Secondly, I realized that over the past year I've learned a bit about pectin. The jam we made last year, while delicious, was a bit thin and watery. I wasn't about to make that mistake again. Pectin causes jams and jellies to gel, and some fruits have more and some have less. Apples, it turns out, have a lot of pectin. So when you are trying to make jam with fruits that are low in pectin, like berries or plums, it's a good idea to use an apple, peel and all (trust me, you will never know it's even in the jam). Also, you need the right balance of pectin, acid, and sugar with the fruit to make it all balance and gel correctly.
Anyway, here in our new house we have not only one, or two, but four different plum trees. Lucky for the trees that I love plum jam too, since they've been somewhat neglected over the years (we unfortunately moved just after the plum harvest last year and missed the whole thing). And lucky for me that my husband is tall and can reach all those rogue plums, even though we still wake up every morning to a smattering of plums.
Makes: 13 half-pint jars
About 12 cups (about 6 lbs) pitted and roughly chopped plums
1 or 2 green apples
Juice of 1 lemon or lime
2 lbs granulated sugar
2 small plates in the freezer
1. Cut up all the plums and put them into a heavy, 5-quart stockpot.
2. Grate the apple, skin and all (but not the core), on a box shredder-grater. Add the apples, lemon or lime juice, and sugar to the plums.
3. Stir the plum mixture thoroughly, place over medium-high heat, and bring to a boil.
4. Lower the heat to medium and let boil, skimming the foam occasionally and smashing the fruit as it cooks. Boil for about 20 minutes.
5. When the jam starts to look thickened, start testing it by spooning a small amount onto one of the chilled plates. This will chill it quickly and let you see how thick it is. Keep testing it until the jam is thick enough, but don't cook it longer than about 30-35 minutes. If it's not thick enough for your liking, next time add an extra apple. Don’t worry, the jam will still be great.
6. Once the jam has thickened, get your clean jars set up. You can re-use the glass jars, but you should get new lids and rings each time. If you have a canning funnel, it makes your life a lot easier for filling jars.
7. Fill each jar to about 1/2 inch from the top, leaving a little breathing room. Screw on the lid, but not too tight.
8. Turn the jars over at once so they stand upside down on their lids and let them cool to room temperature. This should seal the lids. If the lids are sealed, the top will be indented. You can store the jam in a cool dark place for up to a year. If not, store in the refrigerator and use within about 1 month.