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Two tubs, same shelf, same size, same shape. Pick them both up and one is nearly twice the weight of the other. Nothing is missing from the light one. It is full to the lid — of ice cream and of air.
Next time you're near a freezer cabinet, do this. Take two tubs the same size and hold one in each hand.
Sometimes they feel much the same. Often they don't — one is noticeably, obviously heavier, and it isn't close.
Open the light one and nothing is wrong. It is filled right up to the lid. There is no gap, no half-empty tub, nothing anyone could complain about.
So where did the weight go?
The assumption almost everyone makes is that a tub of a given size holds a certain amount of ice cream, and the difference between tubs is what the ice cream is made of.
The second part is true. The first part is not.
Frozen dessert is sold by volume — by how much room it takes up. Not by weight. The label on the front tells you how many millilitres or fluid ounces are in there, and that is the number you are paying for.
And here is the thing about volume: some of it can be air. Air takes up room exactly as well as ice cream does, and it is free.
So two tubs can honestly hold the same volume while one contains far more actual food than the other. Nobody is cheating. It's simply that the label is measuring the wrong thing to tell you what you want to know.
Ice cream is not made and then frozen. It is frozen while being churned, and churning does two jobs at once.
The first job is texture. Beating a freezing liquid keeps the ice crystals tiny, and tiny crystals are why ice cream is smooth instead of being a solid block you'd need a chisel for.
The second job is that churning whips air in. Millions of bubbles, so small you can't see them, spread all through the mixture.
That air is not a mistake, and taking it all out would not give you better ice cream — it would give you something closer to a frozen brick, too hard to scoop and unpleasantly cold in the mouth. Every ice cream on Earth has some air in it.
How much extra volume the churning adds has a name: overrun. It's measured as a percentage of the mixture you started with.
Start with 1 litre of mixture, churn until you have 1.5 litres, and you have added half a litre of air to 1 litre of mixture. That's 50% overrun.
Overrun is measured against the MIXTURE, not against the finished tub. This trips up nearly everyone, and it is worth slowing down for.
Overrun and 'how much of this tub is air' are two different numbers, and they never match. This chart converts one into the other. Find 100% along the bottom before you go on.
| Overrun — the extra volume churned in, as a percent of the mixture | Air as a share of what's in the tub |
|---|---|
| 0% | 0 |
| 20% | 17 |
| 40% | 29 |
| 60% | 38 |
| 80% | 44 |
| 100% | 50 |
| 120% | 55 |
| 150% | 60 |
Look at where the line sits at 100% overrun, and notice how many people get this wrong.
"100% air" sounds like an empty tub, which is obviously silly, so people assume the number must mean something else and stop thinking about it. It doesn't mean the tub is all air. It means the churning doubled the volume — and a doubled volume is half original mixture and half air.
Notice too how quickly the line climbs at the start and how slowly it climbs later. Going from 0% to 20% overrun buys a lot of extra tubs out of the same mixture. Going from 120% to 150% barely moves the needle, which is one reason nobody bothers.
A factory churns its mixture to 100% overrun and fills a 500 ml tub with the result. How much of the original mixture actually ended up in that tub?
If air is free and it's sold by volume, why not churn in three times as much and sell four tubs' worth of foam?
Because at some point it isn't ice cream any more, and in the United States that is not a matter of opinion. There's a federal rule — 21 CFR 135.110 — that sets out what a thing has to be before the word ice cream may go on the front of it.
Most of the rule is about ingredients: at least 10% milkfat, at least 10% nonfat milk solids, and at least 1.6 pounds of total solids in every gallon.
But there's one more line, and it is the one that quietly caps the air:
> it must weigh not less than 4.5 pounds to the gallon.
A rule about weight, in a product sold by volume. That is the whole defence, and once you notice why, you can work out exactly what it protects you from.
Pounds per gallon is no use standing at a freezer. Press through and convert it into a number you could check with a kitchen scale.
You weigh the contents of two 1 litre tubs, both sold in the US as ice cream. One comes to 890 g. The other comes to 570 g. Which of these is true?
Almost nothing in these episodes can be tested at home in ten minutes. This can.
You need a kitchen scale and permission to open the freezer.
1. Find two tubs of the same size — the label will say the volume in millilitres or fluid ounces. 2. Weigh each one, lid on, and write the numbers down. 3. Then weigh an empty tub of the same kind once one is finished, and take that off each number. That's the ice cream on its own. 4. Divide the grams by the millilitres.
A result near 1.00 g per ml would be very dense indeed. Down near 0.54 is right at the legal floor, meaning something like half the tub is air. Most things you find will sit somewhere between.
You now know something about the tub that isn't printed anywhere on it. There is nothing sneaky going on — the volume on the label is honest and the ingredients are listed. It is simply that the number they have to tell you is not the number you wanted, and a scale gets you the rest of the way.
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21 CFR 135.110, the US federal standard of identity for ice cream, requires that the finished food 'weighs not less than 4.5 pounds to the gallon' and 'contains not less than 1.6 pounds of total solids to the gallon', and 'contains not less than 10 percent milkfat, nor less than 10 percent nonfat milk solids' (verified against the CFR text, Cornell Legal Information Institute, 21 CFR 135.110). Overrun is the increase in volume produced by churning air into the mix, expressed as a percentage of the original mix volume. The increase 'is limited to 100 percent by the requirement that the finished product weigh at least 4.5 pounds per gallon' (Encyclopaedia Britannica, 'overrun', food technology). The gram figures in the worked table are unit conversions the reader can reproduce: 1 pound = 453.592 g, so 4.5 lb = 2,041 g; 1 US gallon = 3,785.41 ml; 2,041 / 3,785 = 0.539 g per ml; 0.539 x 1,000 ml = 539 g per litre; 0.539 x 473.18 ml (1 US pint) = 255 g. No trade or brand figure is used. Churning during freezing serves two purposes: it keeps ice crystals small, which is what makes ice cream smooth rather than solid, and it incorporates air. Ice cream with no air incorporated is excessively hard and difficult to scoop (MilkFacts.info, 'Ice Cream Production'). This episode deliberately quotes NO typical overrun figures for particular product categories. The ranges published for premium, economy, gelato and soft serve come mostly from equipment vendors and disagree with one another; the weighing experiment gives a reader a better answer than any of them.