SEM micrographs of biomass particle and cell wall surfaces. (a,a’) Control, (b,b’) ZC, (c,c’) SG, and (d,d’) HS. The left column reinforces the particle size reduction and clumping seen at lower resolution in the stereo micrographs. The right column shows changes in surface roughness of the biomass cell walls caused by pretreatment. Surface roughness measurements were calculated as the standard deviation of greyscale values within six selected regions of interest within three different SEM micrographs (orange square, inset). (e) The mean standard deviation of the greyscale values and standard deviation among the 18 surface roughness values is reported. HS, horizontal screw; SEM, scanning electron microscopy; SG, steam gun; ZC, ZipperClave®.