Process for the conversion of the sodium salt of 2-keto-L-gulonic acid to the free acid

A process for the conversion of the sodium salt of 2-keto-L-gulonic acid, which is present in an aqueous fermentation solution, into an alcoholic solution of the free acid and, if desired, into an alkyl ester of the acid comprises a) recovering the sodium 2-keto-L-gulonate monohydrate from an aqueous fermentation solution by crystallization involving evaporation, cooling or displacement and, if desired, pulverizing the thus-obtained crystallizate by grinding, b1) suspending the optionally ground sodium 2-keto-L-gulonate monohydrate obtained in step a) in a lower alcohol, leaving the crystals to swell and thereafter adding an acid of low water content, whereby the measured pH value should lie above 1.5, or b2) adding the optionally ground sodium 2-keto-L-gulonate monohydrate obtained in step a) together with an about stoichiometric amount of an acid of low water content to a lower alcohol using a wet grinding system, whereby the measured pH value should lie above 1.5, or b3) carrying out a combination of steps b1) and b2) including recycling of product streams, and c) separating the salt of the added acid formed in step b1), b2) or b3) and thus obtaining an alcoholic solution of 2-keto-L-gulonic acid, and, if desired, d) treating the alcoholic solution of 2-keto-L-gulonic acid obtained in step c) with a catalytic amount of an acid or with an acidic cation exchanger in order to esterify the 2-keto-L-gulonic acid with the alcohol to give the corresponding lower alkyl 2-keto-L-gulonate.