Should you buy a banana ripe or unripe? And what about strawberries? Or pineapple, avocado, green beans or Brussels sprouts? Your first reaction might be that it depends. It can depend on when you’ll be eating it. If you don’t plan on using it within a week, you might prefer an unripe banana so it has time to ripe and won’t spoil as quickly. In you’re planning it on eating when walking out of the store though you’d prefer a ripe banana.
However, that’s not all you should take into account. Some fruits and vegetables won’t ripen any further, no matter how long you leave them on your counter top. How do you know which fruits and vegetables do continue to ripe and which don’t? It’s can almost all be explained with a little molecule, called ethylene.
Importance of ripening
Fruits aren’t very appetizing when they’re unripe, they’re not as sweet and soft. If you’ve ever eaten a too green banana, you know what we refer to. Only when fruits have gone through the ripening process they’ll develop their appealing flavor and texture characteristics. This ripening process is the last step of the development of a fruit, after that, it’s only spoilage, and just before ripening is the final growth phase. This should make sense, you’ve probably never seen a banana grow while in your fruit bowl.
The ripening process is a complex sequence of events. Once ripening has set in, all these processes are set in motion, a lot of them executed or aided by enzymes. One of the most important processes is the conversion of those tough dry starches into sweet sugars. Fruits are meant to be eaten be animals and this process is what makes them more appealing. Apart from that, the fruit becomes less acidic and the texture softens and in a lot of cases the colour will change. A banana changes from green to yellow, a strawberry and tomato turn red and a mango loses its greenness.
Initiator of ripening
But how does a fruit know whether it’s time to ripen? This is all governed by that one molecule we mentioned at the start: ethylene. The plant will start producing ethylene and thus triggers ripening.
In some fruits the ethylene will trigger a pretty fast ripening process. Those fruits can often also produce their own ethylene, speeding it up even further. These fruits are called climacteric fruits. Characteristic for these fruits is also a very high respiration rate during ripening.
The ‘opposite’ are fruits which ripen a lot slower. Also, they will do so on the plant and will not continue ripening once harvested. These fruits can produce ethylene, but in a lot smaller quantities and the ethylene serves less as an immediate trigger. These are the non-climacteric fruits.
Handling climacteric fruits
Climacteric fruits are the fruits that will continue ripening on your counter top or in your fruit bowl. This makes processing them a lot easier for farmers and manufacturers, since they can harvest them unripe. Generally, unripe fruits are a lot less vulnerable to transport or processing. They still contain a lot of those sturdy starches and have a tougher structure.
What makes it even easier is that the pretty fast ripening process can be initiated by ethylene. This ethylene doesn’t necessarily have to come from the plant they’re attached to. Any ethylene will trigger the process so releasing ethylene in a room full of unripe climacteric fruits will ripen the fruits.
Bananas are an example of a climacteric fruit. Bananas are harvested when they’re still green and unripe. In the subsequent boat ride the temperature and gas composition are controlled closely to make sure they don’t ripen. Once the bananas arrive at their final destination that are placed in huge storage facilities with close control of the air composition. When they need ripe bananas, it’s a matter of releasing enough ethylene into the area which will ripen the bananas in a very controlled manner.
Other examples of climacteric fruits are pears, avocados and kiwis. Pears are a special one, they are best harvested unripe, ripening on the tree will even decrease their quality due to stoniness. All of these fruits will produce ethylene during ripening. If you store them together, they will actually influence each other’s ripening process! If you want your avocados to ripen faster, store them close to your ripe bananas, or even together in a paper bag, ripening will go a lot faster.
As opposed to the climacteric fruits, the non-climacteric fruits do not significantly ripen after harvest. It’s not useful to buy unripe raspberries or strawberries. They won’t ripen further, they’ll only spoil.
These types of fruits do produce ethylene, however, the levels are a lot lower than they are for climacteric fruits. Also, the ripening of these fruits can’t be triggered by ethylene as can be done with the climacteric fruits. This makes overall handling harder, you have to be more careful with these fruits.
Non-climacteric fruits have a lot lower respiration rate than climacteric fruits and this rate will only decrease after harvest. A slow respiration rate is beneficial when it comes to spoilage since it won’t consume the sugars in the fruit as fast.
Examples of non-climacteric fruits are strawberries, grapes and a lot of citrus fruits.
Other changes induced by ethylene
Just because non-climacteric fruits do not ripen further after harvest doesn’t mean they don’t change anymore after harvest. Ethylene can induce and fasten colour changes of fruits. Oranges for instance, may be harvested green and then ‘made’ orange by releasing ethylene on them. That doesn’t make them ripen more though, so the orange will have to be ripe at the point of harvest.
On Food and Cooking, Harold McGee, 2004, link, p. 352-354
Advances in minimal processing of fruits and vegetables: a review, Wassim Siddiqui et al, 2011, link
Biochemistry in fruit ripening, edited by GB Seymour et al, 2012 link
Fruit processing, D. Arthley et al, 2012, link, p. 41-50
Post-harvest technology of horticultural crops, K.V. Peter et al, Vol. 7, 2007 link, p. 31-33