Acids are indispensable elements in wine. However, acids are inappropriate whether they are too abundant or too scarce. The overall balance is extremely important.
Wine contains multiple acids, but from a winemaking perspective, tartaric acid and malic acid are particularly important. These two acids are especially abundant among the acids contained in wine and have a strong influence on the wine's acidity.
When adjusting the acidity balance to modify taste during the winemaking process, the approach most commonly taken is to reduce the amount of acids contained in the wine.
In recent years, due to the effects of climate change, a phenomenon has become problematic where the acids contained in grapes become excessively low during harvest time. This phenomenon is called "acid drop." Under these circumstances, it would seem unnecessary to reduce the amount of acids during the brewing process. However, among the multiple deacidification methods available, some affect wine beyond simply reducing acid levels.
One of these methods is malolactic fermentation (MLF), which is famous in red wine production. MLF is a brewing technique that uses lactic acid bacteria to convert malic acid contained in wine into an acid called lactic acid. Lactic acid has a distinctive aroma and is characterized by weak acidity. Because lactic acid bacteria are used, this technique is also called lactic acid fermentation.
MLF can be succinctly explained as "lactic acid bacteria make wine's acids milder while simultaneously imparting butter-like or creamy aromas." However, the actual mechanism is more complex.
In the first place, in lactic acid fermentation, malic acid is not directly converted to lactic acid.
This article provides a detailed explanation of the reality of MLF and its surrounding aspects.
History and Overview of MLF
Wine contains multiple acids. Among these acids, malic acid is primarily what lactic acid bacteria can utilize.
Tartaric acid and malic acid constitute the majority of acids contained in wine, but the amounts contained in grapes have different characteristics for each.
The content of tartaric acid is basically hardly affected by weather conditions during the growing period. Malic acid, in contrast, shows completely opposite properties. In years blessed with good weather, the content decreases, and in years with bad weather, the content increases.
The total acid content in grape juice from years blessed with good weather tends to decrease, while the total acid content from years not blessed with good weather tends to increase. In other words, grapes become more acidic in years not blessed with good weather, and behind this lies an increase in the amount of malic acid.
Considering this, MLF, a technique that can selectively remove malic acid—which increases in content during bad weather years and consequently makes grapes acidic—appears to be a very effective method.
MLF became widely known to the world in 1918. In this year, Hermann Müller-Thurgau and others published a paper demonstrating the fact that lactic acid bacteria decompose malic acid contained in wine.
However, as early as 1890, Müller-Thurgau had already suggested the possibility that bacteria could cause acid decomposition. In 1897, Alfred Koch proved this theory. Subsequently, W. Seifert of Austria determined that a type of Diplokokkus was the cause and that malic acid was decomposed into lactic acid and carbon dioxide.
The 1918 paper summarized these studies. At the same time, this paper already pointed out that the use of lactic acid bacteria not only promotes malic acid decomposition but also poses risks of negative effects on wine.
It should be noted that at approximately the same time, the fact that not only lactic acid bacteria but also yeasts are involved in acid decomposition was discovered.
Is MLF Actually Fermentation?
MLF is an abbreviation for malolactic fermentation and is also called lactic acid fermentation. In other words, it is considered a type of fermentation.
Certainly, lactic acid bacteria also produce lactic acid through fermentation. However, the process of converting malic acid to lactic acid is actually somewhat different from fermentation. Strictly speaking, lactic acid bacteria take up malic acid into their cells during metabolism and excrete it as lactic acid.
In other words, the primary purpose of lactic acid bacteria is to obtain energy through fermentation. Taking up malic acid or producing lactic acid from it is not the main purpose. These are separate processes.
Fermentation refers to metabolism by which microorganisms obtain energy necessary for themselves. According to this definition, the conversion from malic acid to lactic acid, which does not involve energy acquisition, is not fermentation.
So why do we call MLF "fermentation"? As mentioned above, it is because lactic acid bacteria also produce lactic acid through fermentation. This is a very complex aspect. Let us organize this somewhat.
