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    Bernhauer  and  Iglauer  (Y),  and  Wells,  Moyer,  and May  (52) reported  results  in  which  the  citric  acid  yields obtained  from  glucose  and  sucrose  greatly  exceeded  the highest  yields  obtainable from these sugars  if  the  initial reactions  followed those  of  the usual  alcoholic fermentation of  yeast. The mechanism  recently proposed by Emde (ag),  in which quinic  acid was suggested  as  an  intermediate, can  likewise be  ruled  out  since both  the yield  of  citric  acid  and  carbon dioxide production  required  by that theory do not agree with experimental data. Any satisfactory  theory  for  the mechanisms of  citric acid formation by fungi must  take into  account  the quantitative data which have been  accumulated, and the  fact  that  citric acid  formation  by  fungi from  2-,  3-,  4-,  5-, 6-,  7-, and  12- carbon compounds  has  been  well  established.  In  spite  of all  the work  done on  this  problem,  the mechanism  of  the mycological production  of  citric acid  is  still obscure, and  no theory so far proposed  accounts for all of  the observed facts. The mycological production  of  citric  acid  has been widely  patented both  in  the  United  States and  in Europe.  However,  the  one successful commercial process in this  country was developed only  after  years  of  intensive biochemical  and  engineering  research;  consequently  the actual details of  operation have been closely guarded as trade secrets.  The process undoubtedly consists  of  a  shallow-pan fermentation  of  pure  sucrose or  of  blackstrap molasses by a strain of  A. niger;  the reaction is probably  completed in less than 9 days. The  patent literature  dates  back  to  1893 when  Wehmer (51)  secured broad patents ewering the process which he had discovered.  The  industrial  application  of  Wehmer’s patent was attempted  in Germany  (28) about forty years  ago, but, because of  the numerous difficulties encountered, it was  soon abandoned. In 1913 Zahorski (56) was granted a patent for the produc- tion of  citric acid from sugars by  the fungus Xterigmatocystis nigra.  A preliminary acclimatization of  the organism to high concentrations  of  citric  acid was claimed  to result in higher yields  of  acid  than  had  previously been  obtained. Falck (23) patented  a process for  the production of  acids based on the  use  of  solid  starchy substrate and  organisms  of  the Aspergillus, Penicillium,  or  Citromyces groups.  The  pro- duction of  citric, malic, tartaric, and succinic acid is claimed, PATENTS. but no details as to methods of  recovery or  inpidual yields of  the acids are given.  The patent  of  Bleyer  (8) specifies a preliminary acidification  to pH 3.3  and  the use  of  an abun- dant  supply  of  air  to maintain temperature  control  and  to remove carbon dioxide. The patent  granted  to Szucs (49)  in 1928 claimed the pro- duction  of  citric  acid from molasses by  fungi  of  the  Citro- myces, Mucor, Aspergillus, or Penicillium  groups.  With  a strain of  Aspergillus  it was claimed  that a temperature around 20 O  C. suppressed oxalic acid formation, inhibited infections, and gave high yields of  citric acid. Fernbach and Yuill  (24) patented  a process in which  the novel  feature  lies chiefly  in the elimination of  sterilization by heat through  the addition of  sufficient hydrochloric or  sulfuric  acid  to  the sugar  solu- tions  to give a pH of  1.2 to 2.5.  Kanhauser  (30) was granted a patent for the production of  citric acid in which  it is claimed that improvements in the process are effected by cultivating the  organisms  in  alternating  stages on  solid  agar media, on media corresponding  to  those  used  industrially, and on vegetable media containing organic acids  and vitamins.  It is  further  claimed  that  the  presence  of  substances  which form complexes of  high molecular weight  with citric  acid  is beneficial. According  to  patent  specifications,  Cahn  (13)  succeeded in producing citric acid in good yields in a short fermentation period  by mold  fermentation  of  sections of  sugar-containing plants, such  as sugar beets and artichokes;  by  fermentation of  plant  residues,  such  as  sugar cane  bagasse  and  spent sugar beet  cossettes,  impregnated with sugar solutions;  and from sections  of  starch-containing material  such as  potato dices.  It was  further claimed  that the  fermentation may take place  in  closed  tanks  or  vats. This feature would  be  a  dis- tinct advantage  over  the  shallow-pan process  in  reducing the  cost  and amount  of  equipment  required  and in  simpli- fying manipulation  procedures.  Lilly  (32)  was  granted  a patent  on  an apparatus  and  method  for  the  production  of citric acid.  The  apparatus  consists  of  a number  of  long, porous,  flat,  hollow  tubes  suspended  within a  closed  outer casing.  The  outer  surface of  the porous  tube is  inoculated with  a suitable organism, and a regulated stream of  nutrient sugar solution  is  allowed  to  flow  by  gravity  through  the tubes.  Oxygen  is  supplied  to the  organism  by  passing sterile air  through  the  outer  casing.  Since no  yield figures are given, it is not possible to determine how  effectively this method operates. The patent  of  Frey  (26) describes an apparatus  for  citric acid production  in which a number  of  superimposed shallow pans are so arranged that the nutrient solution may be drawn off  to a large vessel, the citric acid removed by treating with calcium carbonate, and  the solution recirculated through  the pans.  A yield of  40 per cent citric acid in a period of  15  days is reported. In  1936 Nussbaum  (37)  obtained  a  patent  for  producing citric  acid  in which mixed  cultures,  especially A. niger  and Mucor  piriformic,  are  employed.  The  addition  of  mineral salts  of  uranium, manganese,  and  zinc  in  the proportion  of 0.001 to 0.005 per  cent is  claimed  to  stimulate acid produc- tion.  It is further claimed that addition of  1  to  15 per cent of  citric  acid favors the rapid  development  of  the citric acid enzymes.
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