Wine and pH - What Changes in pH Bring to Wine

One of the most critical factors in winemaking is pH. Many people may recall learning about pH in middle school science or high school chemistry classes. However, understanding how pH relates to wine can be challenging.

When researching the relationship between wine and pH, abundant information is available. However, some sources contain minor misconceptions or require more organized explanations. Due to these circumstances, even winemakers sometimes harbor misunderstandings.

Therefore, this article will provide a comprehensive explanation of the relationship between pH and wine, which is fundamental to winemaking yet extremely important.

Basic Understanding of pH

To grasp the relationship between wine and pH, it is essential to understand the definition of pH. Let me first briefly explain what pH is.

pH is an abbreviation for "potential Hydrogen" and is an indicator of hydrogen ion concentration. Hydrogen ion concentration refers to the amount of hydrogen ions (H⁺) in a solution. This is not the concentration of hydrogen (H) itself, nor does it directly represent the amount of acid.

From a more detailed academic perspective, pH is an indicator calculated from the common logarithm of the reciprocal of the molar concentration of hydrogen ions in one liter of solution, expressed by the following formula:

pH = -log [H⁺]

If understanding this formula is difficult, there is no need to force comprehension. What matters is that pH represents the concentration of hydrogen ions, and H⁺ indicates hydrogen ions.

When hydrogen ions are present in large quantities, the pH value becomes lower. When present in small quantities, the value becomes higher. The characteristic property of hydrogen ions is called acidity, while that of hydroxide ions is called alkalinity. Therefore, liquids with low pH are called acidic liquids.

pH values are typically displayed in the range of 0 to 14. For wine, understanding the range of 3 to 4 is sufficient. pH 7 is neutral, with values smaller than this called acidic and larger values called alkaline. Since all wines are acidic, there is no need to consider alkalinity.

Why Wine pH Changes

Water molecules are represented by the chemical formula H₂O. You may remember learning in school that these water molecules are composed of hydrogen ions (H⁺) and hydroxide ions (OH⁻). When chemical symbols have + or - in the upper right, it indicates that the substance is ionized.

These water molecules, while appearing to be water, do not always exist in the form of H₂O. They continuously undergo ionization and molecularization. This phenomenon also occurs in wine.

Many substances ionize when dissolved in water. By measuring the concentration of hydrogen ions contained in such ionized substances, pH can be determined.

Wine's main component is also water. Water continuously ionizes in wine as well. However, the difference between water and wine is that wine contains components other than water that release hydrogen ions. These components are called "acids."

Here is an important point: It is not because something is an "acid" that it releases hydrogen ions when dissolved in water; rather, substances that have the property of releasing hydrogen ions when dissolved in water are defined as "acids" in the field of chemistry. While this may seem like the same thing, it is an important distinction.

Returning to the topic of pH.

The pH of a liquid is determined by the amount of hydrogen ions present in that liquid. In other words, if a large amount of acid exists in a liquid, the likelihood of increased hydrogen ion release is high. As a result, the pH value becomes smaller, making it more acidic.

This is why many people mistakenly think pH = amount of acid.

Indeed, when a large amount of acid is present, the likelihood of increased hydrogen ion release is high. However, even when acids are present, they do not necessarily release hydrogen ions. This means that even if wine contains abundant acids, if those acids release few hydrogen ions, the pH may remain high.

This is also why each wine shows different pH values. Wine contains various components. Some are acids, others are not. Components that absorb hydrogen ions released by acids are also included.

Even if acids are releasing hydrogen ions, if other components absorb them, they no longer exist as H⁺ in the wine. As a result, the pH does not change.

pH does not directly represent the amount of acid.

Concepts of Acid and Acidity

I explained that acids are substances that have the property of releasing hydrogen ions (H⁺) when dissolved in water. This understanding is not incorrect, but with this understanding alone, compounds that do not originally have hydrogen ions could not become acids. Therefore, a deeper understanding is chemically necessary.

