Heather Smyth’s ice cubes are always salty — not as salty as sea water, but about halfway there.
At least, that’s how they taste to her.
“For years I’d think, ‘how the heck is salt getting into my ice cube tray?'” she said.
But her freezer was working fine. It turns out Dr Smyth, a flavour chemist and sensory scientist at the University of Queensland, is a thermal taster: she perceives taste when her tongue is warmed or cooled.
In her case, she’s a “cold salty”.
It sounds a bit odd, but thermal tasting is surprisingly common: between 30 and 50 per cent of the population experience it, according to Joanne Hort, a consumer and sensory scientist at Massey University in New Zealand.
Despite being widespread, the phenomenon is a relatively new discovery. It was first formally described in 2000 by a pair of Yale researchers.
They, and researchers after them including Professor Hort, found a wide range of thermal taster combinations.
Most common are metallic and bitter tastes appearing when the tongue is cooled, and metallic and sweet tastes when you turn up the heat.
“Different people might taste different tastes with different temperature, but within themselves, they’re very consistent,” Professor Hort said.
You’re probably familiar with the five basic tastes: sweet, salty, bitter, sour and savoury (which you might know as umami).
They’re accompanied by other sensations, such as metallic, fat and starchy tastes.
But it’s a myth that different parts of your tongue are responsible for perceiving specific tastes.
Even though your taste buds are concentrated on your tongue — there’s a few thousand on your tongue right now — they’re also scattered around the inside of your cheeks, upper oesophagus and the back of the roof of your mouth.
Each taste bud contain cells which, in turn, each have loads of different taste receptors embedded in their membranes.
When a molecule from, say, a nice piece of cake latches onto a taste receptor — maybe a sweet receptor — signals travel along nerves to activate cells in the taste cortex of your brain.
That’s when you taste sweetness.
But alongside taste receptor cells in taste buds are thermal receptor cells. These fellas detect changes in temperature and send signals to a different part of the brain called the somatosensory cortex.
It’s the close proximity of temperature-detecting and taste-detecting nerves that Professor Hort thinks is responsible for thermal tasting.
“The temperature is stimulating the temperature nerve, but it’s also stimulating the taste nerve in some way.”
And given that there are loads of different types of thermal taste, there’s probably more than one mechanism behind them.
For instance, we have a sweetness receptor that’s sensitive to temperature.
All in your head
When Professor Hort started looking into the thermal tasting phenomenon, she was sceptical.
“But what convinced me was when we saw the activation in the taste cortex in the brain from temperature stimulation of the tongue,” she said.
She and her colleagues took thermal tasters and heated or cooled their tongue while their brain was being scanned with a fMRI, which measures blood flow to parts of the brain.
When a thermal taster’s tongue was heated or cooled to the point they could perceive a taste, their taste cortex lit up in the same was as if they were actually tasting food.
“We can see in thermal non-tasters, you don’t get that activity in the taste cortex when you heat and cool their tongue,” Professor Hort said.
A matter of taste
Thermal tasters aren’t the only interesting tasters out there.
A quarter of us are “supertasters”, with tongues sporting more tastebuds, complete with more receptors, than non-tasters.
But even nutting out “normal” taste is far from straightforward.
Take bitterness, for instance. It alone is detected by about 25 different receptors.
And let’s not get started on flavour, which is shaped by what you taste and smell.
Aroma is thought to be shaped by tens of thousands of receptors, said Ingrid Appelqvist, a sensory scientist at the CSIRO.
Taste changes with age, too. Children are particularly sensitive to bitterness — which can signal toxicity — and sweetness, which is (usually) a good thing.
For elderly people, their sense of smell goes early. Food becomes less interesting and enjoyable.
So even though it’s early days, food companies are starting to take note of taste research, Professor Appelqvist said.
“Until recently, they have seen the population as a homogeneous mass and taken the average.
By enhancing certain aromas and combining different tastes, they might make nutritious food more palatable for the elderly consumer.
Ideally, companies might one day use information such as genetics and taste preferences to tailor a healthy, nutritious diet that people find delicious, Dr Smyth said.
“It’s early days, and it’s not going to happen any time soon, but companies are starting playing in this space,” she said.
In the meantime, Dr Smyth is keeping her salted caramel intake to cake form only — and definitely not as ice cream.