Detoxification Step of Citrus Pulp Hydrolysate Directs Clostridium Beijerinckii Metabolism Toward Butanol or Lactic Acid

Abstract

This study evaluates the potential of citrus fruit pulp residue for obtaining butanol or lactic acid via fermentation by Clostridium beijerinckii. The citrus fruit pulp residue contains 44.4 ± 1.2% w w− 1 of total sugars and is recalcitrant because it has high lignin content (15.9 ± 0.9% w w− 1). Nevertheless, microwave-assisted pretreatment of this residue at 173 °C for 2.3 min in water recovered 63% of sugars, and subsequent enzymatic hydrolysis with Cellic CTec2 at 10 FPU g− 1 increased sugar recovery to 73%. Further detoxification of the hydrolysate with activated carbon at 2% (w/w) removed 83% of the fermentation inhibitors and caused only 4% of sugars to be lost. Fermentation assays demonstrated that the detoxification step was crucial to direct the fermentative product. Fermentation of the non-detoxified hydrolysate yielded up to 22.9 g L− 1 of lactic acid (yield 0.79 g g− 1), whereas fermentation of the detoxified hydrolysate yielded up to 5.3 g l− 1 of butanol (yield 0.2 g g− 1). Therefore, deciding whether to ferment the non-detoxified or the detoxified hydrolysate of citrus fruit pulp residue allows the final product to be selected on the basis of market demand and resource availability. This innovative approach reduces environmental impact by offering a flexible and sustainable alternative for reusing citrus fruit pulp.