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    Porges ($9)  found  that sodium nitrate was superior to either ammonium nitrate or ammonium sulfate  as a nibrogen source in a concentration  of  0.4 Der  cent. and acid  was  roughly  proportional  to  this ratio.  Doelger  and Prescott  (20) found  a  similar  relation  in studies on  the pro- duction of  citric acid. Large-Scale Operation In  present  industrial  practice  shallow  aluminum pans are used  for the prodnetion of  citric acid from sugars by mold fermentation.  The  pans  must  be  constructed with  high- purity aluminum t.o prevent corrosion and to avoid the harm- ful effect  of other metals such as copper,iron,  etc.  Theproc- ess consists essentially  in  tile  inoculation with  spores of  a suitable organism of  the sterile sugar  solution contained in the aluminum  culture  vessels.  A  continuous  felt  of  mycelium forms over  the entire surface of  the solution within  2  days, and  formation  of  citric acid  occurs  rapidly  aft.er the  fourth day.  The fermentation  is usually complete in 7  to 10 days after  inoculation.  The  solution  is  then  drained  off,  the myeelium  is pressed to  remove any acid present in the  tissues, and  the  acid  is  either  crystallized  directly  after  a  yeast fermentation of  the residual  carbohydrate  or  first separated from  the  solution  as the  calcium  salt  from which  the  acid may  be  recovered  by  crystallization  after  treatment with sulfuric acid.  The yield  of  citric  acid  obtained  is approxi- mately 60 per cent by weight of  the sugar taken, Little  is known  regarding  the  actual details  of  industrial operation  of  the  fermentation,  but the difficulties  involved rnnst  be enormous.  May  et  al.  (S4),  in  semiplant-scale st.ndies on  the production  of gluconic acid, used aluminum pans 43  X  43  X  2  inches  (109  X  109  X  5 em.) which  had an optimum charge capacity  of  48  liters of  culture solution. Based on a  7-day process and  a 50 per  cent weight yield  of acid from 20 per  cent sucrose solutions,  it is calculated  that from  12,000  to  16,000 pans  of  this capacity would  be  re- quired  to manufacture  the  estimated  7,000,000 pounds  of fermentation citric acid  produced  annually in  the  United States.  It is not surprising that the details of such a process are kept secret. MIICHAXISM  STUDIES. Most  of  the  recent  literature dealing with citric acid  has  been concerned with  efforts  to explain  the mechanism  of  the reactions  involved  in  the con- version  of  glucose  and  other  sugars  to  citric acid.  Clial- lrnger  (14)  and  May  and Herrick  (3s) summarized  the various theories  which  had  been proposed  up  to about 1930. The more recent theories have been concerned with mecha- nisms involving  acetic  acid  as  one  of  the important  inter- mediate  substances.  The  acceptance  of  this  acid  as  an intermediate  in  the process naturally  led  to  the assumption that  its formation was preceded  by  a  breakdown of  glucose similar  to that  encountered in  the alcoholic fermentation  of yeast. Chrzaszcz  and  Tiukow  (16) proposed  that  citric  acid  is formed from  acetic acid  through  the following steps:  acetic acid  -+ succinic acid  3  fumaric  acid  -+ malic  acid.  The latter  by  condensation with  another molecule of  acetic  acid then  forms  citric acid,  according  to  their  view.  Bernhauer (6) proposed  a  similar scheme in which succinic acid  reacts with acetic  acid  to form  tricarballylic acid which, on  oxida- tion, yields aconitic and finally citric acid. However,  in spite of  all the favorable evidence to support these  views,  it  has  been shown  that mechanisms  involving acetic  acid  as  an  intermediate  in the  citric  acid process,  in which  it is  assumed  that  the initial  reactions  leading  to  its formation  follow  the general chemical equation which repre- sents  the alcoholic fermentation, can be  ruled  out  on purely quantitative evidence. Butkevich  and  Galvskaya  (la), Chrzaszcz  and  Peyros (15),
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