Malolactic fermentation (MLF) is typically carried out in most red wines and in some white varietals. MLF plays an important role in the finished wine’s feel and taste. MLF reduces acidity and produces flavors often characterized as “soft” or “buttery”. In addition, carrying out MLF before bottling prevents an unintended development of carbon dioxide gas (CO2) due to MLF starting up in the bottle, which can lead to fizzy wine, or worse, exploding bottles!
Malolactic fermentation uses the beneficial bacterium Oenococcus oeni (O. oeni) to convert malic acid to lactic acid, and CO2:
Malic Acid O. oeni Lactic Acid + carbon dioxide
HOOC–CH2–CH(OH)–COOH CH3–CH(OH)–COOH + CO2
Sulfite, as free SO2, inhibits the O. oeni’s activity, so free SO2 levels must be kept low during MLF. This leaves the wine at risk of oxidation and microbial contamination. As soon as MLF is done, then, SO2 should be raised to appropriate levels for protection of the wine. Thus it is important to know when MLF is done, and the best way to do this is to measure malic acid levels in the wine.
Should I do MLF?
These days virtually every winery takes its red wines through MLF. The mellowing tendency of MLF brings out the flavors and complex notes in red wines that are barrel-aged for months and then rested after bottling for a year or more. Increasingly, white and rosé wines are not taken through MLF, to preserve the crisp flavors and floral aromas that would diminish at the higher temperatures (>70F) needed for MLF. However, some winemakers prefer the mellowing, buttery flavors introduced by MLF into, say, a Chardonnay or a Muscat.
If you choose NOT to do MLF, bear in mind that your wine may decide to do it for you at any time. If you keep your SO2 levels properly adjusted, you should not have a problem with spontaneous MLF during bulk/barrel aging. But once you bottle, free SO2 levels are going to slowly drop to where MLF could start up on its own. Unless you have sterile-filtered your wine, it’s always possible that a few stragglers of O. oeni , or of some of its less savory lactobacillus relatives, could wake up to a nice malic acid breakfast and start making CO2 in your bottle.
When to start?
At primary: Some vintners like to inoculate for MLF right at or just after the initiation of primary fermentation. The idea here is that the O. oeni will get going in the presence of yeast and become acclimated to the yeast byproducts and alcohol levels early on. As alcohol levels rise, there is an increased risk of getting MLF stuck, so this avenue may be good for wines whose expected final ABV is going to exceed 15%. Since MLF must be carried out at low free SO2 levels, this approach also has the advantage of minimizing the time that the young wine spends at risk of oxidation.
At secondary: This is the approach taken by most vintners. Once primary fermentation has ceased, the O. oeni doesn’t face a huge population of yeast competing for resources. Also it is possible that in the presence of residual sugar the O. oeni will start to use the sugars instead of the malic acid as a food source, which could lead to off flavors and undesirable affects such as high levels of VA. At our winery we always start MLF after primary fermentation is done.
Protocol for inoculation
If starting MLF after alcoholic fermentation, most winemakers will first rack the wine off the gross lees to minimize undesirable side products such as H2S (hydrogen sulfide, rotten egg smell). Inoculation can be with either of two popular types of O. oeni; one is considered a direct inoculation variant, where the freeze-dried O. oeni are added directly to the wine, and the other type requires a rehydration step in water for approximately 15 mins before adding to the wine. Along with the O. oeni bacteria themselves there is also ML nutrient available that is typically used in the rehydration step.
How to monitor MLF
As mentioned, it’s important to know when MLF is done so you can raise SO2 levels immediately. During MLF, malic acid levels will drop from over 1000 mg/L typically, to below 50 mg/L at completion. Some vintners rely on the pH and TA changes (increase and decrease, respectively) to assess completion of MLF. However, the danger lies in an MLF that goes part way. If you have 100 mg/L of Malic Acid left, that is enough to produce fizziness if MLF starts making CO2. The pH and TA changes are too subtle to be reliable indicators of completion.
There are several ways to monitor the progress of MLF by measuring malic acid levels. Many wine makers rely on the paper chromatography method. This is fairly simple to run, and it is semi-quantitative if you run known standards alongside your samples. The drawbacks are that it involves use of noxious reagents and takes 24 hours generally to complete. But probably the biggest drawback is lack of sensitivity: it’s difficult to assess levels below about 200 mg/L. So again there is a risk of miscalling MLF completion and having fizziness or exploding corks and bottles.
Commercial wine analysis labs will typically use HPLC or enzymatic spectrophotometric methods. These are accurate and sensitive enough, but the instrumentation costs upwards of tens of thousands of dollars.
Accuvin (www.accuvin.com) makes the “Quick Test” kits for a variety of wine tests. Their malic acid test produces a colored spot whose intensity is proportional to malic acid levels. We have found them to be semi-quantitative, but capable of determining completion of MLF. Our assessment of these in comparison to other methods is on our web site https://vinmetrica.com/wp-content/uploads/2014/06/Malic_testing_comparisons_original_SC-50.pdf Note that the version of the SC-50 kit (see below) used in this report has now been updated.
Our SC-50 MLF Analyzer kit https://vinmetrica.com/product/vinmetrica-sc-50-mlf-analyzer-kit/ can be used to determine malic acid concentration in wine. This kit relies on the biochemical MLF reaction caused by enzymes found in certain bacteria, including Lactobacilli and Oenococcus strains, and in the “Biopressure” agent component of the kit. These bacteria live on a variety of nutrients, but their production of CO2 results almost entirely from the enzymatic transformation of malic acid to lactic acid as described back in the beginning. The CO2 creates pressure, which is read by the SC-55 manometer. The CO2 pressure is directly proportional to the amount of malic acid in the sample. The level of malic acid can be calculated from the pressure values by one or more calibrators of malic acid provided with the kit. Detection limit is below 0.04 g/L. The assay takes 45 minutes. Check out our product page here: https://vinmetrica.com/product/vinmetrica-sc-50-mlf-analyzer-kit/