The manufacture  of  citric  acid  in  Italy  is  now  centered mainly in two large modern factories located at Palermo and Messina.  Although  crystallization of  citric  acid direct  from lemon  juice is  apparently  feasible  (I,  4l),  the usual practice in  Italy is first  to separate  it as  the  calcium salt which,  on decomposition  with  sulfuric  acid,  yields  the  free  acid  in crystallizable  form.  The method of preparing crude calcium citrate is briefly  as follows:  The clarified lemon juice, which contains from 4.5  to G  per cent citric acid, is heated  to boiling and  then  neutralized with lime and  calcium carbonate with constant stirring.  The insoluble tricalcium citrate is filtered and  washed  thoroughly with  hot  water until  the washings remain colorless.  The dried salt, which usually contains from 67 to 70 per cent citric acid, is sold on a 64 per cent citric acid basis;  a premium  is  paid for  percentages  in  excess  of  that figure.  Molinari  (36)  states  that  approximately  100,000 lemons are required  to produce  300 kg. of  calcium citrate of this strength, or about 150  lemons per pound. Citric  acid  is prepared  from  the  crude calcium  citrate by adding gradually a slight  excess of  sulfuric  acid  to  the sus- pension of  the salt which  is  stirred continuously.  The mix- ture  is  boiled,  and  the  calcium  sulfate which  separates  is filtered  off  and  washed.  The acid liquors are then  concen- trated  in  vacuum  evaporators  in  two stages;  in  the  first stage  the remaining  calcium  sulfate  is  separated  after  con- centration  to  about  28’  BB.,  and in  the  second  stage  it  is concentrated further to 48-50’  BB. The concentrated liquors are then discharged  and allowed to  crystallize.  The  crude citric  acid  thus  obtained  is  re- dissolved  and treated  to  remove  metallic  impurities  and coloring matter;  then  the purified  liquors  are  concentrated in  vacuum  at 60’  to  65” C. The  pure  acid was formerly crystallized  in lead-lined or wooden containers, but in modern practice  crystallizers  constructed with stainless steel, alumi- num  alloys,  or  rubber are  used.  The  formation  of  normal crystals usually required  from  4  to  5  days.  The  crystals are  separated  from  the mother liquors by  centrifuging  and washing  in  the  centrifuge  with  pure  citric  acid  solution. The  yield  of  acid  from  lemons varies from  15 to  50 pounds per  ton  of  fresh  fruit,  depending  chiefly  on  the  time  of harvest. The methods  employed  in the  California citrate industry were completely described by Wilson  (64), and a description of  the process used  in the West  Indies was  given by Browne (11)  and by Warneford  and Hardy  (49).  In addition to the citric  acid  from  lemons  in California,  some is also  produced from pineapple waste from the canning industry.  According to  Pilhasky  (38), about  250,000  pounds  of  calcium  citrate were produced  in 1926 from this source. The  Hawaiian  citric  acid  industry  is  a  comparatively recent development, although  for  a  number  of  years  about 100,000 pounds  of  calcium  citrate were annually  shipped  to the United States.  However, all of  the available citrate from pineapple waste is now  converted to citric acid  in a modern plant  erected  in Hawaii during 1930 (45).  The total annual production capacity  is  estimated  at  750,000  to  1,000,000 pounds,  Shipments  of  citric  acid from  Hawaii  to  the United States have increased steadily since 1930  and in 1935 amounted  to  approximately  1,000,000 pounds,  or  about  10 per  cent  of  the total production in  the United  States during that year. Future of  the Industry The  fermentation citric  acid  industry  is  now  well  estab- lished.  Manufacturers  consuming  this  acid  are  assured  of adequate  supplies,  regardless of  crop  failures  and other  con- ditions.  The price  trend  since the first  commercial develop- ment  in the United  States has been  steadily downward, de- spite  the fact  that producers are protected  by  an almost  in- surmountable  tariff  barrier  to  foreign  competition.  The present low price of  citric acid has undoubtedly  been respon- sible for  t,he increased domestic consumption of  this product which has taken place during the past few years. Important  new  developments  in  the  industry  can  be  ex- pected  shortly.  The  use  of  citric  acid  in  the  synthesis of certain types  of  plastics  and  other  products  has  been  in- dicated  previously.  Such uses would  provide  an outlet  for large  quantities of  this  acid.  It is  probable  that expansion of  existing  production  facilities  in  the  fermentation citric acid  industry will  take place in the near future.  The use of cheap carbohydrate materials for citric acid manufacture and lower  operating  costs,  due  to  improved technic, may make possible further price reductions which will probably  lead  to greater demand  and  consumption  of  this product.  Al- though citric  acid  production  by  a  true  submerged growth process in yields  high enough  to be  of  practical importance has not  been accomplished, the  possibility  still remains that this problem may be solved. Amelung  (2)  reported  that  19 per  cent  yields  of  citric  acid  from sugar were obtained in  a period  of  40  days  by  employing submerged  growths  of  A. niger.  柠檬酸工业英文文献和中文翻译(8):http://www.youerw.com/fanyi/lunwen_30316.html