Spherification at home: alginate, calcium and the gel skin

Spherification sets a thin gel skin around a drop of liquid while leaving the inside runny. Small drops come out looking like fish roe. Larger spoonfuls become soft, yolk-like spheres that hold their shape on a plate and break open in the mouth. The whole trick is one chemical reaction between a thickener from seaweed and dissolved calcium.

What the powders are

Sodium alginate, listed as E401 on European labels, is extracted from brown seaweeds such as giant kelp and knotted wrack. Its long chains are built from two sugar acids, mannuronic and guluronic acid, in proportions that vary with the seaweed. The European Food Safety Authority’s 2017 re-evaluation of alginic acid and its salts describes the chemistry and notes that alginates pass through the human gut practically undigested, with part of them fermented by gut bacteria. The panel found no need for a numerical acceptable daily intake and no safety concern at reported levels of use. In the United States, sodium alginate is affirmed as generally recognized as safe under 21 CFR 184.1724, with maximum levels set by food category.

Dissolved in a liquid, sodium alginate simply thickens it. When calcium ions arrive, each one can bind to guluronic acid sections of two neighboring chains at once and lock them together. Food chemists call the resulting structure an “egg box,” with the calcium ions sitting like eggs between the chains. The gel that forms is calcium alginate, E404, and it sets without heat.

The calcium comes from one of three common salts. Calcium chloride (E509) is the fastest and the cheapest, but it tastes bitter and salty. Calcium lactate (E327) and calcium gluconate (E578) taste of very little, which matters whenever the calcium goes into the food itself. A 2013 study in the International Journal of Gastronomy and Food Science compared them: calcium chloride brought an alginate gel to full strength in about 100 seconds, calcium lactate in about 500, and calcium gluconate in about 2,000. The finished gels were equally strong. Only the speed differed.

Direct spherification

In the direct method, the alginate goes into the flavored liquid and the calcium stays in a bath of water. The liquid is blended with the alginate and then left to rest, often overnight in the fridge, so the air bubbles whipped in by the blender can rise out. Drops fall from a syringe or squeeze bottle into the calcium bath, set a skin within seconds, and are lifted out and rinsed in plain water.

The skin keeps thickening inward for as long as calcium is around, so direct spheres are best served soon after they are made. Left long enough, a sphere that once burst cleanly sets firm all the way through. The method also fails with liquids that already contain calcium, such as milk or yogurt, because they gel as soon as the alginate goes in. Very acidic juices cause their own trouble. Alginate sets poorly in strong acid, and the usual advice is to raise the pH with a little sodium citrate (E331), a salt with a salty, faintly sour taste of its own.

Reverse spherification

The reverse method swaps the two. Calcium lactate or gluconate goes into the flavored liquid, and the alginate is dissolved in the bath. A spoonful of the flavored liquid lowered into the bath forms a skin on its outside, and the gel grows outward into the alginate, not inward toward the center. Once the sphere is rinsed, gelling stops. Reverse spheres can be made ahead and held, they stand up to larger sizes, and they work with dairy, which brings its own calcium.

The water for the alginate bath matters more than it seems. Hard tap water carries enough calcium to thicken the bath unevenly, so distilled or low-mineral water gives more predictable results.

Another route

Alginate is not the only way to make a bead. Agar, E406, is a seaweed gel that sets as it cools. Dripping a warm agar liquid into very cold oil sets each drop as it sinks. The beads are gelled all the way through, so they have a firmer bite and none of the liquid center, which some cooks prefer for garnish and others find beside the point.