Identification and control of volatile compounds responsible for important sensory attributes
Abstract
This project succeeded in its aim to expand knowledge of several key flavour compounds in wine, including those responsible for ‘stone fruit’, ‘tropical’, ‘green’ and ‘jammy’ characters. Using a wide range of sensory and analytical methods, compounds responsible for flavour attributes in wine were identified. Relationships between wine composition and sensory properties were investigated, as well as the effect of viticultural and oenological techniques on flavour compound formation. New analytical methods were established for a number of compounds, providing an enhanced ability to understand their sensory significance. This research has given producers new knowledge to help them avoid negative flavours and enhance positive flavours.
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.
It also aimed to develop routine analytical methods for flavour compounds that can be applied in research and industry trials.
Compounds responsible for flavour attributes in wine were identified 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.. 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 commercially available wines were used to confirm the importance of compounds, while detailed chemical studies shed light on the formation reactions occurring during winemaking. Use of replicated viticultural and winemaking studies enabled the effects 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 an enhanced ability to measure levels of compounds with context regarding their sensory significance, including flavour contributions. The compounds included those implicated in overripe raisin/jammy flavour, including 1-octen-3-ol, 1-penten-3-ol, 2,4-heptadienal, β-damascenone, 3-methylbutanal, 2-methyl propanal, phenylacetaldeyde, ethyl 3-methylbutanoate, ethyl 2-methylpropanoate, 3-methyl-2,4-nonanedione (MND), 1,5-octadien-3-one, furaneol, homofuraneol, and gamma-nonalactone. A method combining and expanding on the set of monoterpenes and C13-norisoprenoids was developed. The concentration of proline in wines was quantified using a newly developed cost-effective and rapid 1H NMR method.
The sensory interactions of compounds responsible for the ‘apricot’ flavour in Viognier wine (the monoterpenes, geraniol, linalool and nerol with other aroma compounds) were determined, with lactone compounds enhancing this character. Several esters were shown to contribute to ‘peach-like’ aroma in Chardonnay.
Several potent thiol compounds were found to be strongly enhanced in white wines made from grapes where the vineyard had been treated with a simple nutrient foliar spray. Optimal applications were found and trialled with industry partners demonstrating the potential of this simple practice.
Regarding ‘green’ flavour in red wines, stalks in Shiraz and Pinot Noir fermentations were found to give rise to green characters from methoxypyrazines, which were previously thought to be unimportant for these varieties.
Bunch exposure influences were studied in Riesling and Shiraz, with coloured shade cloth found to have a clear effect on aged character in Riesling, and compounds contributing to jammy/raisin characters in Shiraz being identified, together with links of this character to hang-time rather than berry temperature or sun exposure.
A highlight of the project was the recognition of the importance of the previously little-studied amino acids proline and glutamic acid in wine flavour. For proline, there were both direct and interactive effects driving the perception of many positive sensory properties of red wines, notably sweetness, red fruit flavour and viscosity, while diminishing astringency and bitterness. The practical application of this knowledge was demonstrated in a blending study with high- and low-proline wines, showing that wine from warm inland regions can be greatly improved with incorporation of high-proline components. The savoury character in many red wines was shown to involve glutamic acid, which previously had been considered to be at too low a concentration to have a sensory effect in wine.
Rapid and simplified sensory methods were evaluated and found to have utility as substitutes for more complex time-consuming sensory descriptive analytical methods, without sacrificing accuracy or detail. These methods can be used in industry contexts such as evaluating smoke taint or outcomes from viticultural or winemaking trials.
Overall, the results of this research 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. This research has given producers new knowledge to avoid negative flavours and enhance positive flavours in finished wine.