Modern Industrial Chemicals (part 4) - Making gels: Sodium alginate

3.4. Making gels: Sodium alginate

The gels covered so far are all homogenous, in the sense that they are incorporated into the entire liquid and then set with heat. Alginate, however, sets via a chemical reaction with calcium, not heat, which allows for an interesting application: setting just part of the liquid by localized exposure to calcium.

This is done by adding sodium alginate to one liquid and calcium to a second liquid and then exposing the two liquids to each other. The sodium alginate dissolves in water, freeing up the alginate, which sets in the presence of calcium ions, which will only occur where the two liquids touch. Imagine a large drop of sodium alginate–filled liquid: the outside of the drop sets once it has a chance to gel with the assistance of the calcium ions, while the center of the drop remains liquid. It’s from this application that the technique called spherification is derived.

Instructions for use. Add 1.0% to 1.5% sodium alginate into your liquid (use water for your first attempt). Let the liquid rest for two hours or so to hydrate fully. It will be lumpy at first; don’t stir or agitate the liquid, as doing so will trap air bubbles in the mixture.

It’s probably easiest to add the sodium alginate a day in advance and let it hydrate in the fridge overnight.

In a separate water bath, dissolve calcium chloride to create a 0.67% solution (about 1g calcium chloride to 150g water).

Carefully drip or spoon some of your sodium alginate liquid into the calcium bath and let it rest for 30 seconds or so. (You can use a large “syringe” dripper or turkey baster to create uniformly sized drops.) If your shape floats, use a fork or spoon to flip it over, so that all sides of it are exposed to the calcium bath. Remove from bath, dip into another bowl of just water to rinse off any remaining calcium, and play.

Sodium alginate gels firm up over the span of a few hours, so you’ll need to make these near when you intend to serve them.

If your sodium alginate sets without exposure to the calcium bath, use filtered or distilled water. Hard water is high in calcium, which can trigger the gelling reaction.

Uses. The food industry uses alginate as a thickener and emulsifier. Since it readily absorbs water, it easily thickens fillings and drinks and is used to stabilize ice creams. It’s also used in manufacturing assembled foods; for example, some pimento-stuffed olives are actually stuffed with a pimento paste that contains sodium alginate. The olives are pitted, injected with the paste, and then set in a bath with calcium ions to gel the paste.

Origin and chemistry. Derived from the cell walls of brown algae, which are made of cellulose and algin. Alginates are block copolymers composed of repeating units of mannopyranosyluronic and gulopyranosyluronic acids. Based on the sequence of the two acids, different regions of an alginate molecule can take on one of three shapes: ribbon-line, buckled shape, and irregular coils. Of the three shapes, the buckled shape regions can bind together via any divalent cation. (A cation is just an ion that’s positively charged, i.e., missing electrons. Divalent simply means having a valence of two, so a divalent cation is any ion or molecule that is missing two electrons.)

Alginate does not normally bind together (left), but with the assistance of calcium ions is able to form a 3D mesh (right).

3.4.1. Spherification in shapes

Since sodium alginate sets via a chemical reaction, not a thermal one, you can freeze a liquid into a mold and then thaw it in a calcium bath to cause it to partially maintain its shape. The final shape won’t retain the crisp edges of the original frozen shape—it’ll swell and bloat out slightly—but you’ll still get a distinctive shape.

Note:

Note that straight-up lime juice won’t work, because the alginate will precipitate out in the presence of strong acids. If you’re willing to experiment further, try using sodium citrate to adjust the pH.

Try freezing the liquid in a mold before setting the sodium alginate to get more complicated shapes.

3.4.2. Mozzarella spheres

What happens if you want to use sodium alginate in a food that already contains calcium? Depending upon the amount of calcium in the food, adding the sodium alginate straight to it would cause the liquid to set, giving you something similar to a brittle gel.

Swapping the chemicals—adding the calcium chloride to the food and setting it in a sodium alginate bath—doesn’t work; calcium chloride is nasty-tasting. Luckily, it’s the calcium that’s needed for the gelling reaction, not the offensive-tasting chloride, so any compound that’s food-safe and able to donate calcium ions will work; calcium lactate happens to fit the bill. This technique is called reverse spherification.

To create mozzarella spheres, mix 2 parts mozzarella cheese with 1 part heavy cream under low heat. Add around 1.0% of calcium lactate to this liquid and then set it in a water/sodium alginate solution of 0.5% to 0.67% concentration.

Gravy

Flour (roux method)

Create a simple roux by melting 2 tablespoons (25g) of butter in a saucepan and then adding 2 tablespoons (17g) of flour. Stir while cooking over low heat until the roux sets and begins to turn light brown, about two to three minutes.

Add 1 to 1½ cups broth or stock; whisk to combine. Simmer over low heat for several minutes, until the gravy has reached your desired thickness. If the gravy remains too thin, add more flour. (To prevent clumping, create a slurry by mixing flour with cold water, and add that.) If the gravy becomes too thick, add more liquid.

Cornstarch

Create a cornstarch paste by mixing 2 tablespoons (16g) cornstarch with ¼ cup (60g) cold water.

In a saucepan, heat up 1 to 1 ½ cups broth or stock. Add the cornstarch and simmer for 8 to 10 minutes to cook the cornstarch. If the gravy remains too thin, add more cornstarch paste. If the gravy becomes too thick, add more liquid.

Notes

• You can use the drippings from roasted meats, such as turkey or chicken, to bring more flavor to the gravy. If using flour, substitute the fat in the drippings for the butter. If you’re searing a piece of meat, use the same pan for making the gravy, deglazing it with a few tablespoons of wine, vermouth, Madeira, or port to loosen up the fond that will have formed on the surface of the pan.

• Try sautéing some mushrooms and adding them into the gravy as well. Or, if you’re cooking a turkey, slow-cook the turkey neck a day in advance and pull off the meat and add it into the gravy as well.