Mash the ripest fruit you can find
Not overripe, but sweet and full of flavor. If there are a few bruises here and there, just trim them away. Put the fruit in a saucepan—not because it’s going over any heat, but for the flat bottom surface that makes for easy crushing. I like using a potato masher. You can also pulse the fruit with an immersion blender or in a food processor, but take care not to over-process it.
If you’re going to experiment, start out with a small amount, about 2 cups of crushed fruit. If you want to make a larger batch, double this. When trying to figure out how much fruit you’ll need, use this rule of thumb: 1 pound of fruit without pits or 1 1⁄2 pounds of fruit with pits will give you about 2 cups crushed fruit.
Add lemon juice (about a tablespoon per 2 cups of crushed fruit) to low acid fruit like peaches and apricots, both for flavor and to help the jam set.
Sugar or other sweetener
For 2 cups of crushed fruit, use 1/2 to 1 cup of granulated sugar. You can also use brown sugar and honey, but take into account their stronger and sweeter flavors. Some varieties of no-cook freezer jam pectin, including Pomona’s Universal and Ball Instant Fruit Pectin, can also be used with other sweeteners such as Splenda or Stevia, just make sure you use the equal amount to sugar in sweetness, not in volume.
When using Pomona’s, mix the sugar with the fruit. For Ball, combine the sugar with pectin powder first (for two cups of fresh fruit, you’ll need just half of the 1.59-ounce packet) and then add it to the fruit.
Keep it simple. The nice thing about freezer jams is their fresh taste. This is where herbs like chopped basil or mint come in. You can also add a pinch of grated fresh ginger or a dash of cinnamon.
Prepare the pectin
Pomona’s requires you to combine the pectin powder with hot water before adding it to the fruit. For two cups of crushed fruit, use 3 fluid ounces of water and 1 1⁄2 teaspoons Pomona’s pectin. I like to use an immersion blender in this case. Make sure the pectin is fully dissolved and well-blended.
Add pectin to fruit and stir well
Whatever kind of pectin you use, it’s important to blend it well into the fruit, so that you don’t get gummy lumps in your jam.
Pomona’s pectin comes with a packet of calcium powder. Prepare it according to the instructions and add a teaspoon at a time of the calcium water. Stick to the very low end of their recommended amounts if you don’t want a very firm jam. The jam will continue to set a little more after stirring.
Fill clean, freezer-friendly containers
If you plan on freezing the jam, transfer it into clean, dry plastic freezer containers or wide-mouthed glass jars with straight sides (this shape resists cracking when frozen). Leave about 1⁄2 inch of at the top and put on the lids, labeled and dated. Let it set in the refrigerator overnight, up to 24 hours. Then you can freeze up to a year or keep refrigerating it, up to 2 weeks.