Strain cultivation
In the industrial production of citric acid, microbial fermentation is used, and only a few types of aspergillus and yeast are valuable. Among them, Aspergillus niger is a competitive strain in the industry, and the strong competitive yeast in lipolysis Saccharomyces cerevisiae and Ji Yemeng yeast etc.
Aspergillus niger is cultivated on agar, forming a limited colony on the agar, cultured at room temperature for 10 to 14 days, becoming a rich and dense spore stalk, the colonies are black, sometimes dark brown and black. Considering that the citric acid producing bacteria should have the characteristics of strong acid production ability and high concentration of citric acid resistance, the acid filter paper method, the color changing circle method and the single spore transplantation method can be used to separate Aspergillus niger to avoid the interference of other bacteria and make it Aspergillus niger for citric acid production.
Yeast culture can be used for the production of citric acid, yeast candida lipolytica and Candida gerbilii. The former has a strong ability to break down fat, and the better carbon source is n-alkanes. The latter can be produced by fermentation of alkanes to produce citric acid, or by fermentation of sugars to produce citric acid. The pH of yeast fermentation is 3.5 to 4.0.
Fermentation
Since H.A. Krebs proposed the theory of tricarboxylic cycle in 1940, the fermentation mechanism of citric acid has been gradually recognized by people. It has been proved that in the biochemical process of producing citric acid from sugar raw materials, the process of changing from sugar to pyruvate is the same as that of alcohol fermentation, that is, it is hydrolyzed through the E-M pathway (hexose diphosphate pathway). Then pyruvate is further oxidatively decarboxylated to produce acetyl-CoA, and the oxaloacetate produced by the carboxylation of acetyl-CoA and pyruvate is condensed into citric acid and enters the tricarboxylation cycle.
Tricarboxylic acid cycle
Tricarboxylic acid cycle
Citric acid is an intermediate product in the metabolic process. In the fermentation process, when the aconitic acid hydratase and isocitrate dehydrogenase activities in the microorganism are very low, and the citrate synthetase activity is high, it is conducive to the accumulation of large amounts of citric acid.
The fermentation process is divided into surface fermentation and solid fermentation, according to different processes to prepare media of different raw materials, and then steamed. The purpose of steaming is to gelatinize starch and sterilize it. When steaming the material, the material should be heated evenly, the steam is unobstructed, add the material while steaming, add the material to the steaming place, and add it layer by layer. The steamed material should be spread out and cooled. When the temperature drops below 37°C, you can inoculate with water, ferment in a plate, and the end of fermentation is determined by the acidity. The acidity is measured regularly to ensure that the material is discharged when the acidity is highest, so as not to be affected by citric acid. Bacteria break down.
extract
After the fermentation is completed, the fermented aldehyde must be treated. For surface fermentation, immediately separate the cap and fermentation broth, and then wash the cap and shallow dish with a small amount of water. The fermentation broth and the wash water are combined; the citric acid in the solid fermentation cooler must be leached with water, the water temperature is 80 ℃, and leached 2 to 3 times , Merged in water. The fermented acid is filtered with a filter press, and the filtrate and washing water are combined into the filtrate tank. Citric acid reacts with calcium salts 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 much oxalic acid, it can be precipitated in the hot neutralization solution below pH 3, so that the oxalate salt is separated first. The neutralization endpoint is tested with precision test paper, keeping the pH at 6.0~6.8. Stir at about 85°C for 30 minutes to allow calcium sulfate to be analyzed and filtered. Calcium citrate is hydrolyzed with sulfuric acid, and the amount of sulfuric acid is determined according to the citric acid content in the solution. Generally, the excess of sulfuric acid does not exceed 0.2%. After acid hydrolysis, the acid solution is filtered. Purification of citric acid solution removes pigments, colloids and iron ions, calcium ions, copper ions, magnesium ions and other metal cations and anionic impurities such as sulfate ions by adsorption decolorization and ion exchange.
The purification is mostly carried out on the chromatographic column, the decolorizing carbon is GH-15 granular carbon, and the ionic resin is anion and cation resin. The concentration of the citric acid purification solution is only 20% to 25%, and it can be crystallized only after being concentrated to more than 70%. The temperature should not be too high during concentration to avoid the decomposition of citric acid. The concentration of the purification 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 two stages. After the first concentration, it is placed in a settling tank for heat preservation and settlement, and then most of the gypsum is removed. The second concentrated solution contains about 80% citric acid, and is crystallized in time. For the second concentration, a lifting or enclosed plate evaporator can be used to reduce the contact time between the feed liquid and the heat medium, which can improve product quality. Different products can be obtained by different crystallization methods. The crystallization of citric acid monohydrate is 80% solution, the temperature is 55 ℃, and it is cooled naturally in the crystallizer with stirring. When the temperature drops to 40 ℃, add crystal seeds to start crystallization. Control the temperature not to exceed 36℃, and the product is citric acid monohydrate; if the solution is concentrated to 83% at 60℃, cool to 46℃, add seed crystals, maintain the temperature at 40~60℃, slowly crystallize, and finally drop to At 38℃, the product is anhydrous citric acid. The crystal paste is centrifuged densely to obtain crystalline commercial citric acid.
