Effect of pretreatment using ammonia water combined with freezing and thawing on enzymatic hydrolysis and sugar production of sorghum straw

Objective  To reduce the cost of pretreatment and improve the enzymatic hydrolysis effect afterstraw pretreatment, sorghum straw was pretreated with simulated natural low temperature environment andammonia water.

Method  We studied the effects of the liquid-solid ratio in soaking solution, freezingtemperature, freezing time and ammonia content in pretreatment using ammonia water combined withfreezing and thawing on enzymatic hydrolysis of sorghum straw through single factor tests. We optimized the pretreatment conditions using orthogonal test design. The compositions of sorghum straw before and after pretreatment were measured using normal form method, and the physical and chemical structures were investigated using infrared spectrum and X-ray diffraction analyses.

Result  In single factor tests, liquid-solid ratio in soaking solution, freezing temperature, freezing time and ammonia content at different levels all significantly increased the production of reducing sugar through enzymatic digestion(P<0.05). The optimum pretreatment conditions of the orthogonal test were 12 liquid-solid ratio in soaking solution, 12 h freezing time, 10 ℃ freezing temperature, and ammonia content of 8%. Compared with straw without pretreatment, in straw with pretreatment using ammonia water combined with freezing and thawing, the hemicellulose content decreased by 42.42% , the lignin content decreased by 50.76%, the yield of reducing sugar for straw was 302.87 mg·g⁻¹, which was 80.34% higher than that of straw without pretreatment, and the crystallinity of cellulose increased by 57.02%.

Conclusion  The pretreatment using ammonia water combined with freezing and thawing can effectively destroy the original connection structure between lignocellulose of sorghum straw, dissolve hemicellulose, destroy the monomer and polymeric structure of lignin. It improves the yield of reducing sugar by enzymatic hydrolysis of sorghum straw, and also improves the crystallinity of sorghum straw cellulose.