Part 12 - Malolactic Fermentation

Part 12 - Malolactic Fermentation (MLF)

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v 2 significant differences between yeasts and  bacteria
Alcoholic fermentation is done by yeasts. They have mitochondria in cytoplasm. Yeasts can produce 38 ATP from one glucose (see Part 2 – TCA cycle).

Malolactic fermentation is done by bacteria. They have no mitochondria. The lack of mitochondria means that bacteria can only generate 2 ATP from one glucose in cytoplasm (see Part 1 – The alcoholic fermentation). Another difference is that bacteria can grow on glucose as well as on organic acids.

    

                        

v Bacterial growth during fermentation in 3 phases

Lactic acid bacteria (LAB) can grow on glucose and organic acids. The first bacterial growth takes place at the beginning of alcoholic fermentation. But soon, after a few hours, the bacterial growth is brought to a halt by the powerful yeast growth. In the presence of oxygen, yeasts break down glucose in cytoplasm and mitochondria to produce 38 ATP. Bacteria, by lack of mitochodria, can only break down glucose in cytoplasm to produce 2 ATP. The bacteria simply lost the battle to the yeasts.

After the alcoholic fermentation, the bacteria can grow again when conditions allow. In the absence of residual sugar bacteria now live on the malic acids in the wine. That is in fact the malolactic fermentation.

v Factors affecting  MLF

Temperature – The optimal temperature is between 20^o and 25^o C. Under 15^oC and above 30^oC, MLF is practically impossible. By regulating the temperature the winemaker can control if MLF will happen or not.

 Acidity (pH) – At pH below 3.3, MLB can live but do not grow. At pH above 3.6, they grow well and optimally at around 4.5. In general, the pH of red wines is rarely less than 3.4 and whites is rarely higher than 3.6. That partly explains why in red wines almost always MLF occurs while a Riesling, from a cool climate, almost never does. If anyone would apply MLF to a wine with a very low pH, he must first deacidify the wine (e.g. with calcium carbonate).

Sulfur dioxide (SO₂) – The presence of SO₂ can prevent MLF because SO₂ is a very effective inhibition agent against malolactic growth. The dosage is dependent on the pH. In principle: the higher the pH, the more SO₂ required.

v Malolactic fermentation (MLF)

MLF is the conversion of the tart ‘malic acid’ into the soft ‘lactic acid’ by the LAB : Oenococcus-oeni (former name Leuconostoc-oenos).  Oenococcus-oeni  take in malic acid and decarboxylate it with malolactic enzyme into lactic acid and carbon dioxide.

                                     

v Oenococcus-oeni convert malic acid into lactic acid to generate energy (ATP)
In chemistry, acid is defined as a compound which can release proton (H+ ion). When this happens, R-COOH will become R-COO^– (R = the rest of the compound). Malic acid outside malolactic bacteria, can release two H+. Malic acid inside malolactic bacteria, will be converted into lactic acid which can only release one H+. This way, malolactic bacteria create purposely a pH gradient. That means the H+ concentration in the wine is higher than inside the bacteria cell. With a pH gradient, H+ will diffuse from an area of high concentration to an area of lower concentration. It is  called chemiosmosis. This way, malolactic bacteria get the H+ in the ATP synthetase to generate ATP.

The conversion will stop when the H+ concentration in the wine and in the bacteria is at equilibrium point (=in balance condition). That means the bacteria can never convert all the malic acid into lactic acid.

v Oenococcus-oeni also convert citric acid into acetic acid, acetoin and diacetyl
The citric acid is first converted to acetic acid and oxaloacetic acid. The oxaloacetic acid is decarboxylated to pyruvate. The pyruvate binds with ethanal to become acetolactic acid, which will then be decarboxylated and reduced/oxidized to acetoin, butanediol and diacetyl. Acetoin and especially diacetyl give off a buttery smell that may contribute to a wine’s aroma. 2.3-butanediol is virtually odorless (We have seen this already in Part 11).

v MLF : YES or NO

(1) MLF can lead to considerable deacidification. One gram of malic acid is converted roughly into 0.67 grams of  lactic acid and 0.33 grams of CO2. It increases the pH by 0.1 to 0.3 units. MLF is beneficial for high acid wines,  but undesirable for low acid wines which, after MLF, will be more flat and unbalanced. Deacidification is also beneficial

to tannic wines since acids aggravate the astringency of tannins. Reason why most red wines go through MLF.

(2) MLF can contribute “buttery” aroma to wine due to the extra diacetyl and acetoin. This buttery aroma was once the hallmark of the California Chardonnay.  However, for fruity, fresh style wines; like Riesling, Sancerre and most Chablis,  MLF is not desirable as MLF might change their fruity and fresh character.

(3) MLF can improve microbial stability because the lactic acid bacteria have consumed many of the leftover nutrients that other spoilage microbes could use to develop wine faults. However MLF increases also pH level and acetic acid, which might make the wine vulnerable. For unsulphured wines this might not be beneficial.

v Spontaneous MLF or  Use of Selected Starter Cultures

Lactic acid bacteria (LAB) found in wine belong to three genera: Lactobacillus, Pediococcus and Leuconostoc. MLF is mainly performed by Oenococcus oeni (former name Leuconostoc-oenos), a species that can withstand the low pH (<3.5), high ethanol (>10 vol.%) and high SO₂ levels (50 mg/L) in wine.

Induction of MLF can be spontaneous or by the use of selected starters. The latter gives a better control on the fermentation of: the start, its progress and the strain that completes this proces. In fact, the inoculum of selected bacteria prevents the development of bacteria belonging to the genera Lactobacillus and Pediococcus. These are regarded as “bad guys”. These contaminating species can produce high concentration of acetic acid that can impair the organoleptic quality of the wine and substances that may be hazardous to human health (such as ethyl carbamate and biogenic amines)

v Moment of MLF

Traditionally, the MLF occured after the alcoholic fermentation, usually in spring, when temperatures rose again. Problem was that in all that time between, the wine was very vulnerable to all kinds of bacteriological contaminants. Interim sulfite addition was not an option, because sulfite will inhibit the growth of LAB. With the progressive knowledge of winemaking, it is possible to have the MLF taken place right after, or even during the alcoholic fermentation by creating the right conditions or by using a starter culture. This is possible, because LAB can grow on malic acid while yeast grow on glucose. The purpose of this is primarily to neutralize the production of diacetyl. The yeasts will then reduce the buttery diacetyl to the less fragrant acetoin and the virtually odorless 2,3-butanediol.

               

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P.S.

In this post, we see two important USES of WINE CHEMISTRY:

1. WINE CHEMISTRY makes it easier to understand ‘why’ and ‘how’ malolactic fermentation occurs.

2. WINE CHEMISTRY makes it easier to see the differences between wine components: citric acid (with 3 acid groups –COOH) is tarter than malic acid (with 2 acid groups), with lactic acid (with 1 acid group) being the softest.

  

In next and last post, we’ll take a look at how sulfite (= sulphite) works.