leaven

Yeast is the most common type of leaven used in baking bread to make it light and airy.

(Chinese) 酵母是烘焙面包时最常用的发酵剂，能使面包变得轻盈蓬松。

Her witty jokes provided a necessary leaven to the otherwise dull and serious business meeting.

(Chinese) 她机智的笑话为原本沉闷严肃的商务会议提供了一种必要的调剂。