Unlike DAP, dilute alkaline pretreatment, and wet oxidation pretreatment, SPORL was proved to be efficient for softwood species. Zhu et al. [109] investigated the combination of a sulfite treatment with mechanical size reduction by disk refining to enhance enzymatic hydrolysis of SW. This study was the first to establish this novel pretreatment process. Pretreatment conditions of spruce chips (20 %, w/v) that produced optimal cellulose conversion during enzymatic hydrolysis (>90 %) were treatment with 8-10 wt% bisulfite and 1.8-3.7 wt% H2SO4 for 30 min at 180 °C. Nearly all hemicellulose was removed, which exposed the underlying cellulose frac­tion to enzymatic attack. Additionally, furfural and HMF were produced in minimal concentrations, about 1 and 5 mg/g untreated wood, respectively. In addition, similar results were also observed with Lodgepole pine and red pine [109, 120].