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Is MLF testing necessary?


Robert Outenreath — San Rafael, California asks,

I’ve read a lot about malolactic fermentation (MLF) and think I know the basic principles. What I do not understand is the constant recommendation to test for the end of MLF with a relatively expensive chromatography kit.

Obviously, if one is going to bottle within a few months after starting MLF, you want to confirm the end of MLF to avoid exploding bottles later.

However, and this is my question: if you are going to age your wine in a carboy for many months before bottling, won’t it be certain that MLF will be over after sitting in a carboy after six or eight months, and there is no need for testing?


You are absolutely right that most wines, especially those that are inoculated and have favorable conditions, will go malolactic (ML) complete within six or eight months of harvest. Even if your area gets cold and your ML bacteria have to take a bit of a “long winter’s nap,” it’s likely they’ll wake up when the weather gets warmer and will tick your fermentation down to completion. This is especially true for big reds that age even longer, like Cabernets and Merlots.

There are two problems with just inoculating and assuming that your ML fermentation will be fine: 1) It’s possible conditions aren’t favorable and your MLF will stick, and 2) You want to know when MLF is complete so you can rack and start adding SO2 right away in order to protect your wine. I’ll tackle the second point first. Basically, when wine is undergoing an active primary or secondary (MLF) fermentation, it’s protected by the carbon dioxide gas being released by the microbes. When the ML fermentation goes complete, however, your wine becomes extremely vulnerable to oxygen and spoilage microbes that love an air environment, especially Acetobacter, which eat ethanol and transform it into acetic acid, or vinegar. Because of this, commercial winemakers typically test for ML completion every two weeks. The hope is that you “catch” the wine when it becomes vulnerable and hit it with SO2 in order to deter Acetobacter from gaining a foothold in your wine. With regards to the first reason, it’s a good idea to keep track of what’s going on with your ML fermentation because it’s quite possible that either the bacteria are from a bad batch or conditions in your wine are not favorable for completion. If you have a “stuck” or not-happening ML fermentation, as I mentioned before, your wine is hanging out in an extremely vulnerable state.

As many of my readers know, there’s a laundry list of conditions that need to be met for ML bacteria to be content. ML bacteria are happiest under higher pH, lower alcohol, and higher temperature conditions. In general, this means that your pH should be above 3.2–3.3, temperature should be above 55–60 °F (13–16 °C), and the alcohol can’t be over 15%. ML bacteria also need some specific micronutrients that can be present in grapes and wine but that aren’t always, especially if they got eaten up early on by the yeast or by other ambient microbes that may have come in on the grapes. For these reasons it’s important to know what’s happening in the mysterious dark depths of your carboy or barrel. You don’t have to go as crazy as commercial winemakers, but I do recommend checking your ML fermentation at least once a month, especially if it’s been over two or three months since you inoculated and if you believe all of the above conditions are met.

Response by Alison Crowe.