When to add sugar to (tomato) Jam

You can add some additional sugar after the mixture has come to a boil, but you wouldn't want to add it much later or at t he end of the cooking. But you need to take care in reducing the sugar too much or your jam may not gel and set. Tomatos contain a lot of pectin on their own, but making it gel requires heat, sugar, and an acid. If you reduce the amount of sugar too much, the pectin in the tomatos might not form a jam consistency. One way to help reduce the final sweetness might be to include some underripe tomatos. You might also want to actually taste the ones you are choosing to use, as some varieties out there are little sugar cubes in disguise. One other trick to try is to let your mixture sit for a bit, maybe a half hour, before starting to cook, with your reduced sugar level choice. Taste it at that point, once the sugars have had a chance to dissolve, and make your decision as to if you need more. If by some chance yours doesn't gel as you'd like, you can reprocess it and use one of the commercial pectins that do not require additional sugar. I happen to like Pamona's myself.