Development of a continuous fermentation system of lactic acid production for polylactide monomer

Abstract

The production of organic acids at low cost by fermentation makes available an alternative route of chemical feedstock from biomass. Because of the multiple functional groups and high reactivity, organic acids can be useful in carrying out a variety of chemical reactions. Lactic acid, in particular, is considered as the most promising candidate of alternative chemical feedstock for catalytic conversions. However, regarding current selling price of lactic acid (~ 2.5 $ per kg of 85% lactic acid) produced by fermentation, the use of lactic acid derivatives in commodity applications faces with some obstacles including high raw material cost and low production efficiency. Thus, the objectives in this study are to tackle the impediments by development of cheaper fermentation media and highly productive continuous bioreactor. As an effort of replacing yeast extract, the most effective but expensive nutrient source, soybean media were investigated in batch cultures of Lactobacillus rhamnosus. An enzyme-hydrolysate of soybean meal, Soytone, with an adequate supplementation of vitamins was found to be highly effective in supporting lactic acid production from glucose and lactose. The effects of seven selected vitamins: d-biotin, pyridoxine, p-aminobenzoic acid, nicotinic acid, thiamine, pantothenic acid, and riboflavin, on cell growth and lactic acid production were investigated to provide the basis for the optimization of vitamin supplementation to minimize the cost. Pantothenic acid was the most required compound while the other six vitamins were also essential for high lactic acid productivity. As a result of the optimization, 15 g/L yeast extract could be successfully replaced with 19.3 g/L Soytone supplemented with the vitamins resulting in a production of 125 g/L lactic acid from 150 g/L glucose. The volumetric productivity and lactate yield were 2.27 gL-1h-1 and 92 %, respectively, which were higher than those with 15 g/L yeast extract. The raw material cost was ...