A comparative study on the enzymatic digestibility of steam exploded and AFEX-treated sugarcane bagasse and harvest residues

Steam explosion (STEX) and ammonia fiber expansion (AFEX^TM) are two leading pretreatment technologies considered for improving the efficacy of enzymatic hydrolysis and microbial fermentation of sugarcane bagasse and harvest residues from South African sugar mills. In this study, we compared the performance of these two pretreatment technologies for their ability to maximize sugar yields from these agricultural crops, using some of the most efficient commercial enzyme cocktails currently available. 

Pretreatment conditions for STEX and AFEX^TM were optimized for sugarcane bagasse and harvest residues at fixed enzyme loadings, in view of maximizing sugar yields from an ensuing enzymatic hydrolysis step. The optimally pretreated bagasse and harvest residues were thereafter used to determine optimum mixtures of commercial enzyme preparations (Cellic® CTec3, Cellic® HTec3, Pectinex Ultra Mesh) for each substrate using a mixture optimization design. Using optimal enzyme combinations, the minimum enzyme dosage (mg protein/g glucan) required to reach a minimum total sugar yield of 80% from high solids loading (6% glucan loading) enzymatic hydrolysis was established. A subsequent comparison of the enzyme requirements for high solids loading enzymatic hydrolysis was used to determine the effectiveness of the two pretreatment technologies for enhancing the digestibility of both sugarcane bagasse and harvest residues.

These comprehensive mass balances stemming from the optimized pretreatment conditions and optimized enzymatic hydrolysis of sugarcane bagasse and harvest residues will help focus attention on necessary process improvements in view of reducing biofuel production costs in a South African context.