Reasons for separating eggs in ice cream

Milk and cream are essentially made up of water, fat, and protein molecules. Within milk and cream, the fat globules are already emulsified in the surrounding water by casein (one of the proteins found in milk and cream). This means that casein molecules surround each fat globule and prevent them from coming together. This is why milk doesn't separate into fat and water, even though it's made up of two substances that don't naturally mix.

However, casein is a little too good at emulsifying the fat. When we add egg yolks, the lecithin in the yolk displaces some of the casein surrounding each fat globule. This partially destabilizes the emulsion of the milkfat by casein and allows some of the fat globules to partially coalesce when the ice cream base is chilled and aged overnight.

Partially coalesced fat forms a network that traps and stabilizes the air that is incorporated into ice cream while churning, leading to slower melting, better shape retention, and overall improved texture. Lecithin is why we need egg yolks.

Egg whites, however, are mostly water and don't contain lecithin. Adding water to your ice cream base leads to an unfavorable icy texture in the final product and doesn't contribute to partial coalescence of fat. Water is why egg whites are typically left out.

Further reading:

1. Chart demonstrating fat interactions with and without an emulsifier

2. More on factors affecting partial coalescence in ice cream

3. In-depth overview of ice-cream emulsifiers

Egg yolks make ice cream richer, smoother, creamier, and softer. (They are definitely not just for the color.) The whites don't provide the nice textural benefits, so you definitely can't replace two yolks with one whole egg. You also don't need extra ingredients to get air into the ice cream; air is added to ice cream by churning it. So there's no need to include the egg whites at all.

Specifically, egg yolks contain fat, protein, and emulsifiers (lecithin). By keeping the custard emulsified, i.e. avoiding separation even on a small scale into water and fat, they prevent larger ice crystals from forming. Cooking the custard also thickens it, helping give the desired texture.

I think you could add whole eggs to ice cream, but the egg whites don't really provide anything beneficial, so you'd have to for example replace 3 egg yolks with 3 whole eggs. You'd also need to strain it after cooking to avoid bits of cooked whites. (This is a good idea anyway, just with the little bits of white that tend to come with the separated yolks, but with whole eggs it'd be way worse.) And perhaps worst, you'd get a much stronger eggy flavor than you do from the yolks alone.

Note that it's also possible to make ice cream without eggs at all. Ice cream made with egg yolks, milk, and cream is called French-style, while ice cream made with more cream and no eggs is called Philadelphia-style. The Philadelpha-style keeps it soft enough by increasing the cream to milk ratio, i.e. adding fat. It's not a direct substitution, though: French-style is always going to be richer and creamier.

So you can make something good but different by reducing (or omitting) egg yolks and increasing fat in the form of cream, but the yolks are absolutely necessary in order to obtain the desired results in the recipes that use them. You could also add pure emulsifiers (e.g. soy lecithin) to make a smoother ice cream without eggs, but I think that's getting a bit beyond the scope of the question.

And it's no accident that fat is added in the form of cream, not something like butter or margarine. If you add pure fat like that, you won't be able to get it fully emulsified, even if you do have egg yolks. You'll end up with little bits of hard frozen butter/margarine in your ice cream.

Good question, and great answers from Jefromi and Josh. I just wanted to add that in the UK, most recipes for ice cream that I've seen make a "custard" ice cream, using egg yolks and effectively cooking them to make a custard before freezing. However, when I bought the Ben and Jerry's ice cream book, their recipe does call for whole eggs (not just yolks), which are whipped for a couple of minutes to make them foamy (obviously, whipping whole eggs doesn't make them peak in the way that whipping just whites does, but it clearly adds air) and no cooking is involved.

Some time ago I made a batch of "custard" style vanilla ice cream, and the whole-egg-no-cooking style ice cream and did a taste test with my friends. Most preferred the whole egg version, so it's certainly possible (and much easier) to use this method.

In terms of difference, the custard style ice cream is a little "chewier", which some people prefer and some don't, and it does taste a little richer. I think the chewiness was both from it being just yolks, and also that it was denser than adding whipped eggs (so even though it was churned, whipping the eggs did add even more air). And, as Josh said, the whole egg method did lead to a slight crunchiness (ice crystals) when I took the ice cream out of the freezer a week or two later.

For years I have used both whole and yolks combined. Never a problem. For half lte cream, half ltr milk I use 4 yolks and 2 whole eggs plus some invert sugar ( which I make ) Faulous ice cream. The above just as vanilla or coffee or equal parts fruit purée and base.