Ask Wine Wizard

Free SO2 Levels


Greg Farke — Armour, South Dakota asks,

I would like to sweeten some red wine that has gone through MLF. I know that I need to prevent renewed fermentation. Will sulfite prevent renewed fermentation? If so, what concentration of free sulfur do I need?


It seems that you and Craig are going through many of the same issues (see this question and answer). Like I mentioned to Craig, it’s really impossible to add enough sulfur dioxide to retard a yeast fermentation because, contrary to popular belief, yeast just aren’t that sensitive to SO2. Unlike bacteria, yeast can still have a wide range of activity at high levels (even above 50 ppm FSO2) in juices and wines. Depending on your sensitivity, you may start to smell the FSO2 at levels of 30 ppm and above. Don’t forget that even a small amount of carbon dioxide, like that produced by a partially-viable yeast cell, will amount to a “flaw” or defect in your still red wine.

I am a proponent of sterile filtration if you can rent one from a winemaking supply store or share the cost of one with some winemaking buddies. If you want to get beyond making wines that are “primary fermentation dry” it might be a piece of equipment you should budget for. Barring that, I really think adding sorbate might be your only option. Yes, there is a chance of developing a geranium off-odor, but some people don’t think that’s the end of the world. If your alcohol is 12%, you will add 100 mg/L potassium sorbate. If it’s 14% you can get away with 50 mg/L. If you aren’t into dosing with sorbate, you may have to decide if you’d prefer a sparkling red sweet wine.

Response by Alison Crowe.