Ano'ctXr,Pi9oTm'}    Deterioration  of  Artificial  Foods.  479 
free  from  foreign  matter  most  of  them  remain  unchanged,  but  com- 
mercial specimens  gradually  turn  rancid.  This  alteration  is 
generally  attributed  to  the  presence  of  certain  foreign  matters,  such 
as  the  cellular  substance  of  the  animal  or  plant  from  which  the  oil 
was  extracted  ;  volatile  fatty  acids  are  set  free.  Max  Grager  con- 
siders that  rancidity  is  due  to  the  oxidation  of  fatty  acids  and  gly- 
cerin in  presence  of  traces  of  water." 
Decomposition  of  this  kind  is  favored  by  continued  exposure  to 
high  temperature,  such  as  being  placed  on  a  shelf  which  adjoins  a 
chimney  flue. 
Fermentative  changes  and  alterations  produced  by  the  agency  ot 
micro-organisms  are  of  rare  occurrence  unless  the  product  has  be- 
come damp,  either  from  being  packed  in  containers  which  were  not 
thoroughly  dried,  or  by  the  absorption  of  moisture  from  being  kept 
in  a  damp  place,  or  the  packages  themselves  becoming  wet  through 
accident.  Where  the  container  is  air-  and  moisture-proof  these  lat- 
ter causes  are  eliminated  from  consideration. 
Mould  growths  will  take  place  in  the  presence  of  10  per  cent,  of 
moisture,  while  bacteria  will  not  flourish  in  the  presence  of  less 
than  50  per  cent,  of  moisture  except  in  the  presence  of  sugars, 
when  the  limit  is  reached  with  30  per  cent,  of  moisture. 
When  fermentative  changes  have  once  set  in  it  is  difficult  to 
retard  their  operation. 
There  are  some  species  of  bacteria  that  will  flourish  after  having 
been  subjected  to  a  pressure  of  600  atmospheres  for  twenty-four 
hours,  and  on  the  other  hand  many  of  them  will  thrive  better  in  the 
absence  of  oxygen  than  when  freely  exposed  to  the  air.  Fermenta- 
tive changes  alter  the  nature  of  the  product,  but  seldom  evolve  any 
products  of  a  harmful  nature. 
It  is  the  putrefactive  changes  which  are  most  to  be  feared,  for 
they  involve  the  nitrogenous  or  proteid  matter  and  often  produce 
toxic  substances  such  as  ptomaines,  or  the  so-called  cadaver  alka- 
loids. 
The  cases  in  which  putrefactive  changes  have  taken  place  are  of 
rare  occurrence,  however,  on  account  of  the  large  amount  of  mois- 
ture necessary  for  their  successful  accomplishment.  Then,  too,  such 
alterations  are  usually  accompanied  by  the  production  of  sul- 
phuretted odorous  compounds  which  give  warning  of  the  change 
which  has  occurred.    The  first  step  in  putrefaction  is  the  peptoniza- 
