Malolactic fermentation is an integrated part of winemaking which cannot be ignored. It can however be beneficial or detrimental and it is important that winemakers are well informed about it in order to make the right decisions. The execution of the decisions is also important to ensure that the required results are obtained in the wines.
Three different MLF genera, Oenococcus, Pediococcus and Lactobacillus, can convert the malic acid in wine into lactic acid. Only one Oenococcus species, but different Pediococcus and Lactobacillus species occur. Oenococcus oeni usually dominates because it has a higher resistance against the conditions occurring in wine. It is also the most desirable bacteria for MLF in wine because it can develop pleasant characteristics in wine while other species can cause spoilage like mousiness, sweaty and sauerkraut characters.
The end result of MLF is the decrease of the fixed acid concentration and increase in pH of the wine. The pH can increase with 0.3 or more units. The pH increase can however create favourable conditions for undesirable malolactic bacteria. Desirable MLF is usually acceptable in cool regions with a high fixed acid concentration to create a better balance in wines. Seeing that nutrients in the wine are also utilised during the process, such wines will be more stable against potential spoilage bacteria. It is not only as result of the utilisation of the nutrients, but also possibly due to the formation of toxins by the lactic acid, which can inhibit the growth of the spoilage bacteria. As result of the formation of other compounds during MLF the flavour profile and mouth feel of wines can also change. Diacetyl, which exhibits a buttery character, is an important compound formed during MLF. Its formation is influenced by the bacteria species, inoculation concentration of the MLF pure cultures, duration of the MLF, temperature, oxygen, pH and yeast lees contact. Ester compounds which can influence the wine flavour positively can also be formed during MLF.
Malolactic bacteria are however sensitive to the circumstances which occur in wine. It is mainly influenced by the alcohol concentration, sulphur dioxide concentration, pH and temperature. The restricting roles played by these factors are summarised in Table 1.
MLF in process.
TABLE 1. The optimal conditions of malolactic fermentation (AWRI, Bartowsky 2014). TABLE 2. MLF-problems which can occur and their solutions [Amanda Tanga, CHR Hansen (Pty) Ltd].
After the winemaker has decided to allow MLF it must also be decided whether it will be spontaneous or inoculated and when the stage of inoculation will be. If spontaneous MLF is preferred, the greatest risks remain the uncertainty regarding the strain or species which will dominate it, as well as the result of the MLF. If spontaneous MLF is preferred the monitoring of the process is of cardinal importance. It will mainly be sensory evaluation, because the undesirable compounds are not easily analysed. The limiting of temperature fluctuations is an important factor in the management of spontaneous MLF.
If inoculations with MLF pure cultures are preferred it can be done at five different stages: Co-inoculation with pure culture yeast with bacteria inoculation first; co-inoculation with pure culture yeast with simultaneous inoculation of the yeast and bacteria; co-inoculation with pure culture yeast with the yeast inoculation first; co-inoculation with pure culture yeast, but with the bacteria inoculation after completion of the MLF; up to five months after the completion of MLF. The best practice to ensure a fast and sound MLF is to obtain an Oenococcus oeni population of more than 106 per ml as soon as possible. Direct inoculation with a MLF culture is the quickest, safest and cleanest method to obtain it. As result of the influence of different factors on MLF in wine as indicated in Table 1, different problems can occur during MLF. The most common problems and their solutions are summarised in Table 2.
A sluggish or stuck MLF can be treated in two different ways. Firstly the tank can be mixed and inoculated with a double dosage MLF culture after bacteria nutrient was also added to the tank. The second solution is the application of an adjustment procedure. Wine with optimal conditions where MLF has not taken place is inoculated with a suitable MLF culture after bacteria nutrient was also added to the wine. The stuck wine is gradually added to the inoculated wine on a 1:1 basis after MLF has started properly in the inoculated wine (Howard, 2014).
Howard, Cathy, 2014 .Best practice in managing MLF whether adopting a cultured approach or leaving it up to nature. Wine and Viticulture Journal 29(6): 18 – 25.
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