They are two kinds of gels: physical gels and chemical gels. Don’t be afraid of the words. A chemical
gel is one that does not break down upon heating once it is formed. Physical gels are like gelatine
gels: they can melt on heating, but will reform again.
Cooks know both kinds of gels well, but often they do not realise that they know.
Cooking egg white, for example, makes a chemical gel, as does cooking meat. When blood
coagulates it forms a chemical gel. In theses cases, the gelling agents are proteins.
On the other hand, jam is a physical gel, as well as bechamel sauce that is kept in the fridge. In jam,
the gelling agent is called “pectin”. In a cold bechamel sauce, the gelling agent is starch (it would be
more appropriate to say amylose and amylopectin, as we now know that these two molecules are the
two constituents of flour starch).
But these traditional agents are not the only ones to promote jellification. In some countries, it has
been known for a very long time that algae are also able to change water into a gel when they are
cooked in it. This has led to the discovery of a large number of gelling agents, which are being used
more and more in the kitchen today. These include agar, guar and tara gums, carrageenans,
alginates… Want to see more about them?
Other thickening agents
There is a major difference between jellification and “thickenisation” of a liquid. Thickening means that
the viscosity of a liquid is increased, but not to a point of making the liquid a solid (jellification).
Cooks have known for a very long time that adding egg yolks to a light sauce and heating the sauce
slowly can “thicken” it.
Traditionally, the thickening agents were eggs, various starches (potato starch, flour, rice starch…),
vegetable purees etc…
Three main cases should be considered. First, some compounds like guar gum or gelatine when
added to hot sauces increase the viscosity (see this word in the glossary) of the sauce because they
bind to a lot of water molecules. Second, some products such as starch swell in hot water, and the