SO2 Addition Advice
One thing to mention: you cannot tell how much SO2 will bind when making an addition based on current bound SO2. As SO2 does it's thing (binds up oxygen) the bound SO2 will continue to increase over time even if you keep the free SO2 at the same level. This is why you need to add SO2 more than
once to your wine. It would be great if we could add SO2 at the beginning and never have to worry about it again, but the ratio of bound SO2 to free SO2 changes.
So what do we do to determine how much to add? If we have free SO2 present in solution then we have a little to work with. Free SO2 at any level indicates that we have some level of equilibrium in the solution. Let's look at the situation where we do not have free SO2 in the wine first.
If we have 0 PPM free SO2, all of the SO2 we add could theoretically be bound up by aldehydes. In that situation we simply do not have a method (big wineries do because they use a spectrograph) to measure the molecules that bind up SO2. instead we take the empirical approach: add a measured
dose and measure free SO2 the next day to see what happened to the addition, and then make (or not) another addition based on what we found out. I have had to do this up to three times before any free SO2 was present in a wine that I have let go through MLF without adding any SO2.
Once free SO2 is measured in a wine, most commercial winemakers simply assume that 1/3 of the added SO2 will bind up. This is the practical approach to the theory. For example, if we want to bring our free SO2 up from 10 PPM to 30 PPM, we would add 30 PPM, and expect an increase of 20 PPM
free with 10 PPM of the addition binding up.
One other key point is that the .5 mol for red and .8 mol for white guidelines are intended for free SO2 at bottling time, not bulk aging. Most wineries aim during aging to keep free SO2 for wines with a PH below 3.7 PH above 10 PPM for red and above 20 PPM for whites. Then increase the SO2
level to .5 and .8 mol respectively at bottling.
Personally, I add SO2 up to the .5 or .8 mol level when I see it is significantly below that level during aging. However, I don't check the wine very frequently, so I am just buying myself a little extra protection.
