Process for extracting volatile fatty acids (VFA), organic molecules called precursors produced by microorganisms (M) in a fermentation reactor (2) by anaerobic fermentation (3) using fermentable biomass (1), said molecules being fermentative metabolites, comprising at least the following steps: a) selecting an extraction means (8) from extraction means that is, at least, insoluble in the fermentation medium and the processing conditions of which preserve the capacity of the microorganisms (M) present in the fermentation medium to produce the molecules, b) bringing the chosen extraction means (8) into contact (9) with the fermentation medium without interrupting the fermentation (3), c) recovering (12) the extracted molecules, at a pH lower than 4.5, by the extraction means (8) outside the fermentation reactor (2). The invention also relates to an installation for implementing the method.