Ice crystals: why homemade ice cream goes hard and grainy

A smooth ice cream and an icy one can have exactly the same composition. The difference lies in the size of the ice crystals. When they are tiny, your tongue can’t feel them and the ice cream seems creamy. When they grow, the ice cream crunches and feels grainy. This article explains what crystal size depends on and why homemade ice cream loses its smoothness after a few days in the freezer.

How big the crystals are

According to Douglas Goff (University of Guelph), the crystals in typical ice cream are around 50 µm in diameter (micrometres, i.e. five hundredths of a millimetre). The Ice Cream Science blog gives more detailed figures:

Freezing speed

Goff sums it up in one sentence: the rate of freezing determines crystal size. When the mix cools quickly, many nuclei form at once and each stays small. When it freezes slowly, there are few nuclei and water adds on to the ones that already exist.

The composition of the mix (sugars, solids) determines how much water freezes at a given temperature. The freezing speed determines how big the crystals will be.

What affects the speed at home:

• the temperature of the mix when it goes into the machine — the colder it is, the faster it freezes; that’s why the mix is aged in the fridge,
• the power of the machine — a compressor ice cream maker keeps cooling all the time, while the bowl of a freezer-bowl ice cream maker warms up during churning,
• the amount of mix — an overfilled bowl freezes more slowly,
• freezing the bowl thoroughly — David Lebovitz recommends leaving it in the freezer for at least 24 hours.

During churning, crystals constantly form and melt. According to Ice Cream Science, what matters most is therefore the time in the machine and the temperature at which you take the ice cream out — the colder it is, the smaller the crystals.

Hardening

When it comes out of the machine, ice cream has the consistency of soft-serve and needs to firm up in the freezer. During this “static” freezing, no new crystals form — the existing ones just grow, by about 30–40 % according to Ice Cream Science. So the rule is: harden it as quickly as possible.

• Use a shallow metal container — a larger surface and metal conduct heat away faster. Glass can crack in the freezer.
• Put it in the coldest part of the freezer, not in the door.
• Chill the container beforehand.

Recrystallisation: why ice cream coarsens over time

Even once the ice cream is finished, the crystals keep changing in the freezer. With every slight warming, small crystals partly melt and the water from them then freezes onto larger ones. This is called recrystallisation, and it’s the main reason why ice cream coarsens after days or weeks.

Goff gives temperature fluctuation (so-called heat shock) as the most common cause of iciness: opening the freezer, defrost cycles, moving the ice cream around, leaving it out on the table.

How fast this happens depends on the storage temperature:

A home freezer runs at around −18 °C, and the temperature fluctuates when you open it. The legal storage temperature for ice cream in both Slovakia and Czechia is −18 °C or lower.

Why homemade ice cream goes hard and grainy

1. It lacks industrial stabilisers and solids. The US National Center for Home Food Preservation (NCHFP) states that homemade ice cream becomes grainy in storage because, unlike commercial ice cream, it has no added stabilisers and milk solids.
2. The freezer is colder than the recipe was designed for. Gelato balanced for −12 °C will be hard at −18 °C. A higher PAC helps — see PAC and POD.
3. Less air. Home machines whip in less air than industrial ones, so the ice cream is denser.
4. Temperature fluctuation every time the freezer is opened.
5. Without a machine, the crystals are large from the start. No-machine recipes, such as whipped cream and condensed milk ice cream, get round this with a high content of fat, sugar and air from the whipped cream.

How to keep ice cream smooth

• Let the mix age and chill thoroughly before churning.
• Churn until the ice cream holds its shape, then harden it quickly straight away.
• Cover the surface with film or wrap the container — the NCHFP recommends overwrapping the container so the ice cream doesn’t dry out.
• Store it at the back of the freezer, not in the door.
• Don’t thaw and refreeze it — small crystals melt and large ones grow. In Slovakia, refrozen ice cream may not even be sold.
• Make smaller batches and eat them fresh, roughly within 1–2 weeks.
• Add a stabiliser — it slows crystal growth when the temperature fluctuates. See Stabilisers at home.

When crystals are wanted

With granita, large crystals are the goal. The mix has less sugar and freezes slowly in the freezer, with the crystals just broken up with a fork. Example: lemon granita.