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Malolactic Fermentation Tips and Tricks

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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


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  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 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


20 thoughts on “Malolactic Fermentation Tips and Tricks

  1. How long can I keep wine doing an ML without worrying of spoilage

    1. Bill,
      Thank you for contacting us. It can take up to three months to complete ML depending on the conditions. You want to ensure that your wine is progressing through this fermentation stage so make sure to test regularly and when you see that your wine has completed MLF you will then want to bring up the SO2 levels to ensure your wine is protected from spoilage. If you have any testing data or need some additional help, please either email or give our tech support line a call.

  2. I grew up making wine with my dad, and now am attempting this on my own. I am making a zinfandel and a moscat. I pressed grapes approx 3-4 wks ago, have just racked them for first time, and it appears primary fermentation has stopped (no bubbling from air locks). Would I just at the MLF bacteria now or is it too late? I have 2 15 g demijohns filled up, one for each wine, with space at top of demijohn for one more gallon of wine. Any direction would be great. Thanks

    1. John, now is a good time to start MLF. Assuming you haven’t added sulfites since primary fermentation ended.

  3. Thanks for the information as it is very helpful! This is my first attempt at malolactic fermentation, and I have a couple questions. Does it matter what yeast I use for the alcohol fermentation if I plan to do MLF? Is EC-1118 OK? And do I need to do a starter with 20 gallons of wine? How much do I add? Thanks!

    1. Michelle,
      Sorry for the delayed response. Your local wine store should be able to help you determine which yeast to use based on the varietal you are making. We usually always follow the yeast manufacturers instructions about how much to add and which type of yeast is good for which varietals. Your wine shop should also be able to help you pick out any nutrients your might need, like FermaidK and/or GoFerm. It is difficult to tell you exactly how much to add without many of the details about your wine or its chemistry.
      Let us know if you have any other questions.

  4. I am planning to go through malolactic fermentation for the first time with my red wine as soon as it finishes the primary fermentation. I have enough to fill 2 puncheons (500 litres x 2) and my questions is how much should I fill the puncheons? 90% full ? what would be the right gap? or should be full and I insert an airlock?

    1. If you are sure the primary fermentation has completed you can probably fill your tanks about 90-95% full. Make sure to use a fermentation lock on the top of the puncheon so that air can escape. During Malolactic fermentation the wine should not expand too much if at all. Let us know if you have any other questions.

  5. Dr Sportsman, I’m creating a Bordeaux-Cabernet blend. My two wines have completed their primary fermentations and I started ML on them both after each being racked from their respective PF into separate car boys. After 3 days, my pH value on my Bordeaux has risen to an alarming values of 3.71, while my Cab is still at 3.51, allowing me to conclude that ML is underway. My ML temp is at 71 degrees. I’m concerned that my pH is too high, and the stability of the Bordeaux will become problematic. Should I rack it off now to stop the ML? Should I move the Bordeaux to a temp controlled room (55 degrees to stop ML? Should I add SO2 to the wines, and if so, how much (6 gallon carboys). Any help or guidance is appreciated.

    1. Chuck, I would expect your pH to rise if MLF is proceeding or done. So it seems that your Bordeaux is progressing while your Cab is not.
      A pH of 3.71 is not too high.
      Can you test for MLF completion? I would not try to slow anything down until you know. We can do the testing for you, or you can use our kit Obviously if you raise free SO2 above 20 ppm you will likely stop MLF.
      When you do decide to add SO2, use a calculator like this or this one:

  6. I will be coinoculating my mlf, racking the wine 24hrs after pressing into a tank with an airlock.
    Is it an advantage to stir the wine to completion of mlf with a submersible pump. If so how often.
    Jim Deane

    1. Jim, I have never tried the pump idea. It is generally advisable to occasionally stir everything up to keep MLF moving.

  7. If starting MLF later after fermentation is complete, and wine has been on SO2 for protection from spoilage, how low does SO2 level have to be before it’s safe to start MLF?

    1. It may depend on the strain of Oenococcus you are using but as a general rule of thumb we would say that your SO2 level needs to be below 10ppm.

  8. Hello,
    Does the malolactic fermentation need to be started just after the primary is complete, or can one wait a week to initiate it?

    Thank You,

    1. Leonardo,
      You can wait a week if you wish. If you add sulfite in between the end of primary fermentation and the beginning of MLF, make sure your sulfite levels are low enough before you start MLF so that the sulfite does not inhibit the malolactic bugs.
      Let us know if you have any other questions.

      1. If we wait a week at what temperature should the wine be kept? I am waiting for my Acuvin

        1. Ken,
          IF you have a little bit more information about your wine that would help us answer your question a bit better. Are you talking about waiting between primary and secondary fermentation? Let us know.

  9. We have the SC-50 and are wondering about the shelf life of the reagents which are kept in the freezer. They have a printed expiration date but if they are frozen until use – warmed for use – then refrozen … will that work or might that alter the results?

    1. Diane and Ron,
      If you keep all of the reagents in the freezer, thaw to room temperature when getting ready to use and then refreeze them, the reagents should be good for at least a year. This is with regular use. So say if you only used them once during that initial year, and they were kept frozen the rest of the time, they should still be okay. If the reagents are over 2 years old, I would recommend replacing them. Even if they have been in the freezer. Rich may have some more insight on the length of time they can be stored in the freezer, ie: longer than two years, but he is out on vacation until Wednesday. Ill discuss with him when he returns and let you know if he has any other advice.
      Thank you

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