Optimization for biopolymer production by Enterobacter cloacae WD7

Factors affecting the production of exopolysaccharide (EPS) from Enterobacter cloacae WD7 cultivated in basal medium (pH 7.0) using 1% w/v glucose as a carbon source for 5 days at 30 °C were investigated. The EPS yield of 2.23 g L−1 was obtained after 3 days cultivation. Among the carbon sources tested, 3% sucrose was selected and gave an EPS yield of 2.72 g L−1. This indicated that sucrose is a better carbon source than the glucose used previously. The addition of either inorganic nitrogen [(NH4)2SO4, NH4Cl and NH4NO3] or organic nitrogen (polypeptone) sources had no effect on the EPS yield. The optimum concentration of yeast extract was found to be 0.05% w/v. The optimum environmental conditions were: initial pH 7.0, temperature 30 °C, pH control at 7.0 and the aeration rate of 2.0 vvm which elevated the EPS yield to 4.80 g L−1 while further increase in the agitation speed (200 rpm) reduced the production of EPS. Comparison of the kinetic parameters of the shake-flask and batch fermentations revealed that batch fermenter culture gave about 2 times higher values of specific growth rate (μ = 0.15 and 0.29 h−1), product yields (Yp/s = 0.25 and 0.52 g EPS g sucrose−1) and maximum productivity (Rm = 0.04 and 0.07 g of EPS L−1 h−1). Substitution of the analytical grade sucrose by commercial sucrose resulted in 15% lower EPS yield (i.e. from 4.90 to 4.18 g L−1, respectively) but could lower the raw material cost by 98%, and therefore is a suitable component for the production medium. For continuous culture studies, the optimum dilution rate was 0.05 h−1 giving the maximum EPS concentration of 7.28 g L−1 which is 1.5 times higher than the batch culture and a total of 3.2 times higher than the beginning (2.23 g L−1). The critical dilution rate (Dc) was calculated to be 0.49 h−1 with the maximum dilution rate (Dm) of 0.485 h−1, Yx/s of 0.03 g cell/g sucrose and the Rm of 0.06 g EPS L−1 h−1. Batch culture therefore gave a slightly higher productivity and would be the method of choice for scale-up studies.