Monter les blancs en neige
First you must "séparer les blancs des jaunes", then you "monte les blancs".

If you see
"monter les blancs en neige ferme" it means they must be very thick and not liquid at all.
How can I do this ?
Use an electrical whisk. It takes about 4 minutes depending on how many "blancs" you need. Do not "monter en neige" more than 6 whites at a time. It's easier to do this if you take cold eggs.

Then, if you use these "blancs en neige" in a cake, mix them DELICATELY, use a spatula.
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