Production of lactic acid by microbial fermentation of hemicellulose sugars

Abstract

Lactic acid (2-hydroxypropionic acid) is an organic acid which has wide applications in food feed, pharmaceutical industries as well as producing polylactic acid, which is a biodegradable material. Lactic acid is commercially produced by microbial fermentation from starchy materials. Converting pentoses from lignocelluloses into lactic acid by lactobacillus has received much attention in recent years.

The feasibility of improving lactic acid production from pentoses by using Lactobacillus pentosus (ATCC 8041) which is a facultative heterofermentative microbial was investigated. It has been shown that L. pentosus ferments glucose by the homofermentative Embden-Meyerhof-Parnas (EMP) pathway giving lactic acid as the sole product, but it metabolizes xylose and arabinose by the heterofermentative phosphoketolase (PK) pathway, producing equimolar amount of lactic acid and acetic acid.

Immobilization of L. pentosus into calcium alginate beads was performed to increase cell density and reusability. Alginate-silica hybrid beads were prepared to improve the rigidity of the ordinary calcium alginate beads. The lactic acid yield was almost same as the ordinary alginate beads although the rigidity and cell leakage were slightly improved by using the hybrid beads compared to the ordinary alginate beads.

Xylose isomerization was performed to improve the lactic acid production from xylose by L. pentosus. Xylose is converted into xylulose by the immobilized xylose isomerase which helps to promote the utilization of xylose by the L. pentosus. To facilitate the recycle and reuse of the immobilized xylose isomerase, the immobilized enzyme (65 g) was packed in a fixed bed reactor with a permeable wall. The lactic acid productivity and yield in the novel bioreactor was higher than that of the control in both 20g/L and 50g/L xylose concentrations. The cells grew much quicker and better in the novel bioreactor than in the control. The recyclability of the immobilized xylose isomerase was tested and we were able to reuse the enzyme for 5 more cycles without obvious reduction of lactic acid yield and productivity.