Citric acid (CA), also known as citric acid, with the molecular formula C₆H₈O₇, is an important organic acid. It is colorless crystals, odorless, has a strong acid taste, is easily soluble in water, and is a natural preservative and food additive.
At room temperature, citric acid is a white crystalline powder, tasteless and extremely acidic. Dissolved in water. Its crystal form varies with different crystallization conditions. It is breeze in dry air, and breeze in moist air. It is hygroscopic, decomposes into various products when heated, and can react with acids, alkalis, glycerin, etc.
Cultivation of strains
In the industrial production of citric acid, microbial fermentation is used, but only a few kinds of Aspergillus and yeast are valuable. Among them, Aspergillus niger is a competitive strain in the industry. Silk yeast and Ji Yemeng yeast and so on.
Aspergillus niger is cultivated on agar
It forms limited colonies on agar. Cultivated at room temperature for 10 to 14 days, the spore stems are abundant and dense. The colonies are black, sometimes dark brown and black. Considering that the bacteria should have the characteristics of strong acid production and high citric acid resistance, the acid filter paper method, color circle method and single spore transplantation method can be used to isolate Aspergillus niger to avoid interference from other bacteria. It becomes Aspergillus niger to produce citric acid.
Yeasts that can be used to produce citric acid include Candida lipolytica and Candida parasitoids. The former has a strong ability to decompose fat, and the better carbon source is n-alkanes. The latter can be produced by the fermentation of alkanes, or the fermentation of sugars to produce citric acid, and the pH of yeast fermentation is 3.5-4.0.
Since H.A. Krebs put forward the theory of tricarboxyl cycle in 1940, the fermentation mechanism of citric acid has been gradually understood by people. It has been proved that in the biochemical process of carbohydrate raw materials to produce citric acid, the process from sugar to pyruvate is the same as alcohol fermentation, that is, glycolysis is carried out through the E-M pathway (hexose diphosphate pathway). Then pyruvate is further oxidized and decarboxylated to generate acetyl-CoA, and the oxaloacetate generated by the carboxylation of acetyl-CoA and pyruvate is condensed into citric acid and enters the tri-carboxy cycle pathway.
Citric acid is an intermediate product in the metabolic process. In the fermentation process, when the activities of aconitate hydratase and isocitrate dehydrogenase in the microorganisms are low, and the activity of citrate synthase is high, it is conducive to the accumulation of citric acid.
The fermentation process is divided into surface fermentation and solid fermentation. 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 materials, the materials should be heated evenly and the steam should be unobstructed, and the materials should be added while steaming, and the materials should be added to the steaming place, layer by layer. The steamed materials should be spread out and cool. When the temperature drops below 37℃, they can be inoculated with water and plated and fermented. The end of the fermentation is determined by acidity. Bacteria decompose.
After the fermentation, the fermented aldehyde should be processed. For surface fermentation, separate the cap and the fermentation broth immediately, and then wash the cap and shallow dish with a small amount of water. The fermentation broth and washing water should be combined; the citric acid in the solid fermentation should be leached with water at a water temperature of 80°C and leached for 2 to 3 times. Soak in water and merge. The fermented acid is filtered with a filter press, and the filtrate and washing water are combined and poured into the filtrate tank. The citric acid reacts with calcium salt and calcium base to form calcium citrate which precipitates out of the liquid phase and is separated from soluble impurities. If the acid solution contains a lot of oxalic acid, it can be precipitated below pH 3 in the hot neutralization solution, so that the oxalate can be separated first. The neutralization endpoint 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 allow the calcium sulfate to be fully analyzed and filtered. Calcium citrate is acid 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 sulfuric acid should not exceed 0.2%. After acidolysis, the acid solution is filtered. The purification of citric acid solution removes pigments, colloids, iron ions, calcium ions, copper ions, magnesium ions and other metal cations and sulfate ions and other anionic impurities in the solution through adsorption, decolorization and ion exchange.
Purification is mostly carried out on a chromatographic column. The decolorizing carbon is GH-15 granular carbon, and the ionic resin is anion and cation resin. The concentration of citric acid purification solution is only 20%~25%, and crystallization can only be carried out if it is concentrated to more than 70%. The temperature should not be too high during concentration to prevent the decomposition of citric acid. The concentration of the purified liquid can be carried out under negative pressure. In order to save energy, double-effect or triple-effect evaporators can be used. Concentration is carried out in two stages. After the first concentration, it is placed in a sedimentation tank for heat preservation and sedimentation, and then most of the gypsum is removed; the second concentrated solution contains about 80% of citric acid, and the material is released for crystallization in time. The second concentration can use a lift or bracket evaporator to reduce the contact time between the material liquid and the heat medium and improve the product quality. Different crystallization methods can be used to obtain different products. The crystallization of citric acid monohydrate is 80% solution, and the temperature is 55 ℃, and the temperature is 55 ℃, and it is naturally cooled under stirring in the crystallizer. When the temperature drops to 40 ℃, seed crystals are added to start crystallization. Control the temperature not to exceed 36℃, the product is citric acid monohydrate at this time; if the solution is concentrated to 83% at 60℃, cool to 46℃ and add seed crystals, maintain the temperature at 40~60℃ to crystallize slowly, and finally drop to At 38°C, the product is anhydrous citric acid. The crystalline paste was centrifuged to obtain crystalline commercial citric acid.