Achieving high enzymatic hydrolysis sugar yield of sodium hydroxide-pretreated wheat straw with a low cellulase dosage by adding sulfomethylated tannic acid

•Sulfomethylated tannic acids (STA) with different sulfonation degrees were prepared.•STA was used as additive to improve enzymatic hydrolysis of pretreated wheat straw.•The highest yield of glucose reached 97.9% by adding an appropriate amount of STA.•Free protein in hydrolysate increased significantly with STA addition.•Hydrolysis efficiency was closely related to the sulfonation degree of STA.