September 02nd 2016
There is now a sixth taste – and it explains why we love carbs.
A carb craving can be hard to resist, but why? The explanation might lie in new evidence that carbohydrate-rich foods may have a unique taste.
We used to think that our tongues registered just four primary tastes: salty, sweet, sour and bitter. Umami, the savoury taste often associated with monosodium glutamate, joined the list in 2009.
However, this misses out a key component of our diet, says Juyun Lim at Oregon State University in Corvallis. “Every culture has a major source of complex carbohydrate. The idea that we can’t taste what we’re eating doesn’t make sense,” she says.
Complex carbohydrates such as starch, which are made from chains of sugar molecules, are an important source of energy. But food scientists have tended to overlook the possibility that we might be able to specifically taste them, says Lim. Because enzymes in our saliva break starch down into simple sugars, the assumption was that we detect starch by tasting those molecules.
Lim’s team tested this by giving volunteers a range of different carbohydrate solutions. They found they were able to detect a starch-like taste from carbohydrate chains of various lengths. “They called the taste ‘starchy’,” says Lim. “Asians would say it was rice-like, while Caucasians described it as bread-like or pasta-like. It’s like eating flour.”
The volunteers could still make out this floury flavour when they were given a compound that blocks the tongue’s sweet receptors. This suggests we can detect carbohydrates before they have been completely broken down into sugar molecules.
This is the first evidence that we can taste starch as a flavour in its own right, says Lim.
Michael Tordoff at Monell Chemical Senses Center in Philadelphia says the evidence is impressive. “It will surprise a lot of people,” he says. “Many people think there are only five tastes, but a bunch of us think there might be others.”
Researchers are already investigating other potential tastes, such as flavours associated with carbonated drinks, amino acids – the building blocks of proteins – and blood, which has a metallic quality. “We are moving away from the idea of five primary tastes,” says Lim.
But before any new flavours can be enshrined as primary tastes, they must meet a strict list of criteria. Tastes need to be recognisable, have their own set of tongue receptors, and trigger some kind of beneficial response.
Starch doesn’t tick all of these boxes: no starch-specific receptors on the tongue have been identified yet. But another criterion is that a flavour must be useful to us. There’s a strong case to be made for starch here, which is well worth detecting as it is both high in energy and slower to release it than sugar.
“I believe that’s why people prefer complex carbs,” says Lim. “Sugar tastes great in the short term, but if you’re offered chocolate and bread, you’d choose bread as a daily staple.”