If you want to Can your Jam for longer shelf life, you can either pressure can it. (Which requires special expensive equipment.) or you can make Jam with a Ph less than 4.6 and Boiling Water Bath Can it as described in Paige Russell'sCanning and Preserving Class.
So how do you make sure your Jam has a Ph of 4.6 or less?
Just add lemon.
You will need:
Step 1: Add Lemon Juice
Lemons are highly acidic, adding lemon juice to your recipe lowers the Ph of the entire recipe.
As you are cooking your Jam add some Lemon juice to it.
How much Lemon juice depends on Ph level of your jam and how much you are making.
Squeezing the juice from half a lemon should be enough to lower the Ph of one jar of jam from 5 to 4, you will need to check it to ensure you don't get botulism.
Step 2: Test Ph
Make sure you completely stir the lemon juice into your jam mixture before you use either your digital tester or your strips to test the Ph.
Keep adding lemon juice until the Ph is 4.6 or lower.
If you have a digital tester you can be more exact, if you are using the strip tester you will have to add more lemon juice than is needed to ensure you are below 4.6.
Once your jam has a Ph of 4.6 or lower your jam is good to can.