Re: How does jello gel?

Collagen is probably the most ubiquitous protein in animals. When heated, 
it breaks down into the protein gelatin.  Gelatin, like other materials 
that will gel, has the special ability to form large, lose networks that 
trap water - this is the gel form.  The unique aspect of the gelatin gel is 
that it forms the gel at temperatures below body temperature and the gel 
also "melts" below body temperature. So when you eat water gelatin desserts 
(e.g., Jello), the transition from a gel to regular liquid occurs in your 
mouth, which gives you a stronger flavor response than if you just "chewed" 
the material as occurs with dessert gels made with "vegetarian" gums. (The 
process is reversible.)  The protein is still in "solution" in the gel 
form. Gelatin's amino acid structure is unusual in that it has a large 
number of prolines and hydroxyprolines, which are actually imino acids and 
have a structure slightly different from amino acids - so the three 
dimensional structure of gelatin is somewhat different from most proteins. 
Nutritionally gelatin is also a poor protein as it is missing some of the 
amino acids that are essential for human growth.

The standard reference for gelatin is the book the Science and Technology 
of Gelatin by A.G. Ward and A. Courts, Academic Press. New York, 1977.