Sweet, salty, bitter, sour, umami – and “starchy”?
Jessica Hamzelou

New Scientist
5 Sep 2016 - 11:17 AM  UPDATED 5 Sep 2016 - 11:17 AM

As any weight-watcher knows, carb cravings can be hard to resist. Now there’s evidence that carbohydrate-rich foods may elicit a unique taste too, suggesting that “starchy” could be a flavour in its own right.

It has long been thought that our tongues register a small number of primary tastes: salty, sweet, sour and bitter. Umami – the savoury taste often associated with monosodium glutamate – was added to this list seven years ago, but there’s been no change since then.

However, this list misses a major component of our diets, 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. 

Floury flavour

Complex carbohydrates such as starch are made of chains of sugar molecules and are an important source of energy in our diets. However, food scientists have tended to ignore the idea that we might be able to specifically taste them, says Lim. Because enzymes in our saliva break starch down into shorter chains and simple sugars, many have assumed we detect starch by tasting these sweet molecules. 

Her team tested this by giving a range of different carbohydrate solutions to volunteers – who it turned out were able to detect a starch-like taste in solutions that contained long or shorter carbohydrate chains. “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 receptors on the tongue for detecting sweet tastes. This suggests we can sense carbohydrates before they have been completely broken down into sugar molecules.

When the volunteers were given a compound to block the salivary enzyme that breaks long chains of carbohydrate into shorter ones, they stopped sensing the taste of starch when given solutions containing only long-chain carbohydrates. This suggests that the floury flavour comes from the shorter chains.

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 is convinced by the evidence, and says it is impressive. “It will surprise a lot of people,” he says.

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Taste test

The finding adds to growing evidence that human taste is more complex than thought. “Many people think there are only five tastes, but a bunch of us think there might be others,” says Tordoff, who is investigating whether we might be able to specifically taste calcium.

Other potential tastes being investigated are the flavour of carbonated drinks, the metallic taste you get from blood, and amino acids, the building blocks of proteins. Receptors have been found for kokumi, a full-bodied flavour that has been described as “hearty” and is thought to make foods feel richer and more satisfying, and there is some evidence that we can taste the fatty acids that make up fats. “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 useful physiological response.

Starch doesn’t tick all of these boxes yet: Lim and her colleagues are yet to identify specific starch receptors on the tongue. Kokumi has not so far made the list because people who eat it can’t put their finger on a specific taste.

One criterion is that a flavour must be useful to us. There’s a strong case to be made for starch here, which is a valuable slow-release energy source that is worth detecting.

“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 might eat a small amount of the chocolate, but you’d choose the bread in larger amounts, or as a daily staple.”

Journal reference: Chemical Senses, DOI: 10.1093/chemse/bjw088

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