Question
We measured residual SO2 in table grapes using Ripper method. We determined total, free and bound SO2 in ppm by this method. Which parameter (total, free or bound SO2) gives residual SO2? As you know the maximum tolerance limit is 10 ppm for table grapes. Is it total, free or bound SO2? (E.E.C.)
Answer
The Ripper method is an iodimetric reaction (oxidation/reduction) generally associated with free sulfites. The problem is that this method does not measure the presence of bound sulfites (usually addition compounds to carbonyl groups forming hydroxisulfonates) as bound sulfites are considerably more stable than free sulfites
(sum of sulfites and bisulfites).
The official method for sulfite determinations is the Monier-Williams method (acid distillation) which is not fully ideal, but it is one officially on use. They are certified laboratories where samples can be sent for analysis.
My last reference on this is in 1978 (work done at Davis) “SO2 Determination by Aeration-Oxidation:
A Comparison with Ripper”, Buechsenstein and Ough, Am. J. Enol. Vitic.,1978; 29: 161-164
–Carlos Crisosto