Why does such a small amount of gelatin powder make such a huge amount of water “hold together’’ in a block of Jell-O? Nothing like that happens with similar amounts of salt or sugar.
Gelatin (Jell-O is a brand name for gelatin desserts) is derived mainly from the skins and bones of cows, pigs, and horses, which are boiled in water so that the collagen comes out. Collagen is a protein, which is to say that its molecules are made of long chains of amino acids. These are arranged in three mutually twisted chains - a sort of triple helix.
Adding various chemicals will separate the collagen triple helix into individual long chains, which dissolve more easily in water and form what’s known as gelatin. The powder form you buy in the store is dried gelatin.
When dissolved in liquid, the long chains of amino acids form a tangled network reminiscent of a big plate of spaghetti, with water molecules stuck to the spaghetti in layers. The huge surface area of all the strands means they can hold a lot of water, and because the strands are so tangled, you can pick it all up as a lump.
By comparison, sugar and salt are small molecules. They too will be surrounded by water, but they’re not connected to each other in long strings and you don’t get the vast tangled network that is a bowl of Jell-O.
There is an interesting connection between all of this and what fruits you cannot put in gelatin - notably fresh pineapple, papaya, kiwi, and figs. These fruits, if not cooked, contain enzymes that break down protein (hence their usefulness in tenderizing meat - soaking a cheap cut of beef in papaya or pineapple juice will make it much more tender).
If you put them into gelatin, you will break down the long protein chains and the gelatin won’t set.
Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to firstname.lastname@example.org or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.