Electric milk was a famous dish in the El Bulli kitchen – ‘A kind of toast made from dried milk, with Szechuan buttons on the top,’ says Ferran Centelles, the restaurant’s former sommelier who is part of the team at the El Bulli Foundation. ‘Our guests would find it tasted like crunchy milk, then at some point after tasting, [they’d have the sensation of] electricity flowing through the palate.’
Ferran Adrià ‘has been obsessed with the mechanics of that since he did that dish,’ Centelles continues. ‘It’s a challenge, trying to understand all the sensations and perceptions.’
It certainly is. We used to talk about all the sensations that come from food or drink in the mouth as ‘taste’. It is now widely understood that much of what we used to think of as taste is actually smell. But considering only what we know about the taste buds in the mouth and the olfactory receptors in the roof of the nasal cavity does not cover all the bases when you try to pin down the perceptions associated with even some of the most common eating and drinking experiences.
The astringent perception of tannins in wine is thought to have a trigeminal component. Likewise, the burn of alcohol
The fifth cranial nerve – known as the trigeminal nerve – plays an important and increasingly scrutinised role. But what exactly does it do? Can it explain the electric tingle of Adrià’s electric milk? If not, what can? And how do trigeminal sensations contribute to flavour perception?
The functions of the trigeminal, the largest of the cranial nerves, include control of some of the muscles used for chewing and swallowing. Crucially, in terms of flavour perception, the trigeminal can also be activated by chemical agents, temperature and touch to elicit a variety of sensations including heat and cold (sometimes when there has been an actual temperature change and sometimes in an illusory way), as well as pain.
While much specialist taste information is carried by the chorda tympani, a branch of the seventh cranial, or facial, nerve, it’s through the trigeminal nerve that we perceive the irritation of garlic, the fiery sting of capsaicin, the tickle of pepper, the burn of wasabi or ginger, and the coolness of menthol. The astringent perception of the sandpapery roughness of tannins in wine is thought to have a trigeminal component. Likewise, the burn of alcohol. For knowledgeable northern Rhône drinkers wondering about rotundone – the volatile aromatic compound that sometimes brings a peppery note to Syrah – it’s the piperine in black and white pepper that has been found to activate trigeminal receptors.
Unsurprisingly, ‘it’s complicated,’ says Professor Charles Spence, head of the Crossmodal Research Laboratory at Oxford University. ‘It’s clearly not one of the five basic senses. It’s not smell, it’s not touch, it’s not taste – it’s something else. Some people would say it’s not even right to call it “trigeminal”, because the trigeminal nerve also transmits some gustatory information to the brain. Some people say that we should refer to the common chemical sense, or chemesthesis, instead. That might be the better choice.’
As Spence says, referring to the common chemical sense has a logic to it because, for example, the burn of chilli can be perceived through nerve endings in the skin, as well as via the trigeminal nerve in the mouth.
The complications don’t end there. It may be accepted that olfaction butts its way confusingly into the flavour sensations we seem to sense in the mouth, but it is less well known that ‘most olfactory stimuli, or smells, probably have a trigeminal component to them’, says Spence. ‘We find it impossible to localise which nostril a pure olfactant comes in. If we think we can localise where the smell is coming from, that’s because of the trigeminal part of it. Vanillin (the key compound in vanilla) and phenylethyl alcohol [which smells like roses] are two of the very few compounds that have no trigeminal input whatsoever.’
It’s clearly not one of the five basic senses. It’s not smell, it’s not touch, it’s not taste – it’s something else
– Professor Charles Spence
Trigeminal sensations are increasingly used to bring a tingle, buzz or burn to the finish of new breeds of zero-alcohol drinks, filling in (some of) the gap left when the alcohol has been removed. Some winemakers believe trigeminal inputs can be recruited to hone the flavour of certain wines.
In our experience, wines made with more whole-cluster fermentation seem to taste cooler in the mouth, even when served at exactly the same temperature as wines that are 100% destemmed,’ says Chris Mullineux of Mullineux & Leeu Family Wines in South Africa. ‘Our theory is that the wintergreen character that one gets from the stems acts in a similar way to menthol in stimulating (tricking) the TRPM8 receptors in your mouth to “feel” cool.’
TRPM8 is a cation channel (a channel that allows the flow of positively charged ions) whose receptors are expressed by trigeminal neurons. It is activated both by cooler temperatures and by compounds such as menthol, found in mint leaves and eucalyptol, that create cooling sensations. If it is responsible for the cool effect in whole-cluster-fermented wines observed by Chris and his wife Andrea, then it explains why whole-cluster fermentation could be used to bring a sensation of freshness to wines in regions where a warming climate means some of the acidity has been lost.
As those who enjoy spicy food may have worked out, trigeminal inputs might also interact with the gustatory system to have an impact on wine lovers when they are eating and drinking. Capsaicin is the compound in chilli that activates the TRPV1 receptors to create the trigeminal sensation of burning and irritation in the mouth. Research shows that the presence of capsaicin suppresses the perception of sweetness. That’s why it works to drink medium-sweet wine with spicy food.
What of some of the more unusual sensations we experience as tastes? Hospital patients given general anaesthetic are sometimes warned they may experience a metallic sensation in the mouth. Likewise, wine tasters speak of finding metallic flavours in wines, such as those made in the south of France from the grape Fer Servadou.
What is going on in those cases when a stimulus – wine – is present? Could metal be added to the list of basic tastes (salt, sweet, umami, sour and bitter)? ‘The jury is still out,’ says Professor Barry Smith, director of the Centre for the Study of the Senses. ‘It is still unclear that there is a basic taste for metallic; it just may be a combination of a saline taste with an aroma combination, and yet we are fairly convinced we taste it. For example, we perceive the taste of iron in our blood when we lick a cut finger.’
Some research suggests a combination of taste buds, the olfactory system and the trigeminal nerve could be responsible for creating the complex metallic sensation. Metal salts have been found to activate both TRP receptors (in the trigeminal nerve) and one of the bitter taste receptors, T2R7.
It’s through the trigeminal nerve that we perceive the irritation of garlic, the fiery sting of capsaicin, the tickle of pepper, the burn of wasabi or ginger, and the coolness of menthol
But back to that electric milk. Szechuan pepper is known to create a buzzing sensation on the tongue and lips that has been compared to touching the terminals of a 9-volt battery with your tongue. This sensation is created when a compound called hydroxy-alpha-sanshool in the Szechuan pepper acts on a set of potassium channels.
Researchers attempted to quantify the tingling sensation by asking participants in a study to compare the sensation on the lips to vibrations applied to their right index finger and concluded that Szechuan pepper induces a tingling at a perceived frequency of 50Hz. Literally, then, Szechuan pepper makes your lips vibrate – which is a sensation I have yet to experience with any wine.
