Why is it best to use fresh eggs at room temperature when making a sponge cake?

When making a sponge cake eggs are whisked with sugar to increase their volume and lighten them, resulting in the characteristic fine crumb and light texture of this wonderful cake. Fresh, room temperature eggs are best used in sponge making, but why?

Eggs, by nature, are acidic when fresh, and this acidity causes the proteins in the white to be tightly knit. As the egg becomes older it becomes more alkaline and the proteins start to pull away from each other, causing the white to become thinner.

Now, when you whisk egg whites you are actually forcing these proteins apart and then recombining them in a new structure around small bubbles of air, hence forming a foam. With fresh eggs, initially it is a little harder to break these tightly knit proteins apart and the whisking will take longer. But the resulting foam will be more stable than with older eggs.

So to counter-balance the extra time that fresh eggs take to whisk to a foam you can use room-temperature ones that are easier and quicker to transform than chilled eggs are.

Therefore, with fresh eggs that are at room temperature you get the best of both worlds – a stable foam/mixture that doesn’t take ages to reach the desired consistency. So when making any recipe based on whisking egg whites or whole eggs, including those for soufflés, meringue mixtures and some cakes, such as sponges, choose the freshest eggs you can get your hands on and make sure they are at room temperature.