How Is Ice Cream Made? Why It Stays Soft in a Freezer That Turns Water to Stone
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Water frozen at minus eighteen degrees is hard enough to break a tooth, and ice cream at the same temperature can be scooped with a spoon. The difference is that ice cream is only about half ice, and the ice is in crystals too small to feel, held apart by a syrup that never freezes, fat that has been whipped into a scaffold, and enough air to double the volume. Making it is the business of getting those four things into the right arrangement in the few minutes before the water freezes into something else.
The mix
Ice cream begins as a liquid: cream and milk for fat and protein, sugar, sometimes egg yolk, and flavouring. Each ingredient does a structural job as well as a culinary one. The sugar dissolves in the water and lowers its freezing point, so that at minus eighteen a large fraction of the water is still liquid syrup, thick and cold, between the ice crystals; without sugar the whole thing would set solid. The milk proteins and any egg coat the fat droplets and stabilise the foam. The fat, ten to sixteen percent in a premium ice cream, gives richness and, once it is partly frozen, the physical structure that holds the air. The mix is pasteurised, homogenised to break the fat into fine droplets, and left in the fridge for several hours to age, during which the fat firms up and the proteins settle onto the droplets, which makes the next step work.
Freezing fast and stirring hard
The mix is then frozen while being churned, in a machine whose walls are chilled far below zero and scraped continuously by blades. Two things happen at once. Ice crystals form on the cold wall and are scraped off into the mix, and because the freezing is fast and the mix keeps moving, the crystals stay small, ideally under fifty micrometres, below the size at which the tongue can feel grit. And the churning beats air into the mix and drives the fat droplets against each other so that they partly coalesce into a network, like the structure in whipped cream, which traps the air bubbles and holds the whole thing together. The result leaving the machine is a soft semi-frozen foam at about minus five degrees, and it is hardened afterwards in a deep freezer without further stirring. The structure that comes out:
- •Ice crystals, small and numerous, making up perhaps half the volume
- •A concentrated sugar syrup that stays liquid and keeps the crystals apart
- •Fat droplets partly joined into a network, coating the air bubbles
- •Air, anything from twenty percent of the volume in a dense premium tub to a hundred percent in a cheap one, which the trade calls overrun
Why it goes grainy
Ice cream that has melted a little and refrozen turns sandy, because small crystals are unstable: in any slightly warmed tub the smallest crystals melt first, and when the temperature drops again the water refreezes onto the larger crystals that survived, so the average size grows with every cycle of a freezer door opening. The process is called recrystallisation, and it is the enemy of every ice cream maker. Commercial makers fight it with stabilisers, small amounts of gums such as guar or carrageenan that thicken the syrup between the crystals and slow the water's movement, and by keeping the product below minus twenty-five from factory to shop. A home freezer at minus eighteen, opened daily, gives a tub a few weeks before it coarsens.
Sorbet, gelato and the rest
Sorbet has no dairy, only fruit, sugar and water, and depends entirely on the sugar to stay soft; a sorbet with too little sugar freezes to a block and one with too much never sets. Gelato is made with more milk and less cream than ice cream, churned slowly to take in less air, and served a few degrees warmer, which is why it tastes more intense and denser. Soft serve is ordinary ice cream drawn from the machine at minus five before hardening, with extra air. Frozen custard is ice cream with a high proportion of egg yolk. And the modern cook's trick of freezing with liquid nitrogen at minus 196 degrees works because the faster the freeze, the smaller the crystals; the result is the smoothest ice cream there is, for the few minutes before it warms and the crystals begin to grow.
A short history of a cold dessert
Flavoured snow and ice were eaten in Persia and China two thousand years ago, and the Mughal emperors had ice carried down from the Hindu Kush by relay. The dairy version appears in Italy and France in the seventeenth century, once cooks learned that salt added to ice makes a brine cold enough to freeze cream; the salt lowers the melting point of the ice, and the melting draws heat from whatever is in the pot inside. Hand-cranked freezers with a dasher, patented by Nancy Johnson in Philadelphia in 1843, made it a household dish, industrial refrigeration made it an industry, and the cone appeared at the St Louis fair of 1904. The physics has not changed since Johnson's bucket: cold walls, scraping, and speed.
The takeaway
Ice cream stays soft because it is a frozen foam rather than a block: tiny ice crystals formed fast under constant scraping, a sugar syrup that never freezes and keeps them apart, fat droplets churned into a network, and air whipped in during freezing. Slow freezing or thawing and refreezing lets the crystals grow and the texture turn grainy, and every variation from sorbet to gelato to liquid-nitrogen ice cream is an adjustment of sugar, fat, air and speed.