The colour of the peel of citrus fruits is due to carotenoids. In view of the fact that carotenoids do not have any anti-microbial activity, there is no correlation between colour and spoilage by micro organisms.

Spoilage is influenced by a combination of intrinsic and extrinsic factors, such as acid, temperature, nutrients and anti-microbial compounds.

Acid is a major factor in spoilage; acid fruits are generally spoiled by fungi only, as most spoilage bacteria do not grow at a low pH.

Temperature is also a major factor; the higher the temperature, the faster spoilage will occur. As citrus fruits are normally stored under similar conditions, this factor does not play a role in this case.

All citrus fruits are generally rich in nutrients, especially sugar; hence the sugar concentration does not influence a difference in spoilage rate.

The peels of citrus fruits are rich in essential oils. These oils generally contain compounds with anti-microbial activity. The presence and concentration of these compounds thus may play a role in the observed differences in spoilage. There is no relation, however, with the colour of the fruit.

Most citrus fruits are treated with anti-microbial compounds on the peel; the fruits are sprayed, washed, impregnated or wrapped in impregnated paper to prevent spoilage during transport. Differences in this treatment are probably the most important factors that determine the rate of spoilage of citrus fruits, together with the overall storage conditions.