In the field of chemistry, acids and their counterparts, bases, are defined as follows:

An acid is a molecule or ion that accepts electron pairs, and a base is a molecule or ion that donates electron pairs.

While it is not necessarily required to understand this strictly, accurate understanding of wine oxidation and similar processes requires understanding based on this definition. The oxidation of wine by metal ions is a typical example.

Attention is also needed regarding the term "acidity." Acidity has both industrial and chemical meanings, and these two have completely different meanings.

Industrial acidity basically refers to the concentration of acids as substances contained in the liquid. This is the concept of "how many grams of tartaric acid are contained in one liter of wine." On the other hand, chemical acidity refers to the degree of ability to neutralize opposing acids through neutralization reactions, indicating the number of OH that can be dissociated. It is not related to how many grams of tartaric acid are contained in one liter.

However, the acidity used in wine is usually titratable acidity (TA) or total acidity (TA). Both of these have units of g/L, indicating the amount of acid. While these are strictly different, they show almost no difference in wine, so in most cases, titratable acidity is expressed as total acidity.

Effects of pH on Wine

During the winemaking process, pH is typically measured at two times. The first is the pH of the juice before alcoholic fermentation. The second is the pH after alcoholic fermentation is complete and the juice has become wine.

You might think that if pH is measured at the juice stage, there would be no need to measure it again after it becomes wine. This is not actually the case. Since hydrogen ions themselves are unstable entities, pH constantly changes.

For example, skin contact, frequently performed in red wine production, raises the pH of the juice. Additionally, pH continuously rises during alcoholic fermentation. When malolactic fermentation is performed, pH also rises. These factors accumulate, creating a considerable difference between the pH of juice immediately after harvest and the pH of the finished wine.

However, in normal winemaking, unless acid is artificially added, pH rarely decreases. Most changes occur in the direction of pH increase, that is, in the direction where pH approaches 4 in wine.

Wine pH is generally between 2.8 and 3.0. White wines tend to be lower, and red wines tend to be higher. Even if this were to become 4, it might not seem like a major problem, but when pH increases by 1, the amount of hydrogen ions in the liquid decreases to one-tenth. While it appears to be only a difference of 1 numerically, it has a very significant impact. To reiterate, what decreases is the amount of hydrogen ions, not the amount of acid.

There are many effects on wine caused by pH increase. Among these, the four most important are:

• Effects on taste
• Effects on microbiological stability
• Effects on color
• Effects on chemical stability

Each will be explained in detail.

Wine Taste Becomes Mellower

The effects of wine pH increase on taste have both direct and indirect aspects.

To reiterate, pH indicates hydrogen ion concentration, not the amount of acid. Therefore, pH does not directly affect wine taste. Even if pH shows strong acidity, the wine is not necessarily correspondingly sour.

However, hydrogen ion concentration is somewhat influenced by the amount of acid. Since acids release hydrogen ions, this relationship is unavoidable.

pH increase sometimes occurs when organic acids (mainly tartaric acid) present in wine combine with components like potassium and precipitate. During the process of potassium combining with tartaric acid, the hydrogen ion concentration in wine decreases. In other words, pH increases.

Tartaric acid is the organic acid responsible for wine's acidity. When this acid precipitates, the wine's acidity weakens. This is the indirect effect.

However, there are also direct effects.

Verification results show that when pH increases, astringency and stringency are felt less strongly. This relates to color, which will be discussed later, but when pH increases, phenolic compounds including tannins become more easily ionized. As a result, the oxidation tendency of phenolic compounds becomes stronger. In other words, they become more prone to oxidation and polymerization.

When tannin binding increases and molecular weight becomes larger, tannins begin to precipitate. Just as astringency is felt less in long-aged wines, when pH increases, tannins become more prone to oxidation and polymerization, and as their quantity decreases, stringency is felt more lightly.

