- Summer 1994 "Update" Newsletter Article -
Researchers try new method of sulfur analysis
From CATI Publication #940101
Copyright © 1994. All rights reserved.
A new analytical method for determining levels of sulfur residues on wine grapes could provide California wineries with a reliable and less expensive method for ensuring wine quality.
The new method involves the use of an atomic absorption spectrometer that can measure sulfur residues down to the parts per million (ppm), a level of importance since traces of as little as one ppm can affect the aroma and flavor of wine made from the grapes, reported CSU, Fresno Chemistry Professor and researcher Barry Gump.
Treatments of elemental sulfur on grape vines are an economical and effective means for preventing powdery mildew, noted Gump, who is overseeing research of sulfur residues on wine grapes for CSU, Fresno’s Viticulture and Enology Research Center. However, higher levels of sulfur residues also have a reputation for producing a "rotten egg" aroma in some wines. This unwelcome essence results from the formation of hydrogen sulfide and other sulfur compounds during fermentation.
While sulfur residues on wine grapes at harvest are typically so low they have to be measured in parts per million, even these low levels can affect the essence of a wine, so it is critical that winemakers be able to discern these levels accurately BEFORE the fermentation process begins, Gump explained.
Traditional methods for measuring sulfur residues involve first washing the grapes in a special barium (Ba) solution to remove the sulfur, then "actual gravimetric weighting of barium sulfate formed from the sulfur," Gump explained. This method is costly, he noted, and many wineries don’t have the laboratory equipment necessary to perform it.
The new trial method uses a measured amount of barium, which is added to a solution of sulfur residue that has been washed from a grape sample, Gump said. Barium combines with the sulfur to form barium sulfate (BaSO4), which is then removed, or "precipitated" from the solution. The remaining barium is measured using an atomic absorption spectrometer.
"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 (AA)," Gump said. "This indirect procedure avoids the necessity of collecting, drying, and weighing submilligram amounts of precipitate."
Gump said there were some problems with clogging in the flame burner used by the spectrometer, but those were solved by changing the solution used to wash the grape clusters.
Measurements of sulfur residues on the wine grapes were generally confirmed by a panel of wine tasters that conducted sensory evaluations of the subsequent wine samples.{ page top }