What the law says
In the soft serve vs ice cream question, the law is the simplest part. The United States has a standard of identity for ice cream, set out in 21 CFR 135.110. It has no separate standard of identity for soft serve.
So a soft serve that meets 21 CFR 135.110 is ice cream, in the legal sense. Under that section, ice cream must contain at least 10% milkfat. The floor drops to 8% of the whole when bulky flavors such as chocolate, fruit or nuts are added. It must weigh at least 4.5 lb per gallon. And it must contain less than 1.4% egg yolk solids.
Soft serve with less fat than that does not qualify as ice cream. It is still sold, but as a different product. When a shop sells its soft serve as ice cream, it is held to the same rules as a carton in the grocery freezer, egg yolk limit included.
Fat and solids
H. Douglas Goff’s Ice Cream Technology e-Book from the University of Guelph gives example mixes for soft-frozen products. They contain:
- 10.0% milk fat
- 12.0 to 12.5% milk solids-not-fat
- 36.0 to 36.3% total solids
That fat level sits right at the federal minimum for ice cream. Guelph also notes that these mixes keep emulsifier high, because dryness is a concern in soft serve. That is one of the few ways the mix itself is tuned for soft serve.
USDA FoodData Central lists French vanilla soft serve at 222 kcal, 13 g fat and 21.2 g sugars per 100 g. Its record for regular vanilla ice cream shows 207 kcal, 11 g fat and 21.2 g sugars. Keep in mind that this pairs one French vanilla soft serve with one plain vanilla ice cream. It does not describe soft serve in general.
Air, weight and temperature
Air is measured as overrun: how much the volume grows as air is whipped in while the mix freezes. For hard ice cream, Guelph gives about 100 to 120% for standard ice cream and 25 to 50% for super-premium. More air means a lighter pint for the same volume, which is why the federal rule sets a weight floor.
For soft serve, we found no sourced figure for overrun. We also found no sourced serving temperature. What Guelph does say is that soft serve comes straight from the freezer with about half of its water frozen.
Hard ice cream is colder and firmer when you eat it. Guelph gives a typical serving temperature of about -16 °C (3 °F).
How each one is made
Both start the same way. The mix goes into a barrel freezer, where it freezes while being stirred and whipped. Guelph says it leaves the freezer as a semi-frozen slurry, with about half of its water frozen and a consistency like soft serve.
From there the paths split:
- Soft serve is drawn into a cone or cup right at that point.
- Hard ice cream is drawn into packages, then moved to a blast freezer at -30 to -40 °C, where most of the rest of the water freezes.
Guelph puts it plainly: that one step is almost the only thing that sets hard-frozen ice cream apart from soft serve. Same mix, same freezer, different exit.
If you make ice cream at home, you have probably seen this stage yourself. The ice cream that comes out of the machine is soft and spoonable. It firms up only after a stretch in the freezer.
Where the numbers do not line up
Our sources do not agree on how much air standard ice cream holds. Guelph gives about 100 to 120% overrun for standard ice cream. The federal weight rule in 21 CFR 135.110, at least 4.5 lb per gallon, caps overrun near 104% at a typical mix density. That cap is our own arithmetic, not a figure printed in the regulation.
We are not picking one number. Read the Guelph range as a description of products, and the 104% figure as a legal ceiling at one typical mix density.
For soft serve there is nothing to weigh against either figure. We found no sourced overrun for it, so any number you see for soft serve air should come with its own source.
If you want an excuse to compare the two side by side, National Soft Serve Day falls on August 19 every year.