Citric acid is found in citrus fruits like lemon, oranges, tomatoes, etc. It is a weak organic tricarboxylic acid. Aspergillus niger is a fungus that is used to produce citric acid in the industry. Clostridium is used to produce butyric acid while Saccharomyces cerevisiae is used to produce ethanol and Pseudomonas is a bacterium that is used for vitamin B₁₂ production.