Publisher Summary This chapter focuses on the production of organic acid and metabolites by fungi. Organic acids and metabolites play an important role in food processing. They serve as food ingredients or as precursors for food ingredient. The major organic acids primarily function as food acidulant. In western countries and also in orient, the domestic consumption of organic acids is constantly increasing. Fungi produce a wide variety of metabolites, including organic acid, vitamins, lipids, flavors, exopolysaccharides, amino acids, etc. Citric acid is the major organic acid produced by fungal fermentation and the second of all fermentation commodities following industrial ethanol. Commercially, citric acid is manufactured by surface-, submerged- and solid-state- or koji-fermentation processes. The medium consists of a carbohydrate supplemented with inorganic or organic nitrogen along with properly balanced mineral salts. Most industrial processes for citric acid production use Aspergillus nidulans (A. niger). In submerged culture, A. niger grows in the form of mycelial pellets under rigorously controlled environmental conditions. The medium typically contains 15-20% (w/v) decationized sugar solutions, usually consisting of cane molasses, sucrose, glucose or fructose.