Foams (or espumas, as they are sometimes known) are mousses that have been whipped more than usual, giving them varying levels of stability.
Siphons are used to dissolve a gas in a mixture made using any ingredient, making them a particularly flexible tool.
The factors which determine the amount of air in the “mousse” and its stability include the amount of fat it holds, its viscosity, the structure of the gel or emulsion, the amount of incorporated gas and the product’s lifespan once the siphon is pressurized.
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Expansion when whipped
Sprinkle the gelatin onto the smaller portion of blueberry purée (40°F or 4°C), then leave it for 15 to 20 minutes until it is hydrated.
Heat the mixture to between 120 and 140°F (50 and 60°C) so that the gelatin melts, add the sugar, then combine it with the second portion of purée (40°F or 4°C).
Homogenize with a mixer. Pour it into a siphon, put in place 2 gas cartridges and store in the refrigerator.
For an original tasting experience, freeze the espuma after extracting it, and serve it iced