics. This is why malolactic fermentation is not likely to occur in wines with pH values lower than 3.3. Bacterial activity is reduced in low pH wines, and many of the bacterial problems discussed in Chapter 13 become insignificant when wine pH is low.
A variety of chemical reactions take place in wine, and many of these reactions are affected by the total number of hydrogen ions present. For example, wine pH has a direct influence on the hot stability of wine. Under warm storage conditions, protein precipitates out of white and blush wine, and serious haze and sediment problems occur when protein precipitates after the wine is bottled. Consequently, white and blush wines are always treated with bentonite to remove excess protein. Here, pH is an important consideration because bentonite is more effective in removing protein when wine pH is low. As the wine pH increases, bentonite becomes less and less effective, and more bentonite must be used to remove the protein. Excessive amounts of bentonite can strip wines of desirable aromas and flavors, so adding more bentonite is not desirable.
Sauvignon Blanc grapes often contain large amounts of protein, and Sauvignon Blanc wines with high pH values can be difficult to stabilize completely. Sometimes little varietal aromas remain in these wines when enough bentonite is used to remove the excess protein. However, wines with low pH values seldom have this problem.
Wines with low pH values generally have better visual qualities. At low pH values, red wines show more color, and the color is better. Color intensity increases, and the red color becomes more purple at low pH values. Both red and white wines have better color stability when the pH is low. Some important polymeric reactions are accelerated at low pH values, and much of the unstable color pigments precipitate out of the wine early in the winemaking process. After the unstable pigments are gone, wine colors are more stable.
The following Table shows how some major wine characteristics are affected by pH.
| WINE CHARACTERITIC | LOW pH (3.0 - 3.4) | HIGH pH (3.6-4.0) |
| Oxidation | Less | More |
| Amount of Color | More | Less |
| Kind of Color | Ruby | Browner |
| Yeast Fermentation | Unaffected | Unaffected |
| Protein Stability | More Stable | Less Stable |
| Bacterial Growth | Less | More |
| Bacterial Fermentation | Less | More |
| SO2 Effectiveness | More | Less |
SULFUR DIOXIDE
Sulfur dioxide is a colorless gas formed from one sulfur and two oxygen atoms (SO2). It is foul smelling and noxious. The distinctive smell left by a burnt match comes from sulfur in the match reacting with oxygen in the air and producing sulfur dioxide. Sulfur dioxide gas reacts with water and forms sulfurous acid, and the sulfurous acid can be further oxidized into sulfuric acid.
Benefits
Sulfur dioxide has several desirable attributes when added to wine in very small quantities. Enzymes in the grapes that cause browning are deactivated by sulfur dioxide. Sulfur dioxide helps protect wine from excessive oxidation. Sulfur dioxide can reduce the oxidized smell of old wine by reacting with acetaldehyde. Sulfur dioxide is very useful in controlling the growth of bacteria and yeast.
Man has been adding sulfur dioxide to wine for more than a thousand years. A large body of knowledge exists on the use of sulfur dioxide in wine and in many other food products. The benefits of using sulfur dioxide in wine are well documented, and its positive effects are indisputable. Several characteristics of sulfur dioxide in wine are briefly discussed below.
Deactivates Enzymes
Grape juice is in contact with the surrounding air during the crushing and pressing operations, and the juice reacts with oxygen in the air and becomes oxidized. Oxidation causes the juice to darken, and the juice gradually turns brown. Browning is greatly accelerated by the presence of naturally occurring enzymes in the grapes. Polyphenoloxidase is the name of the this enzyme, and it is the same enzyme that causes freshly cut apples to turn an unpleasant brown color. Some grape varieties brown easily, while other grape varieties have little browning tendencies. The differences in susceptibility can be accounted for by the amount of Polyphenoloxidase enzyme that occurs in different grape varieties.
Enzymes responsible for browning are very sensitive to free sulfur dioxide, and the enzymes are deactivated when sulfur dioxide is added to the juice. The quantity of sulfur dioxide needed is very small, so sulfur dioxide is a powerful tool for reducing enzymatic browning in white and blush wines.
