Mouthfeel is tribology!

What makes a Barolo so different from a Spätburgunder? What about distinguishing between a Tannat and a Beaujolais? First and foremost, it’s the mouthfeel — the sensations we have when drinking wine that relate to the way in which our tongue, teeth and palate slide over each other. These are independent from the flavors and aromas of the wine and bring us into the realm of tribology — a word deriving from the Greek root τριβ- of the verb τρίβω, tribo, “I rub” in classical Greek, used to describe the science of friction, lubrication and wear.

Friction is the resistance encountered when one surface slides over another. We experience this in everyday life when we walk on the ground — the friction between our shoes and the ground enables us to propel ourselves forwards. If we walk on ice, the friction drops and walking forwards becomes difficult, since the ice is producing a water layer under our shoes, providing lubrication, which lowers the friction.

Lubrication is therefore a way to reduce the friction between two sliding surfaces. We know that in cars or bicycles it is essential to lubricate them to avoid the high friction (and wear) encountered when steel components slide over each other. Similarly in the mouth, the sliding surfaces are covered with a lubricant layer (saliva), which provides low friction under normal circumstances and helps lubricate food as it enters the digestive tract.

Astringency

Many wines increase the friction between oral surfaces, giving rise to sensations that many wine drinkers find pleasant, in moderation. These sensations are referred to as astringency, which is a key component of wine mouthfeel.  But astringency can be subdivided into many different “subqualities”, such as drying, rough, or puckering.

Astringency in wine is most frequently attributed to the strong interaction of wine tannins with the lubricating components of saliva — binding them up and preventing them from lubricating the oral surfaces. Acidity in wine seems to accentuate this process, but also has a puckering characteristic in the absence of tannin (think austere Rieslings or the pucker induced when eating a lemon).

These subqualities of astringency are familiar to those who taste wine, but scientists have been trying to model their physical and chemical origin using direct analytical methods in the laboratory. While tannin concentration, tannin composition, alcohol concentration, and pH all modulate perception of differing subqualities, challengingly, they also interact — a red wine with the same amount of tannin tasted at two different pH values will be perceived as quite different in astringency, with the higher pH appearing less astringent. Also, a red wine with the same tannin and pH will appear more astringent at lower alcohol concentration than the same wine at higher alcohol concentration. Herein lies one of the many challenges with the creation of the newly emerging lower-alcohol-wine category currently entering the market.

Understanding astringency by measuring friction

It is perhaps no surprise then that scientists have not been able to reliably use a single analytical method to mimic or model all subqualities, suggesting that different mechanisms are at play. However, because wine mouthfeel is mostly related to tribology, friction-measuring methods that have been developed over centuries for engineering research are now being used to simulate some processes occurring in the mouth and the interactions between saliva and wine components. Recent work from Australia describes experiments that strengthen the evidence that astringency subqualities may have independent mechanisms, challenging the long-held view that astringency is solely caused by de-lubrication induced by tannins.

The Australian team designed experiments in which the friction between saliva-coated model surfaces (made of silicone rubber) was measured, before and after adding model wines containing varying amounts of tannins and other key components. Not only was the effect of these other components very noticeable in the measurements, but the friction results correlated impressively with the subqualities detected by a taste panel, presented with the same set of model wines.

Summarising our current understanding of astringency, up to and including the new Australian results, the main actor is tannin but it must also be remembered that this is not a single substance, but a set of large molecules (polyphenols) with similar building blocks joined together in chains of different lengths. They are originally formed in the grape (skins and seeds) but are chemically scrambled during fermentation. On entering the mouth, the tannins bind to the lubricating proteins in the saliva layer, such as the mucins, preventing them from doing their lubricating job and resulting in the sensation of astringency. The tannins also change slowly over time, growing, shrinking, and binding to each other and to the color molecules (anthocyanins) in the wine, forming a complex, interwoven network—this is what is happening as a red wine ages and the tannins are described as having “softened”, but in reality are now just too bulky and inflexible to interact effectively with the mucins, whose lubricating properties are thereby largely preserved.

As for the role of the other wine components, alcohol lessens the astringency effect by weakening an important tannin-mucin interaction known as hydrogen bonding. Polysaccharides, originating from the grape or the fermentation process, can interact with the tannins, reducing their ability to bind to the mucins but also providing new lubricious coatings of their own on the mouth surfaces. And acids in the wine seem to facilitate the breakup of the lubricating layer in the mouth, accelerating the effects of the tannins.

Only a beginning…

These new findings open a door into the understanding of wine astringency and its subqualities, but they clearly only represent a beginning. The number of words used in wine-tasting to describe tannins and astringency goes significantly beyond rough, pucker, and drying, running the gamut from silk to sandpaper. It is not unreasonable to suppose that there are mechanistic explanations behind many of these sensations. The subtlety of the distinctions between “green”, “ripe”, and “smooth” tannins may lie in the effect that these complex molecules exert on the equally complex structure of the salivary layer on the tongue. This is an area under active investigation.

The insights gained from tribological studies of wine may well be able to guide future winemakers in tailoring the mouthfeel of a wine for a particular consumer segment. They may also have an impact on improving the mouthfeel of low-alcohol wines — a challenge that has been largely met in low-alcohol beer, but where wine has been significantly lagging behind.

But in the meantime, next time you take a sip of wine, focus on the mouthfeel and contemplate the many complex interactions and tribological changes taking place in your mouth!

Further reading:

Wang, S., Mantilla, S.M.O., Smith, P.A., Stokes, J.R., Smyth, H.E., 2020. Astringency sub-qualities drying and pucker are driven by tannin and pH – Insights from sensory and tribology of a model wine system. Food Hydrocoll. 109, 106109. 
 
Wang, S., Smyth, H.E., Mantilla, S.M.O., Stokes, J.R. Smith, P.A.,2024, Astringency and its sub-qualities: a review of astringency mechanisms and methods for measuring saliva lubrication. Chemical Senses, Volume 49, bjae016 (https://doi.org/10.1093/chemse/bjae016)

Photo by Wil Stewart on Unsplash