Sodium Alginate

A thickening and gelling agent that forms heat stable gels in the presence of calcium. This property allows cooks to make small gelled spheres, in a technique known as spherification. Sodium alginate has been used in the food industry for many years for the production of gel-like foods – for example, the pimento stuffing in prepared cocktail olives.

What is it?

Alginate, or alginic acid, is extracted from brown algae. Commercially, it is most commonly available as a sodium salt. It is composed of long strands made up of carbohydrate units – these long stands allow it to act as a very efficient thickening agent at low concentrations (e.g. 1%). Gels formed from alginates have the amazing ability of withstanding heating to temperatures as high as 150ᵒC without melting, allowing them to be used in hot applications such as broths.

How does it work?
When alginate is added to a liquid, it will act as a thickener. In the presence of calcium ions, a mixture containing alginate will form a gel. The calcium ions insert themselves between individual alginate strands and will allow them to interlock and form a gel, in an arrangement similar to an egg box.

How should it be used?
Alginate needs to be properly dispersed in a liquid in order to act as a gelling or thickening agent. This is most easily done by adding the powder little by little to the liquid, while mixing constantly with an electric hand blender. Dispersion is best achieved when the alginate is pre-mixed with sugar or a bulking agent such as maltodextrin. If the alginate is not properly dispersed, small lumps will be visible when the solution coats the back of a spoon. Excessive whisking of the base liquid during preparation should be avoided, since the air bubbles tend to stay trapped in the solution due to its thickness.

Alginate should be added to the liquid at a final concentration of 0.5 – 1%. The solution however, does not need to be heated with the additive in order to gel (unlike agar and carragenan) so can be used with raw preparations. When this solution is pipetted into a solution of calcium chloride or calcium lactate (at a concentration of 1-2%), small beads or gelled threads can be formed. These beads can be then be removed from the setting bath, rinsed and then consumed immediately to provide a solid bead with an liquid centre – the viscosity of the liquid inside will depend on the amount of alginate initially added.
The addition of dairy ingredients or tap water to the alginate base should be avoided – dairy ingredients and “hard” tap water contains calcium, which may trigger early gel formation, producing lumps in the mixture and preventing the subsequent bead formation. Similarly, the addition of salt to the alginate base should be avoided – alginates are extracted from seaweeds in a sodium form, so in the presence of salt (which contains sodium ions), the alginate will tend to stay in its sodium form and be less likely to form a gel in contact with the calcium.

Reverse spherification is where the calcium, rather than the alginate, is added to the base preparation and this mixture is pipetted or spooned into an alginate solution (which will be thickened by the alginate). This technique is becoming increasingly popular and allows dairy ingredients to be included in the base preparation.

The ability of an alginate solution to form these beads in contact with calcium is due to their ability to form a gel instantaneously on contact with calcium. This also makes it very difficult to form a solid gel (like those formed with agar and gelatine), because the gel will set immediately on adding the calcium, and before it has time to be decanted into the recipient. This can be resolved by adding to the mixture a calcium sequestrant (this will hold the calcium out of solution for a while, allowing the gel to be placed in the appropriate container, and will gradually start to release calcium allowing an even gel to form).

Who uses it?
The most famous example of using sodium alginate to make spheres was by Ferran Adria, from El Bulli. It has since been used by many chefs around the world.