Expert: J. Peter Clark - 10/19/2005

Question
Hello,
I am an artist working with milk. I have made glue out of
pasteurized skim cow's milk by curdling it with white vinegar
and low heat, then adding baking soda solution. The glue is the
protein, neutralized from the acidity of the vinegar.
Then I decided I wanted to make glue from human breast milk. I
have not been successful. I add white vinegar, and heat over low
heat, as I do so, I can see the milk separating in the pot, but the
curds don't gather and stick together like they did with the cows
milk. There are a number of variables which may be causing the
problem. I wonder if you can help.
Possible problems:
the breast milk I am using has been frozen, then thawed.
I am separating the fat off of the milk myself, and there may be
traces of fat left in the milk.
Human breast milk doesn't curdle like cow's milk?
It contains less protein?
Any clues, or advice on who else to ask, would be greatly
appreciated!
Thanks,
 Zoe Cohen.

Answer
Zoe,

What an intriguing question! You are doing very sophisticated biochemistry, whether you know it or not. When you add acid to a milk, you reduce the solubility of protein so it precipitates. Each protein has a specific pH at which it is least soluble. For most of those in milk it is 4.6. With the addition of more acid, some proteins go back into solution and even begin to break down. Human breast milk contains less protein than cow's milk (about 1.87% vs. 3.2%). The proteins are also distinctly different in that they are more easily digested than are the proteins in cow's milk. That is why unmodified cow's milk is inappropriate for infants. It is entirely possible that the differences in protein chemistry prevents human milk proteins from coagulating though they may still precipitate (the separation you observe). Fat should not interfere nor should freezing matter, in my opinion. You are breaking new ground, as I have not heard of people manipulating human milk as you are trying to do. I can only suggest that you experiment wtih different amounts of acid and then, perhaps, filtering to recover the protein. When you add the baking soda, you are restoring the pH and neutralizing the excess acid. The whole process is just a means of concentrating the protein away from the sugars and salts in the milk. In principle, it should work on human milk, but the conditions have to be determined. Good luck.