The compounds and/or chemical factors responsible for the main tactile sensations of wine, such as astringency in general, dryness or adherence, were the focus of the research carried out by Sara Ferrero del Teso, from the University of La Rioja at the Institute of Vine and Wine Sciences (CSIC-GR-UR). This research also required working on the sensory vocabulary of these sensations with a panel of experts, which made it possible to propose a sensory vocabulary, made up of 18 terms, to name these perceptions.
The thesis, entitled New approaches for understanding the formation of mouthfeel properties in wines and grapes, has been carried out in the Doctoral Programme in oenology, viticulture and sustainability of the UR, under the direction of MarÃa Purificación Fernández Zurbano (UR) and MarÃa Pilar Sáenz Navajas (CSIC), researchers at the Institute of Vine and Wine Sciences (CSIC-GR-UR). It has obtained the highest qualification Cum Laude.
Among the senses that contribute to the perception of wine flavour - smell (aromatic stimuli), taste (acid, sweet, bitter, salty and umami) and tactile perception - the tactile sensation is the least known, which makes its study particularly relevant for the oenological industry. It is mainly produced in the mouth, mediated by the trigeminal nerve, through mechanical and chemical processes.
During the research, sensory and chemical strategies were combined. "We separated the wine components into different parts (fractions), which were evaluated by experts using different sensory methodologies and the chemical compounds found in each one were analysed," explains Sara Ferrero. In this way, we obtained fractions that were simpler chemically and less complex sensorially, which allowed us to study tactile sensations. "We also studied the effect of oxidative and reductive ageing, the influence of grape maturity and other factors".
To determine the tactile sensations in mouth of a wine, a panel of experts tasted the wine and described their perceptions. However, it proved difficult to obtain a complete sensory profile, as the substances that give rise to them are not known and therefore there are no reference materials that illustrate these properties unequivocally. Thanks to this research, it has been possible to isolate groups of compounds that cause different tactile sensations in the mouth. This has allowed the development of an extensive sensory vocabulary describing relevant sensory properties in the mouth, consisting of 18 terms: burning, rough, powdery, drying, drying on the palate, drying on the side of the tongue, fleshy, grainy, rubbery, spicy, pungent, fatty, sharp, sandy, silky, adherant, unctuous, watery.
Of all the families of polyphenols present in wine, it is conventionally known that anthocyanins (pigments present in grape skins) are responsible for the colour contribution of red wines. However, the results of this thesis support that anthocyanins are involved in the tactile sensations in the mouth produced by wines. Thus, the anthocyanin fraction obtained from certain wines was described as drying, bitter and persistent.
Other descriptors were also studied, such as the term "green character" used by technical experts to describe a sensation that is recurrently observed in their wines and which leads to a depreciation of the product, associated with sensory imbalances in the wines. As a result of their study, the isolated fraction of wines described with high "green character" containing anthocyanins was found to be related to the tactile sensation of adherence, described by the experts as drying tannins, while the fraction containing tannins was found to be related to dryness attributes (rough, drying, drying on the palate and persistent) described by the experts as green tannins. Green character was found to be a multidimensional term associated with descriptors of aroma, taste and tactile sensations.
Notably, for tannins (organic substances abundant in grape skins and pips, as well as in barrel wood), novel parameters were studied, such as tannic activity (Revelette et al., 2014), which consisted of measuring the interaction between tannins and a hydrophobic surface, simulating the reaction that would occur with proteins in saliva. This chemical parameter could be correlated with the general sensation of astringency. Tannin activity has been suggested as a very interesting parameter that can help to control grape maturity, especially because it is independent of other chemical variables related to polyphenol content.
Furthermore, as a result of two periods spent at the Centro Ricerca e Innovazione of the Edmund Mach Foundation in San Michele Alládige, Trento (Italy), this work shows an interesting sensochemical strategy to characterise the tactile mouthfeel properties of wine, where non-targeted analytical methods (Untarget) combined with sensory analysis have proven to be a useful tool to elucidate the chemical basis of flavour. It is worth highlighting the results obtained in which sulphonated flavonoids play an important role in reducing the perception of astringency, possibly as pointed out by other authors (Ma, Watrelot, Addison, & Waterhouse, 2018) due to the alteration of the tannin structure.
This research expands our knowledge on the formation of tactile properties in the mouth and reveals the compounds and parameters that guide and modulate this perception in wines and grapes. This information is of great interest to the wine sector, since taste and other intrinsic properties of wine, together with extrinsic factors (label design, packaging, etc.), play an important role in the perception of quality and the greater or lesser acceptance of the product by the consumer.
For the development of her thesis, Sara Ferrero has been awarded an FPI contract by the Government of La Rioja.
Source: ICVV
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