Citric acid is a chemical compound that is used in many different industrys. A fungus produce citric acid, however. While many peoples may believe the compound is produced by squeezing fruit like lemons for instance, a fungus named *Aspergillus niger* mostly produce citric acid.
The *Aspergillus niger* fungus is utilized in the production of citric acid due to the ability of the fungus to be manipulate to overproduce the citric acid. In order to produce citric acid with the *Aspergillus niger* fungus, it is necessary to provide the fungus with a source of carbon that the fungus can digest. Examples of the types of carbon source that can be provided to the fungus include sugarcane molasses, corn starch, cassava, and sugar beets.
How a Fungus Makes Citric Acid
However, sugarcane molasses is often used as an carbon source for the fungus due to the fact that it is a cheap byproduct of the sugar refining process. Before introducing the fungus to the carbon source, the worker adjusts the pH of the carbon source, and the mixture is sterilized to prevent bacteria from entering the environment and consuming the carbon source. The *Aspergillus niger* fungus undergo a specific life cycle in the preparation of the citric acid.
First, the fungus is provided with a period of spore inoculation of the environment, followed by a lag phase wherein the fungus begins to adjust to the environment. After the lag phase, the fungus undergoes an exponential growth phase in which the mycelium of the fungus are growing, followed by the production phase in which the citric acid cycle of the fungus begins to overflow with citric acid, leading to the fungus to release the excess citric acid. Some of the methods of bioreactors that can be used to allow the fungus to grow is stirring tank reactors, airlift reactors, and solid-state fermentation.
Stirred tank reactors are reactors in which mechanical agitation mixes the fungus with the liquid. Airlift reactors are reactors that utilize air bubbles to move the liquid within the bioreactor, avoiding the need to use mechanical impellers to move the liquid. Additionally, solid-state fermentation can be used, wherein the fungus grows on a solid substrate.
The fermentation broth contain the fungus and the citric acid that the fungus produced. In order to separate the two component, the mycelium containing the fungus can be filtered from the citric acid. Additionally, the introduction of calcium hydroxide can precipitate the citric acid.
The addition of the calcium hydroxide will convert the liquid citric acid to a solid compound known as calcium citrate. This solid citrate compound is more easier to handle than the liquid citric acid. Subsequently, the addition of sulfuric acid will convert the calcium citrate to pure citric acid.
After the citric acid is produced in the precipitation step, the citric acid must be purified. One can purify the citric acid by passing it through ion exchange column and activated carbon. These steps will remove the metal ions and colors from the citric acid, ensuring that the citric acid is white and tasteless.
After the citric acid is purified, the citric acid can be evaporated and crystallized into fine grain. The fungus *Aspergillus niger* produces the citric acid that is used in a variety of different ways. For instance, citric acid can be used as an acidulant in sodas.
Additionally, citric acid can act as a chelating agent in laundry detergent. The compound can also be used as a preservative in pharmaceutical products, as well as an anticoagulant in blood banks. Thus, citric acid is utilized in a variety of industry.