Milk  Drying  ]\[achine. 
195 
A mere  mechanical  description  of  the  ])rocess  gi/asi  machine 
fails  to  convey  a proper  idea  of  the  problems  involved,  which 
concern  the  chemist  ami  bacteriolooist  more  than  the  mechanic. 
I therefore  oive  the  followino-  extracts  from  a letter  received 
from  Mr.  Hatmaker  in  reply  to  my  request  for  information:  — 
This  is  tlie  lirst  process  whereby  milk  has  been  dried  rapidly  and  which  has 
employed  a temperature  in  excess  of  212“  F.  rrevions  processes  have  limited 
themselves  to  much  lower  temperatures,  for  it  has  always  been  taught  and 
believed  that  milk  could  not  be  dried  successfully  by  the  use  of  a high 
temperature. 
But  the  truth  is  the  reverse  : milk  cannot  be  dried  successfully  except  by 
the  employment  of  a high  temperature.  Milk  is  a very  changeable  liqui(l, 
which  is  not  at  all  suited  to  slow  metluxls — particularly  to  slow  methods 
employing  heat — for  under  such  conditions  bacterial  development  is  very  rapid, 
and  very  detrimental  changes  occur  in  the  milk  constituents.  For  this  reason 
no  good  dry  milk  has  ever  been  obtained  by  a slow  process.  The  Just-Hatmaker 
process  is  based  upon  the  discovery  that  milk  can  be  reduced  to  dryness  almost 
instantaneously,  without  in  any  way  injuring  the  most  delicate  of  its  con- 
stituents, if  it  is  suitably  exposed  to  the  influence  of  a temperature  in  excess 
of  212“  F.  In  carrying  it  out,  milk  at  an  ordinary  temperature  is  allowed  to 
fall  continuously  in  thin  streams  between  two  slightly  separated  revolving 
cjdinders  having  a surface  temperature  above  the  boiling  point  of  water.  In 
accordance  with  the  principle  of  the  calefaction  of  liquids  the  high  temperature 
of  the  cylinders  causes  the  continuous  formation  of  a layer  of  steam  upon  their 
surfaces,  which  keeps  the  solirl  constituents  of  the  milk  from  direct  contact  with 
the  said  surfaces.  The  escape  of  the  steam  thus  generated  keeps  the  limited 
quantity  of  milk  that  is  maintained  between  the  cylinders  in  violent  agitation. 
The  .somewhat  condensed  milk  is  drawn  between  the  cylinders  as  they  revolve 
and  is  spread  out  in  a thin  uniform  film  and  carried  thereon  until  nearly  all  its 
water  is  driven  off. 
The  exposure  must  not  be  too  long,  for  it  must  not  be  overlooked  that  dry 
milk  contains  a large  percentage  of  milk-sugar  and  also  milk-salts — substances 
requiring  water  of  crystallisation.  To  ensure  the  preservation  of  the  milk  solids 
in  a normal  state  the  hot  film  must  bo  removed  from  the  highly-heated 
cylinders  while  it  is  somewhat  moist. 
The  moisture  disappears  immediately  upon  cooling  and  the  dry  milk  is  then 
easily  reduced  to  an  uniform  powder  by  being  passed  through  a sieve.  This 
milk  is  almost  instantaneously  deprived  of  its  unnecessarj^  water  and  rendered 
into  dry  preservable  form  without  in  any  way  injuring  its  nutritive  qualities. 
The  process  thus  makes  milk  a staple  concentrated  fooil  which  can  be  quoted 
in  the  markets  of  the  world  like  flour  and  other  staple  foods. 
But,  happily,  the  process  goes  further — it  rids  milk  of  all  its  dangers  and 
makes  it  an  absolutely  safe  food  for  infants  and  others  by  completely  destroying 
all  its  germs.  It  has  been  proveil  repeatedly  that  no  germs  can  survive  the 
process.  The  dry  milk  as  it  issues  from  the  drying  machine  is  absolutely 
sterile.  This  means  nothing  less  than  the  absolute  prevention  of  the  communi- 
cation of  all  diseases  through  milk. 
The  commercial  article  as  now  ])laced  on  the  market  con- 
si.sts  of  either  whole  milk,  separated  milk,  or  a mixture  of  the 
two,  the  latter  being  the  one  ])robaldy  most  suitable  for 
reconstruction  by  the  addition  of  water  into  milk  for  general 
use.  As  one  hundred  parts  of  weight  of  new  milk  contain 
twelve  and  a half  parts  by  weight  of  solids,  the  dried  milk 
