People often throw away fruit that has even one soft spot. They’ll spit out in disgust an apple that’s mushy and mealy. Yet fruit at this far limit of ripeness actually makes the best cooked sauces.
When fruit is green, it’s hard and sour. It’s hard because the cells are firmly glued together by pectin. It’s sour because the carbohydrate is starch, which has no sweet taste.
If you were to try to cook green fruit into sauce, it would never get sweet, because cooking doesn’t break down starch into sugar. What you might get, if you drain off the liquid, is a solution of pectin, useful for making jelly. This was the traditional way of getting pectin before it was a commercial product.
As fruit gets ripe, several processes occur. At the surface, the whole fruit changes color. Inside, the fruit tissues produce a mixture of esters, flavor molecules that give the characteristic essence of, say, apple, peach, or pear. If this fruity scent is not there before you cook fruit, there’s no way it can be there after!
The pectin that binds the cells, one to another, begins to break down, so the whole fruit gets softer.
The starch turns to sugar, and goes into each cell’s central vacuole. This vacuole is like a storage tank, which makes up most of the internal space of the cell. The fruit cells become like tiny water balloons, full of flavorful liquid.
When you bite into a piece of fruit that is perfectly ripe, your chewing crushes the cells open to release their sweetness and flavor.
As fruit gets past the point of perfect ripeness, the same processes go further. Esters build up even more, and can give the fruit an “off” flavor, even reminiscent of paint thinner. The starch continues to convert to sugar, and builds up even more in the cell interior.
If there’s so much sugar, why is an overripe apple mealy and bland? The pectin that used to glue the cells together is gone, so the cells come apart from each other when you chew. They roll around in your mouth like little BBs. They don’t pop open and release their sweetness.
If you take courage, and dare to cook fruit that has gone into this unpromising state, the magic happens. The heat drives off excess esters, and tones the flavor down. The cells burst open, and release all their internal sugar.
If there’s any complaint, it may well be that the product is “too sweet”. You can always blend it with something that could use additional sweetness, or use it as a sweetener in its own right. Some years ago, there was a spate of natural products advertised as “pear juice sweetened”. This is where that pear sweetener came from.
In using past-peak fruit, there’s a line between overripe and actually rotten. Apples and pears can be completely brown and translucent, but it’s merely their own ripening process.
This is the same thing that happens, to a different degree, in the fruit of the medlar (Mespilus germanica), closely related to pears and apples. Medlars are not considered ripe until they are “bletted”, which is the word for this stage of being brown and soft.
You can tell by the smell. If it smells like fruit, even very fruity, it’s OK. Your nose knows. But if it smells like vinegar, or mold, or fermented, it’s gone beyond. And of course don’t use it if you can see actual mold.
If you have overripe fruit trickling in piece by piece, you can collect it in bags in your freezer until you have enough for a batch to cook. You can cut it in quarters so it packs better, and to make sure there are no “worms”, as in homegrown fruit.
Damage, such as worms, induce early ripening, so a worm-stung fruit may originally be only overripe, though the hole can introduce rot which quickly spoils the whole thing. Often, especially in pears, a worm does little damage, causing only a hard lump by the core. The major part of the fruit is fine, though unsightly, and perfect for sauce.