Where to start?
Espumas can be made from purees, creams and extracts, from juices and even from soups and sauces. The most important element in creating a foam is the binding agent, also known as the stabilizer. Without this additive, the foam lacks the right consistency and can quickly collapse. Different stabilizers are needed depending on whether you are making cold or warm espumas.
Stabilizers are divided into categories according to the base:
... fat-based stabilizer
This group can be sub-divided into two groups: animal and vegetable fats. The best-known example of a foam bound by animal fat is whipped cream. Foie gras espumas are also bound by their high fat content. Therefore they do not need any additional stabilizers. Many cream substitutes, on the other hand, require the addition of vegetable fats, such as cocoa butter, to create a more stable foam.
... starch-based stabilizer
Potato and vegetable starch, rice and wheat flour can be used for both cold and warm foams. They are suitable for creating soups as well as batters.
... egg-based stabilizer
If eggs are being used as a binding agent, you can create either cold or warm foam. For warm applications, the maximum storage temperature needs to be 65 °C / 149 °F in order to keep the mass from solidifying. You can use egg whites to prepare quick meringues right in the iSi Whipper. Beaten egg whites can also be used to lighten souffles and batter. Popular uses for egg yolk are foamed sauces and creams, such as hollandaise and zabaglione. The sauces are emulsified by the egg yolk. They can then be kept warm in the iSi Whipper, in a water bath at 65 °C / 149 °F without becoming runny. Water-soluble proteins, such as egg white powders, can also be used as stabilizers when they are warm– for instance, in warm chocolate mousse.
... gelatin-based stabilizer
One of the best known stabilizers is gelatin. It can be used to bind purees and juices without affecting their flavor. However, gelatin can only be used for cold applications. It is thermo-reversible; ie: the firmed mass can always be remelted if it is heated up. When it is cooled below 25 °C / 77 °F, the binding agent takes effect again – making a perfect, thick foam. A vegan alternative is Agar-Agar (E 406), which is derived from seaweed. It gels at 45 °C / 113 °F, so it can also be used warm. For the best effect, both Agar-Agar and gelatin need three to five hours to cool. The natural thickening agent Xanthan (E 415) is also an excellent stabilizer. It is heat-stable and binds foods as soon as they are prepared.
The amount of any stabilizer is key for consistency.
The firmness of the foam depends on its ingredients and on the resting and/or cooling time. Depending on how much binding agent was used, the same product can have very different degrees of firmness. They can range from a firm mousse to a creamy sauce or a liquid soup.