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The appearance of a hot sauce or pepper doesn't reveal whether it's mild or likely to scorch someone's taste buds, but researchers have now created an artificial tongue to quickly detect spiciness. Inspired by milk's casein proteins, which bind to capsaicin and relieve the burn of spicy foods, the researchers incorporated milk powder into a gel sensor.

The prototype, in ACS Sensors, detected capsaicin and pungent-flavored compounds (like those behind garlic's zing) in various foods.

"Our flexible artificial tongue holds tremendous potential in spicy sensation estimation for portable taste-monitoring devices, movable humanoid robots, or patients with sensory impairments like ageusia, for example," says Weijun Deng, the study's lead author.

Currently, measuring flavor compounds in foods requires taste testers and complex laboratory methods. As an alternative, scientists are developing artificial tongues, which can measure tastes including sweet and umami, among others.

However, capsaicin in chili peppers, piperine in black pepper, and allicin in garlic produce stinging, tingling or burning sensations that are hard to replicate and measure with synthetic materials. Researcher Jing Hu and colleagues noted that the heat of peppers, for example, can be neutralized when their capsaicin is bound by casein proteins in milk. So, the team wanted to create an artificial tongue by adding casein to an electrochemical gel material and measuring spiciness through an electrical current change that occurs when casein binds to capsaicin.

The researchers created a tongue-shaped film by combining acrylic acid, choline chloride and skim milk powder, and then they exposed the solution to UV light. The resulting flexible and opaque gel conducted an electrical current. Ten seconds after the researchers added capsaicin on top of the film, the current decreased, showing its potential as an artificial, spice-detecting tongue.

Initial tests showed that the milk-containing material responded to capsaicin concentrations ranging from below human detection to beyond levels perceived as painful (called the oral pain threshold). Additionally, the material detected other pungent-flavored compounds found in common hot sauce ingredients, including ginger, black pepper, horseradish, garlic and onion.

As a proof-of-concept, the researchers tested eight pepper types and eight spicy foods (including several hot sauces) on the artificial tongue and measured how spicy they were by changes in electrical current. A panel of taste testers rated the spiciness of the same items. Results from the artificial tongue and the tasting panel matched well.

Therefore, the researchers say, the casein-containing artificial tongue could be used to quickly test a food's spiciness level—without putting one's taste buds at risk.