1. Culture of bacteria
The microbial fermentation method is used in the industrial production of citric acid, and only several kinds of Aspergillus and yeast are valuable. Among them, Aspergillus niger is the most competitive strain in the industry, and the most competitive yeasts are lipolytic pseudosaccharides. Silk yeast and saccharomyces cerevisiae, etc. Aspergillus niger is cultured on agar and forms limited colonies on the agar. After 10 to 14 days at room temperature, it becomes a rich and dense sporozoite, and the colonies are black and sometimes dark brown. Considering that the citric acid-producing bacteria should have the characteristics of strong acid-producing ability and high citric acid concentration, the acid filter paper method, the discoloration circle method and the single spore transplantation method can be used to isolate Aspergillus niger to avoid the interference of other miscellaneous bacteria. Become Aspergillus niger for the production of citric acid. Cultivation of yeast The yeasts that can be used for the production of citric acid include Candida lipolytica and Candida quaternaria. The former has a strong ability to decompose fat, and the better carbon source is n-alkane. The latter can be fermented from alkanes to produce citric acid, and can also be fermented from sugars to produce citric acid. The pH of yeast fermentation is 3.5~4.0.
The fermentation process is divided into surface fermentation and solid fermentation, and the culture medium of different raw materials is prepared according to different processes, and then the material is steamed. The purpose of steaming is to gelatinize starch and sterilize it. When steaming the material, the material should be heated evenly, the steam should be smooth, and the material should be added while steaming. The steamed materials should be spread out and cooled down. When the temperature drops below 37°C, water can be replenished and inoculated, and the fermentation is done. The end point of fermentation is determined by the acidity. Bacteria break down.
Fermentation acid – filtration (removal of bacteria and residues) – neutralization, filtration – calcium citrate – acid hydrolysis, filtration – crude acid solution – purification – concentration – crystallization – centrifugation – drying – packaging – finished product. After the fermentation, the fermented aldehyde is processed. For surface fermentation, immediately separate the cap and the fermentation broth, then wash the cap and the shallow dish with a small amount of water, and combine the fermentation broth with the washing water; the citric acid in the solid fermentation solution should be leached with water at a temperature of 80°C for 2 to 3 times. , immersed in water to combine. The fermented acid is filtered with a filter press, and the filtrate and washing water are combined and put into the filtrate tank. Citric acid reacts with calcium salts and calcium bases to form calcium citrate which is precipitated from the liquid phase and separated from soluble impurities. If the acid solution contains a lot of oxalic acid, it can be precipitated in the hot neutralization solution below the pH value of 3, so that the oxalate can be separated first. The neutralization end point is tested with precision test paper, and the pH value is maintained at 6.0~6.8. Stir at about 85°C for 30 minutes to fully separate out calcium sulfate and filter. Calcium citrate is hydrolyzed with sulfuric acid, and the amount of sulfuric acid is determined according to the content of citric acid in the solution. Generally, the excess of sulfuric acid does not exceed 0.2%. After acid hydrolysis, the acid solution is filtered to purify the citric acid solution by adsorption decolorization and ion exchange to remove pigments, colloids and metal cations such as iron ions, calcium ions, copper ions, magnesium ions and anionic impurities such as sulfate ions in the solution. The purification is mostly carried out on the column, the decolorizing carbon is GH-15 granular carbon, and the ion resin is anion and cation resin. The concentration of citric acid purification solution is only 20%~25%, and crystallization can only be carried out when it is concentrated to more than 70%. When concentrating, the temperature should not be too high to avoid the decomposition of citric acid. The concentration of the purified liquid can be carried out under negative pressure. In order to save energy, a double-effect or triple-effect evaporator can be used. The concentration is carried out in 2 stages. After the first concentration, it is put into a settling tank for thermal insulation and sedimentation, and then most of the gypsum is removed; the second concentrated solution contains about 80% of citric acid, and the material is discharged and crystallized in time. For the second concentration, lift-type or bracket-type evaporators can be used to reduce the contact time between feed liquid and heat medium and improve product quality. Different crystallization methods can be used to obtain different products. For the crystallization of citric acid monohydrate, 80% solution, when the temperature is 55 °C, is stirred and cooled in a crystallizer. When the temperature drops to 0 °C, seed crystals are added to start crystallization. Control the temperature not to exceed 36°C, at this time the product is citric acid monohydrate; if the solution is concentrated to 83% at 60°C, cool to 46°C, add seed crystals, maintain the temperature at 40~60°C and slowly crystallize, and finally drop to 38℃, the product is anhydrous citric acid. The crystal paste is centrifuged to obtain crystalline commercial citric acid.