ferment

Short definition from Wiktionary

Verb

1. 1
   To react, using fermentation; especially to produce alcohol by aging or by allowing yeast to act on sugars; to brew.
2. 2
   To stir up, agitate, cause unrest or excitement in.

Noun

1. 1
   Something, such as a yeast or barm, that causes fermentation.
2. 2
   A state of agitation or of turbulent change.
3. 3
   A gentle internal motion of the constituent parts of a fluid; fermentation.
4. 4
   A catalyst.