Final answer:
The statement that flour for sponge cakes must be weak is true. Weak flour has lower protein content, preventing the development of a strong gluten network and keeping the cake light and airy.
Step-by-step explanation:
The statement is true; flour for sponge cakes must indeed be weak, which generally refers to having a lower protein content. This is because high-protein flour can create a stronger gluten network when mixed, leading to a tougher cake structure.
Sponge cakes are meant to be light and airy with a delicate crumb, and using weak or low-protein flour, such as cake flour, helps achieve this texture.
When making sponge cakes, it's also important to carefully fold the flour into the egg mixture to maintain the volume and to avoid overmixing, which can further develop the gluten and make the cake tough.