Calorimetrically recognized maximum yield of poly-3-hydroxybutyrate (PHB) continuously synthesized from toxic...

The broader usage of poly-beta-hydroxybutyrate (PHB), for instance as bulk plastics, calls for cheap raw materials and greater overall process efficiency. The bacterial synthesis is generally induced and promoted by the limitation of growth via nitrogen, oxygen or phosphate depletion with the simultaneous excess and higher concentration of the carbon substrate. Consequently, toxic substrates have been considered unsuitable for PHB synthesis. Nevertheless, a single-stage continuous process for producing PHB from toxic substrates using microorganisms was developed and is reported here. The maximum heat flux during continuous growth and the maximum yield of PHB versus the substrate consumption rate were found to coincide. This suggests the possibility of controlling the conversion of a growth-inhibiting substrate into PHB and maximizing the process efficiency. The observed correlation occurred irrespective of the substrates investigated (phenol or sodium benzoate), the PHB-producing strain (Ralstonia eutropha JMP 134 or Variovorax paradoxus JMP 116), or the type of limitation imposed. The maximum PHB yields obtained comprised up to 50% of cell dry mass.