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:


Chart demonstrating fat interactions with and without an emulsifier
More on factors affecting partial coalescence in ice cream
In-depth overview of ice-cream emulsifiers