Research by the State University in Brazil shows that the fruit, available from June to November, contains up to 12% more vitamin C in this condition.
The spotted skin is a biological response to sunlight.
Papaya exports are booming having grown from 8,000 tonnes to 40,000 tonnes in the last 15 years and the fruit is gaining in popularity in the UK.
It is regularly stocked by most retailers, in conjunction with lemons or limes in a dual prepack. Retail value, according to trade sources, is £5m a year.