SO2 in Wine
SO2 is a naturally-occurring preservative which is very effective against wine spoilage organisms. Some wine yeasts will naturally produce small amounts of SO2 during fermentation as a means to control the growth of other yeasts.
SO2 exists in wine in several forms. Winemakers are most often concerned with the level of Free SO2 (FSO2) in a wine. FSO2 can be easily measured in a lab and is considered to be the SO2 that is available to participate in chemical reactions (like protecting against spoilage organisms.)
The portion of FSO2 responsible for protecting against spoilage organisms is called Molecular SO2 (MSO2). MSO2 makes up a tiny fraction of the FSO2, so it is difficult to measure directly. Instead we can calculate it by knowing the FSO2 and the pH of the wine.
The final form of SO2 of concern is Total SO2 (TSO2), which is a combination of FSO2 and bound SO2 (SO2 that has bonded to other chemicals in the wine such as aldehydes, proteins and pigments.)