For many years, scientists agreed that human tongues perceived four basic tastes: sweet, sour, salty and bitter. Then in 2002, receptors were confirmed for a taste called umami -- first proposed by a Japanese chemist in 1908 and commonly described as meatiness or savouriness -- and it became widely accepted as the fifth basic taste.
Since then, molecular biologists have theorized that humans may have as many as 20 distinct receptors for such tastes as calcium, carbonation, starch and even water. The data supporting each vary widely, but one contender for a sixth taste has begun to stand out from the rest: fat.
The growing evidence is intriguing to scientists and food developers, who hope a better understanding of our perception of fat will have applications in health and obesity management.
Currently, the debate is still over whether fat is a taste, and studies are increasingly likely to say it is.
In 2010, for example, researchers at Deakin University in Australia found people were able to detect the taste of fatty acids. This year, researchers at the Center for Human Nutrition at Washington University School of Medicine in St. Louis said they had discovered some people may be more sensitive to the presence of fat in foods than others.
For the latter study, published in March in the Journal of Lipid Research, 21 people with a body-mass index of 30 or more -- considered clinically obese -- tasted three solutions with a similarly viscous texture and were asked to identify the one that was different.
One of the solutions contained a bit of fatty oil. Participants whose bodies produce more of a protein called CD36 more often picked that solution out of the lineup, suggesting they were more sensitive to the fatty acids.
Researchers speculated this CD36 protein made people more sensitive to the taste of fat, so they perceive it in smaller amounts. If true, that would suggest a genetic basis for why some people crave fat more than others.
"The take-home message is that we now know there is a fat-taste component, and we cannot dismiss it. It exists in humans, but how it influences behaviour and fat intake, we still don't know," said Nada Abumrad, one of the authors of the study and the Atkins Professor of Medicine and Obesity Research at Washington University in St. Louis.
Traditionally, scientists have viewed fat as something humans detect through texture or aroma rather than taste.
To understand this better, a little familiarity with food science is helpful: In this context, taste refers to what we perceive through the tongue alone. Flavour refers to what we perceive through both taste and smell.
For example, to the tongue alone, ice cream mainly tastes sweet. It's not until the nose gets involved that you can experience the flavours -- chocolate or strawberry, for example. This also explains why foods you don't enjoy might go down more easily if you pinch your nose, or why your dinner is less enjoyable when your sinuses are congested during a cold.
But we also experience food through a third sensory system called chemesthesis, which is based on touch and is perceived by the face's trigeminal nerve. Branches of this nerve go to the eyes, nose, tongue and teeth, where they sense pain, irritation and temperature. This includes the burning sensation of chili peppers and the cooling sensation of a minty mouthwash. (Some people have argued that these perceptions should also be considered basic tastes.)
Fat is perceived through the trigeminal nerve, which sends signals to the brain that our mouth is experiencing something viscous, slippery or (as in the case of ice cream) creamy.
"So the question is, are you feeling fat or are you tasting fat?" said Richard Mattes, a professor of nutrition science at Purdue University, who has done extensive research on the topic.
The texture of fat is difficult to distinguish from its taste, because for a chemical to be tasted, it must come into contact with the tongue and dissolve in saliva. The texture of fat is also difficult to imitate, as people dissatisfied with non-fat ice cream can attest.
In the study on CD36, researchers tried to limit visual and olfactory cues by lighting the testing area in red light and having the subjects wear nose clips.
"The evidence is growing (for fat as a taste)," Mattes said. "We have experimental evidence, but we have not definitively eliminated texture."
In many ways, the discovery of receptors for umami and the prospect of receptors for fat, calcium and even carbon dioxide have called into question the traditional notion of basic tastes. Their characteristics are not as simple as, say, sweet or salty. Umami, for example -- a word derived from the Japanese for "delicious" -- is a complex taste whose chief effect is found in the depth of flavour of such foods as Parmesan, mushrooms and tomatoes.
"It might be that we have all these receptors but no words to describe what those receptors are like," said Michael Tordoff, a behavioural geneticist at the Monell Chemical Senses Center in Philadelphia whose research focuses on calcium.
When asked to describe the taste of fat, many study participants said, "I don't know."
"Part of it is a consequence of language," said Jeannine Delwiche, who specializes in sensory science and psychophysics at Firmenich, a Swiss company that makes flavourings and perfumes. "For example, a lot of people confuse sourness and bitterness. You see that a lot with coffee and grapefruit. There's not complete alignment in the way we use these terms."
Taste scientists say discarding the traditional notion of basic tastes would open up more possibilities in taste discoveries. "I'm not sure we need to have a limit to our thinking," Delwiche said.
Isolating the taste properties of fat could have important health implications. Since fat is an indicator of calorie-dense foods, the CD36 protein may help explain why some people consume more fat than others.
"If you taste fat more, you'll be satisfied more quickly than someone who needs more fat to get the sensation," Abumrad said. This might be applied to treating obesity by finding a way to increase sensitivity to fat.
-- The Washington Post