Sponge Cake with Barley Flour

Beating egg yolks with sugar creates a stable foam through mechanical incorporation of air bubbles and emulsification of fats, while the sugar dissolves and helps stabilize the structure. The hot water helps dissolve the sugar more completely and slightly denatures proteins for better foam stability. Folding in stiffly beaten egg whites at the end adds volume without deflating the delicate foam structure, and the slow oven temperature (around 325°F) allows the cake to set gently without collapsing or forming a tough crust.