At What Temperature Does Agar Boil and Set?

Agar does not disperse on its own in a hot liquid; the liquid must reach a boil for the agar to dissolve. It begins to dissolve above about 90°C / 195°F, and after the mixture starts bubbling, keep it there for a few more minutes, stirring continuously. During this time, the powder becomes invisible and the mixture turns smooth. In a mixture that never reaches a boil, the agar dissolves only partially, and the gel may not set at all or may set unevenly. For this reason, agar cannot be added directly to a cold mixture that will not be boiled. If the mixture foams as it boils, lower the heat slightly without stopping the boil completely.

Boiling time varies with the mixture. A few minutes is enough for a thin liquid such as water; thicker mixtures such as milk and fruit purée take a little longer because heat penetrates them more slowly. Keep stirring or the agar will settle on the bottom of the pot and burn there. Choose a heavy-bottomed pot and keep the heat at medium; high heat increases the risk of scorching on the bottom. Agar loses some of its strength in acidic mixtures such as fruit juice, so increase the amount slightly.

The setting behavior is very different from gelatin. Agar begins to gel around 35–40°C / 95–105°F, close to room temperature. It sets on the counter even without putting the mold in the refrigerator; chilling only shortens the time. This quick setting is both convenient and challenging: pour the mixture into the mold while it is still hot, or it will begin setting as you pour and the surface will be rough. For this reason, prepare the molds and line them up on the counter before putting the mixture on the stove.

The melting temperature is also high. Once set, agar gel will not dissolve again until the temperature rises above roughly 80°C / 175°F. This keeps agar desserts from falling apart on the table on hot days; gelatin desserts do not have the same property. However, if you want to melt an agar gel and reuse it, put it in a pot and bring it back to a boil; simply warming it does nothing. The same property makes agar unsuitable for desserts served hot, because the gel does not melt in the mouth.

A mixture that does not set is easy to fix, which is one of agar’s most convenient qualities. Return the mixture to the pot, add a little more agar, bring it back to a boil, and pour it into the mold again. To check the texture, drop one small spoonful of the mixture onto a cold plate; if it sets in one or two minutes, the rest will set as well. This small test costs less than remaking an entire mold. If you chill the test plate in advance, the result appears even faster and you can decide sooner.