NOVELTY - Production of a fermentation product involves hydrolysis of an acidified suspension of particulate plant derived material to form a mixture; separating the mixture; optionally washing the residue, followed by passing an aqueous liquor to produce a fermentation product; and separating the fermentation product. The separation stages are assisted by flocculation of the solid by-product, and employing at least one flocculating agent.    USE - For the production of a fermentation product e.g. ethanol, glycerol, acetone, n-butanol, butanediol, isopropanol, butyric acid, methane, citric acid, fumaric acid, lactic acid, propionic acid, succinic acid, itaconic acid, acetic acid, acetaldehyde, 3-hydroxypropionic acid, glyconic acid, tartaric acid and amino acids (such as L-glutaric acid, L-lysine, L-aspartic acid, L-tryptophan, L-arylglycines or their salts) (all claimed).    ADVANTAGE - The use of special flocculation process in the separation stage gives a consistent high yield of fermentation product. The sugar solution resulting from the first hydrolysis stage is free from extraneous solid material such as cellulosic fibers. The method provides higher cake solids, with less trapped residual aqueous liquor, i.e. a higher proportion of the sugar liquor is available for conversion into the fermentation product. The method requires less wash water. The hydrolysate liquor can be separated particularly rapidly when the flocculation is effected by employing a water-soluble or water-swellable polymer and a charged microparticulate particle. The method achieves fast and efficient separation of solids, in which flocculation is effected by introducing a cationic polymer into the mixture and then reflocculating by adding an anionic microparticulate material.    DETAILED DESCRIPTION - Production of a fermentation product involves:    (1) forming an acidified suspension of particulate plant derived material comprising a first polysaccharide (a) which is more readily hydrolysable and a second polysaccharide (b) which is more difficult to hydrolyze;    (2) allowing (a) to undergo hydrolysis by action of the acid at a temperature of at least 50degreesC under conditions such that (a) is hydrolyzed and forming a mixture of an aqueous liquor containing dissolved sugar and a solid residue containing (b);    (3) subjecting the mixture to at least one separation stage in which the solid residue and aqueous sugar liquor are separated from each other;    (4) optionally washing the residue free of acid and sugar;    (5) adjusting the pH of the aqueous liquor to at least 4;    (6) passing the aqueous liquor from step (iv) into a fermentation stage where the dissolved sugars are acted upon by a microorganism in a fermentation broth to produce a fermentation product;    (7) contacting (b) by an enzyme. The enzyme hydrolyzes (b) to the component sugars, and allowing the component sugar to be acted upon by a microorganism in the fermentation broth to produce the fermentation product; and    (8) separating the fermentation product from the broth.    The separation stage(s) in the step (iii) is/are assisted by flocculation of the solid by-product, and employing at least one flocculating agent(s) selected from water-soluble polymer, water swellable polymer or charged microparticulate material.