Recipe 04 · Polysaccharide · gelation
Seaweed's answer to gelatin, and arguably the better plastic: glass-clear, tougher, vegan, and it sets fast. The trick is patience with the heat — agar needs to be nearly boiling to dissolve, then it gels the moment it cools below body temperature.
Method
Whisk the agar powder into the cold water in a pan until no dry clumps remain. Add the glycerol.
Why: Like starch, agar disperses best in cold water; hot water makes gluey lumps.Heat, stirring, to a gentle simmer (90–95 °C). Keep it there, still stirring, for 2–3 minutes until completely clear.
Why: Agarose double helices only unwind near boiling. Cloudiness means undissolved agar, which becomes weak spots.Take off the heat, stir in colouring, and skim any foam.
Why: Foam and bubbles become opaque flaws in the film.Pour immediately into a tray or silicone mould to 3–4 mm. Work quickly.
Why: Agar starts gelling as it passes about 40 °C, which happens within minutes in a thin layer.Leave undisturbed for 15–30 minutes. It turns into a firm, wobble-free gel.
Why: Chains pair into double helices, and the helices bundle into a rigid network holding all the water.Leave uncovered in a warm, airy place for 2–3 days. It will lose about 90 % of its weight and shrink to a thin, clear sheet. Peel when firm and dry.
Why: Water leaves; the helical network collapses into a dense, glassy film. Glycerol keeps it from becoming too brittle.Under the microscope
Agar is extracted from red seaweeds (Gelidium, Gracilaria). Its useful part is agarose: a long, unbranched polysaccharide of repeating galactose and 3,6-anhydro-galactose units. That odd second sugar puts a kink in the chain that makes it want to coil into a helix.
In near-boiling water the chains are free random coils. As the solution cools, pairs of chains wind around each other into double helices, and those helices then stack side by side into thick bundles. The bundles cross-link the whole solution into a gel — a very firm one, since agarose gels at concentrations as low as 0.5 %.
The striking thing is the hysteresis. Agar sets around 32–40 °C but will not melt again until about 85 °C. Once the helices have bundled, it takes far more energy to pull them apart than it took to form them. That is why agar jellies survive a hot day and why microbiologists grow bacteria on agar plates in a 37 °C incubator without the medium melting.
Dried, the network becomes a hard, clear film that is stronger than gelatin's and, unlike casein or starch, fully plant-based. Glycerol plays the same role as always — spacing the chains so the film can flex — and agar needs rather a lot of it.
Troubleshooting
Keep going
Hard, mouldable casein solid
Clear, flexible protein film
Translucent, tunable starch film
Back to the notebook for the chemistry behind all four.