Rhizopus oryzae for fumaric acid production: optimising the use of a synthetic lignocellulosic hydrolysate

The hydrolysis of lignocellulosic biomass opens an array of bioconversion possibilities for producing fuels and chemicals. 

Microbial fermentation is particularly suited to the conversion of sugar rich hydrolysates to biochemicals. Rhizopus oryzae ATCC 20344 was employed to produce
fumaric acid from glucose, xylose and a synthetic lignocellulosic hydrolysate (glucose-xylose mixture) in batch and continuous fermentations. 

A novel immobilised biomass reactor was used to investigate the co-fermentation of xylose and glucose. Ideal medium conditions and a substrate feed strategy were then employed to optimise for the production of fumaric acid. The batch fermentation of the synthetic hydrolysate at the optimal conditions (urea feed rate 0.625 mg L⁻¹ h⁻¹ and pH 4) produced a fumaric acid yield of 0.439 g g−1. A specific substrate feed rate (0.164 g L⁻¹ h⁻¹) which negated ethanol production and selected for fumaric acid was determined. Using this feed rate in a continuous fermentation a fumaric acid yield of 0.735 g g⁻¹ was achieved; a 67.4 % improvement. Metabolic analysis helped to determine a continuous synthetic lignocellulosic hydrolysate feed rate that selected for fumaric acid production, while achieving co-fermentation of glucose and xylose, avoiding the undesirable carbon catabolite repression. 

This work demonstrates the viability of fumaric acid production from lignocellulosic hydrolysate; the process developments discovered will pave the way for an industrially viable process.