The Handing Over of Temperature

Chapter 7 set out the physics of freezing. Past the freezing point the water freezes in a rush, and the lower the temperature, the gentler the gain. Five to six tenths are already frozen at the exit of the mantecatore, and about eight tenths at the case's −13 to −14 °C.

But that is not all that happens in a shop. Gelato is handed over many times in a day. From the mantecatore to the hardening cabinet, from hardening to the storage cabinet, from storage to the display case. And the spoon carries it to the customer's hand. At every handover the temperature moves. If Chapter 7 took why it happens, this chapter takes how it is run.

Freezing is reversible

One fact comes before the rest. Freezing is a reversible change. Cool it and it freezes; warm it and it thaws. That sounds obvious, and for the running of a shop it carries weight.

A product that loses much heat in freezing draws back just as much as it thaws. So even standing side by side in the same case, products begin to soften at different points. The better frozen it was, the harder it pulls heat from the place it sits in.

And if the point of thawing differs by product, then no single temperature is the best one for every product. Still, selling from one case, a compromise has to be settled somewhere. As a principle, a lower storage temperature is better — that is where it starts.

Hardening — do not touch it, and be quick

Gelato out of the mantecatore stands at −6 to −8 °C, lower being better, and is only about half frozen. Bringing it down from there to −22 °C is hardening.

Two warnings belong to this stage.

One: get it down in the shortest time. Reach the hardening temperature as fast as the machine allows. As Chapter 7 showed, freeze slowly and the crystals grow large. The fineness built in mantecazione is lost here.

Two: do not touch it. No spatula goes in while hardening runs. Stir it and heat enters, and the freezing that was under way is disturbed. Hardening is a step to be left alone.

Hardening and the rapid pull-down are the first gate after mantecazione. Get them wrong, and the best-made gelato is ruined.

Do not go back — the principle of one direction

The most important principle in this chapter is this. A temperature once raised is not returned.

Harden a gelato at −22 °C, store it at −16 °C, and take it back to −22 °C — that is a grave error. Every time the product temperature moves, a little ice melts, and what freezes again is larger (Chapter 7). Each round trip shaves the texture.

Here is what confuses people. Moving from −22 °C to −16 °C is not itself wrong. It is often recommended. What is wrong is going back.

The same −16 °C is right or wrong according to the direction. On the way to the case it is right; turning back to the hardening cabinet it is wrong. Watch only the thermometer and the difference cannot be seen.

Handing it to the case

So how does something hardened and stored at −22 °C reach a case at −13 to −14 °C?

Pass it through a storage cabinet at an intermediate temperature. Set the hardened gelato for a while at around −16 to −18 °C. Let it come back gently, and then move it to the case. The move that follows is quicker and easier for it. Put a −22 °C block straight into the case, and it takes time to become soft enough to sell. While it does, surface and center stand at different temperatures.

This intermediate cabinet is not a place for storage. It is one stage of the handover. The name says storage; the role is closer to a landing on a staircase.

And the night. When the case is emptied for defrosting, where does the gelato go? The answer is the intermediate cabinet, and not the hardening cabinet. Return it to hardening and that is a round trip. All the more so, since it comes out again in the morning. At the end of the day too, the one direction holds.

The spatula tells when the warming is done

So when is that warming finished? To the question "how many hours until something at −25 °C is soft enough to sell", there is no general answer.

There are two reasons. One is that too many things decide the time — the size and material of the tub, the setting of the cabinet. Whether air moves or stands still, the overrun, the solids: change any one and the time moves. The literature, too, writes that no precise indicator exists for when softening begins.

The other is that the warming is not a straight line. And the way it bends has a reason.

First, the speed of warming is proportional to the difference from the surroundings. Set a −25 °C block in a −14 °C place, and the gap is wide at first. So it warms fast, and slows as the gap closes. That is plain physics, the same for cooling and for warming.

