Process for improving the separation efficiency of residual solid matter from the liquid phase of an aqueous acid hydrolysate of a naturally occurring polysaccharide comprising dissolved sugars, and residual acid wherein a flocculating agent(s) is added to the aqueous mixture in an effective amount, and a process of producing fermentation products comprising the steps of, (i) hydrolysing a particulate polysaccharide based plant derived material in an acid medium, and thereby forming an aqueous mixture comprising dissolved sugar and solid matter, (ii) subjecting the aqueous mixture to one or more separation stages in which solid matter are removed from the aqueous phase, (iii) adjusting the pH of the obtained aqueous phase to a pH of at least 4, (iv) fermenting the dissolved sugars of the aqueous phase by a microorganism to produce a fermentation product, (v) isolating the fermentation product, wherein in at least one separation stage in step (ii) a flocculating agent is added to the aqueous mixture in an effective amount.