Like most things we try to classify one way or the other, there tend to be some exceptions or outliers. Apples, for example, ripen after being picked and increase in not just sweetness but also get softer over time. Most people prefer apples underripe (since the varieties generally eaten uncooked tend to be fairly sweet right off the tree) and do not prefer them at full ripeness due to how soft (and mealy) the flesh becomes. (Because apples do ripen after harvesting and we don't often allow them to reach full ripeness, if you've got an apple in your house, it's probably in the process of ripening and therefore outgassing carbon dioxide and ethylene. The latter gas - for reasons not yet fully understood - will trigger and speed up ripening processes in other fruits which can be ripened after picking - see chart below). Another example is the pineapple (a fruit that doesn't ripen after harvest) which does change color after being picked but does not get sweeter or develop a stronger aroma. For this reason, they should be selected not by how golden the skin is but by how fragrant it is.
Since fruits which ripen release ethylene gas, they should not be stored with or near vegetables. The ethylene will cause most vegetables to spoil faster. (Many vegetables can be heat treated - by blanching in boiling water for a minute or two - which deactivates the enzymes which cause wilting and spoilage. It is possible that heat treating could prevent spoilage from ethylene exposure, but I have not found any documentation of this and have not performed the experiment yet.)
Conventional wisdom states that fruits which are ripened on the plant will be superior to those picked early (and possibly ripened later if it is one of those lucky fruits that can continue to ripen). There is much truth in this, but again there are some interesting exceptions. The first is the avocado which does not ripen on the tree at all. This is one fruit which has to be picked (or fall off the tree) to begin ripening. I was surprised to learn during my research that Modernist Cuisine claims pears also do not ripen on the tree. This is not technically true as pears do ripen on the tree but many varieties do not taste good when tree-ripened with particular complaints regarding off-flavors. Pears are one of the few fruits which are best ripened off the tree. Bananas are another interesting case as it is the ultimate example of ripening after harvest - bananas improve in every aspect (sweetness, color, aroma, and texture) dramatically when ripened at home and, by all accounts, is as good as when plant-ripened.
Fruits that ripen after being picked can have their ripening processes accelerated easily in two ways: heat and ethylene. Keeping fruits in a warm environment will ripen the fruit by accelerating the activity of the ripening enzymes. This process speeds up until the temperature rises past that at which the enzymes remain intact (which is lower for fruits from colder weather regions and higher in tropical fruits). This can also increase the rate of spoilage as surface bacteria and microorganisms will generally thrive in warm temperatures up to the deactivation temperature of the enzymes. Washing the fruit with a fruit wash is advisable before attempting to heat ripen it.
Ethylene gas ripening is probably easier since all it takes is an ethylene producer (the fruit itself will work, but an apple is even better). Simply place the fruits in a paper bag with the top folded over. This creates an environment which allows some airflow but will increase the concentration of ethylene gas around the fruit. When fruits which will ripen after harvest are exposed to a high ethylene concentration, ripening is promoted.
In this chart, I have listed fruits in alphabetic order after separating them into two sections. The first section contains those which ripen after harvesting (and can be assumed to produce ethylene gas until fully ripe). The second section lists fruits known to not ripen after being picked (a few of these fruits may change colors but they do not actually ripen). For the fruits that continue to ripen, I have marked which characteristics increase while ripening - sweetness, scent, and/or softness.
|Fruits Which Ripen After Harvest|
|Fruits Which Do Not Ripen After Harvest|
I did find some conflicting information on cherimoyas with some sources claiming they get sweeter and others claiming they do not. I decided to list it as a fruit which does not get sweeter after being picked because that's how it is listed in On Food and Cooking.}?>