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
These compounds arise as primary metabolites of yeast and sugar and the metabolism of amino acids,1,2 and contribute to the pleasant fruity and floral aromas in wines.1 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,3 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.
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.
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.4 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.5,6 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 previously run. Given the complexity of the thiol chemistry in wines, at the moment we have no clear answer to why.
FIGURE 1. Thiol levels (3MH and 3MHA) for the 2015 and 2016 wines as affected by the treatments and the bottle storage.
Additionally, by looking at the overall volatile composition using Principal Component Analysis (PCA), a vintage and age effect occurred for both cultivars. Clear groupings were observed between three and nine months of maturation, and the volatile content gradually changed during this period. Treatment effect was less evident at nine months than at three months maturation, and not a drive for the separation of samples in the multivariate data representation. In other words, the effects of the treatments compared to the control were more pronounced three months after bottling than nine months after bottling. This means that with time, the treatment effects declined.
Sensory evaluation of the wines
For the sensory evaluation of the wines, a free sorting method was used. The judges were presented with all the wines (per cultivar) and asked to group them according to similarity in aroma and describe the groups. Previously, descriptive analysis (DA) has not been successful in this type of work,7 and other research reports did not mention the sensory analysis method used.8,9 Our results showed that the wines could not be grouped according to treatment, and seldom two or three of the four repeats were grouped together. This can be due to the judges not being able to differentiate between the wines, because the wines were very similar in their aromatic expression. The Chenin blanc wines had ‘tropical’ and ‘rich and ripe’ aromas (Table 2). The Sauvignon blanc wines had prominent ‘tropical’, ‘passion fruit’ and ‘grapefruit’ aromas, which are specific for the ‘tropical style’ of Sauvignon blanc wines. During bottle maturation, most fresh notes and aroma characters were maintained, but their frequency of citations (which can be interpreted as intensity of perception) decreased; this is considered normal wine development (Figure 2).
Overall, both cultivars were ester- and thiol-driven. While the S and N+S treatments had some negative (‘cooked vegetables’, ‘sulphur’ and ‘herbaceous’) aromas, which can be linked to reductive sulphur compounds, we observed limited or no negative effects of the treatments overall.
FIGURE 2. Word cloud representation for the top ten most cited aroma attributes of 2016 Chenin blanc wines, regardless of the treatment. Left: 3 months and right: 9 months in bottle.
Although the chemical makeup of the wines changed with the applied treatments as shown by the chemical analysis results, the overall effect was not observed in the aromatic expression of the wines. This can be due to the matrix effect or the interactions of the volatile and non-volatile compounds present in the wines.
To our knowledge, this is the first project spanning over three seasons that examined the effect of N, S, and N+S foliar fertilisation on Chenin blanc. This study contributed to the knowledge on South African Chenin blanc and Sauvignon blanc wines, but also demonstrated that foliar fertilisation can be used to influence the non-volatile (Part 2) and volatile content of wines. This knowledge could ultimately aid researchers and winemakers understand these compounds, produce a specific wine style and produce better quality wines. A better understanding of the factors linking vineyard fertilisation applications, grape heredity, climate, must and wine composition to the sensory expression of wines need to be investigated to fully understand how these different factors can affect the sensory expression of the wines.
Although statistical differences were observed between major volatile and thiol levels of foliar treatments and the control in this study, no differences were observed between the different treatments in sensory analysis. The question thus remains whether it is worth to do foliar treatments for this purpose taking into account the cost involved, if there will be no perceivable difference for the taster (be it consumer or professional)? Foliar treatments can be very beneficial in some cases as seen in previous studies and in the case of low YAN vineyards in this study, and in other cases have limited effect. It must not be a practice that is simply widely followed without careful consideration of the possible outcome.
Chemical and sensory evaluation of wines made after foliar fertilisation showed whether the effects of vineyard treatments lasted during bottle maturation.
Until now, no foliar fertilisation studies have focused on determining the volatile content present in wines during bottle maturation. This aspect is relevant, since the impact of the treatments on the final wines can be temporary and the effect can only last for a limited period. Therefore, the study of the evolution of volatile chemical compounds such as major volatiles and thiols during ageing is needed to better understand the (lasting) effects of the foliar fertilisation treatments. The information gained by performing sensory analysis during different stages of bottle maturation, can add to the knowledge base of the aroma of these wines.
- Swiegers, J.H., Bartowsky, E.J., Henschke, P.A. & Pretorius, I.S., 2005. Yeast and bacterial modulation of wine aroma and flavour. Australian Journal of Grape and Wine Research 11: 139 – 173.
- Henschke, P. & Jiranek, V., 1993. Yeasts – metabolism of nitrogen compounds. In: Wine Microbiology and Biotechnology, 77 – 164. Harwood Academic Publishers, Chur, Switzerland.
- Selli, S., Canbas, A., Cabaroglu, T., Erten, H., Lepoutre, J-P. & Gunata, Z., 2006. Effect of skin contact on the free and bound aroma compounds of the white wine of Vitis vinifera L. cv. Narince. Food Control 17(1): 75 – 82. Doi: 10.1016/j.foodcont.2004.09.005.
- Herbst-Johnstone, M., Nicolau, L. & Kilmartin, P.A., 2011. Stability of varietal thiols in commercial Sauvignon blanc wines. American Journal of Enology and Viticulture 62(4): 495 – 502. Doi: 10.5344/ajev.2011.11023.
- Capone, D., Sefton, M., Hayasaka, Y. & Jeffery, D., 2010. Analysis of precursors to wine odorant 3-mercaptohexan-1-ol using HPCL-MS/MS: Resolution and quantitation of 3-S-cysteinylhexan-1-ol and 3-S-glutathionylhexan-1-ol. Journal of Agricultural and Food Chemistry 58: 1390 – 1395.
- Sarrazin, E., Sinkharuk, S., Pons, M., Thibon, C., Bennetau, B. & Darriet, P., 2010. Elucidation of the 1,3-sulfanylalcohol oxidation mechanism: An unusual identification of the disulfide of 3-sulfa-nylhexanol in Sauternes botrytized wines. Journal of Agricultural and Food Chemistry 58: 10606 – 10613.
- Juhasz, A., 2015. Effect of foliar fertilization on wine aroma for white wine cv. Sauvignon blanc (Vitis vinifera L.).
- Lacroux, F., Tregoat, O., Van Leeuwen, C., Pons, A. & Dubourdieu, D., 2008. Effect of foliar nitrogen and sulphur application on aromatic expression of Vitis vinifera L. cv. Sauvignon blanc. Journal International des Sciences de la Vigne et du Vin 42(3): 1 – 8. Doi: 10.1007/s11032-013-9938-5.
- Geffroy, O., Charrier, F. & Poupault, P. et al., 2016. Nitrogen and sulfur spraying allow to produce wines with enhanced concentration in varietal thiols. Macrowine, 27 – 30 June: 81310.
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