Lactic Acid Fermentation That Does Not Consume Malic Acid
First, lactic acid bacteria perform fermentation to obtain energy necessary for their survival. This fermentation generally consumes glucose. There are two types of fermentation that lactic acid bacteria perform: homofermentative and heterofermentative types. Both types are similar in that they consume glucose.
In homofermentative fermentation, glucose is consumed and approximately 90% is excreted as lactic acid. In heterofermentative fermentation, on the other hand, the glucose taken up is decomposed not only into lactic acid but also into ethanol and acetate.
While there are differences in the products produced by fermentation between the two types, in principle, malic acid is not required for either type of fermentation.
However, lactic acid bacteria have the characteristic of taking up malic acid into their cells during metabolism. Malic acid taken up into lactic acid bacteria cells is converted to lactic acid with half the acidity through enzymatic decomposition. Unlike chemical deacidification methods, in this case, the acid is not precipitated from the wine and removed. However, because the acidity is halved, the measured acid amount is also halved.
Lactic Acid Bacterial Enzymes That Actively Decompose Malic Acid Are the Main Players in MLF
In winemaking, when the purpose is to reduce the acid content of wine, what has the greatest impact is not the lactic acid produced by lactic acid bacteria metabolism (i.e., the fermentation process). Rather, it is the conversion to lactic acid and carbon dioxide that arises from malic acid uptake, which is not involved in fermentation.
So why does this phenomenon occur? It is due to the characteristics of enzymes possessed by lactic acid bacteria.
The enzymes possessed by lactic acid bacteria have higher affinity for malic acid than for sugars. Even when taking up sugars for fermentation, they ignore those sugars and react with malic acid. This characteristic is very welcome from the standpoint of wanting to efficiently reduce the amount of malic acid contained in wine. However, for lactic acid bacteria that originally want to decompose sugars, this could be said to be an unwelcome result.
Regarding lactic acid fermentation, which has homofermentative and heterofermentative types, which type of fermentation is performed is determined by the strain type of lactic acid bacteria. From various conditions, what is mainly adopted in winemaking is heterofermentative fermentation.
Supplementary Note
Originally, there was a question about what purpose lactic acid bacteria serve in taking up malic acid and excreting it as lactic acid. However, recently it has been pointed out that lactic acid bacteria also obtain energy in this process.
Lactic Acid Bacteria Suitable and Unsuitable for Winemaking
The lactic acid bacteria considered most suitable for winemaking is the strain called Oenococcus Oeni. Oenococcus oeni was formerly called Leuconostoc Oenos. However, because it possessed properties greatly different from other Leuconostoc species strains, it is now distinguished as a separate species and called Oenococcus Oeni.
Oenococcus Oeni is a strain that performs heterofermentative lactic acid fermentation. The reasons this strain is suitable for winemaking are as follows: it has tolerance for low pH environments, high alcohol tolerance, and produces almost no substances that cause off-flavors in wine.
On the other hand, Pediococcus species strains that perform homofermentative lactic acid fermentation are suitable for winemaking in that they can survive at relatively low pH. However, they produce large amounts of diacetyl and are considered to have high risk of creating substances that cause off-flavors. Therefore, they represent strains that are never intentionally adopted in winemaking.
Past investigations found that among more than 700 types of lactic acid bacteria detected in wine, those with heterofermentative fermentation patterns were 10 times more numerous than those with homofermentative fermentation patterns. Only three strains lacked the ability to decompose malic acid, and it was reported that all strains could not metabolize tartaric acid.
Through these investigations, it is known that Oenococcus Oeni is currently the strain with the most suitable conditions for winemaking.
Where Do Lactic Acid Bacteria Come From?
Lactic acid bacteria do not naturally exist in grape juice. The reasons lactic acid bacteria exist in grape juice or wine can be attributed to the following three points:
- Those attached to the grape surface are mixed into juice through pressing, etc.
