Calcification

It is possible to use a little culinary trick, calcification, which is the formation of calcium carbonate, to create a shell on the outside of a soft interior. As explained earlier, this can be done with pretzel dough, but it also works with tomatoes that have been blanched to remove their skins. This results in an amusing and surprising mouthfeel. The exterior feels firm and a little crisp, whereas the interior has a totally different structure. The trick is based on a bit of chemistry that involves a reaction between the carbon dioxide in the air and a base, lye, which is used to treat the surface of the ingredient.