Wine and Volatile Acidity: Origins and Impact of Acetic Acid in Wine

Many people who study wine have encountered the term "VA" at some point. Initially, the meaning of this abbreviation may not be immediately clear.

VA stands for Volatile Acid, which translates to "volatile acidity" in Japanese. More precisely, volatile acidity refers to acids that have the property of volatilizing under normal temperature and pressure conditions.

Wine contains many acids, including tartaric acid and malic acid. Among these acids, those that do not volatilize under normal temperature and pressure are called "non-volatile acids." Most acids contained in wine are actually non-volatile, with volatile acids representing only a small portion. The primary exception is acetic acid. Therefore, in the context of wine, volatile acidity is used almost synonymously with acetic acid. VA measurements are also basically recorded as acetic acid content.

The Impact of Volatile Acidity on Wine

Acetic acid is commonly known as vinegar. As anyone who has used vinegar in cooking can imagine, high concentrations of acetic acid have a very strong odor and sharp taste. This same impact applies to wine, where acetic acid aromas above a certain intensity are treated as off-flavors or faults. High concentrations of volatile acidity are considered an indicator of wine spoilage.

Off-flavors caused by volatile acidity are among the most frequently occurring wine faults. While reduction aromas are another common off-flavor, from a winemaking perspective, volatile acidity-related faults are considered more dangerous defects.

Why do volatile acidity off-flavors occur so frequently? And why are they considered more dangerous from a winemaking standpoint? The following sections will explain these reasons in detail.

Standard Values for Volatile Acidity in Wine

Nearly all wines contain some amount of volatile acidity. This is because it is impossible to completely avoid acetic acid production during the winemaking process.

However, the amount of volatile acidity permitted in wine is legally defined in many countries and regions.

Generally, dry wines contain 0.2–0.6 g/L of volatile acidity, but sweet wines tend to have higher concentrations. According to regulations by the OIV (Organisation Internationale de la Vigne et du Vin), the upper limit for regular still wines is 1.2 g/L in acetic acid equivalent, and for extremely sweet wines like noble rot wines, it is 2.1 g/L.

These regulatory values are basically managed by measurements taken at the time of bottling. The volatile acidity content in wine may increase through changes that occur during aging and storage periods. Therefore, long-aged wines may potentially contain volatile acidity levels above the regulatory standards.

Sensory Threshold for Volatile Acidity

The sensory threshold—the boundary at which volatile acidity taste or aroma becomes perceptible—is known to vary depending on grape variety and wine style. Generally, it becomes detectable at concentrations around 0.5 g/L. However, verification has confirmed that even at 0.9 g/L concentrations, it may not necessarily be recognizable as taste or aroma.

Three Reasons Why Wine Contains Volatile Acidity

The presence of volatile acidity in wine during production is not surprising regardless of the amount; rather, it should be considered inevitable. In this sense, volatile acidity can be viewed as an essential component of wine.

There are three main reasons why wine contains volatile acidity: (1) acetic acid production by microorganisms used in winemaking, (2) introduction from rotten grapes, and (3) management deficiencies. Let's examine each in detail.

The Relationship Between Winemaking Microorganisms and Acetic Acid

Multiple microorganisms are essential for winemaking. Among these, yeasts and lactic acid bacteria play particularly important roles.

Acetic Acid Production During Alcoholic Fermentation by Yeasts

Yeasts play the crucial role of fermenting grape juice into wine through alcoholic fermentation.

Winemaking primarily uses a type of yeast called Saccharomyces cerevisiae. This yeast is involved in nearly all fermented foods including bread, sake, and beer, and is also utilized in non-food applications, making it deeply embedded in our daily lives. This extremely important S. cerevisiae produces acetic acid during sugar metabolism. This means that virtually all products involving S. cerevisiae in fermentation contain volatile acidity.

The amount of acetic acid produced by S. cerevisiae varies greatly depending on the strain type and fermentation environment and conditions. In winemaking environments, most cases fall within the range of 0.2–0.4 g/L. However, since acetic acid production by yeasts is proportional to the sugar concentration in the juice, fermenting juice with high sugar content will result in increased acetic acid production.

It should be noted that many other types of yeasts exist besides S. cerevisiae. Among these yeasts, some have been confirmed to produce large amounts of acetic acid exceeding 1 g/L.

How MLF Increases Volatile Acidity Content

Another microorganism likely to be used in general winemaking is lactic acid bacteria.

Lactic acid bacteria are used to convert malic acid in wine to lactic acid, which has lower acidity. This process is called MLF (Malolactic fermentation) or lactic acid fermentation.

