Results and discussion
3.1. Substrate inhibition on cell growth
The influence of substrate content on the specific growth rate of K. oxytoca ΔldhA mutant was investigated by batch fermentations in which the medium contained glucose as the sole carbon source in initial concentrations ranging from 0 to 1000 mM. The values of parameters estimated are listed in Table 1. The specific growth rate was determined by plotting ln(XX) versus time (tt) using the data obtained during the exponential growth phase. The glucose saturation constant (KSKS = 3.318 g L−1) and maximum specific cell growth rate (μmμm = 0.985 h−1) in Eq. (2) were estimated by plotting 1/μ1/μ versus 1/S1/S (r2 = 0.9998) in the Monod equation at low glucose concentration range (0–100 mM). The degree of glucose inhibition (n = 0.650) was estimated by curve fitting the experimental data to Eq. (2) using the least squares method (r2 = 0.940). The glucose concentration where no growth occurs (S*S* = 256 g L−1) was determined by plotting the specific growth rate versus glucose concentration.
As shown in Fig. 2, the specific cell growth rate increased with increasing initial glucose concentration up to a certain point (S = 32 g L−1) where μ=μmμ=μm was observed. Above this concentration, a gradual decrease in the specific cell growth rate was observed. In most fermentation processes, the cells serve as a catalyst for the production of a target product, and therefore, a higher cell concentration leads to a higher overall production rate (g L−1 h−1). It is therefore important to culture a high amount of cells with high specific growth rate by maintaining the optimum substrate concentration in the fermentation broth.