^m$llZ'S£rm'}  Acid  Fermentation  of  Milk.  479 
tion  does  not  help  the  curdling  by  rennet,  but  that  the  presence  of  a 
soluble  calcium  salt  is  necessary ;  it  is  immaterial  whether  the  salt  is 
phosphate  or  chloride,  etc. 
According  to  Mayer  (31ilchzeitung,  x,  36),  when  milk  is  heated  at 
75°  it  undergoes  a  change,  and  at  a  still  higher  temperature,  but  still 
much  below  100°,  it  loses  its  power  of  being  curdled.  Experiments 
made  by  the  author  show  that  milk  does  not  necessarily  quite  lose  the 
power  of  being  curdled  by  being  heated  at  100°,  although  the  time 
required  to  curdle  milk  so  treated  is  much  lengthened,  especially  with 
milk  of  less  than  the  usual  acidity.  The  reason  that  boiled  milk  will 
either  not  curdle  at  all,  or  requires  a  longer  time  to  curdle  than  fresh 
milk,  is  that  a  part  of  the  dissolved  calcium  salt  is  precipitated  as  tri- 
calcium  phosphate.  For  the  same  reason  curdling  of  milk  by  rennet 
is  also  prevented,  or  retarded,  by  adding  more  or  less  alkali.  In 
either  case,  the  property  of  being  curdled  by  rennet  may  be  restored 
to  the  milk  by  adding  acid,  passing  carbonic  anhydride  through  it,  or 
by  the  addition  of  a  soluble  calcium  salt.  The  author  confirms 
Schaffer's  statement  that  boiled  milk  treated  with  carbonic  anhydride 
curdles  more  quickly  than  fresh  milk. 
ON  THE  ACID  FERMENTATION  OF  MILK.1 
By  De.  Fokker. 
It  has  been  well  known  for  some  time  that  the  souring  of  milk  is 
caused  by  bacteria,  and  one  would  expect  that  by  this  time  the  specific 
germ  would  be  sufficiently  well  known.  But  it  is  not  so.  Hueppe 
and  Marpmann,  who  have  studied  the  subject  minutely,  are  of  opinion 
that  there  are  more  germs  than  one  that  possess  the  power  of  souring 
milk,  and  Grotenfeld  has  quite  recently  declared  that  the  milk-sour- 
ing germ  is  a  modification  of  ordinary  saprophytes,  or  that  its  proper- 
ties are  due  to  culture,  and  that  there  is  no  specific  germ  ferment 
causing  the  acid  fermentation  of  milk. 
Our  author  has  spent  some  considerable  time  in  experimenting  on 
the  subject,  and  his  paper  consists  of  a  short  description  of  these  ex- 
periments, and  a  resume  of  the  conclusions  he  draws  from  them.  He 
agrees  with  Grotenfeld,  and  thinks  his  own  experiments  justify  him 
in  maintaining  that  the  role  played  by  the  germ  in  milk  souring  is  a 
1  Fortschritte  der  Medicin,  June  1st,  18S9  ;  translated  by  W.  A.  Stewart  for  the 
Medical  Chronicle,  July. 
