The Reverse Spherification technique consists of submerging a liquid with calcium content in a bath of sodium alginate. If the juice or flavored liquid does not naturally contain calcium, Calcium Lactate or Calcium Lactate Gluconate is added. There is also a variation of this technique, Frozen Reverse Spherification, that reduces preparation time, does not require practice and results in perfectly shaped spheres of consistent size.

Pros of Reverse Spherification

- Reverse Spherification is more versatile than Basic Spherification as it can make spheres with almost any product. It is best for liquids with high calcium content or alcohol content which makes them great for cocktails and dairy products like cheese, milk, cream and yoghurt.

- The resulting sphere is long-lasting and can be stored for later consumption. Contrary to Basic Spherification, the process of jellification can be stopped when the sphere is removed from the sodium alginate bath and rinsed with water. This is very convenient when entertaining friends as you can prepare them ahead of time. This also allows you to macerate the spheres overnight to add some extra flavor (e.g. in aromatized olive oil or truffle water).

- Reverse Spherification results in a sphere with a thicker membrane than with Basic Spherification. Thanks to this, the resulting spheres can be manipulated easily, they conserve their shape better when plated (spheres produced with Basic Spherification flatten and acquire an orb or egg yolk shape when plated) and can be used in more ways (e.g. as fillings in sponge cakes or mousses).

- Jellification still occurs when the main ingredient liquid has some acidity. This is because in Reverse Spherification the jelling process occurs on the surface of the sphere as the sodium alginate fails to penetrate it. A translucent layer of gel is created around the main ingredient. In Basic Spherification, the gelling process occurs internally and has the color of the main ingredient.

- The main ingredient consistency and flavor is not altered by the addition of calcium lactate gluconate and calcium lactate as they have no discernable flavor and dissolve in liquid without altering its density. This is the reason why in Reverse Spherification we don’t use calcium chloride which is very salty.

Cons of Reverse Spherification

- The thicker membrane of these spheres is more evident in your palate. You still get the “pop” sensation but in addition to the liquid you also feel the solid jelly in your mouth.

- The sodium alginate bath needs to rest for 12-24 hours before using it for Reverse Spherification to eliminate the air bubbles created by the process of dissolving the sodium alginate with the immersion blender.

- The flavored liquid may need to be thickened with Xanthan Gum and if air bubbles get trapped in the process, you may need to wait a few hours to eliminate them.

- It is a little harder to get a perfect sphere on the plate with Reverse Spherification than with Basic Spherification. You are pouring the main ingredient into a viscous bath, the spheres tend to stick to each other if you don’t separate them in the bath and the thicker membrane maintains the shape better which is great if you were able to create a perfect sphere but not if your spheres are “deformed” (for optimal results read "10 Tips to Create a Perfect Sphere"). Frozen Reverse Spherification solves this problem if the main ingredient can be frozen. Learn about Frozen Spherification here.

- Reverse spherification is not great for making caviar as the main ingredient needs to be thickened to be able to penetrate the dense sodium alginate bath and cohere into a sphere. (for optimal results read "7 Tips for Making Spherification Caviar")

Preparing the Bath for Reverse Spherification

Dispersing and Hydrating Sodium Alginate

To produce Reverse Spherification, you need a bath solution with 0.5% sodium alginate (0.5 g per 100 g of flavored liquid). Sodium Alginate, like most hydrocolloids, needs to be dispersed in the liquid and hydrated before it can gel in presence of calcium ions. This can simply be done by mixing the sodium alginate with an immersion blender or regular blender but you need to know a few things first to be successful:

- Use distilled water. Water cannot contain free calcium ions or the bath will thicken and gel as the calcium reacts with the sodium alginate before you drop the flavored liquid with calcium. Most tap water and spring water contain calcium ions.

- Water needs to be cold or otherwise the sodium alginate may hydrate and gel before it can get dispersed, resulting in gel lumps.

- To facilitate dispersion you can pre-mix dry sodium alginate with another powder ingredient such as sugar.

1- Blend the sodium alginate in 1/3 of water with an immersion blender until it is completely dissolved. Keep in mind that the sodium alginate is hard to disperse and hydrate in water and this process may take a few minutes. You can also use a regular blender, but include the entire amount of water and slowly add the sodium alginate into the vortex.

2- Then add the rest of the water and keep in the refrigerator for 12 to 24 hours to remove the air bubbles created by the blender.

Removing Air Bubbles

Mixing sodium alginate with any liquid usually results in many air bubbles trapped in the resulting dense liquid. Air bubbles are a problem because they may create weak points in the sphere membrane that will break or leak easily.

Here are a few ways to eliminate the air bubbles:

-Let it rest: this is the most common method. Just let the solution with sodium alginate rest in the fridge. Depending on the density of the liquid, this may take 1 to 24 hours.

-Pass it through a fine sieve: to speed up the process, you can pass the sodium alginate solution through a fine sieve. Let the liquid flow through it on its own without applying pressure. You may have to repeat this process a few times.

-Use a vacuum chamber: these are expensive but if you have access to one, you can place the liquid in the vacuum chamber to eliminate the air bubbles. This is definitely the fastest method.

-Using a magnetic stirrer instead of a blender will prevent the formation of air bubbles.

Preparing the Flavored Liquid for Reverse Spherification

The flavored liquid you use for Reverse Spherification needs to have enough calcium content (free calcium ions) that can react with the sodium alginate in the bath to form a gel membrane around the droplet. If the main ingredient already contains enough calcium, such as milk or cream, you may just need to adjust its density. If it doesn't, you need to increase the quantity of calcium ions in the solution by adding a calcium salt to obtain obtain a solution with 0.18% calcium. You will also need to adjust the liquid density as explained below.

