Time course of HMF and furfural concentrations in SSF of spruce with the strains investigated in the present study. Black squares: CEN.PK 113-7D, red circles: GSH1, blue triangles: GSH1/CYS3, green diamonds: GLR1, brown reversed triangles: GSH1/GLR1, orange stretched diamonds: GSH1/CYS3/GLR1. The filled and open symbols represent HMF and furfural, respectively. The data points are mean values from two independent cultivations. The error bars represent the maximum and minimum values.