"Strictly speaking, taste is a chemical function," Russell Keast, a sensory scientist at Deakin and lead author of the paper, tells The Salt. He says that when a chemical substance – a salt or sugar crystal, for example — comes into contact with sensory cells in our mouths, it triggers a series of reactions. The cells in our mouths tell other nerve cells that they're perceiving something sweet or salty and those nerve cells eventually pass this information on to the brain.
According to the paper, there are five criteria that need to be met to call something a primary taste. It starts with a chemical stimuli (like sugar or salt), which then trigger specific receptors on our taste buds. Then, there has to be a viable a pathway between these receptors and our brains, and we've got to be able to perceive and process the taste in the brain. And finally, this whole process has to trigger downstream effects in the body.
When it comes to fat, scientists know what the stimuli are: fatty acids — the building blocks of oil, butter and lard. And scientists also know that we have taste receptors for these fatty acids in our mouths and intestines.
But researchers have yet to pin down exact how our the receptors on our tongues signal the presence of fat to our brains, though they have some promising leads.
And here's another place where thinking about fat as a taste gets interesting, and controversial: "When we eat something sweet, we have that instant perception of sweetness. With fatty acids, we don't have a conscious perception," Keast says.
In his experiments, Keast says, people usually can't even describe what a solution with pure fatty acids tastes like. "They'll say they know it's different from water, but they don't know why," he says. "We don't have any vocabulary to describe the sensation."
There's one exception to that, says Keast: When food goes rancid, it's usually a sign that mold or bacteria have degraded the triglycerides in oil and lard. We are able to taste fatty acids once they've reached this foul state.
But generally, our inability to perceive fat taste is why some say there's not enough evidence to say fat is a true taste.
If fat is a taste, it isn't like any of the others, says Richard Mattes, a professor of nutrition science at Purdue University, who wasn't involved with the Flavour paper.
"If we confirm that fat is a basic taste quality, it's the equivalent of saying chartreuse is a primary color," Mattes says. "It changes our basic understanding of what taste is."
There's some evidence that fat may meet the criteria of having downstream effects on the body. Fat is a key nutrient that our body wants and needs — and even though we may not perceive fatty acids specifically, there's evidence that the presence of fatty acids on our tongues signals to our digestive system to get enzymes ready to digest fat.
The taste of fat may also signal to our brains and digestive systems that we should eat less, because something caloric and satiating about to make its way down to our guts.
"This may be why low-fat foods have been generally unsuccessful so far," Mattes says. Most low-fat alternatives are designed to emulate the texture of fat, but not the taste — and our bodies aren't fooled by it.
"If we recognize fat as a taste we could start developing better low-fat products," Mattes says, though at this point researchers aren't quite sure what that would entail.
Researchers are also looking into the link between the fatty acid receptors in our mouths and obesity. Preliminary evidence suggests that people who are obese may be less sensitive to the taste of fat, and therefore may feel as satiated by high-fat food.
"We don't fully understand all that yet. But we're close," Mattes says. "The evidence is, in my opinion, quite strong and growing. And I think fat will be accepted as a taste soon."
Copyright 2015 NPR.