14 THE CHEMISTRY OF THICKENING AND GELLING Substances that gel span a wide range of chemical compositions, but they all exhibit similar behavior. They contain long polymer molecules that can link up to create a mesh that traps water molecules. The stages of the process shown below illustrate how alginate gels with a calcium coagulant. Other gels work a bit differently, but always involve interlinking polymer molecules.
Gelling agents contain long polymer molecules. It takes some effort-heat or stirring, or both-to disperse them and surround them with water molecules. a process called hydration. Prior to gelling. the long molecules typically thicken the liquid as they slide past each other like strands of spaghetti in a pot.
In order to form a gel. the molecules must stick to each other, either directly or through a coagulant such as calcium ions (red spheres). Think of the gel molecules as being like egg boxes. and the coagulant molecules like eggs. Without eggs present. the boxes can't stick to each other; with them, the boxes will connect together to form a gel network. Some gels don't need a coagulant-in that case. the egg carton analogy isn't as apt: the molecules just stick to each other.
The gel sets as water molecules become trapped in the molecular mesh like fish caught in a net. The bonds among the molecules are like the knots that connect strands to make a fishnet.
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