pH adjustment increases biofuel production from inhibitory switchgrass hydrolysates

Area of Science:

• Biomass Conversion
• Renewable Energy
• Microbial Fermentation

Background:

• Lignocellulosic biomass, like switchgrass, is a sustainable source for biofuels.
• Drought conditions increase inhibitory compounds in switchgrass, hindering microbial biofuel conversion.
• Engineered yeast (Saccharomyces cerevisiae) and Zymomonas mobilis show reduced ethanol production with drought-stressed switchgrass hydrolysates.

Purpose of the Study:

• To investigate methods for alleviating inhibition in drought-stressed switchgrass hydrolysates.
• To optimize fermentation conditions for improved biofuel production from challenging feedstocks.
• To identify the specific inhibitory mechanisms affecting microbial biofuel conversion.

Main Methods:

• Two pretreatment methods were used: Ammonia Fiber Expansion (AFEX) and Soaking in Aqueous Ammonia (SAA).
• Hydrolysates from different switchgrass conditions (drought vs. typical rainfall) were fermented.
• Fermentation performance was evaluated at different pH levels (5.0 and 5.8).
• A synthetic hydrolysate mimic was used to pinpoint inhibitor effects.

Main Results:

• Increasing pH to 5.8 enhanced fermentation rates and biofuel production for both yeast and Z. mobilis, across all feedstock types and pretreatments.
• Soaking in Aqueous Ammonia (SAA) pretreatment resulted in higher fermentation rates and biofuel yields compared to AFEX.
• pH-dependent inhibition by lignocellulose-derived compounds was identified as a key factor limiting biofuel production.

Conclusions:

• Adjusting hydrolysate pH to 5.8 effectively mitigates inhibition from drought-stressed switchgrass.
• SAA pretreatment followed by pH adjustment offers a significant improvement for biofuel production from recalcitrant biomass.
• These findings enable more efficient industrial microbial conversion of challenging lignocellulosic feedstocks.