From Muppet Wiki

Doozer sticks are the thin, transparent modules produced by Doozers for use in their various construction projects. Building is what Doozers love to do, so they do it all day long. As such, the amount of space they are alloted to build in would be overrun were it not for the Fraggles' propensity to consume them as snacks.

It was revealed in the first season episode, "The Great Radish Famine," that Doozer sticks are made from ground-up radishes. The Fraggles, however, would not realize this until season five's "The Gorg Who Would Be King."