Part II — The Science of Making
Pasteurization and Aging
Pastorizzazione e maturazione
The blueprint called the formulation is drawn; now begin the steps that turn it into an actual mix. Heat for safety, shear to break the fat, cold to let it rest — pasteurization, homogenization, aging. These three decide whether the same formulation becomes a good structure, or does not.
Why the mix is heated
The first gate for the mix is heat — pasteurization (pastorizzazione). The purpose is double. First, safety: the pathogenic bacteria are killed, and most of the spoilage bacteria are removed. Second, quality: as the mix heats, the sugars dissolve, the stabilizers and the proteins absorb water, and the fat liquefies and disperses evenly. Here, for the first time, the components formulated through Part I — the amounts and proportions of the mix — become one body.
Pasteurization is not almighty. Unlike sterilization (sterilizzazione), which kills every microorganism down to the spores, pasteurization lets the spores survive. So after the heat, the mix must be cooled rapidly, passing quickly through the temperatures where the survivors multiply. Bacteria grow best at about 25–40 °C — around the warmth of skin — and the mix must not drift through there. Simply boiling the mix is no substitute, because the cooling that follows is slow.
There are also wrong ways to cool. Leaving the mix at room temperature is out of the question. And putting the hot mix in the refrigerator to cool — that too is to be avoided. The reasons are three. The steam rising from the mix coats the evaporator and weakens the cabinet. The other products inside take a shock of heat. And the heaviest reason: even then, the cooling is too slow. Rapid cooling does not mean moving the mix somewhere cold. It means leaving the danger zone within a short time.
Why is the refrigerator slow? Not because it lacks cold, but because what carries its cold is air. Compared at the same volume, air can remove only one twenty-first of the heat that iced water can. The refrigerator cools with that air — still air, at that. The cooling of a pasteurizer works through a metal wall, the refrigerant drawing the heat directly. Choose the wrong carrier of heat, and no cabinet setting, however low, arrives in time.
Pasteurization is a finishing measure of safety, not a magic that cleanses dirty ingredients. It has meaning only on a footing of sound ingredients and clean handling.
In Japan, the floor is set by law
The temperature and the time of pasteurization are not free choices. Making ice cream products in Japan, the floor is set by the law.
Standards for the methods of manufacture
or pasteurized by a method whose effect is equal to or greater than this
(The English wording above is a reference translation; only the Japanese text carries legal force.)
The standard comes from the Ministerial Order on Milk and Milk Products Concerning Compositional Standards, etc. The order has long been known by its shorthand — the nyūtō shōrei. Break the floor, and the result is not ingenuity but a violation (Chapter 14).
Here is something to notice. This floor of 68 °C is higher than the floor for milk. The same order gives milk 63 °C for 30 minutes, and the category of special milk 63 to 65 °C for 30 minutes. The gelato mix is asked to go higher than milk itself.
So do not import the numbers of the milk world as they stand. "Low pasteurization means 65 °C for 30 minutes" is the common sense of milk. It does not reach what ice cream products require. When the Italian literature lists 65 °C as low pasteurization, that too is the story of that country's framework. As long as the making happens in Japan, the floor is 68 °C.
Three pasteurization cycles
With this floor in place, the pasteurization of the workshop divides into a few combinations of temperature and time.
| Cycle | Heat and hold | Cooled to |
|---|---|---|
| Low pasteurization (Japan's legal minimum) | 68 °C · 30 min | Rapidly to about 4 °C |
| High pasteurization | 85 °C · 30 min | Rapidly to below 5 °C |
| Intermediate cycles | Temperature higher, hold shorter (electronic control; the literature's figure shows 72 °C · 15 min as one example) | Rapid cooling |
The 68 °C · 30 min of low pasteurization is the legal standard itself (the item for ice cream). Its source is the Ministerial Order on Milk and Milk Products Concerning Compositional Standards, etc. The 85 °C · 30 min of high pasteurization, and the cooling targets, are values of the technical literature. The same literature, however, draws 82 °C · 5 seconds in the figure its text points to — the text and the figure do not agree. So read the high-pasteurization row not as "safe at this number" but as the representative value the literature gives in its text. The 72 °C · 15 min of the intermediate cycle is likewise one example shown in the literature's figure. What the law requires is the combination of temperature and holding time. The judgment belongs to those who know the actual profile of their own machine. Heating and cooling times vary with the size of the vat and with the mix.
Whatever the cycle, the skeleton stays the same: heat, hold for a set time, then cool in one stroke. Traced as temperature against time, the path becomes Fig. 6-1.
One caution first. What this table points to is not the program name on the machine, but the temperature and the time the mix actually follows. Confusing the two is dangerous. The intermediate cycles above all. On an electronically controlled pasteurizer, the operator sets only the temperature, and the software assembles the process. The temperature goes up — and in exchange, the hold shrinks. The example the literature shows in its figure is 72 °C for 15 minutes: half the 30 minutes of low pasteurization. "The temperature sits between the two, so the hold must sit somewhere between" — it does not.
