Here's the technique for making jam without the boxed pectin:
For the pectin:
5 tart apples,blossom and stem ends removed, chopped up, core and seeds and all
1 lemon (or 2 limes) choped up, peel and seeds included.
Cook this down until soft, and put through a food mill. I use my Kitchen Aid Fruit and Vegetable Strainer attachment, which is a handy thing to have if you are into canning and already have a Kitchen Aid stand mixer. It's way easier to use, and costs about the same as a hand cranked food mill. Alternatively, you could press it through a sieve with a wooden spoon. This will make about 2 cups of puree.
For the fruit:
8 cups strawberries, halved and hulled and 5 1/2 cups sugar
4 cups blueberries and 3 cups sugar. Use limes in the pectin with blueberries for added flavor
4 cups raspberries and 5 cups sugar
Add the pectin puree to the fruit and sugar and boil until it hits the gel temp at your elevation. For "flatlanders" like me, that's 220 F. Put the jam in sterilized jars and process in a boiling water bath for 10 minutes. You will not be able to taste the apples or lemons (the limes give a nice lime essence to your jam)
I am not a fan of boxed pectin for a few reasons:
It's cheaper to make it from scratch. Pectin can be expensive, especially the low sugar varieties. A product search shows that the popular no sugar required Pomona's Pectin cost almost $5 per box, and regular style boxed pectin (Ball, Sure Jell, etc) runs about $3 per box.
Using apples in with the berries increases the yield of jam made. Apples are generally much cheaper than berries, so adding 2 cups of the puree to the fruit decreases the cost per jar.
The process for making pectin is anything but "natural", despite what some brands advertise. A pectin factory receives apple residue or citrus peels from juice factories. It's mixed with acid to get all the pectin out of the sludge. The solids are separated and then alcohol is added to precipitate the pectin out of solution. Ammonia is added to some kinds to make it work without added sugar normally needed (those expensive brands of pectin that allow you to make jams and jellies without adding sugar), and then it's mixed with dextrose or sugar to stabilize it.
For the no/low sugar kinds of boxed pectin like Pomona's, it's even more of a science project. It's made in the same way as regular pectin, but then some amide groups are then introduced into the pectin molecule during the process of de-esterification (a process by which the pectin is changed from high-methoxyl to low-methoxyl). High-methoxyl pectin requires a sugar concentration above 55% to gel whereas low-methoxyl pectin gels in the presence of calcium ions. So, users of this style of pectin have to make a calcium solution and add it to the fruit. So instead of sugar, you're adding calcium ions, so preservers can use other sweeteners like Splenda. I don't care if they sell Pomona's pectin at Whole Foods, I don't think I want to eat anything that requires "de-estrification" or adding calcium ions.
I don't use Splenda. Natural apple/lemon pectin jams require less sugar than boxed pectin recipes. If you are looking for a less sweet jam, skip the Pomona's/Splenda and just use good old fashioned apples and lemon.
In the fall, I plan on foraging for crab apples and putting up some pectin for next years jam making. It will be a great off season canning project. Happy Can Jam!