Organic Acids

Citric acid is the most important acid produced by fungi and the only organic acid produced almost exclusively through fermentation. Widely used in the food and beverage industry, its pleasant acid taste enhances products such as soft drinks, fruit juices, jams, jellies, candies, prepared desserts, and frozen fruits. Citrates are also efficient buffering and chelating agents and are used by the cosmetics industry and in blood transfusion products, effervescent tablets, detergent manufacturing, electroplating, printing, inks, leather tanning, and a host of other applications. With increasing emphasis on nonpolluting chemical products, the market for nature's acidulant is increasing.

Many molds and yeasts accumulate large amounts of citric acid from glucose. The biochemical basis of this metabolic overproduction has been studied for a century. Names associated with early research include C. Wehmer in Germany, and J.N. Currie and C. Thom in the United States. The first commercial fermentation was perfected by Currie at Pfizer and Company in Brooklyn, New York, during the 1920s and involved surface fermentations of A. niger. This species is still used in many industrial processes worldwide, although submerged fermentations have generally replaced surface and solid-state systems. Production strains are selected not only for their high yield of citric acid, but also for their ability to use cheap raw materials as substrates. Optimal yields of citric acid are associated with low pH (below 2.5), high carbohydrate, and manganese deficiency.

Although a great deal is known about the environmental cues involved in maximizing citric acid production, the exact biochemical basis and the crucial regulatory control points are the subject of continuing research (17,18).

Mycelium is removed by filtering or centrifugation; citric acid is then precipitated. Subsequent recovery steps can include treatment with activated carbon, cation and anion exchanges, and final crystallization. The whole process is highly optimized, with A. niger capable of producing about 90% of the theoretical yield from a carbon source.

Other organic acids produced by Aspergillus fermentation are listed in Table 2. None of these is as economically important as citric acid.

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