Effect of post-treatment process of microalgal hydrolysate on bioethanol production

Gyeongho Seon,Minsik Kim,Jun Muk Cho,Yong Keun Chang,Won-Kun Park,Hee Su Kim

doi:10.1038/s41598-020-73816-4

Abstract

Microalgae accumulate abundant lipids and are a promising source for biodiesel. However, carbohydrates account for 40% of microalgal biomass, an important consideration when using them for the economically feasible production of biodiesel. In this study, different acid hydrolysis and post-treatment processing of Chlorella sp. ABC-001 was performed, and the effect of these different hydrolysates on bioethanol yield by Saccharomyces cerevisiae KL17 was evaluated. For hydrolysis using H2SO4, the neutralization using Ca(OH)2 led to a higher yield (0.43 g ethanol/g sugars) than NaOH (0.27 g ethanol/g sugars). Application of electrodialysis to the H2SO4 + NaOH hydrolysate increased the yield to 0.35 g ethanol/g sugars, and K+ supplementation further enhanced the yield to 0.41 g ethanol/g sugars. Hydrolysis using HNO3 led to the generation of reactive species. Neutralization using only NaOH yielded 0.02 g ethanol/g sugars, and electrodialysis provided only a slight enhancement (0.06 g ethanol/g sugars). However, lowering the levels of reactive species further increased the yield to 0.25 g ethanol/g sugars, and K+ supplementation increased the yield to 0.35 g ethanol/g sugars. Overall, hydrolysis using H2SO4 + Ca(OH)2 provided the highest ethanol yield, and the yield was almost same as from conventional medium. This research emphasizes the importance of post-treatment processing that is modified for the species or strains used for bioethanol fermentation.

Highlights

Microalgae accumulate abundant lipids and are a promising source for biodiesel
ED clearly increased cell growth at 11 h (1.80 g/L for H 2SO4 + NaOH + ED: 62.6% increase; 0.32 g/L for H NO3 + NaOH + ED: 321.6% increase). These results indicate that ED improved the fermentation of hydrolysates and that the HNO3 hydrolysate contained growth inhibitory compounds that were not removed by ED
The effect of a HNO3 hydrolysate that was stored at 4 °C for 1 month on fermentation was tested (Fig. 3b). This medium led to increased cell growth (0.32 vs. 1.64 g/L), sugar consumption (0.26 vs. 1.32 g/L/h), and ethanol yield (0.06 vs. 0.25 g ethanol/g fermentable sugar; Figs. 2 and 3b). These results suggest that the oxidative stress caused by reactive oxygen species (ROS) and reactive nitrogen species (RNS) was partly responsible for the inhibitory effect of HNO3 hydrolysate on fermentation

Summary

Introduction

Microalgae accumulate abundant lipids and are a promising source for biodiesel. carbohydrates account for 40% of microalgal biomass, an important consideration when using them for the economically feasible production of biodiesel. The chemical treatment of these carbohydrates to produce edible value-added carbohydrate products may produce undesirable contaminants due to the adverse effects of organic solvents[11,12] Utilization of these microalgal carbohydrates as a source for microbial fermentation medium has been limited to ethanol p roduction[13,14], there is a need for more progress in this area. In contrast to the lignocellulosic biomass from land plants, microalgae have no lignin (which requires harsh chemical conditions for hydrolysis) and generate little or no toxic compounds during hydrolysis[9,15] For this reason, hydrolysis of microalgal carbohydrates to produce fermentable sugars, and use of these sugars for fermentation by microorganisms to produce value-added products could be an effective approach for enhancing the economic feasibility of the microalgal biofuel industry. It is necessary to optimize the acid hydrolysis and post-hydrolysis treatments[18,19]

Methods

Results

Conclusion