My absolute favorite freezer jam is made with raspberries, rhubarb, and raspberry-flavored Jell-O. It sounds like a crazy combination, but it tastes amazing. I love the "set" of the jam, too- it's a soft, spreadable jam that's just perfect.
This year, we had lots of raspberries, but the rhubarb crop failed completely. I searched for a plain-gelatin based jam for just raspberries, but found very little.
Here is why I like using gelatin in my freezer jams:
1) Gelatin does not need sugar to gel- you can add sugar to taste.
2) It is easily adaptable to how much fruit you have on hand.
3) It is not dependent on certain temperatures to make a successful batch of jam.
Here is why you need to be careful when you use gelatin to set your jam:
1) It can not be canned safely. Don't try. This MUST be a freezer jam. Gelatin has protein in it, and must be kept frozen or refrigerated to be safe.
2) If you keep the jam at room temperature, it will turn runny. It's only firm when it's cold.
3) This isn't vegetarian. Nope. See #1.
I wish I could remember the handful of links I used to cobble this recipe together, but I'm working from a scrap of paper I jotted my cooking notes on. Use this as a starting point, and a reference for ratios- this recipe would work with any juicy fruit. The basic equation is 1 cup of sugar and 2 packets plain gelatin per quart of mashed fruit (measure it after mashing to the texture you want the final jam to be.) The sugar can be to taste. I actually had a bit of cantaloupe leftover from dinner the night before and chopped it up fine before adding it to the pot. See? Versatile.
Raspberry Freezer Jam
5 quarts of raspberry mash
5 cups sugar
10 packets plain gelatin.
Bring it all to a boil, and boil one minute. If your fruit isn't very juicy, let it sit in the pot with the sugar for a bit to macerate, and then add the gelatin before bringing to a boil. Ladle into clean containers, leaving 3/4 inch headspace. Freeze or refrigerate immediately.