barm

n. foam that rises as yeast is fermenting (i.e. in bread leaven or malt liquor)

Barm, a word derived from the old English for yeast, is the foam, or scum, formed on the top of liquor when fermenting. It was used to leaven bread, or set up fermentation in a new batch of liquor. Barm, as a leaven, has also been made from ground millet combined with must out of wine-tubs.
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Noun
1. a commercial leavening agent containing yeast cells; used to raise the dough in making bread and for fermenting beer or whiskey
(synonym) yeast
(hypernym) leaven, leavening
(hyponym) yeast cake

(n.)
The lap or bosom.
  

(n.)
Foam rising upon beer, or other malt liquors, when fermenting, and used as leaven in making bread and in brewing; yeast.
  

bust, bosom