The Structure of Gelato — The Gelato Textbook

What is gelato made of? The answer is ice, air, fat, and concentrated syrup — four phases combined into one of the most complex microstructures in food. The science of formulation begins with an understanding of this structure.

A frozen dessert made of four phases

Look at finished gelato at the microscopic scale, and four phases coexist there. Ice crystals, air bubbles, fat globules, and, filling the space between them, the unfrozen phase. The last is a liquid in which sugar and other solutes stay concentrated, and it does not freeze.

The smoothness of one spoonful is decided by how fine these four kinds of particles are, and how they are distributed. Large ice crystals make the gelato rough. Coarse air bubbles make it hollow. Poorly dispersed fat globules produce lumps like butter. Making gelato is, in short, the work of controlling the size of particles too small to see.

Fig. 1-1　A schematic view of the microstructure of gelato. The unfrozen phase, carrying the fat globules, fills the space between the ice crystals and the air bubbles.

Four states inside the mix

Before it is frozen, in the mix itself, the ingredients are not uniformly "dissolved". Depending on the size of the particle, they exist in the water in different ways.

Tab. 1-1 States of dispersion in the mix, from the smallest particle up
State	Main components	Typical particle size
True solution	Sugars (including lactose), mineral salts	under 1 nm
Colloidal solution	Milk and egg proteins, stabilizers	about 1–100 nm
Emulsion	Fat globules	100 nm and up
Suspension	Insoluble particles of cacao and nut pastes	larger still

The classification and the particle sizes are checked against the technical literature. The boundary at 100 nm is taken from the scale of the source's own chart, not from its running text. The text gives 10 nm as the upper limit of colloids. Yet the same source draws the colloid band up to 100 nm, and sets the emulsion's lower limit at 100 nm on the same page. Taking 10 nm would leave the range from 10 to 100 nm with no state, and the main proteins of milk would fit nowhere.

This classification is not an exercise in tidiness; it maps directly onto practice. The sugars in true solution govern the freezing behavior (Chapter 2). The colloidal proteins and stabilizers hold water and build viscosity. The fat globules of the emulsion help the air bubbles hold through mantecazione, the freezing with agitation. The mix looks white and cloudy because particles of colloidal size and above scatter light.

Milk and cream are themselves oil-in-water emulsions — fat dispersed through water — so emulsification begins the moment they enter the mix. Homogenization then uses mechanical force to break the fat globules finer and disperse them evenly. How well that step is done maps directly onto the structural defects described below.

Approfondimento — deep diveHow do fat globules hold the air bubbles up?↓ Go deeper

What happens in freezing

As the mix freezes during mantecazione, only pure water freezes. An ice crystal is a solid of pure water. Sugars and minerals are shut out of the ice, and they concentrate in the liquid that remains. The concentrated liquid becomes ever harder to freeze, until it reaches equilibrium. That liquid is the unfrozen phase.

"The gelato gets colder" therefore means the same thing as "the share of ice increases". At about −14 °C, the usual serving range, some 80 percent of the water in the mix is ice. The remaining 20 percent, a concentrated syrup, gives plasticity and guarantees the softness a spoon can pass through. The relation between ice and temperature is designed from the formulation — the amounts and proportions of the mix. The tool for that design is PAC, defined in Chapter 2.

The artisan's view

The defrost cycle of the case, or the swing of its doors, moves the product temperature by ±2 °C. The share of ice moves visibly with it. When the gelato seems hard today, suspect the thermometer before the formulation. Understanding the structure makes the fault faster to isolate.

Corpo and struttura

Italian gelato theory separates the texture of gelato with two words. corpo — literally "the body" — is the sense of weight, the density, and the melting behavior. It is set mainly by the formulation: the amount and the composition of the solids. struttura — literally "the structure" — is the fineness and the smoothness of the texture. It is set mainly by the process: the control of freezing, hardening, and storage.

The distinction earns its keep as a diagnostic tool. The common structural defects:

Sandy texture — an excess of lactose. Under poor temperature control in hardening and storage, lactose grows into coarse crystals, felt as sand on the tongue.
Buttery texture — not an excess of fat, but fat globules clumped together through insufficient homogenization, or insufficient cooling before freezing.
Coarse texture — ice crystals grown large through errors in the freezing or hardening step. The gelato feels icy, close to granita.
Heavy or light structure — an excess or a shortfall of incorporated air (overrun).

When the formulation is correct and the gelato is still not good, the cause is more often struttura, not corpo — the process side.

Structure is built in freezing, and fixed in hardening

The structure of gelato is built during the few minutes of mantecazione, and fixed by the rapid hardening that follows. The slower the freezing, the larger the ice crystals grow. The slower the hardening, the more the fine crystals coarsen once made. A good structure therefore cannot be had from a good formulation alone. Formulation (Part I of this book) and process (Part II) are the two wheels of a single purpose: structure.

The chapters that follow take this structure apart one element at a time. First the sugars, which govern the water (Chapter 2). Then the milk solids, which build the frame (Chapter 3). Then the stabilizers and the emulsifiers, which guard the boundaries between the phases (Chapter 4).