But gelato has one more circumstance. Between −15 °C and −2 °C, the ice begins to melt rapidly. The heat entering then is spent not on raising the temperature but on melting ice. That is why the apparent heat capacity makes a large peak across this band. So at the end of the warming — just as the serving temperature comes near — heat goes in and the needle barely moves.

So the warming is not measured by the clock. It is read at the core temperature, or by the feel. The literature sets the move after hardening in temperature, not in time — move it once the core has reached −10 to −12 °C. That is the same circumstance turned around.

And on the floor the surest sign is how the spatula goes in. It flows when pushed, and stops when the force is released — that feel, when it comes, is the end of the warming. This is not intuition. It means the yield stress — the force needed to start it moving — has come to just the right height. When Chapter 4 spoke of what the emulsifiers do as scoopability, that was this quantity. The spatula is the instrument that reads it.

The thermometer tells the state of the cabinet. The spatula tells the state of the gelato. The warming is judged by the second.

If there is no blast freezer

Not every workshop has a blast freezer. What happens if there is none?

The exit of the mantecatore is −6 to −8 °C; the selling temperature is −13 to −14 °C. On the way between them, about a fifth of the water freezes. On the freezing curve of Chapter 7, that is the region where the curve still stands steep. And that fifth, without a blast freezer, freezes slowly — the ice crystals grow large, and the air that was worked in escapes.

This is the most dangerous moment of the day. On this one point, a workshop without a blast freezer stands at a structural disadvantage.

The size of the tub is a question of temperature too

It is easily overlooked, but the size of the container is part of temperature control.

The larger the container, the longer the cold takes to reach the center. The surface has long since hardened while the center still sits in the critical temperature zone — that is what happens. So working with tubs of about 4 to 5 liters is preferable to containers larger than that.

For the same reason, a large container is at a disadvantage on the way back up. By the time the center reaches the selling temperature, the outside has begun to melt. Making the tub smaller means making the handing over of temperature quick, and even.

From when can it be sold?

Without waiting for the full −22 °C of hardening: come to −15 °C and about eight tenths of the water is already frozen. The texture is reasonably stable by then. Depending on the kind of product, it can go out to the shop at that point.

That said, this does not mean −15 °C is enough. To keep it long, the principle is to store at the same temperature as hardening. For a short period it may sit higher — though how much higher is allowed differs by the thawing point of each product.

Distrust the thermometer

The whole day's handing over is managed by temperature. Which means that if the thermometer is wrong, the management itself falls apart.

The thermometer fitted to a machine measures one point somewhere inside the cabinet, and nothing guarantees that its reading is right. So a workshop wants one calibrated thermometer of its own, preferably with a calibration certificate. Take that as the reference. Check periodically how far the thermometers on the refrigerators, the pasteurizer, the aging vat, and the case can be trusted.

This may look like too fine a point. It is not. Every temperature this book has built up — pasteurization, aging, hardening, serving — can be touched only through the reading of a thermometer. 68 °C, about 4 °C, −22 °C, −13 to −14 °C: every one of them is a reading. Let the reading sit two degrees out, and everything built on it sits two degrees out.

The inside of a cabinet is not at one temperature, as Chapter 8 showed. The reference thermometer is also the tool for seeing that unevenness with your own eyes.

The −13 to −14 °C just listed for serving is the structural value, the one Chapter 7 drew from the share of water frozen. Chapter 8 placed −11 to −14 °C as a working guide for practice in Japan, taking in how formulations there tend to run. This chapter has written in the structural value in order to show the line of the handover as a single thread. What to set your own case at is settled by confirming it on your own formulation, starting from that band. The reason is the same as for distrusting the thermometer — a borrowed number is not your number.

A day is a succession of handovers

What this chapter has shown comes down, in the end, to one thing. Cool fast, warm slowly, and never go back.

Build the formulation perfectly, and make ideal crystals in mantecazione. If the handovers that follow are careless, the texture is shaved away all the same. And the shaving does not show at the time. It shows days later, on a customer's tongue. When Chapter 13 began the diagnosis of defects with "suspect the temperature first", it was looking at this succession through the day.