During the production of a product compound by fermentation, the concentration of a precursor compound is maintained within a pre-selected concentration range by having an adsorbent resin in contact with the culture medium. The adsorbent resin reversibly adsorbs precursor compound and, as un-adsorbed precursor compound is converted to product compound, adsorbed precursor compound is released from the resin, thus maintaining the concentration of precursor compound within the pre-selected range.