For the control of Powdery Mildew, the AWRI recommends a thirty day withholding period (WHP) for grapevines which are sprayed with elemental or crystalline sulfur. Use of these chemicals within the WHP may increase the risk of residual levels remaining in must. This may lead to fermentation problems. The WHP is set to allow enough time for elemental sulfur to degrade to levels that minimise the risk of fermentation problems. Research indicates that as the length of time between the application of sulfur in the vineyard and harvest increases, the impact on fermentation declines.
Elemental sulfur residues in must can be converted by yeast to sulfurous off-flavours such as hydrogen sulfide (H2S) during fermentation. During and after fermentation, elemental sulfur may also be reduced to sulfide by enzymatic or non-enzymatic chemical processes.
The main factors that control H2S production from elemental sulfur are:
- temperature (lower production at lower temperature);
- oxidation potential (higher production rate at lower oxidation potential); and
- ethanol concentration (lower production at lower ethanol concentration).
Elemental sulfur can also oxidise to sulfur dioxide (SO2) and have an antimicrobial affect on inoculated yeast, delay the onset of fermentation or cause a slow or sluggish fermentation.
Settling of white musts before fermentation might remove some elemental sulfur in the lees.
Treat the ferment as ‘high risk’, paying attention to temperature, oxidation and fermentation rate.
Ensure adequate YAN (yeast assimilable nitrogen) levels in juice to facilitate good yeast performance. Click here, for recommended levels.
Use of low hydrogen sulfide producing yeast strains could limit the amount of hydrogen sulfide produced.
Addition of greater yeast inoculums as a sacrificial culture could assist with binding of elemental sulfur and generated SO2.
Please click here to download the AWRI stuck fermentation fact sheet.