Abstract

Background: Rapid depletion in petroleum resources coupled with environmental pollution from the burning of coal and oil has led to the search for alternative energy sources. Lignocellulostarch biomasses (LCSBs) comprising peels of root crops and vegetables causes environmental hazards due to their non-judicious disposal. They are highly recalcitrant to breakdown necessitating effective pretreatment to exploit their use as bioethanol feedstock. Although microwave (MW)- assisted chemical pretreatment has emerged as a cost-effective process for lignocellulosic biomass, there is no information on its efficacy in deconstructing LCSBs that also contain appreciable quantities of starch.

Methods: Response surface methodology was adopted to study the impact of three parameters such as MW irradiation power, dilute sulphuric acid (DSA) concentration and irradiation time on the polysaccharide (cellulose, hemicellulose and starch) and lignin changes as well as the reducing sugar (RS) recovery after pretreatment. The optimized system was saccharified using triple enzyme cocktail to assess the RS recovery and saccharification kinetics.

Results: Microwave power had the greatest influence in deconstructing the LCSBs and maximum quantities of hemicellulose (88%), followed by cellulose (65%), starch (42%) and lignin (32-42%) were removed under RSM optimized conditions (600 W; 0.1M DSA; 7 min irradiation time). Pretreatment Efficiency ranged from 52-62% indicating high RS release from the LCSBs. The residues after optimized pretreatment when saccharified using triple enzyme cocktail containing cellulase, xylanase and Stargen (starch hydrolyzing enzyme complex) for 120 h resulted in very high RS recovery (57-71 g L-1).

Conclusion: The Overall Conversion Efficiency of carbohydrates to RS was very high (85-96%), indicating that the MW-assisted DSA pretreatment followed by triple enzyme-based saccharification system is an effective strategy for enhancing fermentable sugar yield from lignocellulo-starch biomass.

Keywords: Dilute acid pretreatment, lignocellulo-starch biomass, microwave, overall conversion efficiency, polysaccharide changes, saccharification.

Graphical Abstract