leaven

US /ˈlev.ən/
UK /ˈlev.ən/
"leaven" picture
1.

a substance, typically yeast, that is added to dough to make it ferment and rise

:
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The baker added leaven to the bread dough.
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Unleavened bread is made without any leaven.
:
yeastsourdoughferment
2.

an influence that produces a gradual change or development of something

:
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His radical ideas acted as a powerful leaven in the conservative community.
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A little humor can leaven a serious discussion.
:
influencecatalystmodifiertransforming agent
1.

to cause (dough or bread) to ferment and rise by adding leaven

:
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The baker will leaven the dough overnight.
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The yeast helps to leaven the bread.
:
fermentraiserise
2.

to permeate and subtly transform (something) for the better

:
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His optimism began to leaven the gloomy atmosphere.
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A touch of humor can leaven even the most serious topics.
:
permeateinfuseenlivenimprovetransform