Ariana Grande -- Tongues New Boyfriend ... and an Innocent Donut!! (VIDEO)

Ariana Grande's new boyfriend got upstaged by her tongue -- which got dangerously close to some powdered donuts ... pissing off the people who busted their asses making 'em.

Ok, first the BF -- Ricky Alvarez, he's one of Ariana's backup dancers, and you can see them engage in some serious mouth-to-mouth PDA in this security cam video ... which wouldn't have come to light if not for the donut incident.

Sources inside Wolfee Donuts in Lake Elsinore, CA tell us the couple came in on Saturday, and decided to play truth or dare with the goods -- daring each other to lick powdered jelly donuts on the counter.

Hard to tell if their tongues actually make contact, but our sources in the shop say it was definitely too close for their liking -- especially since Ariana pulled the stunt when no one was looking, and they would've tossed the contaminated donuts if they knew.

Then again ... plenty of people might pay extra for a Grande-licked pastry.