Molasses was a viscous byproduct of the refining of sugarcane or sugar beets into sugar.
In 2370, after Keiko O'Brien told her husband Miles that people "naturally… slow down" when they got older, he indignantly replied "like molasses in winter". (DS9: "Rivals")
In the script for the Star Trek: The Next Generation seventh season episode "Attached", Jean-Luc Picard and Beverly Crusher's breakfast is described as "some strange-looking soup with the consistency of molasses". [1]
In a deleted scene from the later episode "Sub Rosa", Crusher includes a dollop of molasses as part of Felisa Howard's recipe for gingerbread. [2]