Ahh Jell-O, (jelly if you're from the UK like me). So tasty in its lovely wobbly form. So disgusting if you start to think about what is actually in it. Yup, jelly is made from cow or pig hooves, bones and connective tissue, all ground up and boiled. Yummy.
The basic recipe for jelly is gelatin, water, sugar, flavorings and food coloring. The gelatin is the important bit. Gelatin is basically a processed version of the protein, collagen. You have probably heard of collagen before - this protein makes up a third of your body and is found in skin, bones and tendons, making them slightly elastic. As you get older, the collagen deteriorates and that's why our joints get stiffer and wrinkles start to appear in our once smooth skin.
So how do they make gelatin from collagen? To start with, they grind up all the animal parts and treat them with a strong acid or base. After this, it all gets boiled and the collagen is partly broken down. The gelatin can be skimmed off the surface of the liquid, dried into a powder and used in jelly.
As I mentioned, collagen is a protein. It is made up of long chains of amino acids, with every third amino acid being glycine. Lots of the other residues are proline or hydroproline. It lacks tryptophan, an essential amino acid that we can't make ourselves, so isn't a very good nutritionally source for humans, but who cares? So these long protein chains exist as a triple helix at room temperature - imagine this like three bits of spaghetti all twisted together. The chains in the helix are covalently linked together. An interesting collagen fact - you know those people who have obviously spent too much time in the sun and have developed nice brown leathery skin? This is caused by the sun adding more crosslinking to the chains of collagen in their skin, making it all horrid and tough.
When you make jelly, you add boiling water that provides the gelatin molecules with enough energy to break the bonds holding the chains together. Now, jelly is pretty amazing stuff as it is capable of holding around 10 times its own weight in water. This occurs because, as the jelly cools down, the helices start to reform and crosslinking occurs. The chains form areas of tangled protein with pockets between them - the water gets trapped in these pockets. This is what gives the jelly its characteristic wobble!
Something I should point out now is that jelly isn't a solution, but is actually a suspension of tiny particles that are in constant motion in the water (this is called Brownian motion if you're interested). You can tell the difference between a solution and a suspension by shining light through them - a solution will not scatter light while a suspension will. The proper name for the state jelly exists in is a semisolid colloid. It possesses an interesting property called thixotrphy - this means that it has a solid like appearance until it is shaken upon which it acts like a semi-fluid colloid. Try the Solid or liquid? experiment and read the experiment's Why does this work? section to find out more about colloids.
I was asked why, when Jell-O is made, you can get a think gelatinous
skin at the bottom of the bowl after it has set? As the jelly
begins to set, most of it is still a liquid while some bits have
started to crosslink. Any big chunks of jelly won't be supported
by the liquid and will sink to the bottom. Bits of jelly that
haven't been properly melted will also sink before the jelly has
a chance to set. I would guess that the skin is a thick layer
of gelatin which has no water trapped in it. Try poking it and
see if it goes 'Moo'.......
