I am puzzled what's the difference in using stiff peaks or firm peaks in a cake recipe? How does it affect the structure and texture of the cake in each scenario? Anyone can advise on this? Thanks
When you whip egg whites you are trapping air inside and getting the eggs to grow in size (they can get up to 6-8 times their size) and to foam and become stiff. It is the main leavening ingredient in foam cakes such as angel food, chiffon, etc.
The difference between firm and stiff peaks is that if you were to turn your whisk upside down:
Firm Peaks: the peaks will hold and be more distinct but the tip will fold back on itself AND;
Stiff Peaks: the peaks hold straight up without folding and the mixture will be quite thick and heavy
You know you have over mixed when the mixture collapses back on itself and looks really dull. It is close to breaking.
The difference, IMO, is all in the language rather than the technique. Some people say stiff, others say firm. As long as you make a firm peak with your eggs you are golden.