Abstract

Lignocellulosic and algal biomass are promising substrates for lactic acid (LA) production. However, lack of xylose utilization and/or sequential utilization of mixed-sugars (carbon catabolite repression, CCR) from biomass hydrolysates by most microorganisms limits achievable titers, yields, and productivities for economical industry-scale production. This study aimed to design lignocellulose-derived substrates for efficient LA production by a thermophilic, xylose-utilizing, and inhibitor-resistant Bacillus coagulans Azu-10. This strain produced 102.2 g/L of LA from 104 g/L xylose at a yield of 1.0 g/g and productivity of 3.18 g/L/h. The CCR effect and LA production were investigated using different mixtures of glucose (G), cellobiose (C), and/or xylose (X). Strain Azu-10 has efficiently co-utilized GX and CX mixture without CCR; however, total substrate concentration (>75 g/L) was the only limiting factor. The strain completely consumed GX and CX mixture and homoferemnatively produced LA up to 76.9 g/L. On the other hand, fermentation with GC mixture exhibited obvious CCR where both glucose concentration (>25 g/L) and total sugar concentration (>50 g/L) were the limiting factors. A maximum LA production of 50.3 g/L was produced from GC mixture with a yield of 0.93 g/g and productivity of 2.09 g/L/h. Batch fermentation of GCX mixture achieved a maximum LA concentration of 62.7 g/L at LA yield of 0.962 g/g and productivity of 1.3 g/L/h. Fermentation of GX and CX mixture was the best biomass for LA production. Fed-batch fermentation with GX mixture achieved LA production of 83.6 g/L at a yield of 0.895 g/g and productivity of 1.39 g/L/h.

Highlights

  • Biomass materials are promising carbon sources for the production of various valueadded products

  • Modified de Man, Rogosa, and Sharpe medium supplemented with xylose instead of glucose was used for bacterial growth and inoculum preparation. mMRS is composed of g/L. Total consumed sugars (g/L): xylose, 22.0; yeast extract, 5.0; peptone, 10.0; beef extract, 8.0; K2 HPO4, 2.0; MgSO4, 0.1; MnSO4, 0.05; sodium acetate, 5.0; ammonium citrate, 2.0, and tween 80, 1.0 mL

  • Biomass hydrolysates are composed of a mixture of sugars; co-utilization of various sugars is essential for economically feasible lactic acid (LA) fermentation processes

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Summary

Introduction

Biomass materials are promising carbon sources for the production of various valueadded products Among these products, lactic acid (LA) is an essential organic acid that provides a broad range of applications in food, cosmetic, pharmaceutical, and chemical conversion industries [1]. Lactic acid (LA) is an essential organic acid that provides a broad range of applications in food, cosmetic, pharmaceutical, and chemical conversion industries [1] It is an important monomer for the production of polylactic acid (PLA) that has the potential to serve as promising biodegradable commodity plastics used for food packaging, textiles, trays, plates, and trash bags [2]. The main expenses lie within the efficient fermentation processes (either microbial strain or utilized substrate) or the downstream processes of LA production [4]

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