leaven
[ˈlɛv&+{ə}n]
n
any substance that produces fermentation in dough or batter, such as yeast, and causes it to rise
a piece of such a substance kept to ferment a new batch of dough
an agency or influence that produces a gradual change
vb
to cause fermentation in (dough or batter)
to pervade, causing a gradual change, esp with some moderating or enlivening influence
现代英汉综合大辞典
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柯林斯