Furthermore, when wine pH increases, malolactic fermentation (MLF) by lactic acid bacteria becomes more likely to occur. Moreover, its state also changes. This also becomes a cause of changes in wine taste and aroma as a result of pH changes.

Risk of Microbial Contamination Increases

The biggest problem with pH increase in winemaking is the increased risk of microbial contamination.

There are two main reasons why wine can sometimes be stored for decades without spoiling and remain drinkable. First, the wine's pH is kept low. Second, sulfurous acid, also called antioxidant, or sulfur dioxide (SO₂) is added.

Most microorganisms cannot survive in acidic environments. Even if they survive, they cannot be active. SO₂ completely suppresses such microorganisms, which is why wine can be enjoyed deliciously for long periods.

However, wine pH increase is the worst situation where both of these powerful suppressive forces are lost simultaneously.

When pH increases and wine changes from strongly acidic to moderately or weakly acidic, microorganisms become able to be active. I will explain using lactic acid bacteria as an example.

Malolactic fermentation (MLF), well known as a method to make wine acidity mellower, is performed by lactic acid bacteria metabolism. There are actually many types of lactic acid bacteria that play the main role in MLF.

Oenococcus oeni is mainly used for MLF, but other strains such as Pediococcus species also exist. While all lactic acid bacteria may seem the same, in winemaking, types of lactic acid bacteria other than Oenococcus oeni often cause off-flavors, which are wine defects. Therefore, it is necessary to thoroughly prevent their contamination or proliferation in wine.

This is where pH becomes important.

Most lactic acid bacteria cannot survive in low pH environments. Even Oenococcus oeni, which is considered relatively tolerant of low pH, requires pH 3.2 or higher. On the other hand, Pediococcus species, which cause major problems in wine, begin activity at pH 3.5 or higher.

In other words, to perform MLF more safely, producers need to manage wine pH within an appropriate range. However, when pH increases beyond this management, situations may arise where MLF occurs due to completely unwanted lactic acid bacteria metabolism, or MLF begins spontaneously even when MLF itself is not desired. Moreover, the higher the pH becomes, the stronger the lactic acid bacteria vitality becomes, and the degree of MLF also increases.

Such microbial activity is not limited to lactic acid bacteria. In high-pH wine, various microorganisms begin unauthorized activity.

The final resort to suppress such unauthorized microbial activity is SO₂. However, the effectiveness of SO₂ depends on wine pH.

Since SO₂ is also a compound of sulfur and oxygen, when dissolved in wine, it combines with hydrogen ions and oxygen ions present in wine and changes to forms like HSO₃⁻ and SO₃²⁻. The proportion of this binding increases as pH becomes higher.

The problem here is that each time SO₂ combines with other molecules, it significantly weakens its effectiveness. Strictly speaking, to be effective in the minute amounts added to wine, the portion that remains as SO₂ itself, not in forms like HSO₃⁻ or SO₃²⁻, is necessary. This becomes almost zero when pH exceeds 3.6.

As mentioned earlier, when pH exceeds 3.5, very troublesome lactic acid bacteria strains begin activity, and other microorganisms also become active. However, the essential SO₂ becomes almost entirely in the form of HSO₃⁻ and becomes powerless against microorganisms.

The yeast that converts grape juice to wine also cannot escape the influence of pH. Yeasts that perform alcoholic fermentation in wine can be active at much lower pH than lactic acid bacteria, but their vitality still decreases in low pH environments. Conversely, when juice pH increases, alcoholic fermentation starts earlier and fermentation speed also increases.

In the case of red wine, the fermentation period becomes the extraction time from grape skins, so pH changes may change the time required for extraction. This may affect wine color and taste.

Wine Color Becomes Lighter

In wine, particularly red wine, wine color also changes when pH becomes higher.

When pH is high at the juice stage, it has been found that the extraction amount of anthocyanins, which are the substances responsible for red wine's red color, decreases. In other words, the absolute amount of pigments that make the color red decreases.

