leaven

leaven (http://definr.com/leaven)

     n 1: a substance used to produce fermentation in dough or a
          liquid [syn: leavening]
     2: an influence that works subtly to lighten or modify
        something; "his sermons benefited from a leavening of
        humor" [syn: leavening]
     v : cause to puff up with a leaven; of dough; "unleavened bread"
         [syn: raise, prove]