Mouthfeel - the unspoken hero

Mouthfeel is an essential part of flavour. It can separate great drinks from average ones by adding complexity, variety, and depth. It’s something that we all commonly talk about too with terms like ‘It’s as rough as hell’, or ‘It’s as smooth as silk’. Terms that are inherently related to textures, and therefore mouthfeel. But even though we love to use such phrases, very few stop to consider how we experience textures, and what exactly mouthfeel is anyway?

Today we aim to set things straight. By the end of this somewhat lengthy read you will have enough knowledge to enthusiastically throw golden nuggets of information at your whisky-drinking, wine-sipping, and gin-tippling friends, whilst they look at you blankly. Nevertheless, your sensory skills and expertise will be elevated to remarkable new heights.

Texture vs Mouthfeel

Mouthfeel is often confused with texture, which is no surprise considering how closely related they are. At one end of the texture scale we have silky, velvety, oily, thick, soft, chalky, creamy and waxy. On the other end we have coarse, gritty, grainy, sharp, firm, dusty, prickly, and granulated. Plus of course the classic ‘rough’ and ‘smooth’. These are all very much touch-related terms, as in textures. However, mouthfeel goes much deeper. So, viewing texture as mouthfeel bypasses a whole world of sensations.

Burning, hot, spicy, tingly, puckering, drying, aggressive, cooling, viscous, and even effervescent. We also have mouth-filling and mouth-coating, which while we may associate them with oiliness, are terms that convey subtle nuances of otherness. Once we begin to consider the question before us, defining mouthfeel is not so simple. However, it’s safe to say that texture can be considered as just one component of mouthfeel.

Explaining the inexplicable

The topic of mouthfeel is so complex that sensory scientists haven’t even come up with a name to describe it yet. Although the flavour wheel created by The Scotch Whisky Research Institute subdivides mouthfeel into mouth warming, mouth coating, and astringent.

When looking at the sensory origins of mouthfeel, we have related terms such as Chemesthesis - the chemical sensitivity of the skin and mucous membranes. The Somatosensory System - the network that is responsible for the perception of touch including Haptic Perception - the sensory detection of friction between two surfaces. Also included is Thermoception - detecting temperature and pain. Lastly there’s the Trigeminal Sense, the resistance feedback of biting and chewing food.

Each one relates to a specific area of mouthfeel, but there’s no single term to describe the sensation of mouthfeel other than the unsatisfactorily sounding - mouthfeel. It demonstrates how, while mouthfeel is perhaps the most used way to describe drinks, pinning down what we are describing is tougher than chewing old leather. Perhaps we need some oil.

Oily textures in whisky

Oily-related mouthfeel textures span a wide range of sensations. Anything from buttery to creamy, from waxy to fatty, and from light walnut oil to thick engine grease. They all play an important role in the style of many whiskies. While the oiliness of a dram may be one of the more creative ways to describe whisky, the mechanisms behind this sensation are quite remarkable.

Textures light up the same areas in the brain that are also associated with taste, such as the primary taste cortex, the orbitofrontal cortex, and the amygdala. The brain has dedicated neurons for specific categories of texture. For example, we have dedicated neurons for ‘thickness’ or 'viscosity'. Other neurons identify fat in the mouth, based on its feel rather than its chemical reaction within the skin or mucous membranes.

It's been discovered that these fat-responsive neurons are concerned with the sliding friction within the mouth rather than viscosity. For example, the force required to slide one surface over another. Therefore, our brains can differentiate between viscosity and fattiness, and have dedicated neurons for identifying each. Interestingly, the degree of friction is also impacted by tannins, which we shall come to shortly.

The point is that while we may feel that the sensation of texture in the mouth is many sides of the same coin, it is in fact many different coins. Each oily texture has its own dedicated neurons to identify it. This is how we can also differentiate between double cream and beeswax. So, what about those tannins.

The secret world of tannins

Tannins are compounds that get much of the attention from wine fanatics and tea gurus. Fine tannins, coarse tannins, grainy tannins, chewy tannins. These are all ways of describing different expressions of tannins. For wine, tannins come from the seeds and skins of grapes. Want to understand tannins? Chew some grape seeds. But they also come from oak.

Therefore, tannins from oak contribute towards aged spirits such as whisky, brandy, and rum, alongside wine. When in balance with other flavours, tannins provide astringency to benefit complexity, body and structure (two other interesting mouthfeel-related terms). In too high concentrations, however, tannins can transform a whisky into a dried-up creek, and a wine into a mahogany desk. But have you ever wondered; how do tannins do that?

As in turns out, tannins bind to proteins within the saliva of the mouth. When this happens the lubing properties of the mouth are reduced, thereby increasing friction. This increased friction creates the drying sensation that we associate with tannins and the ensuring oral desert in extreme cases. There’s more too.

