The most common reason whipped egg whites won't fluff up is even the smallest trace of fat left in the bowl or on the whisk. Fat coats the protein strands in egg whites and stops them from wrapping around air bubbles to form foam; that's why even a bowl that looks clean can ruin a batch if it's carrying a thin film of grease.
Even a tiny bit of yolk mixed into the whites causes the same problem, since yolk contains fat too. Extra care should go into separating eggs so none of the yolk touches the whites; if a bit does get in, it should be carefully lifted out with a piece of shell or a clean spoon.
Does the bowl material make a difference?
Plastic bowls are the least reliable choice for whipping egg whites, since old grease can sit in the microscopic scratches on their surface even after a thorough wash. Glass or stainless steel bowls don't carry that risk and are much more reliable for whipping.
Wiping the bowl with a lemon slice or a few drops of vinegar and drying it with paper towel before use neutralizes any invisible traces of grease and improves the odds of a good whip.
When do egg whites whip up better?
Room-temperature egg whites whip up easier and to a greater volume than whites straight out of the fridge; that's why it's recommended to take the eggs out of the fridge about half an hour before whipping.
Adding a pinch of salt or a few drops of lemon juice while whipping strengthens the foam's structure, helping it stay stable and fluffy for longer.
Is the copper bowl trick actually true?
The claim that egg whites whip up more stable in a copper bowl is scientifically accurate; ions from the copper interact with the protein structure and help the foam hold together better. That's part of why professional kitchens still favor copper bowls.
Without a copper bowl, a similar effect can be mimicked by adding a small pinch of cream of tartar before whipping; this acidic powder strengthens the foam's structure in a way similar to salt or lemon juice.
If neither a copper bowl nor cream of tartar is on hand, careful attention to keeping the bowl grease-free, along with fresh, room-temperature egg whites, still gets a good enough result for most home kitchens.
Sugar should be added to egg whites gradually, in the later stages of whipping, once the whites have already started foaming lightly; adding it too early can slow down how much volume the foam builds. Sugar also helps stabilize the foam's structure, keeping it fluffy for longer.
Using powdered sugar instead of granulated gives a smoother foam texture, since the finer grains dissolve faster; this is especially preferred in delicate recipes like meringue.
Starting the whipping at low speed and gradually increasing it produces a more stable foam with finer bubbles than whipping at high speed from the start; this technique is especially favored for delicate recipes like meringue.