Water is...well, wet, right? So how is that with the addition of a small amount of a chemical, it can become a semi-solid, a gelatinous lump that makes for a great food? The reason for this is gelatin, a curious chemical that seems to defy physics and turn liquids into solids. Let's take a look at the chemistry of how you make Jell-O.
Most of us know gelatin from the dessert Jell-O. This popular treat has been around since the 1890s, and the main ingredient is gelatin, a complex chemical that has some interesting chemical properties. At room temperature, it is a solid. Heat it up to body temperature, and it becomes a liquid. So, when you eat it, it literally melts in the mouth. Gelatin itself has no taste: the flavor of Jell-O and other similar desserts comes from the added flavorings. Gelatin, however, is responsible for the release of these flavors as it melts and releases the liquid that it is holding onto, which contains the flavoring.
Gelatin is made from long strings of amino acids, the fundamental building blocks of proteins, with a bit of hydrogen attached. These long strings, each usually a few hundred amino acid blocks long, are generally fond of each other: at room temperature, they stick together in a formation called a triple matrix. This allows each chain to bond to several others, and form a complex 3D matrix. When you heat the gelatin up, these bonds between the chains loosen, turning the chemical into a liquid as they slide away from each other. But each other isn't the only thing that these gelatin strings like to stick to: they also have an affinity for water.
These gelatin chains have hydrogen atoms attached to their sides.These hydrogen branches can weakly bond with water. This isn't a true chemical bond: the water remains as good old H2O, but the oxygen atom is weakly bonded to the hydrogen atoms on the sticky-out branches of the chain. Chemists call this a hydrogen bond. If you give the water a bit more energy (such as heating it up), this hydrogen bond will break, and the water molecule will drift away. As the water cools, it slows down until this weak bond can be re-established, linking the water to the gelatin chain again.
In a cup of warm water, all of the water molecules are happily bouncing around. Add in some gelatin, and the gelatin will dissolve in the water, while the water molecules stay in motion. But as the water cools, the molecules slow down and start to bond weakly to the hydrogen on the gelatin chains. On each chain, there can be hundreds of these hydrogen branches sticking out, each of which could bond with a water molecule.
As the mix cools, these gelatin chains also connect to each other in a curious triple matrix structure. Eventually, these interlinked chains of gelatin form a huge 3D maze, with most of the water stuck to it. That's the gel form of gelatin and water: a complex matrix of loosely bonded water and gelatin strings, all stuck together to form a semi-solid. This is why a relatively small amount of gelatin can produce a lot of gel: because this 3D matrix can hold a lot of water and still be fairly strong, because the long chains can still connect when they are holding onto many more water molecules. There will still be small amounts of unattached liquid water in between the chains, but not enough to create a liquid.
If you heat the gel by putting it in the microwave or into your mouth, the water molecules get excited, and the weak bond is broken. The water molecules float away, and you get liquid water. The same happens with the bonds between the gelatin chains, breaking down the 3D matrix to form a liquid. So, the gel melts in your mouth.
Gelatin is created by the breakdown of collagen, a protein used by all animals and plants to bind cells together: about 30 percent of your body weight is formed by the collagen in the extracellular matrix that holds your cells together. Collagen is a very long chain of amino acids that bonds to itself in a triple helix structure, creating a flexible link that allows cells to move a little, but still maintains the strength of the overall tissue. It isn't very easily soluble in water, so fluids can move between the cells, carrying the nutrients that the cells need to live.
You go from collagen to gelatin by, to put it bluntly, boiling down the bits of animals that people don't eat. When you boil down tissue and bones, this collagen breaks down into gelatin, which has shorter chains and more hydrogen poking out from the chain (chemists call this process hydrogenation). After much boiling and filtration, this gelatin can be extracted.
This poses a problem for people, like me, who don't eat meat out of choice. Personally, I avoid eating gelatin if possible, as it is a side-product of the meat industry. Others don't feel the same, though, arguing that no animal was killed specifically for it, so it is just a by-product. It is rather hard to avoid gelatin, though, as it is used in many foods and products ranging from the gel caps around prescription meds and food supplements to camera film. That means that most people will come into contact with it on a daily basis. There are alternatives, though: our old friend sodium alginate (which we used in spherification) does the same thing as gelatin, but not as well. There are also carageenans, made from seaweed, and pectin from fruit. The Cooking Issues blog has a good overview of the pros and cons of the various alternatives.
Whether you choose to use gelatin or not, it is a curious piece of chemistry that shows how a slight change to a common chemical can make a big difference to how it acts.
