I’m sure many of you are familiar with the technique of malolactic fermentation (MLF), 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 titratable acidity, increases pH, 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 to convert malic acid to lactic acid, with CO2 being the byproduct of the reaction.
Malic Acid O. oeni Lactic Acid + carbon dioxide
HOOC–CH2–CH(OH)–COOH CH3–CH(OH)–COOH + CO2
Sulfite, as free SO2, inhibits the bacteria that carry out MLF. Therefore free SO2 levels must be kept low during MLF, carrying risks that the wine is left unprotected against 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?
This is really a matter of choice that depends on your taste. 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.
At our winery (Little Oaks Winery) we have made Viognier, rosé of Barbera and of Sangiovese, and Chardonnay. All of these are fermented at 50-55 °F, and no MLF is undertaken. When fermentation is done, the wines are chill-proofed to 30-32 °F for two weeks, then allowed to warm up for 2 days in time for bottling. We carry out the bottling under nitrogen and sterile-filter through a 0.4 micron filter to prevent any bacterial stragglers.
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: Typically 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. In general we find that post alcoholic fermentation is the best method for us.
Protocol for inoculation
If starting MLF after alcoholic fermentation most winemakers will rack the wine off the gross lees before initiating MLF because leaving wine on the gross lees during secondary fermentation could lead to undesirable effects such as H2S (hydrogen sulfide, rotten egg smell). Once racking is complete it is time to inoculate the wine. There are two popular types of O. oeni used for malolactic fermentation; 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.
Our wine is pressed off the grape skins at about 0 degrees Brix and we will rack the wine approximately 24 hours after it has been pressed.. We like to use the direct inoculation O. oeni. We have found that adding ML nutrient before adding the O. oeni to our wine helps promote a successful MLF. We measure out the O. oeni at 1 g/40 gal and for our ML nutrient we use Acti-ML at a rate of 0.84 g/gal. We put our desired amount of O. oeni and Acti-ML in with chlorine free water and let incubate for 15 minutes. We then give the mixture a good stir and then gently add it into the wine. Once it is in the wine container we do not mix it but let it sit for a few days. The O. oeni tend to settle at the bottom of the barrel so it is a good idea to mix/stir the barrel or container 1-2 times per week.
How to monitor MLF
It’s important to know when MLF is done, because you need to bring up SO2 levels as soon as you can to minimize oxidation and spoilage of the wine. 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 pop a cork if MLF starts up again. 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 and having 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 (Napa, CA) 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 shown above:
The CO2 creates pressure, which is read by the MLF Analyzer. The CO2 biopressure signal is directly proportional to the amount of malic acid in the sample. The level of malic acid can be calculated from the digital readout by a calibrator of malic acid provided with the kit. Detection limit is below 0.04 g/L (40 mg/L). The cost of the unit is around $160 and cost per test is about $3.00.
Do you have any tips for starting up stuck MLF? Share them in the comments below.
Dr. Richard Sportsman