I used to make a lot of jam. But it had to be legal, I was selling it.
I remember patiently explaining to people why a certain amount sugar is essential to preserve the jam, and ensure food safety. If you don't want to use sweeteners and preservatives, or have a very short shelf life, you have to have a minimum sugar content of 60% in the finished product.
But people think only of white sugar out of a packet i.e. sucrose. There's no doubt in my mind that it does the job well.
One way of avoiding adding the full amount of added refined sugar, is to increase the amount of fruit sugars within the jam mix. Dead easy if you have a glut of any fruit.
A bit of arithmetic helps here.
For best results to measure the sugar content get a refractometer specifically for jam, 40-90 roughly on the Brix scale. You have to read it at 20 deg centigrade.
Start with your fruit, weigh it. Weigh your water. Weigh your saucepan.
Say you have 1000g of raspberries. There's about 10% "natural" sugar in them. And about 80% water.
Anyhow, you probably have about 100g of sugar to start with in the fruit. Add 50g of water and cook it, just as it is. Microwave is ok for this but you need a big bowl, and don't forget to weigh the bowl.
Cook the lot down slowly without sugar until its really thick. Weigh the lot. Deduct the pan or bowl weight.
If you end up with 335g you've got 335g of cooked fruit STILL with 100g of sugar in it = 30%.
You'd need to add about 200g of sugar, giving a weight of 535g. (If you go back to looking at your starting weight of 1000g of raspberries, you will think you are adding just 1/5 of that weight in sugar)
By the time you've cooked it and lost some more water, assuming the total weight is about 500g the total sugar content will STILL be 300g and at the 60% mark.
But it's unrealistic to expect folk to mess about doing all these calculations at home.
Fruit varies in sugar and water content over different years, let alone different fruits.
The key point you might want to take away from this info is that by cooking the fruit well down to a much thicker consistency- and to about a third of its starting weight - you can reduce the refined sugar that you add drastically, and the taste is out of this world.
Jen