It seems that tannins also bind to proteins in the epithelial cells that line the mouth. This would explain the ‘mouth-coating’ or ‘furry’ sensation that is common with high tannin whiskies and wines. It’s interesting to note that these binding effects appear to increase at a lower pH (more acidic), and temperature plays a role too.

It’s tannins attraction to proteins that explains common food and drink associations. Pairings that seem to work remarkably well through matching high concentrations of tannins with proteins. The two appear to embrace in a merry dance and create an experience that’s greater than the sum of its parts. Think of milk in tea, cream in coffee, red wine with oily meat, or whisky with cheese.

It feels natural to consider tannins as a type of texture through haptic sensation (i.e. touch). But on the one hand tannins function through our chemical sensitivity pathways instead. However, on the other hand, the way that tannins react with saliva proteins to create a unique sensation is distinct from all other components of mouthfeel.

The next component of mouthfeel that we shall look at is more straightforward though, or at least on the surface. Are you ready to feel pain?

Hit me with your cinnamon stick

When we think of pain in relation to taste the first things that may come to mind are chilli peppers. These painful reactions represent a whole world of masochistic experiences in their own right, but they are nevertheless central to mouthfeel.

In wasn’t until 1997 that pharmacologist David Julius at the University of California identified the receptor behind this burn. You will know capsaicin, it’s the molecule that weaponises chilli peppers. The research identified that the TRPV1 or trip-vee-one receptors are the ones that get excited when capsaicin comes along. These receptors are not only responsible for our reaction to capsaicin but also for our experience of extreme temperatures of heat too. But there’s more.

Think of the sensation of cloves, ginger, pepper, eugenol, and the neurotoxins from tarantula venom (we’ve all been there). These are all experienced through the trip-vee-one receptors. Plus ethanol too. Hence the trip-vee-one receptors play a central role in the mouthfeel of whisky and other strong spirits. What would happen if we didn’t have them?

In 2009 Y.A. Blednov and R.A. Harris studied what happens when the TRPV1 gene is deleted in mice. The results were that mice without the TRPV1 receptor actively consumed more ethanol. In fact, given the choice of two bottles to drink from, the mice consistently chose the single malt whisky. (Well it was just ethanol but it’s a fun image). The point is that alcohol was far less burning, even for mice who are not renowned as being hard drinkers!

Menthol mouthfeel

On the other end of the pain spectrum, we have the cooling effects from irritants like menthol and eucalyptus. While menthol and its related minty counterparts are widely enjoyed, in high enough concentrations they become undeniably painful. (Ever overdosed on toothpaste?) The cells in the mouth and throat responsible for this cooling pain are TRPM8 receptors.

Now menthol may not be at the top of your list for classic distilled spirits or wine flavours. But my experience with many cask samples is that the TRPM8 receptors are being stimulated surprisingly often in whisky at least. The sensation can be interpreted in various guises of cooling freshness, from mint tea to chewing gum, eucalyptus to peppermint, muscle rub, Olbas oil and beyond. It’s also common in spicy rye whiskey.

But there’s a different type of mouthfeel that’s a real buzz.

Both the pain-related sensations of capsaicin and menthol invoke highly invigorating responses which may be either a nice tingle or a moment of regret. But there are other ways in which mouthfeel creates a buzz. The first is related to a very specific type of pepper – Szechuan peppers.

Trivia alert: Szechuan peppers are not related to chillies whatsoever, instead they are flower buds from the citrus family.

The remarkable thing about Szechuan peppers is the way in which they buzz. The even more remarkable thing is that no-one really understands how. Rather than chilli pepper capsaicin, Szechuan peppers contain sanshool. The difference is that sanshool creates a buzzy sensation in the mouth that has been very precisely compared to the hum of a 50 hertz vibration. Now all this buzz brings us onto a similar tingle - effervescence.

The mousse is loose

A certain high-altitude climber also happened to be a doctor, and like many climbers he would occasionally use altitude sickness tablets. On one of the climbs, he hauled a few cans of celebratory beer to the summit. However, when high-fiving and cracking open the cans, the beers did not taste as fizzy as they should. Even though they were still fizzy by sight and sound. Why was this?

Alcohol sickness tablets have an inhibiting effect on an enzyme called carbonic anhydrase. This enzyme just so happens to convert carbon dioxide from carbonated drinks into carbonic acid. The inhibition of this enzyme impacts the sensation of fizziness, suggesting that acidity creates the sensation of fizz, rather than the bubbles per se.

