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One of the most attention grabbing modernist techniques is spherification. It's the process of trapping a liquid inside of a gelled sphere. The gelling agents used in spherification only gel in the presence of certain ions, such as calcium or potassium.
There are two main spherification techniques, direct spherification and reverse spherification, depending on where you put the gelling ingredient. If you add the ingredient to the flavored liquid it is "direct spherification" and if you add it to the setting bath, it is "reverse spherification". Making spheres with direct spherification takes more practice than with reverse spherification.
Keep in mind when working with direct spherification, many bottled juices already have potassium or calcium added to them; this will cause the flavored base to set before it is added to the setting bath. For these juices, and other liquids with calcium or potassium present, you will have to use a sequestrant or use reverse spherification.
Sodium alginate is a good gelling agent to have on hand because it is very effective at both reverse and direct spherification. Once set, it also can be heated above the boiling point without melting, making it very versatile.
You can get more information about direct spherification from my guide on how to make direct spherification or from the direct spherification articles below.