What fat, sugar, protein and air do in ice cream

Ice cream looks simple — milk, cream, sugar, a bit of flavour. From the point of view of food science, though, it’s one of the most complex things in the kitchen. Douglas Goff of the University of Guelph describes it as an emulsion and a foam at the same time: a partly frozen foam containing ice crystals, air bubbles and fat globules, with a concentrated sugar solution between them. Each ingredient has its own role in this structure.

Fat

Fat gives ice cream full flavour and creaminess. It acts as a lubricant on the palate, so ice cream feels smooth even though it contains ice. It also helps with the structure: during whipping, some of the fat globules partly join together (scientists call this partial coalescence) and form a network that holds the air bubbles — much as in whipped cream. This gives ice cream body and makes it melt more slowly.

Milk fat melts over a wide range of temperatures. For good ice cream, it’s important that about half to two thirds of the fat is solid at fridge temperature. That’s why the mix is left to age in the fridge — more in the article Pasteurising and ageing the mix.

Too much fat means heavy ice cream, more calories, poorer overrun and the risk that with long churning the fat clumps into butter granules.

Milk solids non-fat (MSNF)

MSNF is everything in milk except fat and water. According to Goff, it consists of about 54 % lactose, 38 % protein and 8 % minerals.

• Proteins (around 4 % in a typical mix) bind water, increase the viscosity of the mix and help emulsify the fat and hold the air. Ice cream with enough protein has a firmer body and is less icy.
• Lactose is milk sugar. It’s barely sweet, but it lowers the freezing point just like ordinary sugar.

Too much MSNF, most often from milk powder, causes sandiness: lactose crystallises into grains that don’t dissolve on the tongue. The Ice Cream Science blog puts the risk at above about 16 % MSNF. You can feel lactose on the tongue once its crystals exceed about 15 µm.

Sugars

Sweeteners make up 12–16 % by weight of a typical mix. They do four jobs at once:

1. they sweeten and bring out the flavour of the other ingredients,
2. they lower the freezing point — part of the water stays unfrozen, which is why ice cream can be scooped,
3. they add solids — they take up space from the water, so the crystals are smaller,
4. they improve texture.

Too much sugar masks the flavour, the ice cream melts quickly, and if the freezing point is too low its crystals grow faster. Too little sugar means hard, icy ice cream. How different sugars work is explained in the article PAC and POD.

Stabilisers

Stabilisers are substances that bind water and thicken the unfrozen part of the ice cream: locust bean gum (carob gum), guar gum, xanthan, alginate, carrageenan, gelatine and, in the home kitchen, starch too. According to Goff, they:

• slow the growth of ice and lactose crystals when the freezer temperature fluctuates,
• prevent the ice cream from shrinking,
• improve melting,
• carrageenan prevents whey separation.

They are used in very small doses — together with emulsifiers, 0.2–0.5 % of the mix. Too much stabiliser gives gummy, stringy ice cream that melts poorly. More in the article Stabilisers at home.

Emulsifiers

Emulsifiers — in the home kitchen mainly egg yolk (lecithin), in industry mono- and diglycerides of fatty acids or polysorbate 80 — displace proteins from the surface of the fat globules. The fat then partly joins into a network more easily during whipping. The result is a drier, smoother ice cream that holds its shape better and melts more slowly.

According to Goff, egg yolk was the first emulsifier used in ice cream. That’s why recipes with yolks, such as vanilla with egg yolks, are thicker than egg-free vanilla. Too much emulsifier leads to churning out butter.

Air

Without air, ice cream would be like an ice cube. Air gives lightness and delicacy, and the smaller the bubbles, the creamier the ice cream. The amount of air is expressed as overrun — more in the article Overrun: how much air is in ice cream.

Too much air means foamy, crumbly ice cream with a weak body that melts quickly.

Ice crystals

Ice is an ingredient too. Crystals give ice cream its structure and refreshing coldness. The smaller they are, the smoother the ice cream — the ideal is 10–20 µm, and above about 50 µm the texture feels grainy. How to influence the crystals is covered in the article Ice crystals and texture.

Overview

So a good recipe is a balance: enough fat and protein for body, enough sugar for softness, but not so much that it masks the flavour. zmrzka shows how the mix stands with every recipe — see how the composition is calculated.