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обычно этот термин относится только к белку и обозначает изменение его структуры во время взбивания http://www.cookies-in-motion.com/Whipping-Egg-Whites.html

to fold in an egg означает осторожно ввести яйцо в тесто. Взбивать яйцо при этом не нужно. Просто хорошо перемешать, чтобы не образовалось комки.

http://www.taste.com.au/recipes/11125/basic cheese souffle
Spoon about 1/4 of the egg whites into cheese sauce mixture and use a spatula or large metal spoon to fold the whites through the mixture until just combined. To fold in, gently run spatula or spoon around side and along base of bowl and use a sweeping action to fold the mixture over on itself, rotating the bowl. Repeat until mixture is just combined.
The folding action is more gentle than a stirring or mixing motion and retains the air beaten into the whites. Adding just some of the whites helps to "loosen" the mixture and makes it easier to fold in the remaining whites.

Overmixing causes the mixture to lose air and the souffle will not rise as well as it should. Undermixing results in clumps of unfolded egg white which cause uneven rising.

Вероятно тут идет перенесение термина не на само яйцо, а на способ его приготовления.