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The MLF manufacturer’s instructions say the wine should be less than 15 ppm. Isn’t that risking spoilage?

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Frank Dimatteo — Kensington, Connecticut asks,
Q

I always put my Zinfandel through malolactic fermentation. The manufacturer’s instructions say the wine should be less than 15 ppm. Isn’t that risking spoilage? I add about 1/4 tsp of sulfite per 5 gallons (19 L) when I crush, and this usually results in about 25 ppm after primary fermentation. So, I usually rack and wait until it lowers to 15 ppm. Is this safe?

A
Since you don’t mention whether the 15 ppm is free or total SO2, it’s tough for me to refer specifically to either the instructions or your protocol. However, I routinely add up to 30–40 ppm total sulfur dioxide to my reds pre-primary fermentation and then inoculate with my ML bacteria after the wine is dry (from a residual sugar point of view). Since free sulfur dioxide binds readily to sugars and aldehydes, by the time primary fermentation is over, your free SO2 is essentially zero. This makes for a relatively safe environment for your ML bacteria of choice, which are not affected by bound SO2 at that concentration. If you’re keeping your pre-fermentation sulfur additions to a reasonable level, there’s no reason to wait, as you suggest above, before you add your ML bacteria because post-primary the levels of free (biologically active and therefore available to interact with the ML bacteria) sulfur dioxide will have dropped to a safe (negligible) level. You’re right to hint that leaving long periods of time at low levels of sulfur dioxide is risky for
Response by Alison Crowe.