Am.  Jour.  Pharm.) 
February,  1920.) 
Yeast  Growth. 
109 
that  the  increase  should  be  the  same  whatever  the  seeding  is.  The 
non-multiplication  in  this  case  is  probably  due  to  lag  in  growth  and 
the  very  rapid  accumulation  of  retarding  influences. 
Again  Stern  shows  that  yeast  crops  are  almost  independent 
of  the  temperature  of  growth.  The  temperature  coefficient  of 
growth  and  that  of  fermentation  over  the  range  of  temperature  in 
these  experiments  are  not  equal,  but  approximate  closely  enough 
to  account  for  the  results  obtained. 
Death  Rates. — When  yeast  growth  has  finally  ceased  the  cells 
suspended  in  the  fermented  liquid  gradually  perish.  If  the  liquid 
is  kept  free  from  other  organisms  a  few  living  yeast  cells  may  still 
be  found  after  many  years'  preservation.  Very  little  information 
regarding  death  rates  of  yeast  cells  is  available,  but  experiments 
with  bacteria  show  that  micro-organisms  under  unfavorable  condi- 
tions usually  perish  at  a  logarithmic  rate  (H.  Chick,  1908,  19 10). 
In  investigating  alcoholic  fermentation  at  high  temperatures 
or  in  the  presence  of  poisons,  death  rates  and  rates  of  inactivation 
come  prominently  into  consideration.  The  process  of  pasteuriza- 
tion, the  preservation  of  pitching  yeast  and  pure  cultures  of  yeast 
depend  on  death  rates  which  have  not  yet  been  investigated. 
Growth  and  Fermentation. — The  main  factors  which  determine 
the  rate  of  fermentation  during  any  of  these  periods  are  the  number 
of  cells  present,  the  fermentative  activity  of  the  yeast  and  the  tem- 
perature. The  rate  of  fermentation  is  independent  of  the  sugar 
concentration  except  in  dilute  solutions.  Sugar  concentrations, 
therefore,  conie  into  consideration  only  at  the  end  of  fermentation. 
Maltose,  the  principal  sugar  in  malt  wort,  is  hydrolyzed  suffi- 
ciently rapidly  by  culture  yeasts  to  supply  adequately  the  yeast 
cell  with  glucose.  Dextrin  is  not  fermentable  by  yeast,  but  there 
exist  in  malt  wort  substances  intermediate  between  dextrin  and  mal- 
tose which  are  hydrolyzed  slowly  by  yeast  and  subsequently  fer- 
mented. Little  is  known  of  the  rate  of  fermentation  of  such  malto- 
dextrins  or  how  they  are  hydrolyzed. 
If  living  yeast  is  introduced  into  wort  or  into  a  solution  of  glu- 
cose, fermentation  starts  immediately.  Quiescent  yeast  cells  are 
usually  rather  more  active  than  the  smaller  growing  ones.  Fer- 
mentation does  not  become  visible  until  the  solution  is  saturated 
with  carbon  dioxide,  and  hardly  shows  on  a  saccharometer  until 
some  gas  has  escaped,  but  proof  of  immediate  action  is  obtained 
by  other  means. 
