Identification and origin of volatile compounds responsible for important wine sensory attributes

Abstract

Knowledge of the flavour compounds that are responsible for the sensory characteristics of wines is of great importance to be able to control and adjust wine aroma properties in production. Compounds not previously recognised as active flavorants in specific varieties have been identified in this project for the first time, including several compounds that give Viognier wines their distinctive ‘apricot’ flavour, and a ‘green’ flavour compound in Shiraz. In addition, numerous compounds not previously understood adequately have been revealed as significant in wine flavour, such as sulfur compounds in Chardonnay being found to confer fruit attributes. Ways of adjusting the contribution of various compounds have also been examined. For example, absorption onto food grade plastic film was shown to be effective in removing ‘green’ character from a red ferment, while avoidance of material from specific wind-break vegetation planted near vineyards reduced ‘green’ flavour in wine. Another outcome of this research has been the development of analytical tools for measurement of key wine flavour compounds, and improved understanding of winemakers’ concept of ‘green’ tannin and flavour with reference to consumer preferences. Studies of Chinese consumer red wine preferences and behaviour have helped shed light on this rapidly growing and important market for Australian wine.

Summary

A good understanding of the relationship between wine composition and wine flavour is important to being able to control grape and wine quality. This research project was carried out to improve knowledge of:

• compounds responsible for key flavours, including the identity of previously unrecognised compounds, especially for the varieties Shiraz and Chardonnay
• the relationships between wine composition and wine sensory properties;
• the effect of viticultural and oenological techniques on the formation of compounds, such as ‘green’ flavour and to develop routine analytical methods for flavour compounds that can be applied in research and industry trials. 

Using sensory-guided chemical methods, gas chromatography-mass spectrometry, in combination with olfactometry (use of the human nose as a detector), liquid chromatography –mass spectrometry, and formal sensory studies, compounds responsible for flavour attributes in wine were identified. Analytical methods using isotopically labelled standards were adopted where possible, to ensure accurate, precise and sensitive analyses of compounds at or below ppb levels. Surveys of commercial wines were used to confirm the importance of compounds, while detailed chemical studies shed light on the formation reactions occurring in winemaking. Use of replicated viticultural and winemaking studies enabled the effect of different production practices to be determined, and formal sensory and consumer preference data were obtained.

Analytical methods were established for a number of key flavour compounds, providing enhanced ability to measure levels of compounds with excellent context regarding their sensory significance, including detection thresholds and flavour contributions. The compounds included 3-mercaptohexanol, 3-mercaptohexyl acetate, 4-methyl-4-mercaptopentan-2-one, benzyl mercaptan, furfuryl thiol and numerous aldehydes and other compounds related to oxidative effects. Being able to measure such compounds allowed greatly improved understanding of the effect of yeast and fermentation, oxygen, nutrients and many other variables through various AWRI research projects, and the methods were used by numerous wine industry personnel, through collaborative studies or in resolution of issues between third parties.

The compounds responsible for the ‘apricot’ flavour in Viognier wine were identified as the monoterpenes, geraniol, linalool and nerol. These are compounds not previously associated with this flavour, and their sensory interactions with other aroma compounds were established. While a series of lactone compounds were found to be generally unimportant to wine flavour, their contribution in combination with monoterpenes to ‘stone fruit’ flavour was indicated. Several esters were shown to contribute to ‘peach-like’ aroma in Chardonnay.

Several potent thiol compounds, previously known in Sauvignon Blanc wines, were determined to be major contributors to Chardonnay fruit flavour, including at high concentrations a Sauvignon Blanc-like ‘tropical fruit’ character. This opens a new area of control of flavour of this important variety. Similarly, the volatile thiol, benzyl mercaptan, was found to be related to ‘flint’/‘struck match’ aroma in Chardonnay, providing a basis of control of this ‘Burgundian’ character in Australian wines.

Regarding ‘green’ flavour in red wines, grapevine proximity to Monterey Cypress trees was shown to be a cause, with grape rachis included in Shiraz ferments also a major influence. ‘Green tannin’ was elucidated to be related to both ‘green’ volatiles and elevated bitterness, probably related to specific tannins.

Studies investigating consumer preference and behaviour of Chinese consumers of red wines gave insight into Chinese-based language to describe wines, appropriate for marketing and communication purposes, with recognition of sensory attributes that relate to preference. 

Overall, the results of this research project have increased knowledge of the main volatile compounds involved in wine flavour attributes, so that the causative compounds for many of the most important sensory attributes of wines are now established and many of the influences on their formation understood. While there are several key sensory attributes of wines where the cause is still not known, this research stream has given producers new knowledge to avoid negative flavours and enhance positive flavours.