- Those attached to and surviving on brewing equipment (wooden barrels, tanks, hoses, etc.) are mixed in through use of such equipment
- Artificial addition
It is known that there is very large variation in the number of lactic acid bacteria attached to grape surfaces in advance. Additionally, the lactic acid bacteria often confirmed in juice immediately after pressing are Leuconostoc mesenteroides and Leucon. paramesenteroides. However, these strains have low tolerance for acids and alcohol and tend to rapidly reduce their population during the brewing process.
In contrast, Oenococcus Oeni, due to its high acid tolerance, can continue to survive throughout the period from the stage of freshly pressed grape juice until it becomes wine. This lactic acid bacteria can also proliferate during alcoholic fermentation, but because competition for nutrients with yeast occurs, the proliferation rate is kept low. The population reaches maximum several weeks after fermentation is completed.
It takes a long time for lactic acid bacteria that have naturally mixed into fruit juice to proliferate and begin metabolism. Moreover, in some cases, lactic acid bacteria may not exist in the fruit juice at all, and MLF may never begin no matter how long one waits. For this reason, many wineries have recently begun artificially adding lactic acid bacteria.
Requirements for Efficient MLF
The requirements for performing MLF efficiently are surprisingly numerous. For lactic acid bacteria to proliferate, they need amino acids, vitamins, and minerals in addition to sugars. However, because lactic acid bacteria are anaerobic microorganisms, oxygen is not necessarily required.
As we have already seen, lactic acid bacteria perform fermentation called lactic acid fermentation. However, what is needed in winemaking is the decomposition of malic acid that occurs in parallel with this process.
The amount of malic acid decomposition by lactic acid bacteria depends on the population of these bacteria. In other words, to decompose larger amounts of malic acid in a short period, larger populations are needed. The amount of sugar needed for malic acid metabolism is not large for most strains, at 0.4-0.8 g/l. However, regarding other nutrients, there are considerable differences in required amounts depending on the type of lactic acid bacteria.
In particular, Oenococcus Oeni, the main strain for performing MLF, requires large amounts of amino acids. It also requires many more types of vitamins compared to other strains, making it a very fuel-inefficient strain. Many of these required nutrients are supplied through the process of autolysis of dead yeast cells that settle after fermentation ends. However, depending on brewing methods, there may be cases that lack sources of these nutrients. Therefore, to perform more efficient MLF, measurement and addition as needed become necessary.
Summary | MLF Effects Beyond Deacidification
While the primary purpose of performing MLF is often thought to be reducing the acid content contained in wine, the effects that wine receives through MLF certainly do not stop there. The reasons for performing MLF include the following:
- Reduction of acid content
- Changes in aroma and taste
- Ensuring microbiological stability
- Reduction of SO₂ requirements
It is well known that performing MLF imparts butter-like aromas and creamy nuances to wine. This is mainly due to the production of diacetyl during the metabolic process of lactic acid bacteria. However, excessive diacetyl is considered an off-flavor for wine. Additionally, depending on the type of lactic acid bacteria strain that was active in MLF, other off-flavors may also arise.
Furthermore, through MLF, lactic acid bacteria consume not only malic acid but also amino acids, vitamins, and minerals—nutrients that other bacterial species need for proliferation—in advance. Therefore, the microbiological stability of wine is improved. Additionally, because lactic acid bacteria consume acetaldehyde and ketone compounds that have strong binding affinity with sulfur dioxide (SO₂), they improve the amount of free SO₂ in wine. These effects can be said to be more meaningful than reducing wine's acid content in some cases.
However, the problem is still the negative effects on wine.
It is out of the question for strains other than Oenococcus Oeni, particularly Pediococcus species strains, to become active. However, even if only Oenococcus Oeni were to be active, the results would not necessarily be positive. This is because MLF is a technique that depends on microbial metabolism, so absolutely reliable results are not guaranteed.
Microbial behavior is extremely delicate. Even slight changes in environmental conditions can cause different results. There is also the risk of causing contamination of unintended wines with lactic acid bacteria through equipment used in the winery.
While red wine tends to be associated with MLF, winemakers need to carefully consider whether MLF is truly necessary for their particular wine.
