Citric acid can be extracted from fruits such as lemons and oranges, as well as synthesized from oxaloacetic acid and ethylene ketone. However, the industrial production methods currently used domestically and internationally are all fermentation methods. There are solid fermentation, shallow plate fermentation, and deep fermentation processes in fermentation methods. More than 80% of citric acid in China is produced from dried potato flour through deep fermentation by Aspergillus niger, neutralization with calcium carbonate, sulfuric acid hydrolysis, refining, concentration, crystallization, and drying.
(1) Fermentation involves mixing dried potato flour into a slurry of 18% to 22%, which is then sent to a steam sterilized (0.2MPa, 40-60min) fermentation tank. Heat with steam to 110 ℃ and sterilize for 20 minutes, then liquefy with 0.05% amylase. After cooling to 35 ℃, inoculate with Aspergillus niger for fermentation. The fermentation temperature is 36-38 ℃ in the early stage and 34-36 ℃ in the later stage; The ventilation rate is 1: (0.1-0.2); The pH value shall not be lower than 2.5; The fermentation cycle is 96 hours. The acid production level is 10% to 14%, and the conversion rate is 95%.
(2) After the neutralization fermentation is completed, heat the fermentation broth to 80 ℃ to eliminate mold, and then send it to the plate frame for pressure filtration to obtain clear liquid. Add calcium carbonate powder to the filtrate at 80 ℃ and neutralize it to a pH value of 5.0-5.5, and keep it warm for 0.5 hours. Then, after vacuum filtration, wash with 80 ℃ hot water to obtain calcium citrate.
(3) Acid hydrolysis and refining are carried out by continuously stirring, adding calcium citrate to water or dilute acid, then slowly adding sulfuric acid until the pH value reaches 1.8, heating to 90 ℃, and determining the endpoint using the double tube method. Then add 1% to 3% activated carbon with citric acid content, keep it warm for 0.5 hours, put it in a suction filter to dry, and wash the acid with 90 ℃ hot water until the residual acid is below 0.5%. The washing water (i.e. dilute acid) is used for the next acid hydrolysis. Finally, use 732 cationic resin for ion exchange to remove various cations.
(4) The concentrated, crystallized, and dried clear liquid after ion exchange contains approximately 15% to 20% citric acid. Concentrate under vacuum at 55-600 ° C to a relative density of 1.335-1.340 and place in a crystallization pot. Cool with frozen saltwater to below 36.6 ℃ to precipitate water crystals. After separating the crystals, wash with a small amount of distilled water until the sulfuric acid content is 200 × Below 10 inches, finally dry with hot air at 35 ℃ to obtain the finished product.