So what must be confirmed is not the temperature alone but the combination of temperature and holding time. The law asks for "equal to or greater than 68 °C for 30 minutes", and equal-or-greater is decided by both. To judge it, know what your machine actually holds on that program — what temperature, for how many minutes. It is also worth checking, now and then, that the displayed duration and the actual duration agree. One failing heater slows the heating, and the mix stays in the danger zone that much longer.
The same goes for high pasteurization. The literature writes 85 °C · 30 min in its text. The figure that passage points to draws 82 °C · 5 seconds — a sharp peak with almost no hold. Treat "high pasteurization" not as the name of one fixed process, but as a family of names whose contents differ from machine to machine.
In crema formulations that carry egg, care is needed when swinging to the high side. Where the yolk is reduced and the water is high, overheating past 83–85 °C makes the proteins release their water. They cluster, and they harden (Chapter 10). The 85 °C of the table's high pasteurization sits against that line.
Choosing the low side also moves something with it. This time it is not the law but the formulation — the stabilizers. As Chapter 4 showed, locust bean gum needs heating past 85 °C to do its full work. The legal floor of 68 °C for 30 minutes does not reach that temperature — run the low cycle, and this stabilizer stays incompletely hydrated. If low pasteurization is the default, either build the blend around guar gum, which hydrates even in cold water (Chapter 4), or reconsider the cycle. The choice of pasteurization cycle and the choice of stabilizers are linked. And here too, the deciding factor is the same — knowing what temperature your own mix actually reaches.
The rules described in this section are Japanese law. Your country or region has its own rules, and they change. Always confirm the current requirements with your local food authority.
Homogenization — breaking the fat globules
Most pasteurizers have homogenization (omogeneizzazione) built in: the step that uses strong mechanical force to break the fat globules fine and disperse them evenly. As Chapter 3 showed, the smaller the globules, the more of them there are and the better their membranes work — the emulsion steadies.
The effects reach wide. Every ingredient disperses evenly, the emulsion of oil and water stabilizes, and corpo improves. The proteins hydrate to the last corner, and the structure gains. Whipping improves, and with it the melt and the feel in the mouth. This step physically prepares the ground on which the emulsifiers work.
Aging — resting in the cold
The mix that has been pasteurized and homogenized is not frozen at once. It rests at about 4 °C for several hours — aging (maturazione). Easily overlooked, it is a step that shapes the structure. Its work gathers into two.
One is the binding of water by the proteins. The proteins of milk, egg, and gelatin absorb water over time, turning water that moves freely into bound water. As Chapter 1 showed, what freezes first and fast is the free water. Water held through aging freezes only slowly, and large ice crystals form less easily. The result is a smooth, scoopable texture.
The other is the crystallization of the fat. In the cold, part of the fat inside each globule crystallizes. Freeze a mix still warm — its fat still liquid — and the fat clumps beyond control, inviting the buttery defect (burrificazione). Aging prevents this, and it lays the ground for the whipping that Chapter 3 described.
The time needed varies with the stabilizers and the proteins in use. Gelatin takes 12 to 24 hours to hydrate fully; alginates and carboxymethyl cellulose manage in 2 to 4 hours. Aging of 48 to 72 hours was once held essential; improved stabilizers have shortened it, today, to 4 to 6 hours.
But what can be shortened and what should be shortened are different things. The literature's "good habit" is a different routine. Pasteurize in the evening; move the mix to the aging vat; find it ready the next morning. That gives about 12 to 14 hours — a somewhat wider aging. Four to six hours is a floor, not a target. If the schedule can give the mix a night, make the night the default.
And where there is a floor, there is a ceiling too. The literature states that a mix aged past its maximum turns rancid. How many hours that limit is moves with the formulation and with hygiene, so this book gives no number. What matters is one point — aging is not a step where longer is always better. The night is the default because it sits above the floor and still far from the ceiling. A mix forgotten past a second night has not aged; it has begun to spoil (rancidity as a defect is Chapter 13).
While it ages, the mix occupies a vessel. Aging inside the pasteurizer is possible, but a night of it keeps that machine from the next batch until morning. The more batches a day brings, the more meaning there is in a separate vessel for aging (Chapter 8).
The artisan's view
The process decides the structure
The same formulation, with a different pasteurization temperature, with or without homogenization, with a longer or shorter aging, gives a different structure. The blueprint drawn in Part I becomes the frozen dessert it aimed at only when these steps are taken rightly.
The aged mix now waits only for the freezer. Freeze while stirring, add air, keep the ice crystals small. That step — and the guarding of the built structure in storage — is the business of Chapter 7, next.