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["Peel, core, and dice the apples, reserving the trimmings (the peels, cores, and seeds). Put as many of the apple trimmings in a jelly bag or 4 layers of cheesecloth as will fit, and tie the bag closed.", "Set the diced apples aside. Don't worry about it if they start to brown a bit; they are going to be turned purple by the blueberries anyway.", "Place two or three small saucers in the freezer. You will use these later to check the set of the jam.", "Put the blueberries and sugar in a wide, 6 to 8 quart preserving pan.", "I use a potato masher to mash up some of the blueberries, about a third of them, but you don't have to do this. I just feel that it gets the juices flowing quicker.", "If you do not have a preserving pan, use the widest pan you have. The wider the pan, the more quickly moisture will evaporate from the mixture, and this is what you want when you are making jelly, jam, or marmalade.", "Bring the berries and sugar to a simmer over medium heat, stirring frequently, then continue to cook until the juices are just deep enough to cover the blueberries, about 5 minutes. Pour the berries into a colander set over a large bowl and stir the berries gently to drain off the juice. Return the juice to the pan, along with the diced apples and the bag with the trimmings, and bring to a boil over high heat.", "Boil, stirring occasionally, until the syrup is reduced and thick. This usually takes about 15 to 20 minutes. Take the pan off the heat and remove the bag containing the apple trimmings from the syrup and place it between two plates. Hold these plates over the preserving pan, and squeeze the plates together to squeeze out any juice which is in the bag. You don't have to try to get every last bit of juice out, just as much as you can get reasonably, and then set the bag aside.", "Return the blueberries and any accumulated juice, along with the lemon juice and zest, to the pan and bring to a simmer. Simmer, stirring frequently, for 5 minutes, and then check the set.", "Use the cold plate test to check set: take the pan of jam off the heat (if you don't remove the jam from the heat while you check the set, it could over-cook and become rubbery or hard, if the jam is indeed already set) place a drop of the jam mixture on one of the saucers you've kept in the freezer, & place the plate back in the freezer for 1 minute. After 1 minute, take the saucer out of the freezer and nudge the drop of jam with your finger. If it ""wrinkles"" when you nudge it with your finger it is done. If the jam is not set, continue cooking over medium-high heat, checking the set again every 5 minutes.", "Once the jam is set, remove it from the heat and stir gently for a few seconds to distribute the fruit in the liquid.", "Ladle the jam into sterilized canning jars, leaving 1/4"" headspace. Wipe the rims of the jars with damp paper towels to remove any jam which got on the rims or the threads. Place the lids and the bands on the jars, just tightening the bands fingertip tight.", "Process in a boiling water bath for 5 minutes, then remove and let sit, undisturbed, for at least 12 hours before checking seals. It is important to let them sit undisturbed for 12 hours because the sealing compound on the lids is still cooling and hardening, completing the seal. While the jars cool, you will hear a ""plink"" type sound from each jar - this is the jars completing the vacuum seal as the final air escapes the jar. After 12 hours have passed, remove the bands and check the lids - press down in the center of the lid. If you cannot push the lid down any further, the jar is sealed. If the lid ""gives"" a bit, and you can push it down, the jar did not seal. You can either put the band back on the jar, and reprocess it for another 5 minutes, or you can just put it in the fridge and use it within 3 months."]
