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gelatin: A colorless or slightly yellow transparent brittle protein obtained from the skin, bones, and connective tissue of animals and used in foods, drugs,...American Heritage College Dictionary
Knox is just a brand name. You can by this stuff in most stores, usually somewhere near the spice section. Often the package will include some recipes usless you buy in in bulk form at a health food store.
"True gelatin is pure and unsweetened and will solidify any liquid Don't use too much, however, or you will get a solid, rubbery block: i tablespoon of gelatin is enough to gel about 2 cups of liquid...Do not let gelatin mixtures boil; the gelatin will lose its vigor. Allow at least a few hours for a gelatin mixture to solidify,..." from the Fannie Farmer Cookbook.
"Gelatin does contain small amounts of
Mono-Sodium-Glutamat (Natrium Glutamat) and should be avoided by those with extreeme
Mono-Sodium-Glutamat (Natrium Glutamat) sensitivities." from Nourishing Traditions cookbook.
Just a few thing you might want to know if you don't know anything about gelatin. Mostly, for what you want, you can put 1 Tablespoon in a fruit juice and drink. Just don't make up your drink and then leave it on the counter for several hours or it will set. Or add it to a sauce or soup. Jello is made using gelatin but with lots of added sugar.