ABSTRACT
Citric acid is an intermediate of the tricarboxylic acid (TCA) cycle. Citric acid is enriched in citrus fruits, such as lemon and oranges, and used to be extracted from them. Several fungi accumulate citric acid. Citric acid fermentation was discovered in a culture of Penicillium glaucum on sugar medium in 1893 (Wehmer, 1893). Subsequently, numerous strains of Aspergillus were found to accumulate citric acid at highly acidic conditions (pH 2.5-3.5) (Currie, 1917). This discovery overcame contamination problems and enabled the industrial production of citric acid, and subsequently citric acid producers with higher yield were screened. The improvement of the strains has been achieved by mutagenesis, followed by the selection of an improved mutant known as classical strain improvement (CSI) (Grewal and Kalra, 1995; Haq, 2001; Soccol et al., 2006). There have been many other attempts to optimize citric acid production with respect to substrates and the fermentation processes (Mattey, 1992; Soccol et al., 2006; Papagianni, 2007). Commercial production of citric acid utilizes Aspergillus niger in an industrial-scale submerged fermentation process (Mattey, 1992). Bulk production of citric acid by fermentation exceeded 1.4 million tons per year worldwide in 2004 and continues to increase each year. Because of the general recognition of its safety to humans, a pleasant acid taste, high water solubility, and chelating and buffering properties, citric acid is frequently added to beverages (soft drinks, syrups, wines, and ciders), to foods (jellies, jams, preservatives, ice creams, processed cheese, and candies), to pharmaceuticals as an effervescent agent in powders and tablets, and to cosmetics and toiletries as a buffering agent (Milsom and Meers, 1985a; Soccol et al., 2006).
