Malolactic fermentation

The process is standard for most red wine production and common for some white grape varieties such as Chardonnay, where it can impart a "buttery" flavor from diacetyl, a byproduct of the reaction.

Today, most Vinho Verde producers no longer follow this practice and instead complete malolactic fermentation prior to bottling with the slight sparkle being added by artificial carbonation.

[8] Malolactic fermentation is possibly as old as the history of wine, but scientific understanding of the positive benefits of MLF and control of the process is a relatively recent development.

Grapes harvested from cooler climates usually have the highest malic content and have the most dramatic changes in TA and pH levels after malolactic fermentation.

When the bacteria move this anion from the wine into higher pH level of its cellular plasma membrane, it causes a net-negative charge that creates electrical potential.

The decarboxylation of malate into L-lactic acid releases not only carbon dioxide but also consumes a proton, which generates the pH gradient which can produce ATP.

[3] However, some studies have also shown that malolactic fermentation may diminish primary fruit aromas such as Pinot noir, often losing raspberry and strawberry notes after MLF.

Several factors influence which species will be dominant, including fermentation temperature, nutritional resources, the presence of sulfur dioxide, interaction with yeast and other bacteria, pH, and alcohol levels (Lactobacillus species, for example, tend to prefer higher pH and can tolerate higher alcohol levels than O. oeni), as well as initial inoculation (such as "wild" ferments versus an inoculation of cultured O.

O. oeni is a heterofermenter that creates multiple end products from the use of glucose with D-lactic acid and carbon dioxide being produced in roughly equal amounts to either ethanol or acetate.

Additionally, while sulfur dioxide levels above 0.8 molecular SO2 (pH dependent but roughly 35-50 ppm) will inhibit the bacteria, O. oeni is relatively resistant compared to other LAB.

[2] Species of Lactobacillus that have been isolated from wine and grape must samples across the globe include L. brevis, L. buchneri, L. casei, L. curvatus, L. delbrueckii subsp.

[5] However, in the absence of glucose, some species, such as P. pentosaceus, begin using glycerol, degrading it into pyruvate which later can be converted to diacetyl, acetate, 2,3-butanediol and other compounds that can impart unfavorable characteristics to the wine.

[2] Most Pediococcus species are undesirable in winemaking due to the high levels of diacetyl that can be produced, as well as increased production of biogenic amines that has been implicated as one potential cause for red wine headaches.

Towards the end of fermentation, while most of the original grape must resources have been consumed, the lysis of dead yeast cells (the "lees") can be a source for some nutrients, particularly amino acids.

Plus, even "dry" wines that have been fermented to dryness still have unfermentable pentose sugars (such as arabinose, ribose and xylose) left behind that can be used by both positive and spoilage bacteria.

While moldy, damaged fruit has the potential to carry a diverse flora of microbes, the LAB most often found on clean, healthy grapes after harvest are species from the Lactobacillus and Pediococcus genera.

[8] In the winery, multiple contact points can be home to native population of LAB including oak barrels, pumps, hoses, and bottling lines.

For practitioners of minimalist or "natural winemaking" who choose not to inoculate with cultured LAB, malolactic fermentation can happen at any time depending on several factors such as the microbiological flora of the winery and the competing influences of these other microbes.

The amount and exact nature of these products depends on the species/strain of LAB conducting the malolactic fermentation and the condition influencing that wine (pH, available nutrients, oxygen levels, etc.).

[12] In wine conditions that have a low redox potential (meaning it is more oxidative such as in a barrel that is not fully topped up), more citric acid will be consumed and diacetyl formed.

In more reductive conditions, such as in alcoholic fermentations where yeast populations are at their peak and the wine is heavily saturated with carbon dioxide, the formation of diacetyl is much slower.

[3] Chardonnay producers desiring to make the high-diacetyl "buttery style" will often do late or "wild" inoculation in the barrel after primary fermentation, allowing the wine to spend several weeks or even months sur lie in reductive conditions that promote diacetyl production.

[7] For early Vinho Verde producers, the slight effervesce that came from in-bottle malolactic fermentation was considered a distinguishing trait that consumers enjoyed in the wine.

Today, most Vinho Verde producers no longer follow this practice and instead complete malolactic fermentation prior to bottle with the slight sparkle being added by artificial carbonation.

[2] Heterofermenting species of Oenococcus and Lactobacillus have the potential to produce high levels of acetic acid through the metabolism of glucose, though with most strains of O. oeni, the amount is usually only 0.1 to 0.2 g/L.

Wines starting out with a high pH levels (above 3.5) stand the greatest risk of excessive acetic acid production due to the more favorable conditions for Lactobacillus and Pediococcus species.

[8] Ferments of high-pH wines (greater than 3.5) that spent time cold soaking prior to yeast inoculations and received little to no sulfur dioxide during crushing seem to be at the most risk for "ferocious" Lactobacillus.

Nicknamed "Fresno mold" due to where it was first discovered, the culprit of this growth was determined to be L. fructivorans, a species which can be controlled by sanitation and maintaining adequate sulfur dioxide levels.

French winemakers had long observed this phenomenon and called it tourne (meaning "turn to brown")[7] in reference to the color change that can occur in the wine at the same time likely due to other processes at work in addition to the tartaric loss.

The compound is produced from the degradation of the amino acid arginine which is present in both grape must and released in the wine through the autolysis of dead yeast cells.

A winemaker running a paper chromatography test to determine whether a wine has completed malolactic fermentation
The Swiss enologist Hermann Müller was one of the first scientists to theorize that bacteria were a potential cause of acid reduction in wine.
The chemical process of malolactic fermentation is actually a decarboxylation instead of a fermentation. The bacterial cell takes in malate, converts it into lactate, and releases carbon dioxide in the process. The lactate is then expelled by the cell into the wine.
The sensory characteristic of "buttery" Chardonnay comes through the process malolactic fermentation.
Oenococcus oeni
Cultured Oenococcus oeni inoculum strain and "Opti-malo" nutrient additive
Lactobacillus from a yogurt sample.
Acrolein taint is a common wine fault that undesirable species of Pediococcus can introduce to wine. Acrolein can interact with various phenolic compounds, imparting a bitter taste to the wine.
When yeast cells die, they sink to the bottom of the tank or barrel, creating the "lees" sediment visible in this picture. The autolysis of the dead yeast cells is a source of a nutrients for lactic acid bacteria.
Winery equipment offers multiple contact points for native populations of lactic acid bacteria to be introduced to the wine.
Some winemakers choose to inoculate for MLF after the wine has been racked off its lees and transferred to a barrel. If malolactic fermentation has previously taken place in the barrel, a "wild" or "natural" ferment will often take off in the barrel without any inoculation needed.
Wines that have not gone through malolactic fermentation will often be "sterile bottled" with a 0.45-micron membrane filter (pictured without housing).
A paper chromatography sheet showing that one wine still has some level of malic present while the other three wines have seemingly gone through malolactic fermentation
Winemakers can encourage diacetyl production by keeping the wine sur lie in the barrel. The barrel on the right has recently been stirred with a bâtonage tool.
To prevent malolactic fermentation from happening in the bottle, wineries must have high levels of sanitation throughout the winemaking process.
Wine grapes infected with Botrytis bunch rot tend to have higher levels of glycerol which can be metabolized by LAB into acrolein. Especially in red wine grapes with their high phenolic content, this can lead to the development of bitter-tasting wines as the acrolein interacts with these phenolics.
Cadaverine is one of the biogenic amines that some LAB species, particularly from the Lactobacillus and Pediococcus genera, have the potential to produce.