The last few days I've been busying myself making jellies. The idea came when we noticed huge amounts of ripe blackberries by the roads when cycling around. So I went blackberry picking and I tried to make blackberry jelly.
Blackberries are relatively low in pectin, especially the very ripe fruit that I picked. So I bought jam sugar which contains added pectin to help the jelly set. Because I wanted to make »reduced sugar« jelly, I only added half the amount of jam sugar to the fruit juice. I boiled it for a while and it didn't seem to set, but I filled it into jars anyway. Of course, by next morning it still hadn't set.
The next day, I opened the jars again and reboiled everything after adding the rest of the jam sugar which brought the ratio to about 1.6 parts of sugar per juice, considerably over 1:1. Again, the resulting jelly didn't set, maybe because I didn't boil it for long enough.
Later that day, we went blackberry picking together. I boiled the resulting juice with the same amount of normal sugar and some lemon juice and also added the first batch of liquid jelly, resulting in a sugar ration of 1.2:1. After boiling for 50 minutes, the jelly finally started to show signs of setting (for example, when dripping from a spoon, the last drop becomes long), so I filled it into jars and the jelly set. Most likely, the reason it had to boil for so long was because the pot was too full, so I had to keep the temperature very controlled in order to stop it from boiling over.
The next day, I went blackberry picking again and also bought a bottle of Certo, which is apple pectin. I picked blackberries and followed the instructions on the Certo, but used only half the amount of sugar. The instructions say to boil the juice for 2 minutes, reduce the heat, add the Certo, and then fill the jars. This mixture didn't set, which might have been because due to the low pectin blackberries need more boiling, maybe also after adding the Certo.
When I reboiled the jelly the next day, I added more sugar to one half, and more sugar and more Certo to the other half. Both mixtures started jelling after boiling for about 5 minutes and the half with the added Certo set considerably faster than the other, but might have turned out too solid (like toffee).
I also bought quinces which I quartered, removed the kernels, and placed in a pot which I filled with water to just cover the fruit and added the juice of half a lemon to keep the fruit from turning brown. The mixture already started jelling while I was boiling it to extract the juice. Therefore, I didn't add any Certo, but just an equal amount of normal sugar and the juice of the second half of the lemon, and the jelly became very solid.
From the remaining fruit pulp I made quince paste by adding half its weight in sugar and boiling it until it became quite hard to stir.
Experiments using less sugar
For blackberries (or low-pectin fruit), it seems to be necessary to add equal amounts of sugar to the juice and improve the pectin content by using Certo or mixing with apples (or quinces?). I still think it may be possible to use only half the amount of sugar when using Certo.
A good strategy is probably to start with half the amount of sugar, adding the Certo at the start of the boiling and then increasing the amount of sugar if the juice does not begin to jell after 10 minutes. Another approach would be to try and make up for reduced sugar by adding more Certo.
A test for pectin is to drop some liquid into ethanol (I used burning alcohol for the trangia). If it forms a single blob, there is sufficient pectin. If it dissolves or forms several strains, more pectin is needed. When I tried this with blackberry juice that contained Certo, or quince juice, it nicely formed a single blob. I have yet to try it with untreated blackberry juice.
For quinces, I also think it might be possible to use much less sugar because the juice seemed to set even while extracting it before adding any sugar.
The other point about reducing the sugar is that the sugar binds excess moisture in the resulting jelly, thereby preventing the growth of microbes. Jam sugars for smaller amounts of sugar per fruit don't just contain special modified pectins (PDF) but also added preservatives. Jellies made with less sugar may therefore have a reduced storage life.