When  the  composition  of  flour  is  critically  considered,  it  may  fairly  be
asked  whether  indeed  there  is  real  necessity  for  such  substances  as  yeast
No  Need  for  a  foods.  From  a  large  number  of  analyses  it  is  found  that
Yeast  Food.  flour  contains  an  average  of  2  to  3  per  cent  of  sugar,  which
is  said  to  consist  of  maltose  and  cane  sugar  in  about  equal  proportions.
Now  Dumas  has  shown  that  the  amount  of  carbon  dioxide  required  to
raise  an  ordinary  dough  can  be  obtained  from  less  than  1  lb.  of  sugar  for
each  100  lb.  of  flour.  Again,  the  soluble  proteins  in  flours  vary  between
the  limits  of  l'5-2'5  per  cent;  that  is  to  say,  in  1  sack  (280  lb.)  of  flour
there  is  present  from  4  to  7  lb.  of  soluble  nitrogenous  matter  ready  for
absorption  by  the  yeast  plant.  Apparently,  then,  there  is  sufficient  to
permit  of  a  fivefold  to  sevenfold  increase  in  yeast.  Even  should  this
quantity  of  protein  become  exhausted,  there  remains  the  gluten,  which  we
know  from  experience  is  slowly  converted  into  soluble  forms  during  fer-
mentation, and  may  in  that  condition  become  suitable  for  absorption  by
the  yeast.  The  figures  clearly  show  that  the  crude  materials  required  for
fermentation  already  exist  in  flour  abundantly  sufficient  to  satisfy  the
requirements  of  bread-making.  Why,  then,  add  more?  The  answer  has
been  given  above,  viz.,  for  greater  speed  of  fermentation.
Potatoes

Potatoes  must  be  considered  as  the  earliest  yeast  food.  Where  and
when  they  were  introduced  no  records  now  exist  to  show,  but  it  is  certain
Composition  that  until  recent  years  potatoes  were  universally  in  use  for
of  Potatoes,  ^j^jg  purpose  throughout  England.  The  mean  composition  of
raw  potatoes  is  given  by  Konig  as  follows: —


YEAST  FOODS


227


Water.
Proteins.
Fat.
