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The substitution effects of whey protein isolate(WPI) for egg in the preparation of sponge cake were determined by objective and subjective tests. Milk whey is drained from milk curd as a by-product of cheese manufacture. Whey protein is known as a good nutritional source and a functional material for many processed foods, especially baked goods. WPI contains above 90% whey protein. The specific gravity and viscosity of sponge cakes tend to be affected by WPI substitution. The cooking loss of sponge cakes with WPI substituted for egg(abbreviated as WPI cake) during oven baking was smaller than that made with egg(abbreviated as egg cake) and the specific loaf volume of WPI cake was larger than that of egg cake. The number of pores was highly increased and the size of pores was more uniformly and finely distributed in the cross section of WPI cake than those of egg cake, as observed by scanning electron microscopy(SEM). The hardness, gumminess and chewiness of WPI cake made with 10-20% WPI substitution were the lowest among all the tested cakes, including egg cake, thereby confirming the considerable improvement in their cake qualities. By the results of sensory evaluation, appearance, pore uniformity, softness, chewiness, moistness, flavor, mouth feeling, and overall acceptability of 10-20% WPI substitute cakes were evaluated as being significantly superior to those of all other cakes(p<0.05). These results support the better physicochemical characteristics and sensory evaluations of sponge cake prepared with 10-20% of WPI substitution for egg.