Lactic acid bacteria exist in two types: homofermentative and heterofermentative. The lactic acid bacteria Oenococcus oeni, mainly used in winemaking, is classified as a heterofermentative strain. These heterofermentative lactic acid bacteria are known to produce acetic acid during their metabolic processes.

Acetic acid production by lactic acid bacteria, like yeasts, is greatly influenced by environmental conditions. Generally, they produce around 0.4 g/L, but in some cases can exceed 1 g/L of acetic acid. Additionally, co-inoculation, a technique used in MLF, has been suggested to potentially increase acetic acid production by lactic acid bacteria strains.

Increased Acetic Acid Content Due to Spoilage

Acetic acid production by yeasts and lactic acid bacteria is certainly not negligible. However, since it represents an unavoidable aspect of winemaking, it must be accepted to some degree as inevitable. In contrast, what must be absolutely avoided is increased acetic acid content caused by rotten grapes.

As stated initially, high concentrations of acetic acid serve as indicators of spoilage degree, and acetic acid content has a strong relationship with grape health. This relationship involves acetic acid bacteria.

Types of Acetic Acid Bacteria and Their Impact on Wine

Acetic acid bacteria are a type of microorganism, like yeasts and lactic acid bacteria. As their name suggests, they are bacteria with very strong acetic acid production capabilities and are essential microorganisms for vinegar production. While they are highly useful microorganisms in human life overall, they are consistently treated as harmful in winemaking operations.

While lactic acid bacteria are broadly classified into two types (homofermentative and heterofermentative), acetic acid bacteria are classified into many more types, representing an extremely diverse group of bacteria. Among these, the species most closely related to winemaking operations are acetic acid bacteria classified under two genera: Gluconobacter and Acetobacter.

Both types of acetic acid bacteria exist naturally in the environment, but healthy grape skins are primarily inhabited by Gluconobacter species. In contrast, Acetobacter species become more prevalent on spoiled grape skins. This difference stems from the energy sources each strain consumes.

Gluconobacter species primarily metabolize glucose, while Acetobacter species metabolize ethanol. Though both are acetic acid bacteria, they have completely different ecological characteristics.

Incidentally, Gluconobacter species have low tolerance to acetic acid and their growth is inhibited by low concentrations of acetic acid, making them inefficient acetic acid producers. In contrast, Acetobacter species have high acetic acid tolerance and can efficiently produce acetic acid. Due to these differences, Acetobacter species have a greater impact on wine.

The Complex Relationship Between Botrytis, Wild Yeasts, and Acetic Acid Bacteria

One type of grape where acetic acid bacteria proliferate easily is grapes infected with Botrytis cinerea (hereinafter referred to as Botrytis). Botrytis is known as noble rot fungus, and grapes infected with this fungus become noble rot grapes used as raw material for noble rot wines. However, grape bunches infected with Botrytis often simultaneously contain a considerable number of rotten grapes. One cause of such rotten grapes is Acetobacter species acetic acid bacteria.

When Botrytis attaches to grape skin surfaces, it extends hyphae and creates holes in the grape skin. Not only does juice leak out from these holes, but bacteria can also directly contact the grape flesh and juice through these openings. This is where wild yeasts become active.

Wild yeasts are naturally present on grape skin surfaces. When these yeasts come into contact with juice leaked by Botrytis or enter the holes to directly contact the juice, they begin consuming sugars and metabolizing them. This produces small amounts of ethanol.

Gluconobacter species acetic acid bacteria, originally present on grape skin surfaces, may consume the leaked juice but do not consume the ethanol produced by wild yeasts. However, Acetobacter species acetic acid bacteria do consume this ethanol.

Using the ethanol actively produced by wild yeasts as an energy source, Acetobacter species acetic acid bacteria explosively increase their numbers. Acetic acid bacteria that normally exist at only about 10²–10⁴ cells/ml on healthy grapes have been confirmed to reach massive quantities of up to 10⁷ cells/ml after Botrytis infection.

Grapes with massive acetic acid bacteria proliferation contain large amounts of acetic acid corresponding to their cell numbers. The sharp, sour smell of spoiled grapes is caused by acetic acid produced in this manner.

The Real Problem with Introducing Rotten Grapes

The primary problem with bringing rotten grapes into the winery as harvest material is not so much introducing already-produced acetic acid, but rather introducing large numbers of living acetic acid bacteria that caused the problem in the first place.

As we have seen, Acetobacter species acetic acid bacteria consume ethanol to produce acetic acid. What makes acetic acid bacteria troublesome is that these microorganisms do not die during alcoholic fermentation by S. cerevisiae and can survive until fermentation is complete. This means that once introduced into the winery, acetic acid bacteria can produce additional acetic acid using ethanol created by yeasts after arrival at the winery, in addition to the acetic acid they were already producing on the grape surfaces.

