- Spring 1995 "Update" Newsletter Article -
Sulfur residues
Chemist outlines new method of measuring sulfur residues on grapes
by Bary Gump, Chemistry professor and Interim Director, Viticulture and Enology Research Center
From CATI Publication #950401
Copyright © 1995. All rights reserved.
The application of elemental sulfur has been shown to be an economical and effective treatment for the prevention of powdery mildew on grapes.
However, there has been concern in following this practice with wine grapes, due to the possibility that sulfur residues may produce elevated levels of sulfide compounds in the finished wine. Volatile sulfide compounds in a wine produce objectionable odors, and at detectable levels are considered to be major defects.
The quantitative determination of elemental sulfur at levels expected from "usual" vineyard treatments is difficult. The official AOAC (Association of Official Analytical Chemists) method involves a lengthy digestion, precipitation, and actual gravimetric weighing of barium sulfate formed from the sulfur. An inductively coupled plasma method has been reported to be successful; however, this procedure requires an instrument that is not commonly available.
We have successfully modified the AOAC procedure so that instead of weighing the precipitated sulfur, the barium concentration remaining after precipitation is measured using an atomic absorption spectrometer (AAS). This indirect procedure avoids the necessity of collecting, drying, and weighing sub-milligram amounts of precipitate.
The method has been successfully applied to samples from two years of vineyard trials with sulfur and other fungicide treatments. As expected, those treatments involving sulfur alone showed the highest residue levels. AAS analysis has also been used to monitor sulfur dioxide residues on fumigated table grapes.{ page top }