Egg white in a cocktail contributes almost no taste and transforms the drink anyway. Its proteins build a foam that changes texture, appearance and how the acidity is perceived.
Proteins unfold and trap air
Agitation causes the coiled proteins in egg white to unwind and rearrange, with water-avoiding sections turning toward air bubbles.
The unfolded proteins link into a network around each bubble, which is what holds the foam together after shaking stops.
The same process happens when egg white is whisked in a kitchen, at a smaller scale and inside a cold acidic liquid.
The dry shake exists because ice interferes
Shaking with ice chills the mixture quickly, and cold proteins unfold more slowly and less completely.
Shaking first without ice lets the foam form at a warmer temperature, and the second shake with ice then chills and dilutes it.
Reversing the order produces a thin, short-lived head, which is the most common failure with these drinks at home.
Acidity helps the foam and then limits it
Mild acidity assists protein unfolding, which is why egg white appears almost exclusively in sours rather than in spirit-forward drinks.
Too much acid causes the proteins to aggregate too tightly, producing a coarse foam that drains quickly.
Sugar stabilises the foam by thickening the liquid between bubbles, slowing the drainage that eventually collapses it.
Texture changes how the drink tastes
A foam slows the liquid across the tongue and softens the initial impact of acidity, so a sour with egg white reads as rounder at the same measured sourness.
The foam layer also holds aromatics at the surface, which is why bitters dropped onto the head are so noticeable.
None of this comes from flavour, since egg white contributes little beyond a faint richness that most drinkers cannot identify blind.
The substitutes work by the same mechanism
Aquafaba, the liquid from cooked chickpeas, contains proteins and saponins that foam in a similar way and is the common plant-based replacement.
It produces a slightly looser foam and carries a faint savoury note, so recipes using it are often adjusted rather than swapped one for one.
Commercial foaming agents based on plant proteins give more consistent results, which is why bars serving these drinks in volume tend to use them rather than raw egg.