Ways to solidify liquids
Gels are kind of improbable — turning a liquid into a solid.
Take gelatin for example. Add just 1% of gelatin (by weight) and you can turn water into something solid. A solid consisting of 99% water? Where did the water go? What did such a tiny amount of gelatin do with it?
Gelatin is a multi-stranded polypeptide (assemblies of amino acids). It is formed by the breakdown of collagen. Think of each gelatine molecule as wound up on itself and separate from the others. But when gelatine is added to water and heated, it unravels into a long and complex rod-like triple-helical structure. As it cools in water these long structures get entangled and bind to each other to form a 3-dimensional random cross-linked mesh. This mesh traps water and stops it from flowing. The large size and complex structure of the gelatin means that not a lot of it is needed to trap quite large amounts of water.
So the solid gel is just trapped water. In fact, so well-trapped that even if you cut it, it doesn’t leak. That’s improbable.
Gelatin belongs to a larger class of hydrocolloids (that form suspensions in water). There are a number of hydrocolloids now available to the cook, and they all have different properties.
For example, what makes a getlatin-set gel interesting is that it melts at ~35C, in other words it will begin to melt when placed in the mouth, creating flavour release and a pleasant mouthfeel.
But some gels remain solid until much higher temperatures (agar for example). Other gels will set not by temperature but in the presence of another agent (usually calcium). This is what is happening to the alginate in spherification (the calcium is the intermediary that links the separate alginates). Other gels have differing properties of elasticity, brittleness and hardness. Some can even be frozen and thawed. There is a hydrocolloid (methylcellulose — it’s in Metamucil) that sets when heated and melts when cooled — the exact opposite of what gels are supposed to do — a hot gel. Combining hydrocolloids can lead to multiple or complementary properties.
Using lower concentrations of hydrocolloids thickens fluids rather than sets them. Another range of possibilities.
Some common examples of gels that aren’t obviously gels: pureed potato, marshmallow, yoghurt, cooked pasta, cooked eggs.
But, here’s an application for gels that you might not have expected — clarification.