Pasteurising and ageing the mix: temperatures, times and why to wait

Most ice cream recipes start the same way: heat the mix, cool it and leave it in the fridge overnight. You can skip both stages and still end up with ice cream — but it will be worse, and with eggs riskier too. This article explains what happens in the mix during heating and ageing, and what temperatures and times the sources give.

Why heat the mix

Heating has two aims:

1. Safety. To kill pathogenic microorganisms, especially if there are eggs in the mix.
2. Quality. According to Douglas Goff (University of Guelph), pasteurisation hydrates the proteins and stabilisers and dissolves the sugar. Slower batch pasteurisation also alters more of the whey proteins, which some say improves the body of the ice cream.

Temperatures and times

“85 °C for 15 seconds” is sometimes quoted as the standard for short-time pasteurisation — we didn’t find it in verified sources. What is verified is 80 °C/25 s and 79.4 °C/15 s.

According to Ice Cream Science, at 77 °C the mix is not only pasteurised, but the whey proteins also partly unfold (this happens at 70–78 °C). Above about 78 °C they start to clump together.

Eggs: the lower and upper limits

With mixes containing yolks (custard base (crème anglaise), frozen custard), two numbers matter:

• Lower limit: 71 °C. The US FDA recommends that dishes containing eggs reach 71 °C, measured with a thermometer. The Czech National Institute of Public Health (SZÚ) states that salmonella dies after a few minutes at 60–65 °C.
• Upper limit: around 80 °C. The custard starts to thicken at around 70 °C, and above roughly 80 °C the yolks curdle into lumps. Overshooting the right temperature by 3–6 °C is enough.

The commonly quoted 82–85 °C for custard is the upper limit of pastry-kitchen practice, not a safety requirement — with a lot of yolks there’s a risk of curdling. The safer documented route is a lower temperature with a hold, for example 77 °C for 30 minutes.

More in the article Eggs in homemade ice cream.

No-cook recipes

An egg-free mix made from pasteurised milk and cream can also be prepared without heating — the sugar dissolves with stirring. The MDH recommends using only pasteurised milk and cream. Without heating, however, some stabilisers don’t hydrate (locust bean gum, for example, needs heat) and the mix will have a weaker body.

Cooling

After heating, cool the mix quickly, ideally in a bowl set in an ice bath, and then put it in the fridge. Industry cools pasteurised mix immediately to fridge temperature. Slow cooling at room temperature gives bacteria time.

Ageing

Ageing is the time the chilled mix spends in the fridge before churning. Goff recommends at least 4 hours, ideally overnight, at 5 °C or below (as low as possible, but without freezing).

What happens during ageing:

1. The fat crystallises. Only partly solidified fat can join into a network during whipping that holds air and gives ice cream body.
2. The proteins and stabilisers fully hydrate, and the mix thickens slightly.
3. Emulsifiers (such as lecithin from yolks) displace proteins from the surface of the fat globules.

The result is better overrun, body and texture. Goff writes that an unaged (“green”) mix shows itself as soon as it’s churned — it whips less well and the texture is poorer.

When to adjust the method

• Sherbet — add the acid (lemon juice) only after ageing, just before churning. Otherwise it can curdle the mil
• Frozen yogurt — add the yogurt only to the chilled mix. Heating would kill the culture
• Sorbet — long ageing isn’t needed; it’s enough to chill the syrup with the fruit thoroughl
• Stabilisers — locust bean gum needs heat of around 80 °C for full activation; guar works cold too. Details in the article Stabilisers at home.

Timeline

So in total, heating and ageing add half a day to a day to making ice cream — but most of that is waiting.