The bad news in all of the above is that malolactic bacteria are extremely sensitive to sulfur dioxide. For that reason, it’s critical to not add any SO2 to wine, in any form, between primary and secondary (MLF) fermentation if you want to give your bacteria the best chance of survival. I believe the extra SO2 you’re measuring (even if it’s a rough analysis) came both from the wet storage solution and especially from burning a sulfur wick before filling. Even though you dried the barrels out for a few days before filling, wood is porous and so will often retain a little bit of the sulfur storage solution. Additionally, some of the SO2 gas created by the sulfur wick certainly will transfer into the wine as sulfur dioxide. While it’s probably safer to store your barrels with sulfur solution rather than with nothing, you may want to avoid burning sulfur wicks in a barrel before transferring in wine that you intend to go through MLF.
As for how to get your wine to go through MLF again, I’ll provide a list of tips. To have successful fermentations, the most important thing is to “think like a bacterium.” Just like us, yeast and bacterial cells like conditions to be nice and comfortable for them to do their best work. As a winemaker, your job is to provide your ML bacteria with an optimal environment so they can get down to business. To that end:
• Keep temperatures of your wine above 60 °F (16 °C) if you can. Too cold and they’ll slow down. Try an electric blanket on your barrels or an aquarium heater in the bung.
• Make sure the pH isn’t too low. ML bacteria don’t like high acid conditions. You’ll have best luck if your pH is above 3.20.
• Minimize SO2. Never add SO2 to wine between primary and MLF as ML bacteria are very sensitive to SO2. In your case, I would not have sulfured the barrel headspace with the sulfur wick.
• Pick your grapes early enough so the alcohol stays under 15%. This means not going much above 24.5–25 °Brix, depending on your alcohol conversion rates. ML bacteria have a harder time working if the alcohol is too high.
• Feed with MLF micronutrients. You can buy ML bacteria micronutrient mixes at home winemaking supply stores and online. ML bacteria are what we call finicky feeders and if conditions aren’t right and they lack some key mineral or vitamin they won’t operate at their best.
• Make sure you purchased your strain from a reliable source and that it’s fresh. I prefer to use the freeze-dried powdered cultures instead of dealing with messy liquid cultures that I have to grow. Make sure you’re buying a fresh packet every season and that it’s not through its expiration date. Store freeze-dried cultures according to instructions and definitely do not try to store an opened packet of ML bacteria from year to year. Freeze-dried, liquid, or on a slant (live culture growing on media), be sure it’s fresh and from a supplier with high turnover who has stored the “bugs” correctly.
• Consider re-inoculating. Try to change any of the above conditions that you can and then re-introduce ML bacteria to the wine. SO2 will dissipate with time so after a few weeks you might want to re-test your SO2. Hopefully it’s dropped a bit and you’ll have a better chance of getting your new culture on the right foot.
