Natural citric acid is widely distributed in nature. Natural citric acid is found in the bones, muscles, and blood of fruits such as lemons, citrus, pineapples, and animals. Synthetic citric acid is produced by fermentation of sugary substances such as sugar, molasses, starch, grapes, etc., and can be divided into anhydrous and hydrated. Pure citric acid is a colorless transparent crystal or white powder, odorless, and has an attractive acidity.
The principle of fermentation method
Since H.A. Krebs proposed the theory of tricarboxylic acid cycle in 1940, the fermentation mechanism of citric acid has gradually been recognized. It has been shown that in the biochemical process of producing citric acid from saccharide raw materials, the process from sugar to pyruvic acid is the same as alcoholic fermentation, that is, glycolysis by the E-M pathway (hexose diphosphate pathway). Then pyruvate is further oxidized and decarboxylated to form acetyl-CoA, and oxaloacetate formed by acetyl-CoA and pyruvate carboxylation is condensed into citric acid and enters the tricarboxylic pathway.
Citric acid is an intermediate in the metabolic process. In the fermentation process, when the activity of aconitate hydratase and isocitrate dehydrogenase in the microorganism is very low, and the activity of citrate synthase is high, it is beneficial to the accumulation of citric acid.
The citric acid production is divided into two parts: fermentation and extraction
There are three methods of fermentation: solid fermentation, liquid shallow fermentation, and deep fermentation.
Solid-state fermentation is based on potato dry powder, starch mash, and starch-containing agricultural by-products. After the medium is prepared, it is cooked under normal pressure, cooled to the inoculation temperature, inserted into the seed koji, and loaded into a curved plate at a certain temperature and humidity. Fermentation under conditions. The production of citric acid by solid state fermentation is simple and easy to operate.
Most of the liquid shallow-dish fermentation uses molasses as raw material. The production method is to transfer the sterilized culture liquid into a fermenting tray through a pipeline, and connect the strain, and then add the sugar liquid to ferment after the bacterial body is propagated to form a bacterial membrane. Sterile air is required to be introduced into the fermentation chamber during fermentation.
The main equipment for the deep fermentation production of citric acid is a fermenter. Microorganisms are propagated and fermented in this closed container. A general fermenter is now used. Its main components include tanks, agitators, cooling units, air distribution units, defoamers, shaft seals and other attachments. Fermentation tank diameter is generally 1:2.5, should be able to withstand a certain pressure and has a good seal.
In addition to the universal fermenter, a lift tank, a tower fermenter, and a spray-type self-priming fermenter can be used.
In order to obtain excellent strains of citric acid production, it is usually separated from the soil collected from different regions or from the decayed fruits, and then the strains are selected by physical and chemical methods. For example, the dry fermentation of citric acid from dried potato powder is obtained through continuous variation and selection. The strain is suitable for fermentation at a high concentration, and the acid production level is high.
The fermentation of citric acid varies depending on the strain, process, and raw materials, but it is also necessary to master certain conditions such as temperature, ventilation, and pH during the fermentation process. It is generally believed that Aspergillus niger is suitable for acid production at 28 to 30 °C. The excessive temperature will cause the bacteria to multiply, the sugar is consumed a lot so that the acid production is reduced, and more oxalic acid and gluconic acid are produced. When the temperature is too low, the fermentation time is prolonged. Microbial production of citric acid requires low pH, and the optimum pH is 2 to 4, which not only facilitates the formation of citric acid, but also reduces the formation of sour acids such as oxalic acid, and at the same time avoids the contamination of bacteria. Citric acid fermentation requires strong ventilation conditions, which is beneficial to maintain a certain amount of dissolved oxygen in the fermentation broth. Ventilation and agitation are the primary means of increasing dissolved oxygen in the medium. As the cells are formed, the dissolved oxygen in the fermentation broth gradually decreases, thereby inhibiting the synthesis of citric acid. The method of increasing the air flow rate and the stirring speed to make the dissolved oxygen in the culture solution reach 60% saturation is advantageous for acid production. The formation of citric acid is closely related to the morphology of the bacteria. If the normal bacteria sphere is formed in the late stage of fermentation, it will help to reduce the viscosity of the fermentation broth and increase the dissolved oxygen, so the acid production will be high; if the mycelium is abnormal, the bacteria will multiply. , causing a decrease in dissolved oxygen, causing a rapid decline in acid production. The content of metal ions in the fermentation broth plays a very important role in the synthesis of citric acid. Excess metal ions cause a decrease in acid production rate, and iron ions can stimulate the activity of aconitate hydratase, thereby affecting the accumulation of citric acid. The molasses raw material for citric acid fermentation must be used by ion exchange method or potassium ferrocyanide de-ironing because it contains a large number of metal ions. However, trace amounts of zinc and copper ions can promote acid production.
In the citric acid fermentation broth, in addition to the main products, it also contains other metabolites and some impurities, such as oxalic acid, gluconic acid, protein, colloidal substances, etc. The composition is very complicated, and citric acid must be extracted by physical and chemical methods. Most plants still use the calcium carbonate neutralization and sulfuric acid hydrolysis process to extract citric acid. In addition, studies have successfully studied the extraction of citric acid by extraction, electrodialysis and ion exchange.