Emprego do glicerol como fonte de carbono na produção fermentativa de 2,3-Butanodiol por Enterobacter aerogenes ATCC 13048

Abstract

In this work, the use of glycerol as carbon source for cell growth and 2,3-butanediol/acetoin production in the in cultivation of Enterobacter aerogenes ATTT 13048 was studied. The influence of initial substrate concentration (S0) and oxygen transfer rate (OTR) was assessed in batch-mode experiments. Initially, cultivations with S0 of approximately 20 to 180 g/L, impeller speed of 500 rpm and specific air flow rate of 0.50 volume of air per volume of medium per minute (vvm), corresponding to characteristic OTR values between 10 and 13 mmol/L/h approximately, were performed. In the sequence, runs with S0 in the range 40 to 180 g/L, with varying impeller speed (650 to 750 rpm) and specific air rate (0.50 to 0.87 vvm) were carried out to maintain the dissolved oxygen concentration close to 30% of saturation as long as possible during the process. Characteristic OTR values for the conditions with the upper-limit impeller speed (750 rpm) and specific air flow rate (0.87 vvm) were about 34 and 24 mmol/L/h for the lowest and highest glycerol concentrations, respectively. In batch mode, for any OTR evaluated, S0 values up to 60 g/L did not result in growth inhibition of E. aerogenes, with maximum specific growth rates (μX,m) of 0.70 and 0.73 h-1 being measured for low and high OTR, respectively. On the other hand, an S0 of about 180 g/L led to a strong cell growth inhibition, especially under low OTR condition, and glycerol consumption was incomplete. In the group of runs performed with low OTR, the largest final 2,3-butanediol/acetoin final concentration (63.6 g/L) was achieved with a measured S0 of 146.4 g/L, but the highest yield was attained with S0 = 92.7 g/L. In the cultivations under high OTR, the maximum final product concentration (46.8 g/L) was obtained with S0 = 92.7 g/L, whereas the maximum yield of 70% was achieved with S0 = 102.5 g/L. Increasing OTR values favored productivity and cell yield but, however, resulted in decreasing conversion of glycerol to products. Afterwards, two fed-batch cultivations were done to avoid the strong inhibition due to high glycerol concentrations, and to allow that elevated concentrations of 2,3-butanediol/acetoin could be achieved. The first fed-batch run was carried out at the high-OTR conditions previously defined and the second one with impeller speed of 750 rpm and specific air flow rate of 0.50 vvm. The mass of glycerol employed corresponded to that needed to have an S0 of 240 g/L in batch-mode. With high OTR, a final product concentration of 90.4 g/L was quantified after 56 h of process, that means a yield of 83.2% and a productivity of 1.61 g/L/h,. In the experiment with 750 rpm and 0.50 vvm, 102.6 g/L of products were obtained after 74 h, with a yield of 94.7% and a productivity of 1.37 g/L/h. During the final hours of the fed-batch cultivations, increasing oxygen concentrations, decreasing oxygen uptake rate and incomplete glycerol consumption were observed, these data indicating the occurrence of inhibition of the bacterium respiratory activity by the high product concentrations which were present in the medium. The results allow concluding that glycerol is an adequate substrate for the production of 2,3-butanediol/acetoin by E. aerogenes.