WINETECH Technical Yearbook 2019

ASTRID BUICA, ALÉTA BRUWER & WESSEL DU TOIT: Department of Viticulture and Oenology, Stellenbosch University, Stellenbosch KEYWORDS: Foliar fertilisation, sulphur, nitrogen, major volatiles, thiols, sensory evaluation. OCTOBER 2019 NITROGEN AND SULPHUR FOLIAR FERTILISATION (PART 3)

In this three part series, we explore some fundamental aspects of sulphur and nitrogen foliar fertilisation and the chemical and sensory results of a recent project that took place in South Africa. This last part of the series focuses on the effects of the nitrogen and sulphur foliar fertilisation on wine volatile composition and evaluation timing. VOLATILES IN WINES Major volatiles These compounds a r i se a s pr ima r y metabolites of yeast and sugar and the metabolism of amino acids (Swiegers et al ., 2005; Henschke & Jiranek, 1993) and contribute to the pleasant fruity and floral aromas in wines (Swiegers et al ., 2005). Major volatiles, such as esters, fatty acids and higher alcohols, were affected the most by S-containing treatments (S or N+S), for

both Sauvignon blanc and Chenin blanc. As expected, the overall ester content increased over time for most treatments for both cultivars (Selli et al ., 2006), but no obvious trends were observed. The summary of effects is presented in Table 1; the treatments mentioned showed an increase compared to the control, marked significant or non-significant. Methoxypyrazines The levels of IBMP and IPMP in the Sauvignon blanc wines were very low (below 3 ng/L) and not influenced by the treatments. Leaf removal practices performed after véraison could have influenced the IBMP and IPMP levels already in the grapes to a larger extent than the foliar treatments. Volatile thiols 3MHA and 3MH showed a vintage, vineyard

and treatment effect for the cultivars (Figure 1). 3MHA levels decreased significantly for Sauvignon blanc in 2015 during ageing due to hydrolysis and oxidation (Herbst- Johnstone et al ., 2011). 3MH levels increased significantly for S and N+S treatments and during bottle maturation, a treatment effect can be observed with all the treatments for Chenin blanc (2015) and N+S treatments for Chenin blanc (nine months, 2016) and

Sauvignon blanc (nine months, 2015 and 2016). The increase was possibly due to the hydrolysis of 3MHA to 3MH, to the production from thiol precursors present in the wines, or by the breakdown of 3MH disulphide present in the wines (Capone et al ., 2010; Sarrazin et al ., 2010). We have also observed an increase in 3MHA with storage in some cases. This has not been reported in the literature, but it is line with other experiments we have