Malolactic fermentation (MLF) is an oenological technique actively employed to reduce wine acidity and impart distinctive aromatic characteristics and nuances. MLF represents one of the microbial-mediated approaches in winemaking, and its appeal as a more natural method that does not rely on chemical interventions contributes to its widespread adoption.
MLF utilizes specific strains of lactic acid bacteria. These bacteria are widely recognized for their positive associations—they are employed in traditional Japanese fermented foods such as miso, soy sauce, and pickles, and are present in yogurt where they provide beneficial effects on intestinal health.
However, both lactic acid bacteria and yeast, being living microorganisms, do not exhibit the predictable behavior of mechanical systems. Like humans, microorganisms alter their behavior in response to changing environmental conditions.
The challenge in utilizing microorganisms during winemaking lies in the fact that such behavioral changes can occasionally inflict substantial damage on wine. The negative impacts produced by microorganisms manifest as unpleasant odors and off-flavors in wine.
This article examines the adverse effects that lactic acid bacteria can impose on wine.
MLF Elevates the Risk of Microbial Contamination
The hygienic integrity of wine is maintained by its low pH (hydrogen ion concentration index). Most microorganisms cannot function actively in the highly acidic environment characteristic of wine, nor can they proliferate effectively.
Furthermore, sulfur dioxide (SO₂) exhibits enhanced efficacy in lower pH environments.
pH represents an extremely critical parameter for wine quality. However, deacidification procedures including MLF possess the effect of elevating pH concomitant with the reduction in total acidity. Additionally, because implementing MLF necessitates raising the wine storage temperature to a certain degree, conditions that favor microbial proliferation become even more pronounced.
Once MLF has been completed, even with elevated pH, the risk of microbial contamination is suppressed because other conditions necessary for microbial proliferation (such as nutrient availability) deteriorate. However, this does not apply during the MLF process. Particular caution is required during stages when residual sugar—sugar that remains unfermented—persists in the wine.
The microorganisms capable of damaging wine under these conditions include lactic acid bacteria.
When deacidification is desired while protecting wine from microbial influence, it becomes necessary to perform appropriate SO₂ additions while employing chemical deacidification methods that do not involve microorganisms.
Microbial Contamination Begins from the Start
Microbial assault on wine commences from the moment grapes are harvested.
The damage is not limited to visibly affected grapes infected with gray mold (Botrytis), powdery mildew (Oidium), or downy mildew (Peronospora). Even grape clusters that appear healthy harbor various microorganisms on their skin surfaces. When such grapes are harvested together with others, placed in the same containers, and transported to the winery, crushing under their own weight allows the microorganisms attached to the skins to obtain sugar from the juice and commence vigorous activity.
Once microorganisms begin their activity, arresting their proliferation is not straightforward. Without early intervention, microorganisms multiply while expanding their range of activity.
Microorganisms proliferate while extending their activity range within the press used to crush harvested grapes, through the pumps and hoses that transport the extracted juice, and ultimately within the tanks they reach. The vitality of microorganisms persists until wine has completed fermentation and sugar has been consumed—in some cases, it continues unabated even after sugar depletion.
Appropriate addition of sulfites (SO₂) proves effective in suppressing such microbial proliferation. Conversely, without such appropriate measures, wine remains perpetually exposed to the risk of relentless microbial attack.
Moreover, damage to wine from bacteria often proves difficult to detect from external appearance, and progression can occur without the winemaker’s awareness. Additionally, once such damage has occurred, complete removal of its effects becomes virtually impossible.
Among the troublesome microbial contamination of wine, contamination caused by lactic acid bacteria holds particular significance. This is partly due to the numerous species of lactic acid bacteria, but also because many other microorganisms have lost much of their former impact through the modernization of winemaking techniques and the thorough implementation of winery sanitation management.
Damage to wine caused by lactic acid bacteria includes various off-flavors such as acetic acid odor, excessive lactic acid odor, and mousy taint, as well as increased bitterness and viscosity, reduction in color intensity in red wines, and accumulation of biogenic amines.
A typical characteristic of wine damage from lactic acid bacteria is that many of these defects occur not in isolation but in combination with multiple defects present simultaneously.
Representative Example of MLF-Induced Off-Flavors | Diacetyl
The majority of off-flavors caused by lactic acid bacteria are generated during the MLF process—that is, during the metabolic conversion of malic acid to lactic acid. Meanwhile, the sensory characteristics of malic acid and lactic acid are virtually indistinguishable.
Thus, most of these off-flavors are not attributed to lactic acid itself but rather originate from by-products generated during the MLF process. The archetypal example is diacetyl (2,3-butanedione).
Diacetyl is a compound possessing characteristic aromas reminiscent of butter, cheese, or butterscotch with added sweetness. While considered an indispensable aromatic component in fermented dairy products such as cultured butter and certain cheeses, it is frequently treated as a cause of off-flavors in the fermented beverage industry.
In wine, tolerance varies according to wine type. Structurally robust red wines such as Cabernet Sauvignon tolerate up to approximately 3 mg/L, whereas other red wines such as Pinot Noir tolerate less than 1 mg/L, and white wines tolerate only approximately 0.2 mg/L. Sensory evaluation when diacetyl is present in wine similarly exhibits substantial variation, with red wines generally receiving more favorable reception.
Diacetyl is known to be produced by multiple microorganisms.
While diacetyl constitutes one component of human skin emanations, in winemaking, yeast and lactic acid bacteria represent the primary producers.
Diacetyl detected in wine, particularly red wine, has long been considered to originate predominantly from the MLF process. However, in 2016, Sapporo Breweries, in collaboration with Sapporo Holdings, announced that certain anthocyanins (pigment compounds) present in red wine act upon yeast to increase diacetyl production.
