leavening

(noun) an influence that works subtly to lighten or modify something. Synonyms: leaven. Example: "His sermons benefited from a leavening of humor."

(noun) a substance used to produce fermentation in dough or a liquid. Synonyms: leaven.

(verb) cause to puff up with a leaven. Synonyms: raise, leaven, prove. Example: "Unleavened bread."