Increasing final concentration of l-lactic acid via fed-batch fermentation by Bacillus aerolacticus BC-001

Abstract

Polylactic acid (PLA) now gains many interests due to its versatile applications. Nonetheless, cost competitiveness with the existing petroleum based plastic is the key issue in biobased plastic industry. One plausible way to reduce production cost of biobased plastic can be accomplished by low cost feedstocks. Therefore, high final product titer at the end of batch run is necessary. However, batch fermentation using high substrate concentration usually suffers from low yield and productivity due to substrate repression. In this work, we tested for the ability of Bacillus aerolacticus BC-001 to ferment glucose at high concentration up to 200 g/L and study using fed-batch fermentation in both flask and 5 L stirred fermentation cultures. In flask, the results revealed that B. aerolacticus BC-001 was able to ferment glucose at 200 g/L at the close yield and productivity to those obtained from 100 g/L glucose. Moreover, final lactate titer be increased up to 192 g/L with the acceptable yield of 0.90 g/g and productivity of 3.55 g/L×h by intermittent fed-batch fermentation. In case of fermentation in 5 L fermentor, via batch fermentation with sole glucose solution, 104.24 g/L lactate titer with the productivity and yield of 5.79 g/L×h and 0.85 g/g respectively was obtained. When increasing the initial glucose concentration to 125-200 g/L, substrate repression was appeared. To solve this problem, different fed-batch fermentationห were conducted. The most effective feeding program was intermittent fed-batch fermentation. The final lactate titer of 145.08 g/L with productivity of 3.72 g/L×h and yield of 0.95 g/g was obtained. Moreover, adding glucose solution supplemented with (NH4)2SO4 at the C/N ratio of 40:1 was able to improve final lactate titer to 160.84 g/L with 3.58 g/L×h and 1.02 g/g. Therefore, compared with typical batch fermentation, intermittent fed-batch fermentation with glucose solution supplemented with (NH4)2SO4 at the proper C/N ratio produced higher final lactate concentration for 54.30% with the increasing yield by 11.76%. Most importantly, this technique could prolong high production rate at 11.70 g/L×h until 18 h fermentation.