MLF: new things to consider

MLF is much more than the decarboxylation of L-Malic acid to L-lactic acid. It decreases the wine’s acidity, improves the organoleptic quality of the wine and increases microbiological stability. This is widely known. Here are three things that are less widely known…

Malolactic conversion can be stimulated by non-Saccharomyces yeasts

A study in Spain found that two of the most popular non-Saccharomyces strains used during alcoholic fermentation (AF), Torulaspora delbrueckii and Metschnikowia pulcherrima, can be used to make conditions more favourable for spontaneous malolactic conversion.
As MLF – the decarboxylation of L-malic acid to L-lactic acid – usually takes place after AF, the lactic acid bacterium Oenococcus oeni is highly affected by the metabolism of the previous fermenting yeasts. Therefore, the inoculation of certain yeasts will impact the development of MLF.
Inoculating Torulaspora delbrueckii and Metschnikowia pulcherrima along with Saccharomyces cerevisiae has been growing in popularity because these two non-Saccharomyces yeasts have been found to liberate aroma compounds, lower ethanol concentration and increase glycerol and mannoprotein concentration.
In the study in Spain, the researchers examined their effect on MLF in the production of white (Macabeo) and red (Cabernet Sauvignon) wine. They were inoculated 48 hours after the must’s inoculation with S. cerevisiae.
The researchers discovered these two non-Saccharomyces strains can reduce the concentration of compounds that have an inhibitory effect upon O. oeni. In the wines obtained with these two non-Saccharomyces, a reduction in medium chain fatty acids was observed.And a significant 0.5% reduction in ethanol content was obtained in Cabernet Sauvignon wines fermented with M. pulcherrima.
These two compounds, together with sulphur dioxide, are the most toxic to O. oeni in wine. Sulphur dioxide can be exogenously added, and it can also be produced by yeasts during AF. In this regard, T. delbrueckii wines had lower total SO2 concentration (more than a 50% reduction) than those just fermented with S. cerevisiae.
The study also found that the aroma of wines after AF was highly influenced by the use of non-Saccharomyces. The volatile profile was dependent on the inoculation strategy, whereby the use of non-Saccharomyces increased the concentration and type of aromas. After MLF, the wines were homogenised in terms of MLF strategy. Spontaneous MLF in white wine production thus resulted in the lowest aroma profile, while in red wine production it produced the most aromatic wines.
The researchers also say: “Furthermore, the use of non-Saccharomyces somehow helped polyphenolic extraction and enhanced the anthocyanin concentration of red wines, with values ranging from 388 mg/L in S. cerevisiae wines to 451 and 426 mg/L in T. delbrueckii and M. pulcherrima wines respectively.”
They concluded: “Wine comprises a complex microbial environment in which nutrients are very limited. Under these stressful conditions, the reduction of inhibitor compounds directly affecting O. oeni may stimulate MLF. As a result, those wines fermented with non-Saccharomyces had a higher consumption rate of L-malic acid (L-malic acid g/L per day) than in S. cerevisiae wines. In particular, T. delbrueckii wines showed the most favourable conditions for MLF performance, which is reflected in its quick MLFs. In addition, it promoted O. oeni diversity, which could be useful when a spontaneous MLF is desirable.
“We can therefore conclude that M. pulcherrima and particularly T. delbrueckii seem to promote MLF.”

It’s a similar story for Lachancea thermotolerans

Some strains of the non-Saccharomyces yeast Lachancea thermotolerans, which shows great potential for bio-acidification in warmer regions and vintages and as an alternative pathway to lower ethanol production, can be used to promote MLF while others inhibit it.
With L. thermotolerans known to produce L-lactic acid during alcoholic fermentation (AF), a study in Australia set out to explore the impact of sequential cultures of L. thermotolerans and Saccharomyces cerevisiae on MLF performance in white and red wines. Four L. thermotolerans strains were tested in Sauvignon Blanc from the Adelaide Hills with sequential S. cerevisiae inoculation, compared to an S. cerevisiae control and the initially un-inoculated treatments.
The L. thermotolerans wines showed large differences in acidification, and progression of MLF depended on lactic acid production, even at controlled pH. The highest and lowest lactic acid producing strains were tested further in Merlot fermentations with both co-inoculated and sequentially inoculated O. oeni. The low lactic acid producing strain enabled successful MLF, even when this failed in the S. cerevisiae treatment, with dramatically quicker malic acid depletion in O. oeni co-inoculation (after 48 hours) than in sequential inoculation (after alcoholic fermentation).
In contrast, a high lactic acid producing strain inhibited MLF irrespective of the O. oeni inoculation strategy. In a follow-up experiment, increasing concentrations of exogenously added lactic acid slowed MLF and reduced O. oeni growth across different matrices, with 6g/L of lactic acid completely inhibiting MLF. The results confirm that certain L. thermotolerans strains can produce sufficient quantities of L-lactic acid to inhibit O. oeni and MLF even in the presence of little or no SO2, thereby offering reduced processing time and preservative use. The study also highlighted the potential of other L. thermotolerans strains to promote MLF.
In particular, the study found the use of low lactic acid producing strain Concerto (CHR Hansen, Denmark) was conducive to successful and timely MLF, even when prolonged or unsuccessful in the Saccharomyces cerevisiae monoculture (Zymaflore Spark, Laffort, France).

MLF influences the colour of red wines

Oenococcus oeni and other lactic acid bacteria (LAB) involved in MLF appear to have an influence on the colour and astringency of red wine. In general, it is believed that MLF reduces the colour of wine, although there are some results that prove the opposite.
But it depends on the LAB strain used and the variety of Vitis vinifera involved (varieties with a short vegetative cycle, such as Pinot Noir, or varieties with a long vegetative cycle and with a higher phenolic content, such as Cabernet Sauvignon). It also depends on the vinification conditions, the maturity of the grapes and the container in which MLF is carried out (stainless-steel tanks or barrels). The micro-oxygenation that occurs in barrels favours the polymerisation reactions between the anthocyanins. Under these conditions, acetaldehyde is generated, which acts as a link for the formation of polymeric compounds. The result is colour stabilisation and decreased astringency.
A working group in Argentina carried out an experiment using two strains of Oenococcus oeni (O. oeni ATCC 27310 and UNQOe19), two strains of Lactobacillus plantarum (Lb. plantarum ATCC 14917 and UNQLp11), and two varietals of Vitis vinifera (Pinot Noir and Merlot). The aims of the experiment were to evaluate the behaviour of the different LABs strains used and the possible colour changes produced after MLF. The results confirmed that the survival of bacteria and the decarboxylation of L-malic acid differ depending on the LAB strain inoculated and the variety of wine. Furthermore, the working group found that O. oeni can survive in wine even when L-malic acid is not consumed.
Regarding the chromatic characteristics, they found some correlations between MLF and colour-related parameters in Pinot Noir, but not for Merlot, supporting the theory that the colour changes may be due to the LAB strain and its behaviour in a certain variety (or matrix), which will also depend on the technological process carried out in the vineyard and winery.