Comparison of different in situ product-removal methods to enhance the production of the aroma compound 2-phenylethanol

There is an increasing interest by the flavor industry for producing natural aroma compds. through biocatalysts. L-Phenylalanine is transformed by Saccharomyces cerevisiae into 2-phenylethanol, which has a rose-like odor. Unfortunately this product inhibits growth even at low concns. and therefore its max. concn. is 3.9 g/l. This problem of the inhibitory effect of the product on the cell can be tackled by different in-situ product removal (ISPR) techniques. The hydrophobicity of 2-phenylethanol differs from those of the precursor and culture medium. Therefore, extn. into a second org. phase seems to be an appropriate ISPR-technique. This method keeps the 2-phenylethanol concn. in the aq. reaction phase below an inhibitory level in order to sustain a high productivity of the system. In this work, three different integrated processes involving the solvent extn. of 2-phenylethanol from the fermn. broth are evaluated. The first case is a two-phase fermn., where a water-immiscible fatty acid, which shows neither a mol. nor a phase toxicity, is added directly into the reactor vessel. The inhibitory 2-phenylethanol preferably partitions in the org. phase and therefore its concn. in the aq. phase is reduced. In this way, the final concn. of 2-phenylethanol is increased up to 9.1 g/l. The second approach uses an ester as second water-immiscible phase that has more a favorable partitioning for 2-phenylethanol but reveals a phase toxicity towards the cells. Thus the yeast is immobilized in chitosan-alginate beads to protect them from the toxic extractant. The third integrated bioprocess, completely novel, uses alginate capsules that contain the extractant in the core. The capsules are added to the fermn. broth and ext. the inhibitory 2-phenylethanol while it is produced. In this way the encapsulated solvent is not in direct contact with the cells and can prevent the cells from the phase toxicity. Conclusion: Solvents that do not show a phase toxicity can be added directly into the reactor vessel, whereas solvents having a phase toxicity can be encapsulated in alginate capsules which prevent the direct contact of the cells with the solvent. [on SciFinder (R)]