You can play around with the set you want, since this jam doesn't include any added pectin. After overcooking a jam earlier in the summer, I wanted a fairly soft set here. At room temperature, the jam is a little too liquid, which had me worried, but after a night in the fridge it had the perfect soft, spoonable set I was looking for. You can cook it to your liking by testing bits on chilled plates (I pop mini plates into the freezer before I start cooking) -- you can always cook it longer for a firmer set.
One warning - use a big pot. I used my second-biggest large pot (which I usually use for making huge batches of soup), and it still almost boiled over. Once the berries break down, the jam will about double in size once it starts to boil.
Strawberry Jam with Aged Balsamic and Black Pepper (from the Blue Chair Jam Cookbook)
Makes about eight 8-ounce jars with a 6-8 month shelf life
Scant 5 ounces strained lemon juice
1 to 1-1/2 ounces aged balsamic vinegar
4 pounds hulled strawberries, very coarsely chopped (from about 4-1/2 pounds whole berries)
2 pounds 10 ounces sugar
Freshly ground black pepper
1. Place a saucer with metal spoons in the freezer for testing jam.
2. Measure out the lemon juice in a glass measuring cut, and add enough vinegar to bring it up to just over 6 ounces.
3. In a large pot, combine the berries with the sugar and half the lemon juice mixture. Place over medium-low heat and cook, stirring constantly with a rubber spatula. After a few minutes, when the juices start to run, gradually raise the heat to high, stirring often. Add several twists of black pepper to taste.
4. Boil vigorously for 20-30 minutes, gently scraping the pan every few minutes. If the jam sticks, decrease heat slightly but remain at a boil. After 15 minutes, taste the jam and add more pepper as needed. Continue to cook until the foam subsides and the jam is dark and shiny, about 25 minutes total.
5. Remove from heat and do not stir. After a moment, stir in the remaining lemon juice mixture. Return to medium heat and cook, stirring frequently, lowing the heat as necessary to prevent scorching. After 3-5 minutes, the jam should look glassy and dark. Remove from heat and test for doneness, and scrape off any foam while you test. You may need to cook the jam for a further 10 minutes or more, depending on the set you want. When jam is ready, skim off any remaining foam, stir well, and process in jars for 15 minutes.