I froze half a pack of the Two Chicks egg white in 10ml doses in an ice cube tray, and then tested defrosted cubes against the never-frozen product in a cocktail head-to-head. There was no noticeable difference in the foam, not visually, in mouthfeel nor longevity.
Freezing works.

(It seems, sod's law, that every recipe I looked at thereafter asks for 15ml eggwhite, cf 10ml or 20ml, but 10ml works well enough for me)