Sequential optimization of the fermentation factors with integrating seed culture adaptation for increased biorefinery of beet molasses to lactic acid

Microbial bioconversion of agro-industrial wastes to green chemicals has become an important objective in industrial biotechnology for lowering the product costs. Therefore, in this work, beet molasses was used as a potential raw material for lactic acid (LA) production by a newly isolated bacterium, Enterococcus faecium S6. The pretreatment of molasses with EDTA has exhibited the highest effectiveness for bioconversion to LA. The optimal medium components and culture conditions were established. Supplementation of molasses by yeast extract (YE) and ammonium chloride have achieved LA production of 11.90 g/L at a yield of 0.71 g/g_{-consumed sugars} and a productivity of 0.33 g/L/h. Various approaches were used for the optimization of relevant factors affecting l-LA production in batch fermentations. Using classical “one-factor-at-a-time” approach, the optimal factors were as follows: sugar concentration, 40 g/L; pH, 7.0; 40 °C; and inoculum size, 10% (v/v) achieving LA at 29.7 g/L. The effects of these five fermentation parameters were further investigated by response surface methodology (RSM) to maximize LA production. The optimized fermentation conditions by RSM were as follows: sugar concentration, 65 g/L; YE, 0.625 g/L; temperature, 42 °C; pH, 6.75; and inoculum size, 9.5% (v/v) that achieved LA production of 41.5 g/L. Further improvements were achieved using repeated batch fermentation that has conducted for 16 runs. Using seed culture pre-adapted to the actual fermentation conditions in repeated batch fermentation led to enhanced LA production with a shorter process time compared to batch fermentation. These results achieved a maximum LA of 64.7 g/L with high yield (0.94 g/g of consumed sugars) and productivity (2.16 g/L/h). This study presented a cost-effective and long-term fermentative production of LA from beet molasses.