Agar is like carrageenan in that it’s extracted from seaweed. Only where carrageenan was used mostly in Ireland and Scotland until about 1930, agar was the go-to gelling agent in southeast Asia. Indeed “agar”, or actually “agar agar” is the Malaysian word for “jelly”. Makes sense to me! Since it’s “discovery” by Western scientists in 1882 it has been the preferred medium for growing bacteria in petri dishes. There’s a little bit of trivia you can throw out at the dinner table this evening!
Despite its Asian roots it’s actually the French who use the most agar these days, in both home and industrial kitchens. The thing that makes agar so attractive as a thickener is that it’s easy to incorporate, you just sprinkle it into cold liquid and heat the mixture to near boiling. At that point the agar dissolves and sets into a sparkling clear gel as the mixture cools down. Like carrageenan, guar gum and xanthan gum, it’s about six times as potent as cornstarch. Agar’s most unique feature is that once it sets it will remain gelled up to 185 degrees Fahrenheit. Cooks also like it for making so-called “liquid gels” which are firm when at rest but which liquify when stirred or chewed.