ON  FERMENTATION. 
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sugar  by  fermentation  with  beer-yeast,  M.  Pasteur  found  that  it 
contained  mannite  and  the  viscous  substance  before  mentioned, 
and  he  invariably  found  that  sugar,  brought  in  contact  with  the 
lactic  ferment,  is  transformed  into  products,  among  which  man- 
nite is  always  present ;  but  this  is  the  case  only  when  the  liquid 
rapidly  becomes  acid,  for  when  the  liquid  was  kept  neutral  by 
the  addition  of  chalk,  all  other  circumstances  remaining  the 
same,  neither  gum  nor  mannite  were  produced ;  or,  at  least, 
neither  of  these  substances  remained  in  the  liquid  unaltered. 
M.  Berthelot  has  already  shown  that  by  substituting  mannite 
for  sugar  in  the  lactic  fermentation,  that  substance  undergoes 
fermentation.  It  is  easy  to  ascertain  that  in  the  numerous  cases 
of  fermentation  of  mannite,  it  is  due  to  the  production  of  the 
lactic  ferment.  When  a  solution  of  pure  mannite  is  mixed  with 
chalk  and  some  fresh  washed  lactic  ferment,  at  the  end  of  a 
quarter  of  an  hour  disengagement  of  gas  commences,  and  the 
chemical  change  of  the  mannite  takes  place,  the  products  being 
carbonic  acid,  hydrogen,  alcohol,  lactic  acid,  butyric  acid,  and 
all  the  other  products  of  the  fermentation  of  mannite. 
With  regard  to  the  production  of  butyric  acid,  experience  has 
shown  that  the  lactic  ferment  acts  directly  upon  lactate  of  lime, 
converting  it  into  carbonate  and  butyrate  of  lime.  But  the  action 
of  the  ferment  is  first  exercised  upon  sugar,  and  when  it  is  pre- 
sent in  the  liquid,  it  undergoes  fermentation  in  preference  to 
lactic  acid. 
M.  Pasteur  has  also  studied  the  alcoholic  fermentation  experi- 
mentally, in  the  same  manner.  In  reference  to  this  phenomenon, 
he  distinguishes  as  two  principal  cases,  the  action  of  beer-yeast 
upon  a  solution  of  pure  sugar,  and  its  action  in  the  presence 
of  albuminous  substances.  In  the  former  case  the  yeast  is  ex- 
hausted, and  becomes  incapable  of  inducing  further  fermentation. 
In  the  latter  case  the  activity  of  the  yeast  is  not  destroyed.  In 
fact,  the  quantity  remaining  after  the  fermentation,  is  greater 
than  that  used.  It  is  therefore  regenerated,  or  rather  it  is 
exhausted,  as  in  the  former  case  ;  but  since  fresh  ferment  is 
produced,  the  consumption  is  disguised  by  the  production.  The 
amount  of  ferment  that  is  consumed,  is  estimated  in  reference 
to  the  sugar,  at  about  one  per  cent,  of  dry  substance. 
The  decomposition  of  ferment  accompanying  its  action  upon 