Inhibits Oxidation
The great French scientist, Pasteur, observed ". . . oxygen is the ardent enemy of all wine." Air is always present, and oxygen in the air is always ready to react with unprotected juice or wine. Grape juice and wine contain a variety of materials, and many of these substances are adversely affected by oxidation. Unpleasant, bitter, off-odors and off-tastes can be produced when these materials oxidize.
All wine components are subjected to small amounts of oxygen throughout the lengthy winemaking process. Many of the desirable changes that take place during bulk aging are oxidation reactions, so oxidation does not necessarily produce adverse changes when small amounts of oxygen are introduced very slowly. However, wine quality is reduced quickly when oxidation becomes excessive.
When small quantities of sulfur dioxide are added to grapes or wine, roughly half the amount added quickly combines with other wine constituents. The uncombined half remains in the wine in a free state. Only the uncombined or free sulfur dioxide is effective. In the free state, some of the sulfur dioxide combines with any oxygen that may be present before any of the other wine constituents become oxidized.
Sulfur dioxide is one of the most effective methods available for controlling oxidation, and most winemakers add enough sulfur dioxide when the grapes are crushed to give 30 to 50 milligrams of SO2 per liter. The recommended amount of sulfite powder is shown in Table below.
| Lbs. of Fruit | Sulfite (grams) |
| 100 | 3 |
| 200 | 6 |
| 300 | 9 |
| 400 | 11 |
| 500 | 14 |
| 600 | 17 |
| 700 | 20 |
| 800 | 23 |
| 900 | 26 |
| 1000 | 29 |
| 1500 | 43 |
| 2000 | 58 |
Twice as much sulfur dioxide is sometimes used when the grapes are very warm, or when they contain rot. This initial dose of SO2 deactivates the browning enzymes, and it helps prevent oxidation during the crushing and pressing operations.
Considerable oxidation takes place when wine is bottled, and oxidation at this time can be very detrimental. Newly bottled wine will be short lived unless adequate sulfur dioxide is present, and winemakers raise the free sulfur dioxide content of their wines to about 30 milligrams per liter just before bottling.
Removes Oxidized Smell
Acetaldehyde is the material responsible for the characteristic smell of sherry wines, and acetaldehyde can be thought of as oxidized ethyl alcohol. Although desirable in sherry, this distinctive odor is not desirable in table wines. Acetaldehyde is produced when wine oxidizes, and too much acetaldehyde is one of the more common defects in homemade table wines.
Acetaldehyde is an intermediate product when sugar is converted into alcohol, and practically all of the free sulfur dioxide disappears during fermentation by combining with acetaldehyde. Since very little free sulfur dioxide remains in the wine, additional sulfur dioxide must be added when fermentation is finished. The recommended practice is to add enough sulfur dioxide to combine with any remaining acetaldehyde and leave 20 to 30 milligrams of SO2 per liter of wine. Most winemakers routinely add about 50 milligrams per liter of sulfur dioxide to newly completed fermentations. Then about 30 milligrams per liter of free SO2 is maintained in the wine during the lengthy clarification, stabilization and aging period.
Inhibits Bacteria and Yeasts
The initial dose of 30 to 50 milligrams per liter of SO2 added at the crusher also provides the winemaker an effective way of controlling fermentation. Most commercially prepared wine yeasts have considerable tolerance for sulfur dioxide, but the activity of wild yeast is greatly diminished by small amounts of SO2. When small quantities of sulfur dioxide and commercial wine yeast are used to start fermentation, the inoculated yeasts multiply quickly, and the commercial yeasts dominate the wild yeasts throughout the fermentation period.
Small quantities of sulfur dioxide can eliminate many undesirable bacteria. When used at reasonable concentrations, SO2 helps control vinegar bacteria, and protection against vinegar bacteria is very important in all wineries. Sulfur dioxide can also inhibit malolactic bacterial activity, so the winemaker can use sulfur dioxide to help control malolactic fermentation.
Sulfur dioxide can exist in wine as free sulfur dioxide or fixed sulfur dioxide, and the effectiveness of sulfur dioxide in controlling wine microbes depends primarily on the form of sulfur dioxide. Fixed SO2 is not effective. Only free SO2 is biologically active.
