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Calculating Residual Sugar


Frank Dawson — Southlake, Texas asks,

I am looking for an equation for calculating the residual sugar (RS) in store-bought wine when I know:
1. Temperature of wine (from instant-on thermometer),
2. Specific gravity of wine (from hydrometer),
3. % ethanol of wine (from bottle label)

That’s certainly an interesting question and one for which the short answer is “no such equation exists.” The longer answer attempts to help explain why, even though you think you should have all the parts that you need to solve for unfermented sugar. As I was considering whether to answer your question for this issue of “Wine Wizard” or not, I sent your question to WineMaker magazine Technical Editor and columnist Bob Peak. I was debating including the question in this column because I was afraid I would have to give you an unsatisfactory answer: It just can’t be done. However, as I read Bob’s response I changed my mind and realized that you and your fellow readers might be interested in why the answer is “impossible.” Before I give you my opinion, I’ll quote Bob’s response in full because it’ll help illuminate the challenge you pose above. To quote Bob: “What an interesting question! The simple answer from me is, no, I don’t know an equation (for calculating RS based on the above given information) and I don’t think
Response by Alison Crowe.