An Etymological Dictionary of the German Language/Annotated/gären

by Friedrich Kluge

gären, verb, ‘to ferment, effervesce, bubble,’ a combination as to its form of a strong verb Middle High German gëren; jësen; Old High German jësan, ‘to ferment, foam,’ and the corresponding factitive Middle High German *jern (unrecorded, but Old High German jęrian occurs), ‘to cause to ferment’; Old High German jësan is a strong verb, and jęrjan a weak verb (compare ginësan strong verb, and ginęrian, weak verb). Noun derivatives of the Teutonic root jes retain their s (before t) even as late as Modern High German; see Gischt, under which the cognate nouns from the other Old Teutonic dialects are brought together. The root jes, yes, occurs also in Indian and Greek; compare Greek ζεσ-τός, ‘boiled,’ ζέσ-μα, hence also ζέω for *ζέσω (perfect ξεσ-μαὸ), ‘to boil, bubble’ (ζ for earlier j, y as in ζυγόν, see Joch), Sanscrit root yas, ‘to seethe, boil.’ Considering this agreement of forms with initial j and y, Modern High German gären with g is remarkable; so too Old Icelandic gerþ, ‘yeast’ (but English yeast).