GB799752A - Citric acid production - Google Patents

Abstract

Citric acid is obtained from an impure carbohydrate-containing material by deionizing an aqueous solution of the carbohydrate-containing material to reduce the ion concentration thereof to the equivalent of from 5 to 10 parts per million of sodium chloride as measured by conductivity methods, acidifying the de-ionized solution to pH 2.2 to 2.8, fermenting the solution with citric acid producing fungi under submerged fermentation conditions, removing the fungi by filtration to yield a citric acid containing liquor, heating the liquor to concentrate the citric acid therein by evaporative removal of water, and cooling the concentrated liquor to crystallize solid citric acid. The fermentation medium may be derived from invert molasses, corn starch hydrolysate or beet molasses and is suitably an aqueous solution of carbohydrate material in an amount approximating to about 10 to 15 per cent of sucrose, dextrose, levulose or glucose. The impure carbohydrate solution may be submitted to a preliminary treatment for removal of impurities by the addition of lime or an insoluble hydroxide of a heavy metal and the deionization may be effected by subjecting the impure solution to the action of a cation exchange resin, e.g. one operating on the hydrogen cycle, and this treatment should be continued until the Fe concentration is below 4 parts per million whilst other cations such as those of lead, mercury, copper and cobalt are simultaneously removed. In those cases where the total concentration of ions is high treatment with an anion exchange resin is also necessary in order to remove interfering quantities of anions but with purer carbohydrate sources a simple removal of cations by the use of a cation exchange resin may be sufficient. To such a purified carbohydrate medium various nutrient salts are added in the following amounts: 0.15 to 0.2 per cent of ammonium carbonate, 0.01 to 0.02 per cent of potassium acid phosphate, 0.08 to 0.15 per cent of hydrated magnesium sulphate and 0.0002 to 0.0004 per cent of zinc in the form of zinc chloride or other soluble zinc salt. This medium is then brought to a pH of 2.2 to 2.8 by the addition of hydrochloric acid and is then introduced into a fermenter the contents of which are sterilized by bringing the temperature of the medium to 180 DEG F. for two hours. After allowing the medium to cool to about room temperature any fungi capable of producing citric acid are added in concentrations of about 5 million to about 25 million spores per litre of fermentation medium, a vast preponderance of the spores being disposed beneath the surface of the medium. Best results are obtained by using selected mutants and wild strains of Aspergillus niger. Sterile air is then introduced into the inoculated medium, between one tenth volume and one volume of air per volume of medium per minute being generally employed. Anti-foam agent is added as necessary and the temperature is maintained at 24 DEG to 34 DEG C. during the fermentation, 8 to 12 days being generally required to complete the fermentation. On completion of the fermentation the contents are withdrawn and the mycelium which has grown from the spores is removed by filtration which optimumly will include a clarifying filtration using a commercially available activated carbon. The filtered liquor which normally contains about 10 to 12 per cent of citric acid is then concentrated by evaporation at elevated temperature and preferably under reduced pressure, until its volume is reduced to about one tenth to one fourteenth of its volume after fermentation and on cooling and stirring the citric acid will crystallize and can be removed by centrifugation or filtration. The recovered crystals may be further purified by recrystallization by cooling a hot saturated aqueous solution thereof. Examples are given for the crystallization of citric acid from fermented liquors prepared from a corn sugar solution and from a beet molasses fermentation liquor, the carbohydrate medium being first deionized and acidified to a pH of 2.2 to 2.8 as above.