Is partially rotten fruit still edible?

Many people simply cut off the rotten parts of the fruits and eat the rest. Seems prudent, because eating the rotten parts is actually pretty unsafe.

Oops, part of my honey pomelo is spoiled! Should I eat the rest or discard it?

The answer is simple, don’t eat it! Rotting happens mainly because of microbes. They are keen on the carbohydrates and moisture in fruits; as one can easily tell, fruits are sweet and juicy, providing energy for the breeding of microorganisms. There are several hundreds of bacteria, fungi and viruses that can cause fruit rotting, many of them are poisonous or bad for your health.

Normally, fully-ripened fruits deteriorate most quickly, because when a fruit is fully-ripened, its complex carbohydrates such as starch are hydrolyzed rapidly into free sugars accompanied by decreasing acidity and firmness. With more sugar, lower acidity and softer peel, rotting happens at a much higher speed.

Two types of ripening

Fruits are divided into two types: for the first type there is a peak of post-harvest metabolism called “respiratory climacteric” with consequent post-ripening, while fruits that have no respiratory climacteric do not experience post-ripening.

A large proportion of fruits belong to the climacteric group: pear-like fruits (apple, hawthorn, quince, etc.), peach-like fruits (apricot, plum, prune, etc.), melon, avocado, mango, banana, kiwi, pitaya, papaya, durian, cherry tomato, persimmon/kaki, pomegranate, etc. The non-climacteric fruits are: berries, cherries, citrus (orange, lemon, mandarin, grapefruit, pomelo, etc.), pineapple, litchi, coconut, loquat, and so on.

Climacteric fruits and non-climacteric fruits ripen at different paces. The respiratory climacteric process is triggered by ethylene. Ethylene is a plant hormone and the threshold is 0.1ppm. Both endogenous and exogenous ethylenes are valid. Ethylene drives climacteric fruits to their full maturity within 1-2 days, which are then sweet, tender, aromatic and juicy. Maturity of non-climacteric fruits remains more or less the same as when they were harvested, i.e. they hardly become sweeter during storage.

Venting: the key to storage

To store fruits properly, venting is crucial. Venting not only reduces relative humidity, thus repressing microbial activity, but also prevents ethylene accumulation.

Please remember that a fully ripened climacteric fruit will emit ethylene; ethylene accumulation will obviously reduce life of other fruits wrapped within the same bag or container. Therefore, when a climacteric fruit becomes aromatic (an easy sign for optimum maturity), eat it. Collect climacteric fruits into separate bags instead of one bag.

For non-climacteric fruits, since they almost reach optimum maturity when harvested, you’d better buy only a small amount. Refrigerating (around 39°F), of course, protects most fruits. But there are several exceptions: cool places around 50 °F are suitable for bananas, mango, pitaya, papaya and pineapple.

Among climacteric fruits, apples are an exception for their exceptionally long shelf life (up to one month; varying among varieties) and a wise choice for bulk purchase at an economical price. That’s due to the wax outside its dense peel and the fresh-keeping agents on its surface.

Beware of those rotting fruits

When a fruit is rotting and a mildew spot is found, that fungal hypha and its byproducts have already reached every corner of this fruit regardless if we see them or not. Always remember that by-products of microbes can be poisonous and the hazard is unforeseen. For instance, patulin and ochratoxin A are two widely existent toxins in rotten fruits that originate from many pathogens. Patulin is poisonous to the human stomach and gut, while ochratoxin A is classified by the International Agency for Research on Cancer (IRAC) into Group 2B for its toxicity. Here we also draw your attention to the notorious aflatoxin: aflatoxin can be found in spoiled shell-fruits and dry fruits and it is a Group 1 cancerogen; a mere 20 mg dose is fatal.

Rotting fruits are undoubtedly NOT edible. Once a fruit becomes slimy, mushy, smelly, discolored or moldy, please discard it with no hesitation and clean its stain it left in your kitchen if there is any, to avoid contaminating other fruits. Physical damage itself is harmless but it will rot very quickly; therefore we’d better deal with damaged fruits in time. Rotting fruits also can NOT be used for compost.

If you just happen to have excess fruits, there are several ways to deal with them: add them to your smoothie/baking/jam recipe; make dry fruits (either sun-dried or freeze-dried); freeze them for summer; or simply share with neighbors.