The specific growth rate of Alcaligenes eutrophus ATCC 17697 was observed to depend upon the ratio of ammonium sulfate (nitrogen) and fructose (carbon) concentration used for production of poly-β-hydroxybutyric acid (PHB). The growth rate was stimulated at low concentration ratios and inhibited at higher ratios. A mathematical expression was then proposed to fit the substrate inhibition kinetics:
$$\mu = \mu _m \frac{S}{{K_s + S}}\left[ {1 - \left( {\frac{S}{{S_m }}} \right)^n } \right]$$
Together with the Luedeking-Piret product formation equation, the proposed model describes the experimental data as well as literature data, for substrate consumption, growth of A. eutrophus, and poly-β-hydroxybutyric acid formation.