A method is disclosed for reducing the mechanical strength of solid biomass material, in particular ligno-cellulosic biomass. The method comprises heating the solid biomass material to a temperature in the range of 105° C. to 200° C. The heat treatment, which is referred to as “toasting”, significantly reduces the mechanical energy required for reducing the particle size of the solid biomass material.The method is particularly suitable as a pretreatment step to a conversion reaction of the solid biomass material.