Moreau and Vinet studied the antiseptic properties of sulfur dioxide in wine, and they concluded that molecular SO2 was the effective form. Fornachon studied the characteristics of both fixed and free SO2 in Australian wines, and he showed several types of wine bacteria, including Lactobacillus, could be controlled by very small quantities of molecular sulfur dioxide. Several other sulfur dioxide studies have been done, and they clearly show 0.5 to 1.5 milligrams per liter of molecular sulfur dioxide can provide good microbial stability in both dry and sweet wines.
Today, most winemakers feel that 0.8 milligrams of molecular sulfur dioxide per liter of wine provides adequate protection for dry table wines. Consequently, most commercial wineries maintain at least 0.8 milligrams per liter of molecular sulfur dioxide in their wines from the completion of fermentation until the wine is bottled. Since molecular sulfur dioxide is the biologically effective form, winemakers are always interested in how much of the sulfur dioxide in a wine exists in the molecular form.
pH AND SULFUR DIOXIDE
When sulfur dioxide is added to wine, some sulfur dioxide combines with other materials in the wine and becomes fixed, the remainder of the sulfur dioxide remains in a free form. The free sulfur dioxide exists in three different forms, the molecular form, the bisulfite form and the doubly ionized sulfite form. The fraction of free sulfur dioxide that exists in the molecular form is strongly dependent upon the pH of the wine. Since only the molecular sulfur dioxide is effective, winemakers are always interested in how much of the free SO2 exists in the molecular form.
The amount of free sulfur dioxide in a wine can be measured easily. On the other hand, the fraction of free sulfur dioxide that exists in the molecular form is difficult to measure. Fortunately, the amount of molecular sulfur dioxide can be calculated easily when both the free sulfur dioxide content and the pH of the wine are known.
The free sulfur dioxide needed to produce 0.8 milligrams per liter of molecular SO2 for different values of wine pH is shown in the Table below. The free sulfur dioxide is given in milligrams per liter (mg/l). For example, the Table below shows that 32 milligrams of free sulfur dioxide per liter of wine will produce 0.8 milligrams per liter of molecular sulfur dioxide in a wine having a pH of 3.4.
| Wine pH | Free Sulfur Dioxide for 0.8mg/l Molecular SO2 |
| 3.0 | 13 |
| 3.1 | 16 |
| 3.2 | 21 |
| 3.3 | 26 |
| 3.4 | 32 |
| 3.5 | 40 |
| 3.6 | 50 |
| 3.7 | 63 |
| 3.8 | 79 |
| 3.9 | 99 |
| 4.0 | 125 |
These data clearly show the amount of molecular sulfur dioxide in a wine is strongly dependent upon wine pH. These data also show that small quantities of free sulfur dioxide will produce enough molecular sulfur dioxide to provide good microbial stability when wine pH is less than about 3.6. However, when the pH exceeds 3.8 or so, significantly large quantities of sulfur dioxide are required. At high values of wine pH, prohibitively large quantities of free sulfur dioxide are needed to produce 0.8 milligrams per liter of molecular SO2.
SUMMARY
Titratable acid is a measure of the total quantity of all the acids in a wine, and pH is a measure of the number of hydrogen ions present in a wine. Several factors influence wine pH. Wines containing little acid and lots of potassium have high pH values. More tartaric acid, less malic acid, less potassium and greater titratable acid result in smaller pH values.
Low wine pH values inhibit wine bacteria, but wine yeasts are not affected. When wine has a low pH, sugar fermentation progresses more evenly, and malolactic fermentation is easier to control. Bentonite is more effective in removing excess protein from wines with low pH values. In addition, red wines with low pH values have more and better color, and white wines do not brown as easily.
The situation is much different when wine pH values are high. Bacteria multiply rapidly in high pH wines, and unwanted bacterial fermentations become more troublesome. High pH wines are less biologically stable, and they have poorer chemical stability. Red and white wines have poorer color when the pH is high. Wines with high pH values always require more attention and greater care than wines with low pH values.
Only the molecular form of sulfur dioxide is effective against wine microbes. When wine pH is low, very small additions of free sulfur dioxide give winemakers an effective tool for managing wine microbes. In wines with high pH values, excessive quantities of sulfur dioxide are needed to control microbes effectively.
Controlling microorganisms is very important, so winemakers maintain 20 to 30 milligrams per liter of free sulfur dioxide in their wines from the completion of the fermentations until the wine is bottled. However, such small quantities of free sulfur dioxide will not be adequate unless wine pH is low.
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