Why Do We Smell Wine?

[paul-dewsbury]

Firstly, I must confess, I do it, I see lots of people do it and I know of tournaments where the competitors are blindfolded and do it, so why do we smell wine?

Not being a connoisseur, I will not even attempt to go into the technicalities here, but let us consider some of the compounds that affect the nose (or aroma) of a wine and how they are analyzed.

Phenols in Wine

The complexity of wine taste and aroma derives from a huge number of compounds generated from different varieties of grapes during yeast and malolactic fermentation, possibly extracted from oak during storage, and then transformed through various complex reactions during maturation. The contributions of many individual compounds to taste and aroma are well-known, but ultimately it is the fine balance of compounds and their relative concentrations that determine whether it is just an ordinary table wine or exalted as a great vintage.

Of the vast array of substances, phenolic compounds can positively contribute to wine taste and aroma, but conversely, at the wrong levels they can lead to highly undesirable off-odors.  The ability to quantitatively monitor phenolic compounds in wine adds hugely to understanding and controlling wine quality.

Fast Automated Analysis of Phenolic Compounds in Wine

In a recent publication in the Journal of Chromatography Part A, Barnaba et al., have developed and validated a new method involving on-line SPE clean-up coupled to LC-high resolution mass spectrometry for the quantitative analysis of fifty-six simple phenols in wines, spirits and vinegars. Samples were filtered, internal standards added and then automated on-line clean-up was carried out using a UHPLC, equipped with two pumps and a Rheodyne 6-port automated switching valve.  Employing two independent fluidic systems, one was dedicated to on-line SPE sample clean-up, whilst the other controlled the LC separation.  Identification and quantification employed a Q-Exactive™ hybrid quadrupole-Orbitrap™ mass spectrometer equipped with heated electrospray ionization (HESI-II) and operated in negative ion mode. Full mass spectra were acquired at mass resolving power of 140,000 measured at full width at half-maximum at m/z 200.

The method was optimized in terms of selection of SPE column, UPLC separation and LC-MS operating conditions to minimize any matrix effects and maximize recoveries and sensitivity.  Validated for fifty-six phenols ranging in size with [M-H]- ions of m/z=93 for phenol to m/z=289 for catechin, average levels of each phenol were reported for red and white wines, spirits such as brandy, rum, calvados, armagnac, whisky, cognac and grappa, common and balsamic vinegars.   Interpretation of such complex patterns of data is far from simple, especially as descriptions of different aroma sensations is such a subjective process.

My Wine Is a Bit Too Spicy

In general, phenolic compounds, such as 2-phenylethanol and methyl anthranilate, are said to give a peppery sensation. Phenolic compounds have an influence on the perception of sweetness and acidity, whilst they also have direct impact on wine body and balance.

Hydroxybenzoic acid derivatives, particularly ellagic acid, occur frequently in wines aged in oak barrels.   Thus, in this study ellagic acid levels averaged 76 μg/mL in red wine but were only 0.3 μg/mL in white wine.  Degradation of oak lignins can also liberate various cinnamaldehyde and benzaldehyde derivatives which were found in red wines. Hydroxycinnamic esters are generated during fermentation or again derived from oak treatment, but can be metabolized by spoilage microbes to volatile phenols such as 4-vinylguaiacol, 4-vinylphenol, 4-ethylphenol and 4-ethylguaiacol. These are described as having spicy, pharmaceutical, clove-like odors with smoky, phenolic notes.

Not all these aromas are desirable and can become off-odors, when 4-ethylphenol or 4-vinylphenol levels become excessive. Eugenol, another clove-like derivative, can also occur in wine adding a general spicy note, whilst guaiacol has a sweet, smoky odor but can be the source of some cork-derived off-odors.

Overall being able to measure and monitor these phenolic compounds in wines is a huge step forward in improving our understanding of wine taste and aroma and ultimately in improving quality control through routine testing.

Interested in learning more applications in wine analysis? Check out the dedicated pages for Wine Safety, Wine Quality and Wine Authenticity Testing along with Process Monitoring in winemaking at www.thermofisher.com/winesolutions.