Low-sugar fruit preserves
I made great strawberry jam a while back that called for very little sugar and lots of cooking at low heat. The preserves were a little runnier, almost a compote, but they were really good. I'm trying to do the same thing with blueberries, plums, peaches, apricots, etc. Any tips or recipes out there?
Of the fruits you list, I've only gotten plums to gel. If this isn't a concern, preserve away.
If you add lemon juice or lemon rind which is high in pectin, it will help the preserves to gell. You can remove the rind if you wish, personally I like the brightness the lemon adds.
The thing is, strawberries are unusually high in pectin among the summer fruits, so what works with them won't work as well with other fruits. (Blueberries aren't very high in pectin but they are dry, so that's probably your best bet among the fruits you list.)
If want something more than fruit-in-thick-syrup, take a look at Pomona's pectin (the only retail brand of low methoxyl pectin I know of.) Unlike Certo and Surejell, it doesn't need any sugar at all to set, so you can use it to make low or even no-sugar jams/jellies/preserves.
I second the recommendation to check out Pomona's pectin. I used to use Certo, but now I prefer Pomona's. It is true that with Pomona's you can cut down on the amount of sugar and still turn out a good product.
I think of jelly-making as a process of controlling the crystallization of sugar, just as in candy-making. In my experience, there is a trade-off between how much sugar you use and how long you have to cook the fruit and/or evaporate out the excess liquid. I prefer to cook the fruit no longer than necessary, because I like my jams to taste as fresh and uncooked as possible. Pomona's pectin allows me to reduce the sugar without having to cook the jam forever. I've never tried completely eliminating sugar, because I think I would then have to cook it longer than I would like.
Oops, as Cheryl alludes to, if they're not overripe, plums should have a fair amount of pectin, too. Plum preserves never struck me as very appetizing so that's off my radar...
You can make a delicious pear butter with no added sugar, but it is incredibly expensive. I got one half-pint jar out of two pounds of pears.
You can use apples to up the pectin content of other fruits.
I use what commercial jam makers use for sugar free preserves- polydextrose.
If your goal is only 'low sugar' and not sugar free, certo sells a low sugar pectin. It contains some sugar (enough for the pectin to work), but it's not a great deal.
Btw, apricots, plums and peaches are all high sugar fruits. A high sugar fruit makes a high sugar preserve.