The process for producing organic molecules from fermentable biomass includes a step of anaerobic fermentation (5) producing volatile fatty acids (6), these precursors being transformed into final organic molecules by non-fermentation means. It also includes at least the following steps: a) extracting (9) at least one portion of the volatile fatty acids from the fermentation medium in such a way that the production of fermentation metabolites by the microorganisms (M) is not affected, and introducing a portion of the liquid phase (11) containing microorganisms from the extraction (9), b) synthesizing (13) organic molecules from the fermentation metabolites or from the volatile fatty acids extracted in step a)-c) continuing steps a) to b) until the final molecules are obtained, in terms of amount and quality. The invention also relates to an installation for implementing the process.