Custard Apple covers several types of fruit - Bullock's Heart, Cherimoya, Sour Sop and Sweet Sop. Custard apples are tropical fruits. Cherimoyas have a pineapple-like flavour. Sweet sop is popular in the Caribbean and has a sweet, custard-like flesh. Sour Sop has a white flesh and a more sour taste that the other types of custard apple. The flesh of the bullock's heart is more solid and sweet, getting its name from its dark brown colour and heart-like shape.
Dowell, P., Bailey, A. (1980) The Book of Ingredients, Dorling Kindersley. ISBN 0718119150.