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What are the main wine compositional parameters that inhibit MLF?

Alcohol, pH, temperature and sulphur dioxide are the main wine compositional factors that determine the successful induction and completion of MLF.  Each has a range, over which MLF is favourable, but outside of their respective ranges MLF becomes increasingly difficult or inhibited.  Because these factors essentially work synergistically it is difficult to consider them independently of each other.  This means that as one or more parameters becomes increasingly unfavourable, MLF will become increasingly difficult (i.e. the factors are additive).

Conditions for MLF are more favourable at higher pH and become less favourable at lower pH.  However, it must be remembered that wines are more susceptible to spoilage at high pH (favours growth of spoilage lactic acid bacteria, such as Pediococcus) and SO2 becomes more toxic at low pH, as the SO2 equilibrium shifts to provide more molecular SO2. Therefore, a pH in the range 3.3-3.5 represents a balance between low and high pH.  Within this pH range, the free SO2 should be less than 5-10mg/L and the total SO2 should be less than 30-40mg/L.  The addition of a maximum of 50mg/L total SO2 before crushing is considered not to adversely affect MLF.  The absolute lower limit for MLF is around pH3.0 when all other factors are highly favourable.

It is important to note that some strains of yeast produce significant concentrations of SO2 and this needs to be taken into consideration along with added SO2 (i.e. choose a low SO2-producing strain if other inhibitory factors are also present).  Furthermore, some strains of yeast produce more SO2 when significant diammonium phosphate (DAP) has been added.  Although all SO2 exists in the bound form (mostly to acetaldehyde immediately after fermentation, the total SO2 is still inhibitory to MLF because bacteria metabolise the acetaldehyde fraction, releasing the SO2 as inhibitory free SO2.

Although the optimum growth temperate for lactic acid bacteria (LAB) in grape juice is around 30˚C, as the ethanol concentration increases the optimum temperature falls sharply due to the increased toxic effects of ethanol at higher temperatures.  Consequently, the optimum temperate for Oenococcus oeni growth and malic acid metabolism in wine is in the 18-22˚C range, with the lower end being recommended in the alcohol is 14% v/v or higher.  Inoculation temperature is most important because it is the growth stage that is most sensitive to sub-optimal temperature; once growth has occurred, MLF can continue down to around 16˚C but at a much lower rate.

In addition to the parameters mentioned above, pesticide residues from the vineyard can also inhibit the development of LAB, with the effect being enhanced by the presence of ethanol.  High residual copper can also pose a problem.

Favourable and unfavourable conditions for MLF:

Parameter Favourable Unfavourable
Free SO2 (mg/L) <8 >10
Total SO2 (mg/L) <30 >40
Alochol (%v/v) <13 >14
Temperature (˚C) 18 – 22 <16. >25
pH 3.3 – 3.5 <3.1

Note that if any one parameter is on the cusp of being unfavourable, MLF will be slow even when all the other parameters are favourable.  If all parameters are nearing unfavourable conditions, the risk of inducing successful MLF is greatly reduced.

The full version of this article contains answers to the following further questions:

When is the best time to inoculate with malolactic bacteria? …
I’m thinking of co-inoculating with yeast and bacteria this year. My ferments usually go up to 32˚C, will this be OK? …
I have a wine that has unfavourable wine compositional parameters.  Is there anything I can do to maximize the chances of completing MLF? …
What change in titratable acid can I expect after MLF? …
At what level of malic acid can MLF be considered complete? …
Should I sterile filter at bottling a red wine containing residual malic acid? …

The full version of this article was originally published in the September 2012 addition of the magazine.

For further information

Further information on Malolactic information can be found at