When wine has a thirst for research. © FOTOLIA, Fotolia

When wine has a thirst for research

Tasting as the centre of attention

Robe, colour, tannin, structure, body, astringency... the vocabulary used to describe wine is particularly rich because all the senses become involved when tasting it.   Although colour intensity, clarity and brightness all contribute to a wine's robe, its body provides fullness and density in the mouth.  But what can we perceive within this complex mixture?  And how do we perceive it?

Updated on 06/20/2017
Published on 11/08/2013

From chemistry to subjectivity

Black glasses for sensory analysis. © INRA, Yves Cadot
Black glasses for sensory analysis © INRA, Yves Cadot
Examining vintages, defining organoleptic characteristics1, measuring the consequences of a process change, optimising assemblies, forming a tasting panel and measuring the impact of a packaging system... Several teams at INRA devote their efforts to sensory analysis, particularly with respect to wine.  Thanks to panels of people who are selected and trained in tasting, their research enables a clearer definition of the sensory profiles of wines;  it also enables a better understanding of the mechanisms related to pleasure or rejection, which vary from one individual to another and contribute to defining the quality of a product.  Since the 1970s, specialists have sought to identify the aromas in wine, their elemental flavours and their molecular origins.  But the use of analogies (fruits, flowers, spices, etc.) still remains necessary.

 
Thus flavour descriptors such as "banana", "floral" or "fruity" indicate butyl acetate and "apricot" or "honey" suggest ethyl beta-phenylacetate, while "truffle" tends to correspond to dimethyl sulphide.  Although describing the nose of a wine is often at the centre of a tasting, our senses still tend to privilege sight.  One subterfuge consists in colouring white wine as red, so that the tasters start using a vocabulary associated with red wines (red fruits), even though they should be using descriptors for a white wine (honey, apricot, apple, banana)!  Wine tasting thus induces activation of the visual cortex.  It is to iron out this bias that black-tinted glasses are often used for sensory analyses.
Ultimately, it is the consumer who has the last word, which is highly subjective.

The mouth, nose or brain... wine for all the senses

Hundreds of chemical compounds have been identified in grapes and wine.  However, only a tiny number are responsible for a wine's bouquet, or in other words all its aromatic components.  Recent studies performed by INRA have shown that odorant substances arising from oak barrels, even at very low concentrations (sometimes below the threshold of detection), may have an effect on a wine's bouquet.  Another experiment: researchers presented 30 volunteers with mixtures of one woody odorant substance and another fruity substance, either directly via the nose or through the mouth (the retronasal route). The first observation was that depending on whether the odorant compounds entered via the nose or mouth, there were differences in perception.  For example, the scientists showed that the perception of a fruity odour was enhanced when the woody odour in the mixture was present, even at a barely perceptible level.  By contrast, when the woody odour was more intense, perception of the fruity note diminished.  Such sensory effects result from perceptive interactions during the integration of olfactory information by the brain, which occurs as soon as a mixture of odorant substances reaches the olfactory mucosa.

A considerable number of odorant compounds have been identified in wines and new ones are still being discovered.  Human perception of them is not solely the result of their individual concentrations but of their aromatic power and combinations.  For this reason, human "sensors" are the only ones capable of integrating all perceptions, translating them into descriptors and then quantifying them.

Fruity notes that are difficult to perceive

Red wines from the Bordeaux region are well known for their fruity notes, evocative of red or black fruits.  However, no "key" compound at the origin of this fruity descriptor has been identified (in other words, at a level markedly above its threshold of perception).  The source of these fruity nuances has been the subject of several studies.  Thus numerous compounds are known to markedly influence the fruity expression of red wines, even though they are present in quantities far below their level of perception.  One example is ethyl leucate, which has proved to be a remarkable enhancer of black fruit or fresh fruit notes.  Inversely, other compounds (acetoin, acetic acid, γ-butyrolactone) diminish the aromatic intensity of a wine and its fresh, fruity nature.  Obtained by a Université Bordeaux Segalen and INRA team, these results highlight how difficult it is to precisely determine the bouquet of wines.  They also call into question the traditional approach which was to consider the contribution of a compound to aroma as a function of its threshold of perception.

1 Appearance, odour, taste, texture or consistency are the organoleptic characteristics of a food or drink, as they reflect everything that is likely to act on the senses.

Do experts have special noses?

Examples of the olfactory sensitivities of two experts to ten key components in  wine. © INRA, INRA
Examples of the olfactory sensitivities of two experts to ten key components in wine © INRA, INRA
Expert wine tasters are not always unanimous in their comments and judgements.  Based on this observation, a Université Bordeaux Segalen and INRA team focused on the olfactory capacities of these experts and demonstrated considerable variations between individuals.  Their study also revealed that tasters with "perfect" olfactory sensitivity do not exist! To improve sensitivity to a particular odour, the researchers therefore proposed specific training and learning methodologies; these produced positive results following repeated exposure to an odorant substance and, more unusually, using mental olfactory imaging.

Diagram: Examples of the olfactory sensitivities of two experts to ten key components in wine.  On these diagrams, sensitivities labelled 10 correspond to a subject who is highly sensitive to the odorant substance symbolised by an emitting source, while 0 corresponds to someone who is little sensitive.  Expert A was virtually unable to smell banana (a fruity odour that is notably characteristic of young wines) or coconut (characteristic of barrel-aged wines). Expert B was very little sensitive to cork taint, which is nonetheless a well-known defect in wines.  The grey area contains 80% of the olfactory sensitivities of the expert population.