Solventogenic Clostridium species have a biphasic metabolism composed of an acidogenic phase and a solventogenic phase.  During acidogenesis, these bacteria are able to convert several carbon sources into organic acids, commonly butyrate and acetate.  As acid accumulates, cells begin to assimilate the organic acids to solvents. In Clostridium acetobutylicum, a model solventogenic Clostridium species, a combination of low pH and high undisociated butyrate, referred to as the "pH-acid effect", triggers the metabolic shift from acidogenesis to solventogenesis.