Reverse spherification was invented by El Bulli in 2005 as a way of spherifying liquids containing calcium such as milk and yoghurt. Instead of adding the calcium to the water bath, they instead added it to the flavoured liquid itself which was then 'cooked' in a sodium alginate bath. It has since become the preferred method of spherification for many molecular chefs. This is because, unlike basic spherification, where the spheres continue to gellify over time until they are solid, reverse spherification only affects the outside of the liquid, creating a gellified surface which contains a liquid centre.
Here are 6 key things you need to know to get to grips with reverse spherification.
1) Reverse spherification uses calcium lactate instead of calcium chloride
Calcium chloride has a slightly bitter taste which could ruin the flavour of the spheres. In basic spherification this isn't a problem as it is added to the water bath and only a tiny amount touches the spheres that are consumed. However, in reverse spherification the calcium is added to the flavoured liquid, and as such a flavourless compound is preferred such as calcium lactate.
2) Reverse spheres last longer and can be macerated overnight
As mentioned above, reverse spherification create spheres that last much longer than basic spherification spheres. This has many advantages, not only the ease of preparing them in advance. For instance, instead of storing the spheres in water, try macerating them overnight in another liquid to give just a hint of another flavour on the outside of the spheres. This would work well with strawberry spheres macerated in balsamic vinegar, or parmesan spheres macerated in truffle oil.
3) Reverse spherification can cope with acidity
Unlike basic spherification which will not work if the acidity of the flavoured liquid is higher than PH3.6, reverse spherification can be used with acidic liquids. This comes in immensely useful when trying to spherify any kind of fruit puree.
4) Use frozen reverse spherification for perfectly round spheres
Reverse spherification creates a slightly thicker membrane to start off with than basic spherification. This makes it easy to give a domed shape to the surface of the sphere, like an egg yolk. To create more rounded, perfect spheres, frozen reverse spherification is also an option. Pour the liquid into a half-sphere mould and freeze until solid. When it comes to 'cooking the spheres', heat the alginate bath to 65°C to allow the sphere to defrost whilst it is submerged. It can take 2-5 minutes to 'cook' the spheres, depending on their size. Remove the spheres using a spherification spoon and wash them gently in warm water.
5) The flavoured liquid may need thickening with xanthan gum
Sodium alginate makes the bath very viscous. This means that the flavoured liquid also needs to be viscous in order to penetrate the bath without breaking up. This is particularly true when creating caviar. Add 1-1.5% xanthan gum to the mixture after you have added the calcium lactate - whizz in a food processor for 1 minute to disperse evenly. The liquid should have the viscosity of double cream.
6) Prepare the bath and the liquid the day before
Patience is key - both the bath and the liquid will need to rest in the fridge overnight to eliminate the air bubbles. Otherwise, the liquid may float in the bath and not be fully submerged. It is also more likely to break up if there are air bubbles affecting the viscosity.