PURPOSE: To obtain a 2-ketopolyhydroxy-6C-carboxylic acid, useful for producing vitamin C, simply and profitably without producing a by-product salt by subjecting a waste fermentation liq. after the removal of a biomass and calcium ions to extraction with a higher alkanol in the presence of an amine under specified conditions.

CONSTITUTION: A waste fermentation liq. is used. For instance, calcium ions in the liq. are precipitated with dil. sulfuric acid. The liq. contg. no calcium ion is subjected to extraction in the presence of an amine (e.g. trioctylamine or trilaurylamine) with a 4-8C alkanol (esp. methylisobutylcarbinol) in an amt. of 1.5-8 pts.wt., pref. 2.5-5 pts.wt., (based on 1 pt.wt. amine) at 10-35°C and then to decomposition, etc. Thus is obtd. the objective compd. A suitable amt. of the amine used is 0.1-2.5 mol. based on 1 mol. of the 2-ketopolyhydroxy-6C- carboxylic acid.