Biobutanol production using unhydrolyzed waste acorn as a novel substrate

Abstract

Substrate cost and availability are the major bottlenecks that limit the commercialization of next generation biofuels. The possibility of using waste acorn as an abundant and inexpensive feedstock in Iran for acetone–butanol–ethanol (ABE) production with Clostridium acetobutylicum was investigated. No solvent was produced using untreated acorn due to its high contents of growth-inhibitory tannic acid. To circumvent this problem, a simple extraction in close-to-boiling water was employed to remove tannic acid prior to using acorn powder in fermentation. This pretreatment significantly improved the fermentability of acorn powder by C. acetobutylicum. The maximum yields of solvents (g ABE per g substrate) achieved in serum-bottle experiments were 0.43 ± 0.02, 0.40 ± 0.02, and 0.34 ± 0.02 for glucose, pure starch, and treated acorn starch, respectively. Volumetric ABE productivity with tannin-free acorn powder was 0.21 ± 0.01 g L⁻¹ h⁻¹, slightly higher than that achieved with pure starch (0.18 ± 0.05 g L⁻¹ h⁻¹). When scaled up in 5 L fermentors, ABE production using pretreated acorn powder and pure starch resulted in solvent profiles and yields similar to those observed in the serum bottle experiments. This study demonstrates that acorn powder can be a viable feedstock for biobutanol production in Iran. Future work warrants optimization of a two-step process including extraction of tannic acid as a valuable by-product followed by fermentation of tannin-free acorn powder to improve the economic feasibility of acorn to biobutanol conversion process.