Conversely, yeast reduces diacetyl during alcoholic fermentation through acetoin to 2,3-butanediol. Consequently, diacetyl perceived as an off-flavor in wine is, in most cases, attributable to lactic acid bacteria.
Other Off-Flavors Originating from Lactic Acid
One cause of aromas similar to diacetyl imparted to wine is ethyl lactate, an ester formed from lactic acid and ethanol. The threshold (minimum detectable concentration) of this compound is reported to be approximately 60–110 mg/L, and it becomes increasingly recognizable as an off-flavor when concentrations reach approximately 80–130 mg/L.
Cases also exist where excessive buttery aromas from diacetyl deteriorate further.
The cause of this deterioration is the continued metabolic activity of bacteria.
In such cases, wine acquires acetic acid-like nuances in both aroma and taste, developing a sharper or sweet-and-sour impression. The critical issue with this off-flavor is that while remedial methods remained available during the diacetyl-only stage, at this stage the causative factors cannot be removed, leaving wine disposal as the only option.
Fatal Off-Flavors from Acetic Acid
Among the relatively common off-flavors in wine are those caused by acetic acid.
While acetic acid bacteria represent the classic causative agent of acetic acid formation, this compound is actually produced by multiple microorganisms.
Acetic acid contamination of wine is recognized as fatal. Once it occurs, complete subsequent removal becomes impossible. In most countries and regions, wine contaminated with acetic acid is not only prohibited from sale as wine but is also forbidden from conversion into distilled spirits. Such wines can only be converted into wine vinegar through the addition of acetic acid bacteria or be discarded.
This is because acetic acid contamination is defined as food spoilage.
In wine, acetic acid content exceeding 1.08 g/L in white wines or rosés, or 1.2 g/L in red wines, serves as the criterion for determining spoilage. Wines containing such concentrations are no longer suitable for drinking enjoyment.
The exception is wines of extreme sweetness.
For instance, German Eiswein and Beerenauslese permit acetic acid content up to 1.8 g/L, while Trockenbeerenauslese permits up to 2.1 g/L. Similarly, certain sweet wines from France and Italy permit 1.5 g/L, and Switzerland permits up to 1.6 g/L as exceptional allowances.
Microorganisms That Produce Acetic Acid
The most representative microorganism producing acetic acid is acetic acid bacteria. Acetic acid bacteria generate acetic acid through alcohol oxidation. However, most acetic acid that becomes problematic in wine is not caused by acetic acid bacteria. The predominant causes are yeast and lactic acid bacteria.
During the fermentation of grape juice, wild yeast primarily serves as the principal source of acetic acid production. However, post-fermentation, the spoilage yeast Brettanomyces assumes this role. Nevertheless, lactic acid bacteria produce greater quantities of acetic acid than these spoilage yeasts.
Multiple species of lactic acid bacteria exist. Among them, the Oenococcus oeni strain, considered most adapted to winemaking conditions, is classified as a heterofermentative type. This heterofermentative type produces greater quantities of acetic acid.
D-lactic acid, metabolized from glucose during the MLF process, is also considered to cause wine spoilage when concentrations exceed 1 g/L. The reason lactic acid bacteria hold paramount significance in acetic acid-related wine off-flavors is that these bacteria produce not only acetic acid but also D-lactic acid simultaneously.
Many acetic acid-based off-flavors are attributed to the combined effects of acetic acid and D-lactic acid.
Other Effects from Lactic Acid Bacteria
Wine damage caused by lactic acid bacteria extends beyond diacetyl and acetic acid. Lactic acid bacteria inflict additional damage on wine.
Specifically, these include mannitol taint, excessive increases in viscosity, reduction in color intensity in red wines, mousy taint, and biogenic amine production.
While mousy taint frequently originates from Brettanomyces, cases caused by lactic acid bacteria also occur. Particularly in cool-climate regions, mousy taint is predominantly attributed to lactic acid bacteria rather than Brettanomyces.
Conclusion | Can Lactic Acid Bacteria Be Properly Controlled?
MLF is recognized as a critical process in winemaking, particularly in red wine production. This recognition extends beyond winemakers. Consumers similarly often consider the presence of buttery nuances in certain red wines as natural and expected.
Indeed, MLF imparts distinctive nuances to wine. Light-bodied wines frequently gain substance through MLF, with their character undergoing substantial transformation.
In this sense, MLF represents, alongside oak barrel usage, an important winemaking technique for introducing variation in wine character.
However, the lactic acid bacteria that play crucial roles in MLF are microorganisms incapable of communication with humans. Certainly, when implemented under similar conditions, they exhibit broadly similar behavior. Yet achieving completely identical results is impossible, and attaining complete uniformity is unattainable.
Furthermore, microorganisms remain invisible to the human eye. While microscopic examination can confirm their presence, observing a mere 1 mL from a tank exceeding 1000L provides no comprehensive understanding. Introducing microorganisms into the winemaking process equates to introducing numerous unstable variables.
Lactic acid bacteria and other microorganisms do not cooperate with humans to improve wine quality. Microorganisms operate for their own survival and proliferation. Beneficial outcomes for humans arise merely as incidental byproducts of these processes. We humans cannot selectively extract results from microbial activity. The only options are accepting everything or rejecting everything.
Under such circumstances, achieving true, appropriate control of lactic acid bacteria proves virtually impossible.
The only recourse is to establish an environment that maximizes the probability of obtaining desirable results, then hope that events proceed in that direction.
Some may find the accumulation of such contingencies enjoyable, while others may perceive it as an accumulation of risk factors. If the wine at hand cannot afford failure, refraining from uncertainty-laden microbial utilization likely increases the probability of ultimately obtaining higher-quality wine. Winemakers would be well-advised to recognize this reality.