Keep in mind that different calcium salts contain different amounts of calcium ions and therefore the amount you use to create the calcium bath will be different too. Also, since you are going to be adding the calcium salt to the liquid that is going to be inside the sphere, you need to make sure it doesn't ruin the taste. The most common calcium salts used in spherification and the concentrations to create the flavored liquid with enough calcium are:

-2% Calcium Lactate Gluconate: the preferred calcium salt for Reverse Spherification because it has no discernible flavor. Use 2 g per 100 g of water to create a 2% solution of Calcium Lactate. You can stir it or mix with a blender. Calcium Lactate Gluconate is included in the Molecular Gastronomy Essentials Kit, the Molecular Gastronomy Premium Kit and the Molecular Gastronomy Ultimate Kit.

-1% Calcium Lactate: mostly used for Reverse Spherification because it has better flavor than calcium chloride. Use 1 g per 100 g of water to create a 1% solution of Calcium Lactate. You can stir it or mix with a blender.

-0.5% Calcium Chloride: this is never used for Reverse Spherification since calcium chloride has a very salty and bitter flavor. But if you just want to practice and don't care about the flavor, use 0.5 g per 100 g of water to create a 0.5% solution of Calcium Chloride. Calcium Chloride is very hygroscopic (absorbs water in the air quickly) so close the container quickly, store it in a dry place and consider using a desiccant packet if the humidity in the air is high.

1-Add the Calcium Lactate Gluconate to 1/3 of the main ingredient to be used. Blend with an immersion blender until it is completely dissolved. Add the rest of the main ingredient.

Adjusting the Flavored Liquid Density

When you pour the liquid into the bath, the spherical shape forms thanks to surface tension. The density of the flavored liquid and the bath affect the surface tension and the resulting shape. If the flavored liquid is to thin, it will fail to penetrate the dense bath surface and will spread in the bath and fail to hold its shape. If it is too thick, it will fail to round evenly to form a nice sphere. The right consistency is that of thick cream. Xanthan Gum is usually use to thicken the flavored liquid until the appropriate viscocity is obtained.

1-Add Xanthan Gum to thicken the liquid and blend using an immersion blender. Let the Xanthan Gum hydrate for a minute or so. Start with small amounts until you obtain the right consistency. Usually concentrations of Xanthan Gum range between 0.2% and 0.5%.

2-Let it rest for a few hours so that it loses part of the air created by the mixer. Use the techniques explained above if you want to expedite the process.

Creating the Spheres

1-Get the flavored liquid with calcium content from the fridge, the sodium alginate bath and the measuring spoons to make large spheres to resemble ravioli, gnocchi, etc. The measuring spoons are included in the Molecular Gastronomy Essentials Kit, the Molecular Gastronomy Premium Kit and the Molecular Gastronomy Ultimate Kit.

2-Prepare another bowl with plain water that you are going to use later for rinsing the spheres to remove the excess of sodium alginate.

3-Now grab the measuring spoon of the desired size; fill it with the flavored liquid and carefully pour it into the sodium alginate bath. Wipe the bottom with a paper towel, place the spoon over the bath slightly touching its surface and flip it to pour the liquid into the sodium alginate bath.

4-Stir the bath gently with the slotted spoon without touching the spheres. If you let them sit in the bottom of the bath, they will flatten and if you let them float, the top won't be covered with the sodium alginate solution and won't gel. Make sure the spheres don’t touch each other or they will stick. Start with one sphere at a time until you get used to the process.

5-Wait for about 2 minutes. The longer you wait the thicker the gel that will form. In general, for a good eating experience, you want the gel layer surrounding the sphere to be as thin as possible but it also needs to be strong enough to hold the shape and allow for careful handling. If the membrane is too fragile, they may easily break when you remove them from the bath or place them on the serving spoon.

6-Carefully remove the sphere from the sodium alginate bath using a slotted spoon and rinse it in the bowl with clean water. The special slotted spoon is included in the Molecular Gastronomy Essentials Kit, the Molecular Gastronomy Premium Kit and the Molecular Gastronomy Ultimate Kit.

TIPS: I recommend you always start with one sphere first to adjust the pouring process and the time in the sodium alginate bath. If the sphere membrane is too subtle and the sphere easily breaks when handling it with the slotted spoon carefully or when plating it, extend the time in the calcium bath until you get the desired strength. Keep in mind that the thinner the membrane the better experience people are going to have when eating it.

Storing the Spheres and Preserving the Flavor

One of the biggest benefits of Reverse Spherification is that you can store the spheres to be consumed later. Unlike in Basic Spherification, the gelification process stops as soon as you remove the spheres from the sodium alginate bath. This is because there is no more sodium alginate to react with the free calcium ions inside the sphere and all the sodium alginate molecules are already bound in the gel.

If you leave the spheres in contact with air, the gel will start to dry and eventually break. You need to store them in a liquid bath. However, since the gel membrane around the sphere is permeable to small molecules, osmosis will occur if submerged in water and dilute the flavored liquid inside the sphere. To preserve the flavor, store them in a bath of the same flavored liquid in the fridge.

Alternatively, depending on what you are trying to achieve, you can macerate them for a day in olive oil, truffle water or your favorite liquid. For example, the famous Spherical Olives by Chef Ferran Adria are macerated in olive oil infused with lemon peel, garlic, thyme rosemary and peppercorns.