Sodium Alginate is a natural polysaccharide product extracted from brown seaweed that grows in cold water regions. In presence of calcium, sodium alginate forms a gel without the need of heat.
In Basic Spherification, the gelling occurs thanks to the diffusion method in which the crosslinking calcium ion diffuses from an outer reservoir into an alginate solution. Gels form when a calcium salt is added to a solution of sodium alginate in water. The gel forms by chemical reaction, the calcium displaces the sodium from the alginate, holds the long alginate molecules together and a gel is the result. No heat is required and the gels do not melt when heated. The gel coating is formed inside the droplet. Because the calcium ions continue diffusing towards the center of the droplet even after removing the sphere from the calcium bath, the gelification process continues and will eventually form a solid gel sphere.
In Reverse Spherification, the calcium ions diffuse from the droplet into the alginate bath, forming a gel coat outside the droplet of flavored liquid. Because the calcium ions are diffusing from the inside out and no alginate molecules are getting into the droplet, the gelification process stops as soon as the spheres are removed from the alginate bath. This allows you to store the spheres for later use.