This idea has been supported by a study that involved sampling fizzy drinks whilst being inside a hyperbaric chamber. The pressure within the chamber prevented any bubbles from fizzing and popping, as the carbon dioxide remained dissolved in the drinks. To the taste though, the fizziness remained as per normal. So rather than bubbles creating fizziness, it’s actually carbonic acid stimulating another group of receptors called TRPA1.

While whisky is not fizzy in a carbonated sense, could its occasional effervescent sensation come from certain acids stimulating the TRPA1 receptors? Scotch whisky contains organic acids such as gallic acid, lactic acid, and acetic acid. It has been demonstrated how weak acids such as acetic acid stimulate the TRPA1 receptors. It’s an interesting thought.

What about wasabi?

Along with TRPA1 taking centre stage in the sensation of fizz, it’s also the cause of our experience of other types of heat beyond capsaicin. These include cinnamon, mustard and wasabi, which are all commonly experienced in single malt whiskies and rums. The next time you have a dram, consider what type of burn you feel and see if you can match it to a particular pain receptor. A fun idea for the super geeks anyway.

The 6th taste

The tastebuds cannot pass by without a mention. These tongue-based champions of taste are responsible for the experience of salty, sweet, sour, bitter and umami. Each of these has connotations for mouthfeel, and umami in particular has close relations to texture with its savoury characteristics. However, there could be at least one other taste.

Richard Mattes is a leading expert on the relationship between fat and flavour. Although inconclusive, he has identified how free fatty acids generate signals along neurons containing taste fibres. The implications are that we could experience fattiness both as a texture, and also as a taste through two entirely separate pathways. So a whisky could have a fatty taste, but not a fatty texture. Or vice versa. Crazy right?

Wash your mouth out with soap

Soapy notes in spirits are not desirable and are widely accepted as a flaw from production. But they do pop up now and again. However, they can also form when an abv is reduced through adding water too rapidly and haphazardly, causing esters in a spirit to be cleaved back into hydroxylated fatty acids. A process called saponification. Interestingly, saponification is also how cold pressed soap is made, the combination of oils with sodium hydroxide.

Soapy, as a flavour, has received only a small amount of scientific attention. The main focus has been on why some people experience coriander as undesirably soapy. This phenomenon has been traced to variations in olfactory receptor genes, but soapy also has a mouthfeel sensation that is quite unique.

Without any specific studies to date, one can only hazard a guess that soapy mouthfeel is combination of sensations as opposed to individual receptors. An amalgamation of rancid fats, oils, wax and bitterness that our brain patches together with childhood memories of bathtime. With the discovery of coriander-related olfactory genes, it also points to how aroma plays a role in the sensation of mouthfeel too.

Aroma also impacts mouthfeel

Just to drive the concept of mouthfeel deeper down the rabbit hole of bafflement, aroma also plays a role that cannot be ignored. A wonderful example of this is vanilla (vanillin), which as an oak-derived molecule is common in whiskies around the globe. We associate vanilla with creamy textures, but it doesn’t taste creamy in itself.

In fact, vanilla does not taste of anything at all. The experience of vanilla is all about its aroma. However, when the smell of vanilla is combined with a texture in the mouth, the perception of creaminess is enhanced. Even when the vanilla aroma does not emanate from the texture in the mouth. The brain joins the dots together.

Would custard have a less creamy texture without vanilla aroma. Of course. Our brains make similar associations with other aroma/mouthfeel combinations too. Spicy aromas increase mouth burn. Citrus aromas increase acid tingle. And buttery aromas increase fattiness. It’s a phenomenon that’s utilised by food manufacturers, chefs, and sensory marketers to great effect.

The unreal mouthfeel

The long and short of all this is that mouthfeel is a multi-sensory extravaganza that deserves far more recognition and understanding than it commonly gets. Reducing mouthfeel to textures alone is like only reading the first and last chapters of a book. The role that it plays in the overall experience of flavour is as significant as aroma and taste.

As with the other senses, mouthfeel works as part of a reciprocal system that influences the other senses, and in turn is influenced by the other senses. What sets mouthfeel apart from other areas of flavour, is its diversity. Whereas aroma and taste are specialists in their fields, mouthfeel is a combination of touch, taste, aromas, heat, cooling, textures, and things we don’t even have a name for.

Understanding mouthfeel, as promised, will take your tasting expertise to a whole new level. Even simply being able to differentiate between some of the various components of mouthfeel when sipping your favourite tipple is great practice. It’s by being conscious of such nuances that we can take sensory experiences to new heights. And therefore, get much more from each sip.

Want to learn more about the science of flavour? Visit our Flavour Crusaders Substack page here to view more articles and research on topics as diverse as how microbiota impact flavour perception and why we love disgusting flavours such as mouldy cheese.