how to make skim milk soluble?

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An old J. Bacteriology article (1922 or 1923) by Brown and Howe describe 
a transparent milk media composed of 1 part skim milk and 2 parts water 
to which 0.4% sodium citrate has been added.  The media can be autoclaved 
if pH is adjusted to 6.8.  I have prepared a similar media using non-fat 
dried milk powder.  The recipe is 4% dried milk powder, 0.4% sodium 
citrate. Bring to volume and mix.  Let set until proteins go into 
solution.  The media turns a yellowish green.  Add any other ingredients 
such as agar, etc and adjust ph and autoclave