Stabilizers and Emulsifiers

Two supporting players, two separate jobs

From Chapter 2 to Chapter 3, the leading players of gelato — sugar, water, milk solids — have had their turn. The stabilizers and emulsifiers of this chapter are used at under 1 percent of the formulation — the amounts and proportions of the mix. Stabilizers alone mostly stay at 0.2 to 0.5 percent. Yet without these two, gelato degrades quickly in storage.

What matters is not to confuse them. Stabilizers (addensanti) work on the water: they bind the free water and hold back the growth of ice crystals. Emulsifiers (emulsionanti) work at the boundary of fat and water, and govern how the fat globules behave. Stabilizers bind the water; emulsifiers steer the fat — hold on to that division of labor first.

Stabilizers — binding the water

As Chapter 1 showed, the feel of gelato is decided by the fineness of its ice crystals. And with every swing of temperature in storage, the crystals melt, refreeze, and grow a little larger. Slowing this coarsening is the stabilizer's largest job.

Stabilizers bind water strongly and give the mix viscosity. Their work divides usefully in two.

The first part is what happens with any stabilizer. These effects follow from the rise in viscosity, whatever the type. Melting slows; the seeping of water — syneresis (sineresi) — recedes; air is held more easily; the feel in the mouth turns smooth. Any means of raising the viscosity would do.

The second part is what differs from stabilizer to stabilizer — and the slowing of ice-crystal coarsening belongs here. An explanation offers itself: in a viscous unfrozen phase, water molecules move less freely, so they reach the surface of the ice more slowly. As physics, the line holds. But it is not so simple. That stabilizers slow recrystallization is certain; why they do so is still not settled. One view has them attaching directly to the crystal surface and obstructing its growth, viscosity aside. If viscosity alone explained the effect, this work would sit on the first side — the side that needs no particular type. The fact that it does not sit there is itself the sign that viscosity is not the whole story.

The work of the stabilizers, gathered up:

• Hold back the coarse growth of ice crystals; keep the texture fine
• Make the ice crystals already formed harder for the tongue to notice
• Give the unfrozen phase viscosity; prevent syneresis
• Slow the melting at service; hold the shape
• Raise scoopability (spatolabilità)
• Help the intake of air; steady the structure

The second item is easily missed. A stabilizer does not only keep crystals small — it also makes what is already there go unnoticed. Crystals of the same size are harder for the tongue to pick out when a viscous phase wraps them.

More is not better, though. Overdone, the gelato turns gummy and will not melt away in the mouth. A stabilizer is a material of restraint — not "make it work" but "keep it from working too hard".

Approfondimento — deep diveWhy can a polysaccharide hold water at so slight an amount?↓ Go deeper

The principal stabilizers

Most stabilizers are polysaccharides, drawn from plants and seaweeds. They differ in the temperature at which they hydrate, in their tolerance of pH, and in the habits they lend the texture. In practice the standard is therefore a blend — several combined, covering one another's weak points, their synergy giving a steady viscosity from small amounts.

On the shelf it is a whitish, refined powder, but its origin is the seed of this tree. Crack the hard pod, and inside the harder seed coat lies a translucent endosperm — ground, that is locust bean gum.

Locust bean gum (farina di semi di carruba, from the seeds of the carob) is the most used stabilizer in gelato, and the most versatile. It holds 80 to 100 times its own weight in water, and it serves from acid to weakly alkaline. But it dissolves only partly in cold water; to work fully it must be heated past 85 °C. Pass the mix through the temperatures of pasteurization, and the condition is met on its own. The drawback is the price — high, and given to large swings.

Guar gum resembles carob in character, but it hydrates fully even in cold water, so it also serves preparations that are never heated. It is far cheaper than carob, too. Used alone, though, it leans toward syneresis, and pairing it with carob or another partner is the standard advice.

There is a reason the pairing works. Mix two stabilizers, and more happens than either one predicts. The viscosity climbs past the sum — or two components that never gel alone will gel together. This is why designing the stabilizers is a job of combining, not of choosing one.

Pectin comes from fruit and citrus. It works in acid surroundings, which suits fruit gelato and sorbetto. The degree of esterification splits it in two. High-methoxyl (HM) pectin gels where sugar and acid are present; low-methoxyl (LM) pectin gels only where calcium salts are. Not "no sugar or acid needed" — calcium is needed instead. And the type that serves in a gelato mix is the LM type — the milk brings the calcium. Hydration also differs by type: the HM type dissolves even in cold water. This is why pectin alone is handled differently in Tab. 4-1.

