Usage examples for fermenting:

• Here in Jacksonville the agitation of the slavery question was frowned upon; but it was fermenting under the surface of southern sentiment. "Children of the Market Place" – Edgar Lee Masters

• Since Luther's return to Wittenberg a democratic agitation had been fermenting amongst the people. "Pictures of German Life in the XVth XVIth and XVIIth Centuries, Vol. I." – Gustav Freytag