4.2.1.c What conditions influence malolactic fermentation?

What factors hinder malolactic fermentation?

Generally speaking, environmental toxicity will affect malolactic fermentation mainly by influencing the bacteria's membrane fluidity. At this stage, various interdependent criteria are present that can impede fermentation:

• The ethanol content of the environment is a parameter that impacts cell viability. The greater the alcohol content, the more difficult fermentation becomes. This is obviously not specific to lactic acid bacteria but values above 14% by volume make it more difficult to initiate fermentation.

• pH is also an obstacle if it is too low. Below pH 3.2, MLF is more difficult. One solution to this problem is to slightly deacidify a portion of the batch to initiate the MLF and then reincorporate it into the rest of the batch once MLF is underway. Given the current pH of wines, this practice is no longer really relevant.

• Temperature is a key parameter as it is easily controlled. An ideal fermentation temperature is usually between 18 and 25°C. Below 16°C, MLF becomes more difficult. Above 25°C, ethanol toxicity has a major impact. However, the ideal MLF temperature is strongly dependent on alcohol content and pH.

• Polyphenols: these can have a bacteriostatic effect in certain cases. This is highly dependent on the type of tannins encountered (condensed, polymerized, etc.) and therefore on the grape variety. In Bordeaux for example, Merlot is known to be more difficult to ferment than Cabernet Sauvignon.

• Short and medium chain fatty acids that are produced by yeast during alcoholic fermentation. Octanoic and decanoic acids are the most inhibiting acids. It is considered that above 25 mg/L of octanoic acid and 5 mg/L of decanoic acid, MLF becomes difficult. One way to eliminate these inhibitors is by stirring the lees, to help adsorb these compounds, or by adding yeast hulls.

• The initial malic acid concentration. A value that is too high or too low will make it more difficult to start MLF. The ideal value is between 2 and 3 g/L. It is generally accepted that MLF is facilitated between 1.5 and 4 g/L.

Here are some examples of temperature/ethanol/pH interactions:
If the alcohol level is above 14%, the ideal temperature is between 20 and 22°C.
For a wine with a low pH, it is better to use higher temperatures, whereas for a pH over 3.6, a temperature of 18°C is largely sufficient.
The level of total SO2 is also a factor influencing the lactic acid bacteria. If the total SO2 content is high, MLF is more difficult. Particular attention should be paid to sulphiting too early and too intensely.

A very easy-to-use scale enables a rapid evaluation of the medium's fermentability. It is based on four criteria:

What factors encourage malolactic fermentation?

The factors that hinder MLF onset were mentioned in the previous section.
To summarize, it is necessary to have a combination of the following factors:

• Low SO2 concentration.

• A temperature between 18 and 25°C

• An environment depleted in short chain fatty acids (only if there are too many initially)

• Other secondary factors favouring MLF have not been mentioned although they can have an effect:

• An adequate sterol and long chain unsaturated fatty acid content. These contribute to the membrane's rigidification which enables it to cope with high levels of ethanol. They are therefore survival factors.

• A minimal organic nitrogen presence in the environment so that bacteria can grow. However, they are unable to break down ammoniacal nitrogen. Most of the time, the amino acids released by yeast autolysis are sufficient for the needs of lactic acid bacteria, but this is not always the case and sometimes an addition can be useful.

• A slight aeration seems to favour the start of MLF.

What are the potential deviations with indigenous lactic acid bacteria?

Caution: these risks, while real, remain quite rare:

Lactic spoilage

This occurs when the lactic bacteria break down residual sugars. The consequences depend on the type of strain. Homofermentative strains will break down the sugars into lactic acid, which is not very problematic. Heterofermentative strains will transform the sugars into lactic acid and acetic acid. To confirm instances of lactic spoilage, it is necessary to measure the D-lactic acid, which is only produced when sugars are broken down. The breaking down of malic acid results in L-lactic acid. It is then necessary to stop the MLF with a light sulphiting to give the yeasts time to finish breaking down the sugars.

Mannitol deviation

This originates from the breaking down of fructose into mannitol. It gives a sour taste to the wine in excess of 100 mg/L. As with lactic spoilage, it is important to avoid starting MLF before the end of alcoholic fermentation.

Bitterness taint

This deviation is caused when glycerol is broken down into acrolein, a bitter compound. The strains responsible for this defect are often of the Lactobacillus genus.

Mousiness

Mousiness has been on the rise since the reduction or elimination of sulphites; it is caused mainly by the breaking down of amino acids (lysine and ornithine) and the formation of tetrahydropyridine derivatives.
Note that these compounds can also be produced by Brettanomyces bruxellensis during alcoholic fermentation.
The aromatic descriptors of mousiness are very varied (sausage skin, urine, mouse, tortilla, etc.) which makes it all the more difficult to identify, especially since it is not perceptible on the nose but rather in the mouth when the pH of the wine increases on contact with the saliva.
There is currently no way to eliminate mousiness.

Ropiness

Also known as oiliness, it is caused by lactic bacteria often of the genus Pediococcus that are capable of breaking down sugars into glucose polymers. The wine has a slimy, oily appearance. To avoid it, the sugars must be totally broken down before the start of MLF.

Production of biogenic amines

Four biogenic amines are found in wine, in order of importance:

• Histamine produced from histidine
• Tyramine produced from tyrosine
• Cadaverine produced from lysine
• Putrescine produced from ornithine or agmatine
  Histamine and tyramine in high concentrations can lead to migraines, nausea, and vomiting in sensitive people.
  Putrescine and cadaverine are aromatic masks.

To avoid the appearance of biogenic amines, a quick, vigorous start to MLF is required. It is also necessary to avoid an excess of amino acids in the environment (but there should not be a deficiency either) and/or to use strains that do not produce biogenic amines.
The threshold in many countries is generally 10 mg/L but it can be as low as 2 mg/L.