It should be noted that this situation is not limited to grapes infected with Botrytis. The same thing can occur with grapes whose skins have been damaged by insect damage or rainfall.

Management Deficiencies That Make Wine Sour

While less commonly heard recently, many people have probably heard the saying that "wine turns to vinegar if left alone." In reality, stored bottled wine almost never becomes complete vinegar. However, the possibility of developing sour flavors is not zero.

Such flavor changes are mainly due to the action of the aforementioned acetic acid bacteria. While one might think that if acetic acid bacteria are working, the wine should turn to vinegar, there are significant differences between the conditions where acetic acid bacteria are most active and the conditions where wine is generally stored, so acetic acid bacteria rarely work enough to turn wine completely into vinegar.

Activity Conditions for Acetic Acid Bacteria and Their Relationship with Oxygen

One condition that allows acetic acid bacteria to work more actively is the presence of oxygen supply.

Acetic acid bacteria are called aerobic bacteria—a type of bacteria that requires oxygen for activity. During wine fermentation, carbon dioxide is constantly expelled by yeasts as fermentation gas. This action maintains an anaerobic environment inside the fermentation vessel—a state with almost no oxygen. In such environments, while acetic acid bacteria can survive, they cannot actively function, and acetic acid production by acetic acid bacteria is minimal.

Additionally, when bottling finished wine, filtration is often performed beforehand to remove microorganisms from the wine. This process also removes acetic acid bacteria, reducing the likelihood of them remaining in the bottle. The dangerous period is the time in between.

The Importance of Management During Aging

Wine that has completed alcoholic fermentation may be stored in tanks or other containers for a certain period before bottling. Aging in barrels called barriques is a typical example of this. During this period, wine is no longer protected by fermentation gases and becomes exposed to various microorganisms including acetic acid bacteria.

During aging, containers holding wine must be maintained at full capacity at all times. While this requirement is often explained in connection with wine oxidation, maintaining full capacity is also extremely important for suppressing the activity of aerobic bacteria like acetic acid bacteria.

The Special Durability of Acetic Acid Bacteria

One reason wine is recognized as a sanitary beverage despite not washing the grapes before processing is that wine is protected by low pH and relatively high alcohol content, creating an environment difficult for general microorganisms to survive. However, acetic acid bacteria are exceptional microorganisms that can survive in such harsh conditions. Depending on the condition of harvested grapes, such acetic acid bacteria may be introduced into the winery in large numbers. In such environments, if proper wine topping management is neglected, the probability of dormant acetic acid bacteria suddenly becoming active becomes very high.

Acetic acid can serve as an indicator not only of grape spoilage degree but also of wine management quality.

Summary: Controlling Volatile Acidity in Wine Requires Cause-Specific Approaches

While there are multiple causes for volatile acidity in wine, the important thing in addressing them is to first clearly recognize that some influences can be reduced to zero while others cannot. Influences that can be reduced to zero are factors not directly related to fermentation. In contrast, influences that cannot be completely eliminated are factors directly involved in fermentation.

As we have seen, yeasts that perform alcoholic fermentation produce acetic acid during the fermentation process. While production amounts can be reduced by adjusting yeast types and fermentation conditions, they cannot be completely eliminated. When conducting MLF, the same applies to lactic acid bacteria.

On the other hand, acetic acid production by so-called spoilage bacteria, including acetic acid bacteria, can be avoided with proper effort. This requires various tasks such as vineyard work, sorting harvested grapes and other preprocessing, improving winery cleanliness and management standards, but these are not impossible undertakings.

To control volatile acidity levels in wine, it is effective to first firmly establish control over manageable areas, then attempt to control difficult-to-manage areas as much as possible.

Appropriate Approach to Volatile Acidity

Like other off-flavors, volatile acidity is extremely difficult to completely remove once it has been incorporated into wine. While it may not be detectable up to a certain proportion, reducing the amount to that level is also not simple.

However, while volatile acidity becomes an off-flavor when concentrations increase, its characteristic taste and aroma mean that as a wine component, it is not necessarily better to have none at all. Rather than aiming for complete elimination, the correct approach to this compound is determining how to maintain appropriate concentrations.

To make such adjustments, it is necessary to know how to reduce volatile acidity that has already been incorporated into wine. Additionally, knowing such methods provides reference for conducting fermentation that does not increase volatile acidity.

Methods for readjusting the concentration of volatile acidity once it has been incorporated will be explained in separate articles.