Foliar application of nitrogen and sulfur in the vineyard appears to be a promising technique for increasing tropical characters in white wines, new research suggests.
The study – conducted by a research team from the Australian Wine Research Institute (AWRI) and supported by Wine Australia – found that a double application of both sulfur and nitrogen as a foliar spray over the veraison period resulted in increases in potent compounds known as thiols and tropical sensory attributes for Chardonnay.
“The results are exciting because they provide a promising, relatively easy to implement technique for increasing tropical characters in white wine – adding to other techniques already available to winemakers,” said lead author Dr Josh Hixson, AWRI Research Scientist.
Foliar applications of nitrogen and sulfur were tested on a vineyard block in the Barossa Valley planted with both Chardonnay and Shiraz in vintage 2018.
Mixtures of wettable sulfur and low biuret urea (nitrogen) were sprayed onto the canopies at two different dose rates. The foliar applications were made twice during the growing season: at the start and end of veraison. Non-sprayed vineyard areas were used as the controls.
The fruit was then hand-picked and made into small-lot wine (30–40 kg ferments). Six months after bottling, the wines then underwent chemical and sensory analysis.
“Sensory results for both varieties showed an increase in the intensity of ‘grapefruit’ attributes with increasing foliar dose. The Chardonnay wines showed a similar trend for the ‘passionfruit’ attribute,” said Josh.
“For the Chardonnay wines, the only other aroma attribute that differed among the treatments was ‘honey’, which showed decreased ratings with increasing foliar spray dose.”
Josh said the results for the Shiraz wines were slightly more complicated, however, with an increase in a ‘drain’ aroma attribute seen in the foliar spray treatments, along with a small decrease in ‘dark fruit’ and ‘sweet spice’ attributes.
In the chemical analysis results, the Chardonnay wines showed a significant dose-response for thiol compounds, which reflected the increases in tropical attributes noted by the sensory panel.
Josh said while the results were exciting in terms of adding to other techniques available to winemakers, there was still much to learn.
“These results are only from a single season, and from a single Barossa Valley vineyard; although the study was repeated during the 2019 vintage and preliminary assessment suggests a similar pattern of thiol response. However, current studies will expand knowledge of how widely applicable this technique may be for Australian vineyards.”
Josh said the research’s ultimate aim was to be able to provide the Australian wine sector with tools that would allow producers to make relevant choices to achieve desired products for their target markets. For example, there is currently interest from some producers in creating highly tropical blending parcels for Sauvignon Blanc.
“Importantly, this study not only provides a tool that can be used to influence wine styles, but it also helps to close the loop in terms of the yeast-induced changes and balance of sulfur-related aromas that we are investigating across the AWRI research program.”