The viscosifying power, degree of acetylation and molecular mass of the alginate produced by Azotobacter vinelandii in shake flasks are determined by the oxygen transfer rate

The objective of this study was to evaluate the influence of the specific power input and oxygen transfer rate (OTR) on changes in the alginate quality, in terms of the viscosifying power, molecular mass and degree of acetylation, of the polymer produced by Azotobacter vinelandii in shake flasks. Decreasing the shaking frequency from 200 to 100 rpm decreased the maximal volumetric power input (P/V)max from a maximal value of 1.5 kW m−3 to a minimal value of 0.23 kW m−3. As a result of the decrease in the maximal volumetric power input, the maximum oxygen transfer rate (OTRmax) decreased from 6 mmol L−1 h−1 to 2.6 mmol L−1 h−1 and the RQ (respiratory quotient) increased from 0.95 to 1.21 for the lower and higher power input, respectively. A lower RQ value was associated with the highest conversion value of carbon source (sucrose) to alginate. In contrast, at lower OTRmax values, culture broths with a higher viscosity were obtained. Thus, for the same alginate concentration (∼3.0 g L−1), the viscosity of cultures at an OTRmax value of 2.26 mmol L−1 h−1 was twice as high with respect to the viscosity observed in the culture broth obtained at a higher power input (OTRmax = 6.0 mmol L−1 h−1). In addition, under lower power input (lower OTR), the molecular mass and the degree of acetylation of the polymer were the highest. Additional experiments in a stirred bioreactor under constant DOT and agitation rate showed that under non-oxygen-limited conditions (5% of DOT), the alginate yield and the molecular mass of the polymer are determined by the OTR and are independent of the agitation rate of the culture.