Scrolling through the restaurant reviews on TripAdvisor is to enter a grumbly old world. Everything seems to be lukewarm: the service, the coffee and, worse of all, the food. I must admit that if I'm in an unfamiliar place that has yet to earn my trust, a tepid cooked meal does inject an undercurrent of unease into proceedings, as I wonder how long my food has sat around gestating germs, or if it has it been poorly reheated. But in terms of the actual eating, I don't find piping-hot food that tasty. Sure, it will smell exquisite as the aromatic molecules evaporate, but once a morsel hits your mouth, everything tenses up. My friend who has lost his sense of smell doesn't taste a thing until the food has cooled down. And besides, scalding your tongue will erode your taste buds.
Why high temperatures stifle taste
This is a trick heading, I'm afraid, because no one has figured precisely how this works, physiologically. But we can speculate. According to Karel Talavera Pérez, professor of molecular and cellular medicine at the University of Leuven in Belgium, studies recording the electrical activity of taste nerves demonstrate that "the perception of taste decreases when the temperature rises beyond 35C". With very hot food, he hazards, it is possible that the burning feeling "masks" taste sensations, because it works as an alarm signal to warn us about the danger hurting ourselves. "Perhaps we do taste at such temperatures," he says, "but we don't pay attention to it because we become worried about the burning feeling."
How heat alters flavour balance
A 2005 paper published in the Journal of Sensory Studies found that the serving temperature of cheddar cheese affected how its taste was perceived. The cheese was served at 5C, 12C and 21C and sourness increased as the temperature rose. The tasters also found the warmest cheese more difficult to evaluate. Talavera Pérez, meanwhile, discovered in the same year why ice-cream gets sweeter when warmer. It's true: melted ice-cream is too sickly to drink, whereas when cold, it is pleasantly sweet. Beer, on the other hand, tastes more bitter as it gets warmer. Ham tastes saltier when cold and more savoury when warm. Some of these effects, such as the over-sweet melted ice-cream, occur because the taste receptor TRPM5 (which picks up sweet, bitter and umami tastes) sends a stronger electrical signal to the brain when food is warmer. However, our full explanation, so far, as to how temperature affects the balance of tastes is infinitely more complicated and nuanced, with the concentration of taste compounds in the food another key factor, along with variations in taste sensitivity among individuals and the fact that other sensory components of the tongue are heat-sensitive, too.
Temperature's taste spectres
Heating or cooling certain parts of the tongue can create the illusion of certain tastes. A study published in the journal Nature in 1999 found that, for example, warming the front edge of the tongue (where the chorda tympani nerve is), from a cold temperature, can evoke sweetness. Cooling the same area conjures sourness and/or saltiness. Then, at the back of the tongue (where the glossopharyngeal nerve is), a different set of effects occur. The Yale researchers concluded that thermally sensitive neurons form an everyday part of our sensory code for taste.
Think before you drink
It follows that the temperature of what you drink while eating will also affect the food's taste. North American people, on the whole, like ice-cold water at mealtimes, whereas Europeans are happy with not-far-below room temperature, and Asian people often drink hot water or tea while eating. Research published in June this year found that eating immediately after drinking cold water decreased the perception of sweetness, chocolate flavour and creaminess, and the researchers are now wondering whether the preponderance for iced water among Americans contributes to their preference for highly sweetened food.
The intricate manner in which our senses mingle never ceases to amaze. A brand-new study by Charles Spence at Oxford has found that, more often than not, we can tell whether a liquid is hot or cold just from hearing it being poured into a cup. Where do you stand on tea temperature: scalding hot or cool enough to glug? Do you throw a wobbly if your dinner isn't sizzling? Does the idea of a cold soup such as gazpacho leave you lukewarm? And have you ever noticed foods changing taste as they get warmer or cooler?