x1 ferments [undergoes fermentation] with fermenting agent x2.
Proposed combining form -faj-. For agentive 'x1 ferments substance x2', use farjygau (or even shorter, fajgau). This word was made to split the semantic spaces of 'ferment' (farje) and 'rot' (fusra); kukfusra 'delicious-rot' for 'ferment', while technically correct, is silly.