In a foam cake, what turns whipped egg bubbles into a sliceable sponge?

Answer: Coagulated egg proteins

Melted sugar crystals — Sugar helps foam cakes, but melted sugar is not the main solid frame. Sugar can stabilize, sweeten, brown, and delay setting, which is a lot of work for one ingredient. The final sliceable network still depends heavily on proteins unfolding and bonding around bubbles. Think of sugar as timing and tenderness, not the beams of the sponge.

Extra baking steam — Steam expands bubbles and gives some lift, but it disappears as pressure falls. A sliceable cake cannot be built from vapor alone; it needs bubble walls that survive after cooling. That is why souffles and foam cakes are so timing-sensitive: expansion is temporary unless heat turns the protein film into a firm network. Steam is the push, not the skeleton.

Coagulated egg proteins ✓ — Right: egg proteins denature and coagulate, making a network around the foam. Whipping first unfolds proteins at air-water surfaces; baking then makes those proteins bond more permanently. That turns a bowl of fragile bubbles into a crumb you can cut. The surprising part is that the airy texture comes from a solid protein cage, not from air alone.