Improvement of l-lactic acid fermentation by Lactobacillus rhamnosus immobilized in polyvinyl alcohol/Fe3O4 composite using sweet sorghum juice

Abstract

To improve the utilization of sweet sorghum juice (SSJ) in L-lactic acid fermentation, an efficient producer of L-lactic acid with high optical purity was immobilized in the matrix including polyvinyl alcohol (PVA) and Fe3O4 magnetic nanoparticles (MNPs). In addition, the Lactobacillus rhamnosus LA-04-1 (L. rhamnosus LA-04-1) used was an invertase-active strain, and the direct conversion of sucrose which accounted for 80.14% of the total sugar from SSJ was facilitated. Fe3O4 MNPs with the size of 100 nm was obtained, and the co-immobilization of free cells and Fe3O4 MNPs was confirmed. Excellent properties of PVA/Fe3O4/LAB composite (PFLC) in mass transfer and structural stability were achieved with 10% (w/v) PVA, and stable fermentation performances were obtained within 3 freeze-thaw cycles. For batch fermentations, the productivity from SSJ was increased by 39.36% comparing with that of mixed sugars. Productivity was further increased by 3.25 times in the 12-cycle repeated-batch fermentation, during which the titer and yield of L-lactic acid reached 113.6 g L−1 and 98.97%, and the in situ separation of immobilized cells and simultaneous metabolism of fructose, glucose and sucrose were achieved. The results indicate a potential way for improving L-lactic acid fermentation by adopting PFLC and using low-cost renewable substrates.