Where the fruit is sharply acid, tolerance of pH decides the choice of material. Sodium alginate precipitates readily in acid. The reason is plain: add acid, and it returns to alginic acid itself, which does not dissolve in water. On top of that, the calcium of milk brings its gelation on too early. As it stands, it does not suit an acid sorbetto.

But each obstacle has a way around it. Against the acid, the propylene glycol ester of alginic acid stays stable below pH 5. Against the calcium of milk, a chelating agent that seizes and holds it. Commercial alginates for ice cream usually come blended with phosphate or citrate salts. "Unsuited" is not "unusable". Once it is clear what stands in the way, a means of removing it is usually at hand.

What, then, for a sharply acid sorbetto? Xanthan gum is one answer. A polysaccharide made by bacterial fermentation, it dissolves well in cold water and works in acid. The interesting part is how it flows: press it, and it suddenly turns loose; let go, and the viscosity returns at once. That behavior suits sauces — flowing thinly while poured, stopping the moment it lands on the gelato. But it is expensive, and hard to call widely used in gelato itself.

CMC (carboxymethyl cellulose) is a more tangled case. Made from the cellulose of wood or cotton, it dissolves in cold water and in hot — an easy material to handle. Yet expose a formulation that contains milk proteins to high heat, and CMC invites syneresis instead. Its use has therefore stayed almost entirely with fruit flavors. This is why its cell in Tab. 4-1 is written differently — what binds CMC is not acid, but the pairing of milk and heat.

To tangle matters further, CMC is also reported to hold back the precipitation of casein. It looks like a contradiction, but the conditions differ. Protecting casein that acid is trying to clot, and splitting a heated milk formulation into phases, are separate stories. The same material helps or hinders, with the conditions. It has also been noted that a name which sounds like a chemical has made users hesitate.

Still, as with the acid coagulation of casein met in Chapter 3, acid keeps returning as a constraint on the design of the formulation.

Approfondimento — deep diveWhy does casein set with acid? — The micelle and the interface↓ Go deeper

Emulsifiers — steering the fat

An emulsifier molecule has one end that takes to water (hydrophilic) and one that takes to fat (lipophilic). Because of this double nature, emulsifiers line up at the boundary between the two phases that refuse to mix, and mediate between them.

The role of the emulsifiers is not only to keep the fat evenly dispersed. In gelato, the more important work is to control the event that Chapter 3 described without naming it. Under the shear of mantecazione, the membranes of the fat globules partly break. The exposed fat builds its network around the air bubbles, supporting the structure from within. The name of that event is partial coalescence. The emulsifiers keep the process moving — in measure. Held to that measure, it brings a smooth, dry texture, good whipping, and scoopability.

Approfondimento — deep diveWhy does an emulsifier weaken the emulsion? — The paradox of partial coalescence↓ Go deeper

Natural and synthetic emulsifiers

Emulsifiers divide broadly in two, by origin.

The face of the natural emulsifiers is lecithin (lecitina), carried in egg yolk. Traditionally the first emulsifier ever used, it is amphiphilic — at home in water, soluble in fat — and it mediates between the two. In crema formulations built on egg yolk, this work comes folded into the ingredient itself. Purified lecithin on its own is little used — the cost, and a strong flavor of its own. As egg yolk, though, it remains in service (the handling of eggs themselves is Chapter 10).

The standard among synthetic (industrial) emulsifiers are the mono- and diglycerides. They are kin to the fats themselves, the triglycerides. Their character changes with the number of fatty acids bound to the glycerin — one (mono) or two (di). Monoglycerides, with one, lean hydrophilic and suit formulations rich in water; diglycerides, with two, take more readily to fat. The two are blended to match the use.

Refined and synthesized, these emulsifiers are regulated as food additives — in Japan, under the Food Sanitation Act. Rules govern their use and their labeling (Chapter 14). Egg yolk as an ingredient stands outside that frame. Either way, knowing exactly what was used is what connects, later, to labeling and to quality control (Chapter 12).

The knack of using them

Stabilizers and emulsifiers alike work in small amounts and backfire in excess. Overworked, they turn to gumminess and weight, and the melt that belongs to gelato is lost. And for either to work fully, the aging (maturazione) after pasteurization must give them time to hydrate and to interact. That is why the mix rests after pasteurization — the meaning of that step is the business of Chapter 6.

And the constraint that keeps returning is pH. Build on sharply acid fruit, and the acid-weak stabilizers and emulsifiers drop out of the choice. The pairing of ingredients binds the choice of additives. Formulation is nothing but the work of untying such constraints, one at a time. The procedure that gathers the whole into numbers is Chapter 5, next.