Malolactic fermentation (MLF) is one of the more challenging processes that we take on as winemakers. It is the bacterial fermentation that converts malic acid (sour tasting) to lactic acid (smooth) and creates a buttery smelling biproduct (diacetyl). Unfortunately the bacteria that do this conversion don't always like to play nice. Often times they just won't start, or will start and stall out before completion, leaving you with a tart tasting wine. Sure, you can just try again but the bacteria is not cheap and the wine could succumb to other spoilage bacteria if you drag the process out too long, so it is much preferred to get it right on the first try.
When to add the Bacteria
In most cases we will add malolactic bacteria to the wine, instead of relying on a wild strain to do the job. It is most commonly added to the wine after completion of primary fermentation when ethanol is high and nutrients are low, which adds to the challenge.
Co-fermentation or adding your bacteria alongside your yeast is an option which I have found to be easier but comes with it's own risks. While, I have not experienced this myself, it is possible for the bacteria to create excess acetic acid (vinegar) when fermenting in the presence of sugar. When co-fermenting, there will also be considerably less of your buttery aromas, since the yeast will irreversibly break down diacetyl. For these reasons, I rarely co-ferment.
Making a Malolactic Bacteria Starter
While some manufacturers will suggest direct inoculation, I have found making a malolactic bacteria starter to be much more reliable. To do this, you will need the following "ingredients".
Step 1: Mix the Acti-ML (if using) and distilled water until all of the chunks are dissolved. It is a fine powder, so it needs very vigorous stirring.
Step 2: Heat up your water and Acti-ML blend in the microwave to 75-78°F
Step 3: Mix in your bacteria
Step 4: After 15 minutes, add around 5mL of apple juice, wait 5 more minutes and add 20mL more. Doing it this way prevents any major pH shifts.
Step 5: After 15 more minutes, add 25mL of your wine to the starter and let stand for at least 15 more minutes.
Step 6: gently add the starter to your wine. Don't stir.
Tips for Success
Anything you can do to make the wine more friendly to your bacteria will increase your chances of a good, complete malolactic fermentation. Here are some tips for success.
If your pH is below about 3.4 on a red wine, you may want to adjust it up to about 3.45 with potassium bicarbonate before beginning MLF. For a white wine this is a little too high, so I like to use an acid tolerant strain like CH35.
If you know your alcohol levels will be high (above 14.5%), you may choose to co-ferment vs waiting until after primary fermentation.
Adding oak cubes can help trap the bacteria and keep it in suspension, so now is a good time to add the first dose.
Aside from adding sulfite at the crusher, don't add further sulfite until after malolactic fermentation is complete. Here is a video I have on how to test for completion.
I like to gently stir up the wine once a week for the first 2-3 weeks of MLF, so that the bacteria don't get buried in the lees. This also helps to churn up nutrient for the bacteria.
Keep the temperature of your wine between 68°F and 73°F until MLF is complete. Seed heaters under or around carboys are a great tool for gently increasing and maintaining temperature.
For more info on malolactic fermentation, checkout my YouTube videos below on the topic. For those who want to dive deeper into the mechanics of winemaking and wine growing, please checkout my Patreon site, patreon.com/makewine
If you have any comments on the topic, I would love to hear them below!
Cheers,
Rick
