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Commercial Production of Citric Acid What is citric acid? • Citric acid is a 6 carbon tricarboxylic acid. HOOC-CH2C(OH)(COOH)CH2COOH Other names: 2-Hydroxy-1,2,3-Propanetricarboxylic Acid beta-Hydroxytricarballylic Acid. Citric Acid Production and uses • Citric acid is widely used in the food industry as an acidifier and antioxidant. • It also has use as an environmentally friendly and biodegradable cleaner • It serves as a metal chelator in the lab • About 300,000,000 kg are produced annually worldwide, almost all of it (>90 %) from aerobic fermentation of Aspergillus niger. Costs and Manufacture • A commodity chemical, citric acid sells for less than $1.00/lb, perhaps as low as $1.00/kg. • The media used to produce CA is typically derived from a low cost sugar source, most typically sugar cane waste streams or partially processed starch. – (Divalent ions (Fe, Mn, Zn,Cu) play important roles in CA production and typically must be removed from these crude media before fermentation) Biochemistry • Citric acid is found in almost all living organisms • CA is a component of the TCA cycle. – Formed from the condensation of oxalacetate (OAA) with AcetylCoA. – Normally CA is then converted to CO2 by passing through the TCA cycle (next slide) TCA Cycle Citric Acid Overproduction • In high producing lines of A. niger, several things conspire to force high production of CA. – First, the isomerization of CA to isocitrate is slowed by lack of Fe+2 ions, which inhibits the enzyme aconitase. – Second, isocitrate dehydrogenase, the enzyme catalyzing conversion of isocitrate to a-oxoglutarate is inhibited by high citrate levels. – Third, a-oxoglutarate dehydrogenase (a-oxoglutarate to succinyl CoA) is inhibited by high glucose levels in the media. These factors contribute to a blockage of metabolism at the citrate step and accumulation and excretion of citrate. Other Biochemical factors • At high pH (>4) oxaloacetate is broken down (undesirable) to acetate and oxalic acid. This makes it unavailable for CA formation. Low pH (~2) limits this reaction • In order to produce the intermediates of the TCA cycle needed for cell growth and maintenance, cell produces OAA by one of two other routes and “runs the TCA cycle backwards” to supply malate, fumarate, succinate, and succinyl CoA to the cells.
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