leaven

v. add a leavening agent to a dough or batter; influence, cause gradual change, alter or transform

n. substance (such as yeast) which causes a batter or dough to ferment and rise; something that causes gradual change, something which alters or transforms

A leavening agent (sometimes called just leavening or leaven) is a substance used in doughs and batters that causes a foaming action. The leavening agent reacts with moisture, heat, acidity, or other triggers to produce gas that becomes trapped as bubbles within the dough. When a dough or batter is baked, it "sets" and the holes left by the gas bubbles remain, giving breads, cakes, and other baked goods their soft, sponge-like textures.
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Noun
1. a substance used to produce fermentation in dough or a liquid
(synonym) leavening
(hypernym) substance, matter
(hyponym) baking powder
(derivation) raise, prove
2. an influence that works subtly to lighten or modify something; "his sermons benefited from a leavening of humor"
(synonym) leavening
(hypernym) imponderable
Verb
1. cause to puff up with a leaven; "unleavened bread"
(synonym) raise, prove
(hypernym) raise, lift, elevate, get up, bring up
(cause) rise, prove
(derivation) leavening

(v. t.)
To make light by the action of leaven; to cause to ferment.
  

(v. t.)
To imbue; to infect; to vitiate.
  

(n.)
Anything which makes a general assimilating (especially a corrupting) change in the mass.
  

(n.)
Any substance that produces, or is designed to produce, fermentation, as in dough or liquids; esp., a portion of fermenting dough, which, mixed with a larger quantity of dough, produces a general change in the mass, and renders it light; yeast; barm.
  

Various substances were known to have fermenting qualities; but the ordinary leaven consisted of a lump of old dough in a high state of fermentation, which was mixed into the mass of dough prepared for baking. The use of leaven was strictly forbidden in all offerings made to the Lord by fire. During the passover the Jews were commanded to put every particle of leaven from the house. The most prominent idea associated with leaven in connection with the corruption which it had undergone,a nd which it communicated to bread in the process of fermentation. It is to this property of leaven that our Saviour points when he speaks of the "leaven (i.e. the corrupt doctrine) of the Pharisees and of the Sadducees," (Matthew 16:6) and St. Paul, when he speaks of the "old leaven." (1 Corinthians 5:7) (Another quality in leaven is noticed in the Bible, namely, its secretly penetrating and diffusive power. In this respect it was emblematic of moral influence generally, whether good or bad; and hence our Saviour adopts it as illustrating the growth of the kingdom of heaven in the individual heart and in the world at large: because (1) its source is from without; (2) it is secret in its operation; (3) it spreads by contact of particle with particle; (4) it is widely diffusive, one particle of leaven being able to change any number of particles of flour; and because (5) it does not act like water, moistening a certain amount of flour, but is like a plant, changing the particles it comes in contact with into its own nature, with like propagating power.-ED.)