Managing fermentation nutrients
Background
The chemical composition and sensory properties of wine are in large part determined by the fermentation yeast(s) used. Thus, many selections of yeast which produce different sensory profiles, such as estery, neutral or varietal character enhancing, are being used to complement grape aroma and to improve wine flavour. However, many of these sensory benefits can be compromised by the concomitant formation of negatively perceived flavour compounds when the fermentation nutrient environment is not optimal. Although diammonium phosphate (DAP) is widely used as a fermentation stimulant and in an attempt to control hydrogen sulfide production, other benefits and disadvantages of DAP addition are largely unknown. Commercial nutrients prepared from inactivated yeast are also becoming available but their impacts on fermentation are not yet well characterised either.
The aims of this project are to focus on key nutrients (e.g. yeast assimilable nitrogen [YAN] and DAP use) as regulators of wine appearance, aroma, flavour and mouth-feel. By combining genetic, metabolomic and sensory approaches this work will generate better information on how nutrients affect the flavour profile of wine and enhance varietal character and also provide winemakers with new tools to optimise varietal character and manage wine style.
A variable proportion of Australian grape musts contain a sub-optimal amount of yeast assimilable nitrogen, which is believed to limit development of desirable sensory properties in some wine types during fermentation. Fermentation of low nitrogen juices and musts can produce wines with lower concentrations of desirable aroma compounds and higher concentrations of less desirable compounds. White wines show less fruity and more complex aromas whereas the impact of low nitrogen ferments on red wines is still to be determined.
Progress
Recent work has focussed on the role of yeast assimilable nitrogen in modulating wine flavour in white wines. By working with highly clarified, low nitrogen juices and model media we have shown that nitrogen supplementation prior to inoculation not only improves fermentation kinetics but can impact on aroma profile to change the style of wine. Low nitrogen ferments produce wine with a less fruity and more complex aroma, whereas wines from high nitrogen ferments can develop a high ester taint, often described as volatile acidity.
On the other hand, the effect of must nitrogen content on red wine composition and sensory attributes made by fermentation of red musts by maceration on skins is unknown. Taking advantage of a vineyard nitrogen supplementation trial, a series of fermentation trials is being carried out to investigate the influence of DAP addition on Shiraz wine volatile composition and sensory properties. Results to date highlight the complexity of flavour formation, variation in content of tannins and wine colour in relation to DAP supplementation and nitrogen effects on wine ageing.
These results suggest that DAP supplementation can be an effective tool to modulate the aroma composition of red wine as found in white wines. Further work is in progress to understand better the relationship between these changes and the sensory properties of wine and to establish which wine properties are better managed in the vineyard or in the winery. Work is also in progress to understand yeast metabolic responses to nitrogen.
Highlights
- Moderate supplementation of low nitrogen juices and musts with diammonium phosphate moderates the balance of desirable and undesirable flavour compounds. White wines show cleaner and fruitier aromas.
- High concentrations of nitrogen supplements can produce wines with an estery off-flavour which masks varietal characteristics.
Project leader: Dr Paul Henschke
Project team members:
Publications:
875 Bell, S.J., Henschke, P.A. 2005. Implications of nitrogen nutrition: for grapes, fermentation and wine. Aust. J. Grape Wine Res. 11:242-295 (click here to order).
1022 Ugliano, M., Henschke, P.A., Herderich, M.J., Pretorius, I.S. 2007. Nitrogen management is critical for wine flavour and style. Aust. N.Z. Wine Ind. J. 22:24-30 (click here to order).
1093 Ugliano, M., Siebert, T., Mercurio, M., Capone, D., Henschke, P.A. 2008. Volatile and color composition of young and model-aged Shiraz wines as affected by di-ammonium phosphate supplementation before alcoholic fermentation. J. Agric. Food Chem. 56(19):9175-9182 (click here to order).
1001 Vilanova, M., Ugliano, M., Varela, C., Siebert, T., Pretorius, I.S., Henschke, P.A. 2007. Assimilable nitrogen utilisation and production of volatile and non-volatile compounds in chemically defined medium by Saccharomyces cerevisiae wine yeasts. App. Microbiol. Biotechnol. 77:145-157 (click here to order).