There are three common production methods of citric acid, which are direct extraction, chemical synthesis and biological fermentation. Direct extraction method refers to the direct extraction from vegetables and fruits with high citric acid content. Due to the disadvantage of high cost and low yield, this method has not been widely used. Chemical synthesis method is rarely used in industrial production because many chemical reagents are toxic, coupled with expensive chemical raw materials and cumbersome synthesis steps; Now the most used in industry is microbial fermentation, microbial fermentation can be subdivided into surface fermentation, solid fermentation, liquid deep fermentation, etc., of which the most used is liquid deep fermentation.
At present, the most widely used citric acid high-yield strain is A. iger. A. iger is a common Aspergillus fungus in nature, which is widely found in soil, air and food crops. It is a common fermentation strain in industry and is often used to produce citric acid, amylase, cellulase, acid protease, glucose oxidase, pectinase, gallic acid and gluconic acid. The optimum temperature of Aspergillus Niger is 37℃, and the optimum relative humidity is 88%. Aspergillus Niger has a strong acid production capacity, and it can still produce a large amount of acid citric acid even under acidic conditions. The acid production process does not produce toxic by-products, and it can use a variety of carbon sources, so that it has a competitive advantage that other strains cannot match.
Taking the liquid fermentation technology of Aspergillus Niger to produce citric acid as an example, starch is used as raw material, and bran is activated to make spore suspension. Starch is liquefied and saccharified to get saccharifying solution. The whole process takes about 24h, and then a certain concentration of nitrogen source and inorganic salt is added to make basic medium, and the nitrogen source is generally corn pulp and yeast extract. The basic medium was pasteurized at high temperature and cooled, then spore suspension was added. Aspergillus Niger spores were expanded in seed tank and inoculated to fermenter for fermentation. The batch fermentation cycle is about 60~80h. After the fermentation, citric acid products are obtained through the bacterial liquid separation, precipitation and refining process of the fermentation liquid.
Normal citric acid fermentation medium in addition to the fermentation strain Aspergillus Niger sticky other microorganisms, called staining bacteria. In the process of citric acid fermentation, with the accumulation of the product citric acid, the pH value of the fermentation liquid is quickly reduced to below 2.0, and many microorganisms cannot reproduce under acidic conditions, but the value of citric acid in the early stage of fermentation is high, and it is rich in nutrients and easy to be infected with bacteria. Once infected, the production of citric acid is very harmful. In recent years, with the continuous development of citric acid industry, higher requirements have been put forward for the work of fermentation anti-staining bacteria. The following reasons, types and influencing factors of citric acid bacteria were analyzed
Causes of infection:
(1) Operation of bacteria: operators should have aseptic awareness, to master the basic knowledge of microbiology, can be skilled in disinfection and sterilization operations. Citric acid fermentation requires a variety of sterilization operations, such as regular empty fermentation tank, each tank of media to be eliminated, whether it is empty fermentation tank or media elimination, the operator must strictly follow the sterilization temperature and pressure operation.
(2) Seed tank contamination: Citric acid fermentation is mostly secondary culture, and the original seed needs to be expanded through the seed tank first, and the seed tank is also susceptible to microbial contamination during the culture process. Under normal circumstances, the seed tank should be checked for miscellaneous bacteria before being connected to the fermenter, and the seed tank infected with bacteria should not be used.
(3) excipients into miscellaneous bacteria: The main excipients in the process of citric acid fermentation are urea, ammonium sulfate, vesicolor, glycosylase, etc. General nutrients and other auxiliary materials are added before the elimination of the medium, so that when the medium is eliminated, the miscellaneous bacteria in the auxiliary materials can be killed. In the process of citric acid fermentation, due to the characteristics of the medium and ventilation culture, the fermentation liquid is easy to bubble, and the tank will be overflowed in serious cases, especially in the logarithmic phase of Aspergillus Niger growth, Aspergillus Niger metabolism is strong, and generally every few minutes to add a defoamant. The defoamer needs to be added during the fermentation culture process, so if the sterilization of the defoamer and related pipes is not thorough, it is easy to cause fermentation bacteria.
(4) Air into miscellaneous bacteria: citric acid is aerobic fermentation, the fermentation process needs continuous ventilation to ensure that the dissolved oxygen in the fermentation liquid can meet the growth needs of Aspergillus Niger. Therefore, the compressed air needs to be filtered and sterilized before passing into the fermenter, and improper installation and use of the filter or incomplete sterilization before use will cause fermentation bacteria.
(5) Equipment leakage point and hygiene: citric acid fermentation process produces a lot of heat, fermenter generally uses a built-in coil, the use of circulating water for fermentation temperature control, due to the tank vibration, in the process of use, circulating water coil weld will appear welding or leakage, resulting in circulating water into the fermentation fluid, and then contaminate the fermentation fluid, resulting in fermentation bacteria.