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Sweet, Sour, Salty, Bitter, Umami … And Fat?

Remember the tongue maps that showed where we perceive different tastes? Sweet on the tip, bitter in the back? The ones you can still find in textbooks across the country? They’re not accurate. We have receptors for all tastes spread around our tongues — for the tastes we know about, at least.

To date, those include salty, sweet, sour, bitter and umami, the tongue-coating, pleasantly savory flavor most commonly associated with monosodium glutamate, or MSG. Umami was the most recent addition to the group, after unique receptors were identified in the early 2000s. Since then, scientists have wondered what others they might be missing, and a growing body of evidence suggests that fat is the leading contender.

A paper published early this month by Australian researchers in a special edition of the journal Flavour highlights recent breakthroughs in our understanding of fat as a taste. Citing dozens of studies, it describes what is understood about the chemical and electrical pathway between fat in the food we eat and our brains.1

Although taste has been studied and contemplated since the time of Aristotle, there’s no textbook definition of what makes a taste. In science, “taste” is the perception of certain chemicals on the tongue, while “flavor” is the combined experience of taste and smell.2 Fat definitely induces responses based on its smell and texture, but over the past decade, evidence has been mounting that it may also have a taste component.

The most well-known criterion of taste is the presence of dedicated receptors on the tongue, which are understood to act like gatekeepers; they let us know what has come in, how much of it, and whether it should stay (or be spit out). Bitterness is often found in things that are poisonous, and sweet can alert us to high-calorie foods. Our systems need salt and sour (acid), but not too much. Umami seems to signal to us when we’re eating protein and may help us feel satiated.

But for a chemical to qualify as a taste, researchers also look for evidence of a chain of events that turn the food to an electric signal that’s transmitted to the nervous system with a resulting physiological effect. There’s also a fairly opaque idea that the taste must have perceptual qualities independent of other tastes. That’s where we get to the most controversial and confusing of the criteria when it comes to fat. The authors of the Australian paper describe how trying to understand whether we have a unique perception of the taste of fat makes for complicated research (not to mention some bizarre culinary creations).

Fat has a unique texture that’s difficult to separate from taste when trying to understand how it’s perceived. To get around the textural challenge, one study cited in the Flavour article (also conducted by one of its authors) served participants custard with varying quantities of fat, gum acacia and liquid paraffin to try to produce perpetually identical textures. Richard Mattes, a professor of nutrition science at Purdue University, cautioned me to remember that fat and water don’t mix. “It takes quite a bit of science to get where fat stays in solution and you’re not actually making a distinction based on tactile cues rather than taste,” he said, explaining that research methodology may still be lacking when trying to separate taste from texture. In his own research, Mattes used potato flakes, butter and fake butter in one experiment and red light, nose plugs and 5 percent gum acacia in another in an attempt to make textures indistinguishable between foods with varying amounts of fat.

What does seem apparent is that, because we don’t have a vocabulary to describe the taste of fat, it’s harder to perceive it. Mattes said people tend to describe fat as bitter or sour because it’s unpleasant, but that seems to be based on similar hedonic traits rather than the actual taste. Interestingly, umami had such a problem for many years, when, lacking the lexicon to differentiate, people identified it as salty.

Nada Abumrad, a professor of medicine and obesity at Washington University in St. Louis who has researched the chemical triggers caused by fatty acids in the mouth, said we have few answers to how taste works at all, including the basic question of whether sensitivity to a taste causes people to eat more or less of it. But learning more may be key to understanding our motivations with food and the obesity epidemic.

“How many people do you know that only eat when they’re hungry?” she said. “We are motivated by pleasure.”

Many studies involve animals in a lab, but Abumrad said we need more research on food intake in human subjects going about their daily lives if we want to understand how taste influences behavior. Clinicians are wary of the work, however, which is notoriously difficult and expensive. Among other reasons, people are horrible at recalling (or admitting to) everything they’ve consumed in a day.

Meanwhile, Mattes pointed me to one practical reason for understanding whether fat is a taste. “Fat replacers,” products used to mimic fat in food to reduce calorie count, are designed based on texture. If there is a taste component, it likely isn’t being captured, which could explain why products with fake fat don’t taste as good. (No, fat-free half and half is not as good as the real thing.)

So, enough evidence to call fat a taste probably exists, but it’s unlikely there will be a welcome party for this prospective member of the taste club anytime soon. “I think that science is accumulating and accumulating quickly,” Mattes said. “Many people have begun to accept the concept, but this is a fundamental change [in science], so it’s not going to be without problems.”

Footnotes

  1. This process is complicated but interesting, and not yet fully understood. Lipase enzymes are capable of breaking down the fat in foods to fatty acids (the substance we would actually be tasting), and it’s believed that this process does occur in human mouths, though researchers don’t fully understand where the lipase originates. Evidence suggests that the fatty acids are then transported by the protein CD36, which is found on our taste buds, beginning a cascade effect in conjunction with other receptors to generate an electrical signal. That signal triggers the perception of the fatty acids and probably stirs other physiological reactions. CD36 is also found in other parts of the digestive system, playing a role in fat absorption in the intestines and digestion in the stomach, adding to the complexity of the relationship between the protein and fatty acids.

  2. A third, related experience is chemesthesis, which describes the sensation and stimulation of the trigeminal nerves, often by chemical irritants, and is how we explain our reaction to chile peppers and onions. Fat also triggers the trigeminal nerve.

Anna Maria Barry-Jester reports on public health, food and culture for FiveThirtyEight.

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