Egg whites should usually be whisked just before you need to use them; once whisked they should not be left to sit for any length of time or they will separate and begin to collapse.
1. Using a large fine balloon whisk, start to whisk the egg whites in the bowl. Lifting the whisk up and over the whites is more effective and incorporates more air than just a stirring action.
2. As more air is incorporated the whites will become slightly foamy, opaque and very thin. Continue whisking and they will increase in volume, becoming white and foamy. Early on, if you lift the whisk vertically upwards and then turn it upside down, the whites will not hold their shape and will fall from the whisk.
3. Continue whisking and the whites will become a little paler and stiffer.
4. As the whites stiffen a little more, test them again by lifting the balloon whisk vertically, then turning the whisk upside down. If the whites cling to the whisk and start to create a ‘peak’, but the peak falls over on itself, the egg whites have reached the ‘soft peak’ stage.
5. If firmer whites are required, whisk for a little longer then test again by lifting the whisk; the whites will definitely cling to the whisk and, as it is pulled up vertically and turned upside down, the clinging whites will start to fall over onto themselves, then stop halfway. This is known as the ‘medium peak’ stage and is ideal for soufflés and mousses.
6. Continue to whisk again; the whites will become very stiff and when tested the peak will hold its vertical position and not fall over on itself. This is known as the ‘stiff peak’ stage and is the required consistency for meringues and for sweet soufflés where the egg whites are meringued. At this stage there is still some elasticity in the whites.
A note on over-whisking…
Further whisking would cause the whites to become too stiff and they would ‘break’ when pulled away on the whisk.