Furthermore, as seen in the taste effects section, when pH becomes higher, phenolic compounds become more easily ionized and more prone to oxidation. Anthocyanins are one of the representative phenolic compounds. When anthocyanins oxidize, the wine color first becomes brownish, and when oxidation progresses further, they precipitate and the wine loses color.

Even without this, anthocyanins have the property of changing color according to pH. In highly acidic liquids, anthocyanins develop red color. However, as pH increases, they increasingly show blue tones, and when pH becomes even higher, they become almost colorless with a slight yellow tint. This occurs because anthocyanins change their molecular structure according to pH. While grouped together as anthocyanins, multiple types with different molecular structures exist, and these simultaneously dissolve in wine to create the wine's color.

This is like mixing multiple red paints of different tones to create one red color.

Red wines with low pH tend to show higher color density and saturation, while conversely showing lower brightness. However, as pH becomes higher, color becomes lighter, saturation decreases, and conversely brightness increases.

Color development and density are important items when evaluating wine. When these become light and transparent, wine quality judgment is also affected.

Wine Becomes More Prone to Cloudiness

You might find it confusing to hear that wine becomes cloudy immediately after discussing how higher pH makes wine color approach transparency. Color becoming lighter is mainly a story about red wine, while this is mainly a phenomenon in white wine.

The cause of white wine cloudiness due to pH increase is protein.

Wine contains proteins. Some are grape-derived, but their origins vary, including yeast-derived and other microorganism-derived proteins. The main reason such proteins can dissolve in wine is pH.

Proteins have the property of dissolving only when pH is compatible. Conversely, when the pH of a liquid in which they were dissolved changes and becomes incompatible, they can no longer remain dissolved and precipitate. Proteins that precipitate in this way may combine with phenols contained in wine, becoming like white haze and making wine cloudy.

The amount of protein contained in wine changes during the brewing process.

Even if proteins were removed to some extent from juice using clarifying agents before alcoholic fermentation, the content may be higher than expected when alcoholic fermentation is complete. It is impossible to predict this accurately. Proteins that have increased unnoticed suddenly make wine cloudy when wine pH increases.

Importance of Acid Amount and pH in Winemaking

Wine pH has wide-ranging effects. Due to this fact, pH measurement is recognized as an extremely important act in winemaking.

On the other hand, if considering the overall picture of the finally completed wine, juice sugar content and acid amount are also very important elements. There is no superiority or inferiority among these three elements, and all are equally important without doubt. During grape harvest, it is essential to measure these elements thoroughly and understand their condition.

However, the question of which is truly most important does arise. When all cannot be measured for some reason, questions about how to set priorities are surprisingly common.

While approaches vary by person, I personally consider the order as: juice sugar content > acid amount > pH. I do not consider pH as something that must absolutely be measured. With some experience, the approximate range can be predicted. In this regard, juice sugar content is indispensable for adjusting wine alcohol content and residual sugar content, and acid amount is also essential for determining the overall picture of wine taste.

Of course, this order does not mean undervaluing the importance of pH. If measurement is possible, it is absolutely an item that should be measured. This is simply a matter of attention degree considering measurement frequency and work efficiency.

What is important is not the pH value itself, but what might happen as a result of that value and how to deal with it. Without considering this point, simply noting that pH has increased or decreased has no meaning.

Conclusion

The relationship between wine and pH is a fundamental yet extremely important element in winemaking. Changes in pH have direct and indirect effects on wine taste, microbiological stability, color, and chemical stability.

The understanding that pH indicates hydrogen ion concentration and does not directly represent the amount of acid is essential for winemakers. Based on this basic understanding, by performing appropriate wine production management, high-quality wine production becomes possible.

Most importantly, it is not the pH numerical value itself, but accurately understanding the situation that numerical value indicates and taking appropriate measures. Through this basic knowledge and practical application, winemakers can produce better wines.