io6 
Yeast  Growth. 
(Am.  Jour.  Pharm. 
\    February,  1920. 
containing  all  foods  necessary  for  yeast  growth  and  also  large  amounts 
of  fermentable  sugars,  the  yeast  cells  bud  and  grow.  If  the  num- 
ber of  cells  is  plotted  against  the  time  a  curve  of  the  type  given  in 
the  diagram  is  obtained. 
After  a  certain  initial  disturbance  (lag-phase)  the  cells  multiply 
regularly,  the  number  increasing  logarithmically  with  the  time. 
All  the  usual  equations  employed  to  calculate  the  rate  of  unimolecu- 
lar  chemical  reactions  can  be  applied  to  yeast  growth  over  this 
period,  if  note  is  made  that  the  reaction  is  increasing  in  rate  instead 
of  decreasing  as  is  the  case  with  ordinary  chemical  reactions.  There 
are  several  methods  of  measuring  this  logarithmic  constant  of  growth 
(K.)  or  the  generation  time  (G.  T.)  which  is  a  number  inversely 
proportional  to  K.  (G.  T.  X  K.  =  log.  2).  Some  of  them  de- 
pend on  counting  yeast  cells  under  the  microscope,  and  others  on 
measuring  rates  of  fermentation  from  which  rates  of  growth  can  be 
calculated. 
The  results  show  that  when  all  necessary  food  is  supplied  in  suffi- 
cient concentration  yeast  cells  develop  at  a  rate  determined  by  the 
temperature  and  the  rate  of  the  yeast  used. 
Temperature  coefficients  vary  greatly  with  the  temperature.  At 
25°  C.  the  rate  is  usually  about  four  times  that  at  15°  C.  The 
shortest  generation  time  so  far  observed  is  about  i  hour.  The  re- 
tarding influence  of  carbon  dioxide  and  alcohol,  and  the  necessity 
of  oxygen  for  yeast  growth  can  be  demonstrated  and  measured  by 
these  methods. 
Lag-phase  in  Growth. — If  yeast  cells  from  an  old  culture  are  in- 
troduced into  fresh  wort  a  period  of  quiescence  is  observed  before 
budding  takes  place.  When  growth  has  once  started  it  continues 
at  the  normal  rate.  A  bakers'  yeast  at  30°  C.  gave  a  period  of  one 
hour  before  buds  appeared  and  then  all  the  cells,  except  the  dead 
ones,  budded  irregularly  during  the  next  hour.  Older  cultures 
showed  more  dead  cells  but  not  a  longer  period  of  quiescence. 
The  lag-phase  in  the  growth  of  bacteria  has  been  examined  by 
Penfold  (1914)  who  used  the  method  of  "planting"  to  estimate  the 
number  present,  and  many  important  observations  were  made. 
Some  of  the  results  have  been  submitted  to  mathematical  analysis 
by  Ledingham  and  Penfold  (1914).  The  matter  has  been  further 
discussed  (Slator,  191 7),  but  an  investigation  of  the  lag-phase  in 
yeast   growth  (191 8),   leads   one   reluctantly   to  the  conclusion 
