The white or yellow areas on your tomatoes are due to sunscald. Sunscald occurs on fruit exposed to the sun during periods of extreme heat. Initial symptoms of sunscald are the development of shiny white or yellow areas on the fruit. Later, the affected tissue dries out and collapses, forming slightly sunken, wrinkled areas. Secondary organisms invade the affected areas causing the fruit to rot.
Losses due to sunscald can be reduced by growing tomatoes in wire cages. Cage grown tomato plants provide good foliage protection for the fruit. Also, control Septoria leaf spot and other foliar diseases which defoliate plants and expose the fruit to direct sunlight.