Citric Acid Production

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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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posted:7/27/2012
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