Enhanced Cellobiose Production from Cellulose by CaCl₂-Phosphoric Acid Pretreatment for the Efficient Preparation of Astragalin in Recombinant Escherichia coli

Cellulose, as the most abundant and cheap renewable resource in nature, is of great importance for its utilization. An enzymatic cellulose solution, mainly containing cellobiose and glucose, was utilized to produce astragalin instead of cellobiose in the recombinant strains. However, the crystalline structure of cellulose affects the production of cellobiose, resulting in a low astragalin yield. In this study, microcrystalline cellulose was pretreated with phosphoric acid and CaCl₂ to enhance the hydrolysis yield of cellobiose. By pretreating with CaCl₂-phosphoric acid, the crystallinity of cellulose significantly decreases, while the specific surface area increases, which leads to a significant increase in the production of cellobiose. When cellulose was treated with CaCl₂-phosphoric acid at 50 °C and hydrolyzed with 30 FPU/g cellulose, the maximum cellobiose production with yeast extract added reached 1.60 g/L, which was 200% of the control. The maximal astragalin production reached 2038.3 mg/L with the cellulose enzymatic solution. Finally, 100 g of cellulose can produce 34.67 g of glucose and 26.67 g of cellobiose. The recombinant strains produced a total of 25 g of astragalin from these sugars, which is 322% of that from the untreated cellulose. This work provides an economical and efficient method for the production of astragalin.