Am.  Jour.  Pharm.) 
February,  1920.} 
Yeast  Growth. 
107 
that  Ledingham  and  Penf old's  equation  is  not  of  general  applica- 
bility. 
There  is  no  doubt  of  the  interest  and  importance  of  this  period 
of  growth  and  yeast  cells  are  well  adapted  for  the  purpose  of  further 
investigation. 
Phase  of  Retarded  Growth. — As  yeast  develops  in  malt  wort  the 
first  retarding  influence  which  comes  into  play  is  that  due  to  carbon 
dioxide.  Continental  investigators  have  rightly  laid  stress  on  the 
preservative  action  of  carbon  dioxide;  in  this  country  the  influence 
has  been  recognized,  but  the  effect  has  been  attributed  to  the  ex- 
clusion of  oxygen  rather  than  to  a  direct  poisoning  effect. 
The  retarding  influence  of  the  gas  can  be  detected  when  the  yeast 
concentration  has  developed  to  about  a  million  cells  per  Cc. 
Oxygen  is  necessary  for  yeast  growth,  and  lack  of  oxygen  soon 
makes  itself  felt  and  is  the  main  retarding  influence  when  yeast 
cells  develop  from  a  few  million  cells  per  Cc.  to  the  maximum  growth 
of  about  100  million  per  Cc. 
A.  J.  Brown  (1905)  was  the  first  to  show  that  arrest  of  cell  re- 
production under  these  conditions  is  due  to  this  lack  of  oxygen. 
H.  T.  Brown  (19 14)  further  investigated  the  matter  and  inter  alia 
obtained  the  important  result  that  yeast  growth  increases  propor- 
tionally with  the  amount  of  dissolved  oxygen  initially  present  in 
the  wort.  For  the  production  of  10^^  yeast  cells,  i .  7  Cc.  of  oxygen 
are  required. 
Measurements  of  the  logarithmic  constants  of  growth  under 
anaerobic  and  aerobic  conditions  have  been  made.  In  these  ex- 
periments malt  wort  behaved  as  if  it  contained  a  certain  amount 
of  combined  oxygen  available  for  yeast  growth.  The  results  are 
not  in  agreement  with  H.  T.  Brown's  conclusion  (19 14)  that  growth 
under  anaerobic  conditions  is  due  entirely  to  oxygen  previously 
absorbed  by  the  yeast. 
Alcohol  also  acts  as  a  retarding  agent,  but  usually  growth  stops 
in  a  fermenting  solution  before  the  alcohol  concentration  is  sufficient 
to  have  much  influence. 
Yeast  Crops. — The  cause  of  the  final  cessation  of  yeast  growth 
in  malt  wort  is  usually  lack  of  oxygen,  but  it  is  evidently  possible 
to  arrange  conditions  under  which  the  yeast  crop  is  determined  by 
other  limiting  factors. 
