Ever bite into a delicious-looking apple, only to be rewarded with a chunk of mealy pulp? Or take a swig of milk, and get something lumpy in your mouth? Yeah, yuck.
Well, it turns humans have company in associating the texture of food with how tasty it is. In a new study published in the journal PLOS Biology, researchers found that when it comes to food preferences, texture can be just as important as taste in fruit fly maggots.
The lead author and neurobiology Ph.D. student at the University of Fribourg in Switzerland Nikita Komarov, says the findings go beyond flies. He hopes the deeper understanding of how texture plays into the whole experience of eating might one day provide insights into eating disorders among people.
"Understanding the function of sensory cells is important in then evaluating how we perceive things and how that perception can go wrong," he says. On the flip side, it may also inform how we might tweak the texture of foods — healthy or otherwise — to make them more palatable.
In addition, Devasena Thiagarajan, a neurobiologist at the Max Planck Institute for Chemical Ecology who wasn't involved in the research, says, "this study could help us understand what drives pests to different crops, and then how maybe that can be used to manage their influence on agriculture" by creating texture-based traps.
The persnickety persimmon
The persimmon was Komarov's favorite fruit when he was growing up in Moscow. "When they're properly ripe," he says, "they're delicious and sweet and soft."
Just before that, though, they're wretched. Komarov learned to tell from the first bite whether it'd be bliss or blah based on the texture of the fruit — even before he tasted anything.
"I knew that if it was slightly too hard," he says, "I was prepared for my whole mouth to be just this mess of this bitter, tarty taste."
The reason is that taste signals take a bit of time to register in the brain, "whereas with physical attributes of food, for example, texture — this is an instant form of feedback when we put something in our mouth before we even perceive what we're tasting," says Komarov.
This sense of mouth feel — is a food crunchy, soft, chewy, or gooey — provides important information about food safety and quality, helping the eater avoid consuming something that might be dangerous.
Very little is known, however, about how animals perceive food texture, so Komarov and his colleagues turned to study the phenomenon in a species that's easy to manipulate — fruit flies. Specifically, they looked at the larvae, which eat constantly.
"The way it finds the appropriate food sources directly impacts its adult stage and therefore its life cycle," says Komarov. "So it really, really, really cares about food."
The magnificent maggot mouth
The question was, which aspects of rotting fruit do these maggots like and dislike? First, Komarov engineered larvae without their taste organ, which would otherwise be located in each of their "cheeks." He left everything else about them untouched.
"We gave the animals a choice between a harder and a softer substrate where they were allowed to freely move to which one they like," he says.
Normal larvae prefer to eat the softer substrate, that which more resembled rotting fruit. "They are very, very picky," Komarov says. But the larvae without the taste organ ate both offerings. "They suddenly stopped caring."
Since the only difference in the food was the texture, Komarov suspected an intact taste organ also allows a larva to detect that delicious not-too-hard, not-too-soft rotting fruit texture. It's similar to human tongues being able to register both taste and texture.
"In the maggot," says Komarov, "what was thought to be an exclusively taste organ is actually also a texture organ as well."
Texture, it turns out, is as important a signal to the maggot as how bitter or toxic a food is. Komarov then examined one of the neurons within the taste organ. He found it responds to sugar, acid, salt, carbon dioxide (which signals how fermented a piece of fruit is) and texture.
"Seems to be a neuron that sort of does a little bit of everything," Komarov says. "It's chemically multimodal, it is mechanically sensitive and it is a carbonation sensor. The more we look at it, the weirder it gets. So it really opened our eyes to the capabilities of sensory neurons."
In other words, not all neurons respond to just one thing and relay it up to the brain. It's way more complex — both in what neurons sense and how they integrate that information. In this case, they're detecting a mix of flavor and feel.
Komarov puts it this way: "The maggot perceives the food in this multi-dimensional space."
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