barm vs leavening

barm and leavening both are nouns.

Word	Noun	Adjective	Verb	Adverb
barm	Yes	No	No	No
leavening	Yes	No	No	No

As nouns, leavening is a hypernym of barm; that is, leavening is a word with a broader meaning than barm:

barm: a commercial leavening agent containing yeast cells; used to raise the dough in making bread and for fermenting beer or whiskey
leavening: a substance used to produce fermentation in dough or a liquid

Other hypernyms of barm include leaven.

barm (noun)	leavening (noun)
a commercial leavening agent containing yeast cells; used to raise the dough in making bread and for fermenting beer or whiskey	an influence that works subtly to lighten or modify something
	a substance used to produce fermentation in dough or a liquid