Persistence of Elemental Sulfur Spray Residue on Grapes during Ripening and Vinification

Abstract

Elemental sulfur (S⁰) is commonly used to control powdery mildew in vineyards, but S⁰ residues in musts have been correlated with increased H₂S and sulfurous off-aroma formation during fermentation. As a consequence, S⁰ is often used sparingly late in the season, but defining appropriate pre-harvest intervals for S⁰ sprays has been challenging due to limited data on S⁰ persistence in vineyard s and during pre-fermentation operations. Utilizing a new quantification method, S⁰ residues were monitored in the vineyard over 3 years of field studies. Treatments varied in commercial formulation, application rate, and timing of the last application before harvest, all of which affected S ⁰ concentrations on the fruit at harvest. Residue levels generally were lower for a wettable powder versus a micronized formulation applied at the same rate and timing, and increased proportionally to the application rate when timing and formulation were constant. In all years, ceasing application ≥35 days prior to harvest resulted in S⁰ residues below the 10 μg/g concentration associated with increased H₂S production in several previous studies. S⁰ residues >1 μg/g correlated with increased H₂S production in our current work and were observed on all fruit sprayed within 56 days of harvest. However, clarification decreased S⁰ in must by >95% prior to fermentation in all treatments. Furthermore, fermentation on treated skins increased H₂S formation nearly 3-fold relative to fermentations without skin contact. Collectively, these results indicate that S⁰ residues are likely of low concern in white winemaking whereas residue levels in red fermentations can exceed levels associated with increased H₂S production when some S⁰ sprays are applied within 8 weeks of harvest.