We recently received a new publication “Advances in Wine Science” – celebrating 50 years of the Australian Wine Research Institute. The book reviews current knowledge of chemicals contributing to wine aroma, the nature of tannins, role of yeast and bacteria in modulating wine flavour, olfaction and taste, cork taint, and effect of wine on health, and more. It is interesting to reflect on how our scientific understanding of wine and winemaking actually impacts on the product we make.
The aroma and flavour profile of wine results from an innumerable number of variations in grape and wine production resulting in a unique characteristic. More than 680 different compounds have been identified that contribute to wine aroma. We know that specific varieties are defined by their predominant aroma compounds – for example muscat varieties by the floral monoterpenes and sauvignon varieties by the fruity volatile thiols. Fermentation creates major changes in aroma and flavour, contributing new compounds and altering others. Winemakers select strains of the traditional wine yeast Saccharomyces cerevisiae that produce desirable sensory results for a given wine.
Advances in biotechnology allow scientists to use DNA technology as a research tool to gain understanding about wine yeast. Whilst GMOs are not used in the wine industry, the underlying science allows us to unravel fundamental aspects of the growth of these organisms. For example this is allowing researchers to better understand how yeast convert the flavour precursors present in juice to the compounds known as volatile thiols which are responsible for the passionfruit, guava, and cats pee aromas characteristic of sauvignon blanc. We now have available several commercial yeast strains with the necessary metabolic activity to achieve higher than normal rates of conversion resulting in increased intensity and varietal character.
We understand the metabolism of citric acid by malolactic bacteria produces diacetyl, responsible for the buttery flavour seen in complex barrel fermented chardonnays. Researchers are examining means of selecting strains with more or less ability to produce diacetyl. In addition researchers have demonstrated how final diacetyl concentration can be manipulated by winemaking techniques such as lees stirring and sulphur dioxide levels.
The compounds in oak that contribute to wine flavour have also been studied. We know that oak species, source, seasoning, and coopering all contribute to the effect on wine flavour of the oak. The authentic source of any oak can now be confirmed analytically. Coopers are now offering a variety of options to winemakers based on this knowledge. The aim of this is to allow winemakers to more accurately predict the effect of oak on quality and style, and for coopers to produce the desired oak characteristics more consistently. Winemakers can now select for oak that is smoky, vanilla, spicy, or contributes berry like characters.
“I think 1932 was the turning point when after accumulating a reasonable amount of knowledge I realised I knew nothing worthwhile. And when you realise you know nothing you start to learn.” Jack Mann 1906-1989
The removal of haze forming proteins from white wine is routine. The traditional treatment is to fine the wine with bentonite – a clay (yes clay is added to all white wines!). This is effective but not specific, and generally removes some flavour and produces waste lees. Researchers continue to seek alternatives to bentonite, and whilst they are closing in on the problem, a lot of work is still required.
Yeast nutrition has a significant effect on fermentation. Work is being carried out on juice nitrogen levels to allowing winemakers to more effectively manage fermentation, and in particular to avoid “stuck” or incomplete fermentation. This research has also lead to improved understanding of the role of nitrogen compounds on flavour and aroma. Amino acids regulate yeast cell metabolism and consequently the formation of many flavour active compounds during fermentation. It is already well known that a low level of some amino acids during rapid growth can induce hydrogen sulphide (rotten egg gas) synthesis.
Besides aroma and flavour compounds, phenolic compounds contribute significantly to wine taste. Phenolic compounds include the tannins and pigments in wine, as expected, in much higher levels in red wines. The understanding of the nature of red wine colour is growing. We now know that the grape pigments are highly unstable and must react with stable tannins to form coloured compounds that will give the wine stable colour over the long term. Managing the extraction and incorporation of the grape pigments into stable colour during the early stages of fermentation has become an important goal of winemakers. Continuing research into the mechanism of these reactions will improve the ability to maximise red wine colour.
Similarly tannin has an important role in mouthfeel and wine style. Managing tannin formation in the grapes and extraction during skin contact is crucial. The tannins are a vast and complex group of compounds and researchers are working steadily on quantifying them, assessing their role on mouthfeel, and developing techniques to manipulate their levels in wine.
Despite all of this accumulated learning winemaking hasn’t fundamentally changed over the last 50 years. Winemakers still have to make judgements regarding the effect of grape and wine handling on flavour, balance, extract, and style. There is no magic, no winemaking machine; wine quality is still immeasurable and subjective. “Advances in Wine Science” is a fantastic achievement, and the Institute has made a massive contribution to the development of our industry. As our understanding of the science of wine increases, our winemaking evolves. But great wines have always been made and so the skill and craft of winemaking remains, even if some of the mystery disappears.
“Winemaking is entirely a natural process. And no person has the right to assume the title winemaker. Nature is the winemaker and man, if he is skilled, gives nature the opportunity to perform to the best advantage. That is the art of winemaking” Jack Mann