How Is Ice Cream Made? Why It Stays Soft in a Freezer That Turns Water to Stone
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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, adding anything from twenty percent to the mix's 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 was popularised at the St Louis fair of 1904. The physics has not changed since Johnson's bucket: cold walls, scraping, and speed.
Why homemade ice cream sets like a brick, and how to fix it
If you have made ice cream at home and found it rock hard the next day, the problem is usually the syrup. How far a dissolved ingredient lowers the freezing point depends mainly on how many dissolved particles it adds, not on its weight. Ordinary sugar, sucrose, is a fairly large molecule, so each spoonful adds relatively few particles. Dextrose, the form of glucose sold for baking and brewing, is roughly half the size, so gram for gram it lowers the freezing point nearly twice as much, though it tastes less sweet. Swapping some sucrose for dextrose keeps a tub scoopable without making it cloying.
Alcohol works the same way even more strongly, since its molecules are small and it freezes far below any home freezer's temperature. A spoonful of a spirit or liqueur softens a batch noticeably, and too much stops it setting at all, which is why boozy ice creams are often sold in modest strengths. Other tricks follow the same logic: a little honey or invert sugar adds more dissolved particles than plain sugar, and extra milk solids help hold water in place. Letting a very hard tub sit in the fridge for ten minutes before serving simply allows part of its ice to melt back into syrup.
What the cold does to taste
Taste ice cream after it has melted and it is startlingly, almost sickly sweet. Nothing in the mix has changed except its temperature. The sensitivity of the cells that detect sweetness falls as they get colder, and the aromas that make up most of what we call flavour evaporate less readily from a cold mouthful, so less of them reaches the nose. Recipes compensate with more sugar and stronger flavourings than would taste right in a warm dessert. That is also why ice cream served straight from a very cold freezer tastes faint until it softens.
Cold also explains brain freeze, the sharp ache behind the forehead that comes from eating too fast. Its medical name is a cold-stimulus headache, and it is triggered when something very cold touches the roof of the mouth or the back of the throat. The leading explanation is that the sudden chill makes blood vessels there constrict and then rapidly widen, and the nerve that carries sensation from the area, the trigeminal nerve, reports the change as pain in the forehead. It usually fades within a minute, and pressing your tongue to the roof of your mouth to warm it can help.
Ice cream in a bag, with the numbers
You can make ice cream without a machine or a freezer, using the salt-and-ice trick in its simplest form. Seal a small bag holding about 200 millilitres of sweetened milk and cream, put it inside a larger bag of crushed ice mixed with a few generous spoonfuls of salt, and shake or knead it for five to ten minutes. The salt dissolves into the thin film of water on the ice and lowers its melting point, so the ice starts melting even though it is below zero. Melting soaks up heat, and much of that heat is drawn out of the mix in the inner bag.
Every gram of ice that melts absorbs about 334 joules, enough to cool four grams of water by twenty degrees. Chilling 200 grams of mix from fridge temperature to a few degrees below zero and freezing about half of its water takes roughly 30,000 joules, which the melting of about 90 grams of ice can supply, though warm hands and air steal some, so use several hundred grams. A strong brine can fall well below minus ten, cold enough to hurt bare skin, so wrap the bag in a towel. The kneading does the job of a churn, keeping the crystals small.
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.