With a gel membrane enclosing a thimbleful of brown liquid, spherified rum looks a bit like a dark chocolate Werther's Original candy or an oversized fish oil gel capsule. The difference, of course, is in the bite: popping the gel membrane on spherized food unleashes an explosion of flavor that's surprising even when you know it's coming. Food spherization seems like a quintessential concept for modernist cooking--it's as much about presentation as it is taste--but the technique has been around since the 1950s. Thanks to molecular gastronomy experimentation at El Bulli, a famous Spanish restaurant, food spherification has become an awesome (and totally weird) way to science up a dish.
Food spheres aren't really that tricky to create, as delicate and complex as they seem: with a few special ingredients available online you can easily turn a liquid into something you can hold in your hand. Even cooler: making those liquids out of solid foods, like mangos or olives or yogurt. Spherification started with simple liquids in the early 2003s; as El Bulli's chefs pioneered the spherification process, they tried out different sphere sizes and nicknamed them raviolis, mini raviolis and caviar.
The spherification process presented a problem for foods that already contained calcium: when dipped in a calcium bath, the filling would jellify just like the outer skin. No liquid, no explosion. El Bulli found a way to avoid that pitfall with reverse spherification, the process we saw demonstrated at the San Francisco Exploratorium's Gastronomy event. Reverse spherification works with foods that already contain calcium, like yogurt, and is thankfully easy to pull off at home.
A reverse spherification kit containing calcium and alginate costs $20. It includes small quantities of the necessary ingredients, but Modernist Pantry sells spherification ingredients in larger batches as well. The actual spherification process couldn't be much simpler, either: blend water and alginate to form an alginate bath, add calcium to the liquid mixture, and spherize.
It might be tempting to spherize, well, everything, once you get the technique down, but tutorials and recipes online point to some creative starters. Yogurt. Root beer noodles. Mozzarella. Olives. And then try the rum.