Process for isolation of 2-keto-L-gulonic acid

Conversion of the sodium salt of 2-keto-L-gulonic acid, present in an aqueous fermentation solution, into an alcoholic solution of the free acid and optionally into an alkyl ester of the acid, comprises: (a) recovering sodium 2-keto-L- gulonate-monohydrate by evaporation, cooling or displacement crystallisation and optionally comminuting the crystalline material by milling; (b1) suspending the recovered sodium 2-keto-L- gulonate-monohydrate from (a) in a low alcohol, allowing the crystals to swell and then adding an acid (I) with a low water content such that the measured pH is > 1.5; (b2) incorporating the recovered sodium -2-keto-L-gulonate-monohydrate from (a) together with a stoichiometric amount of an acid (I) with a low water content into a low alcohol using a wet milling procedure, the measured pH being > 1.5, or (b3) a combination of steps (b1) and (b2) with recycling of the materials streams; (c) separating the salt of the acid (I) formed in stage (b1),(b2) or (b3) to yield an alcoholic solution of the 2-keto-L-gulonic acid, and optionally (d) treating the alcoholic solution of the 2-keto-L-gulonic acid formed in (c) with a catalytic amount of an acid or an acid ion exchanger to esterify the 2-keto-L-gulonic acid with the alcohol to form the corresponding 2-keto-L-gulonic acid low alkyl ester. Stage (a) is preferably accomplished by continuous evaporative crystallisation. The low alcohol used in stage (b) is e.g. methanol, ethanol, propanol or glycol, especially methanol. The acid (I) is e.g. concentrated sulphuric acid, phosphoric acid, hydrochloric acid or nitric acid or gaseous HCl, and especially > 95 % H2SO4. The pH in stage (b) is 2.5-3.5. The temperature in stage (b) is 20-70 degrees C. The salt formed during stage (b) is removed e.g. by filtration and/or centrifuging, especially centrifuging.