Etymology
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yeast (n.)
Old English gist "yeast, froth," from Proto-Germanic *jest- (source also of Old Norse jastr, Swedish jäst, Middle High German gest, German Gischt "foam, froth," Old High German jesan, German gären "to ferment"), from PIE root *yes- "to boil, foam, froth" (source also of Sanskrit yasyati "boils, seethes," Greek zein "to boil," Welsh ias "seething, foaming").
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eczema (n.)
1753, from Greek ekzema, literally "something thrown out by heat," from ekzein "to boil over, break out," from ek "out" (see ex-) + zein "to boil," from PIE root *yes- "to boil, foam, bubble" (see yeast). Said to have been the name given by ancient physicians to "any fiery pustule on the skin" [Chambers' "Cyclopaedia"].
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baking (n.)

late 14c., "process of making bread," verbal noun from bake (v.). Baking powder "yeast substitute" is from 1850.

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barm (n.)
Old English beorma "yeast, leaven," also "head of a beer," from Proto-Germanic *bhermen- "yeast" (source also of Dutch berm, Middle Low German barm), from suffixed form of PIE root *bhreu- "to boil, bubble, effervesce, burn."
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ferment (n.)

early 15c., from Old French ferment (14c.), from Latin fermentum "leaven, yeast; drink made of fermented barley;" figuratively "anger, passion" (see ferment (v.)). Figurative sense of "anger, passion, commotion" in English is from 1670s.

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biotin (n.)
vitamin of the B group (also sometimes called vitamin H) essential for the growth of yeast, 1936, from German Biotin (1936), from Greek biotos "life" (variant of bios, from PIE root *gwei- "to live") + chemical suffix -in (2).
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*bhreu- 
also *bhreuə-, *bhreəu-, Proto-Indo-European root meaning "to boil, bubble, effervesce, burn," with derivatives referring to cooking and brewing.

It forms all or part of: barm; barmy; bourn (n.1) "small stream;" braise; bratwurst; brawn; brawny; braze (v.1) "to expose to the action of fire;" brazier; Brazil; bread; breed; brew; broth; broil (v.2) "to quarrel, brawl;" brood; effervesce; effervescence; effervescent; embroil; ferment; fervent; fervid; fervor; imbroglio.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit bhurnih "violent, passionate;" Greek phrear "well, spring, cistern;" Latin fervere "to boil, foam," Thracian Greek brytos "fermented liquor made from barley;" Russian bruja "current;" Old Irish bruth "heat;" Old English breowan "to brew," beorma "yeast;" Old High German brato "roast meat."
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