Freezing and Storage

Freezing while adding air

The freezing step is called mantecazione — freezing with agitation — and it performs two jobs at once. It chills the mix and turns the water to ice, while the stirring adds air, finely divided. The "ice crystals" and the "air bubbles" of Chapter 1 are built together, in this one step.

Chapter 1 defined gelato as four phases. Part I designed three of them — the unfrozen phase through the sugars, the fat, and the boundaries between them. The air bubbles alone could not be designed there — Tab. 5-1 has no row for air. What Part I prepared was the support. The fat holds the bubbles (Chapter 3); the emulsifiers and the stabilizers help the intake of air (Chapter 4). The fourth phase is decided here. The amount of air incorporated during freezing — the overrun — is the work of this step.

At the exit of the mantecatore — the machine that performs the step — the mix stands at about −6 to −8 °C. Of its water, 50 to 60 percent is frozen. The surface looks dry; in this state the gelato is drawn from the machine and sent on to hardening. What matters is the fact that at the moment of extraction, only about half is frozen. How the remaining water freezes is the subject of the next step.

Approfondimento — deep diveWhy does it look dry? — Shear and partial coalescence↓ Go deeper

Crossing the critical temperature zone fast

The size of the ice crystals is decided by the speed of freezing. The faster the freeze, the smaller the crystals, and the smoother the result. Most important of all is the band from the freezing point (about −2 to −3.5 °C) down to −8 °C. This band is the critical temperature zone, where ice crystals grow most readily. Crossing it as fast as possible is the key to a fine texture. This is why a mantecatore of high performance is required.

How fine is fine enough? The diameter of the ice crystals and the feel in the mouth correspond as follows.

A difference of a few tens of micrometers separates "it melts" from "it grates". Controlling this dimension, too small for the eye, is the purpose of the freezing step.

Hardening — freezing the rest in one stroke

Fresh from the mantecatore, the gelato is still only about half frozen. Freezing this remaining water fast, and so fixing the structure, is hardening (indurimento). A blast freezer (abbattitore) takes the gelato down to about −22 °C in one stroke. A gentle descent — one degree per hour, say — is not enough. Faster than that: the point is to cross the critical temperature zone quickly.

What if hardening is skipped, and the gelato goes straight from the mantecatore to the case? The remaining water freezes slowly, over two hours and more. The ice crystals grow large in that time, and the fine structure built in mantecazione is lost. Such a gelato does not keep; it must sell out within hours. Hard products like the torta gelato and the tartufo need about 90 percent of their water frozen, and rapid hardening is all the more indispensable.

Storage — low, and steady

Hardened gelato, if it is to keep long, is best stored at the hardening temperature itself — about −22 °C. Freezing is a reversible event. A product that took much heat to freeze absorbs much heat as it thaws. So, as a rule: the lower the storage temperature, the more stable the product.

The case for selling, meanwhile, sits somewhat higher — about −13 to −14 °C. At this temperature about 80 percent of the water is frozen. The remaining fifth — a concentrated syrup — gives the softness a spoon can pass through. When Chapter 1 said "some 80 percent at about −14 °C", it was this selling temperature.

How much is frozen at which temperature has been measured, degree by degree. The whole process deserves one look as a single curve.

This curve has three things to read.

First. Most of the freezing is over within the first few degrees. Between the freezing point and −5 °C, nearly half of the water turns to ice. By the end of the critical temperature zone, two thirds. The few minutes in the mantecatore decide the structure — this book has repeated it, and the reason comes down to this one point. The fineness of the crystals made there becomes the fate of the product.

Second. The lower the temperature, the weaker the effect. From −5 to −10 °C the share grows by 25 points. From −10 to −15, by nine. From −15 to −20, by five. The curve lies down. Harden to −22 °C, and the ice added by pushing on to −30 °C is slight. Driving the hardening temperature ever lower returns little to the texture.

Third. Here is why "keep it low" works. A curve lying flat means that in the low range, the same swing of temperature moves less water. Swing between −15 and −20 °C, and 5 percent of the water melts and refreezes. Swing between −25 and −30 °C, and the traffic is 3 percent. Less water on the move means slower coarsening of the crystals. In "low, and steady", the low earns its place because this curve lies down.

Shock — temperature swings — the greatest enemy

What storage must guard against most is variation in temperature. Each time the temperature rises and falls, the ice crystals melt a little, and grow a little larger as they refreeze. This repeated growth is recrystallization (ricristallizzazione). It breaks the texture, and leaves gritty particles of ice.

Approfondimento — deep diveWhy does swinging temperature grow the ice? — Ostwald ripening↓ Go deeper

Above all to be avoided is the round trip — gelato hardened at −22 °C, stored at −16 °C, brought back to −22 °C. The more the product temperature moves, the more the structure suffers. However fine the crystals made in the making, careless storage wastes them all. Keep the cold chain low from end to end, and steady — this is the last work of guarding the structure that was built.

From the formulation of Part I to the process of Chapters 6 and 7, the chain is now connected. Next, Chapter 8 turns to the machines that carry this freezing and hardening — the mantecatore and the blast freezer. How they work, and how to choose them, on a maker-neutral footing.