Welcome to my life freezer jam. So much easier, and in my opinion tastier. Because you are not cooking the fruit the jam can't be stored at room temperature, instead you store it in the freezer (hence the name). When you take a jar out to be used it can be stored in the fridge for up 6 weeks. I guarantee it won't be around that long anyways. It is still a good idea to use smallish containers, the 1 liter jar may be a little much jam for 6 weeks.
You need to add something to help gel the jam, I bought a package at the grocery store specific for freezer jam. There are a few kinds out there. I chose this one because I liked that the fruit to sugar ratio was a little higher. It is no secret that there is a lot of sugar in jam. It helps to preserve the jam along with making it taste sweet. Freezer jam has less sugar than regular jam, gotta' like that. I also find that freezer jam tastes more like the actual fruit because it isn't cooked. You do have to use great tasting, ripe fruit though because there is less sugar and cooking to hide behind.
4 cups of fruit
1 1/4 cups of sugar
I used raspberries so to prep them I just mashed them in a large bowl with a potato masher. I left some bigger chunks though. If you are using other fruit like strawberries or blueberries just chop finely (don't puree). Stir in sugar and let stand for 15 minutes. Slowly sprinkle in gelling powder while stirring for 3 minutes. Allow to stand for 5 minutes. Stir again for 1 minute. Add jam to jars, leave enough room for expansion. It's ready to eat! Can be stored in the freezer for up to a year.
Your package might be a little different, but that is the jist of it. My package called for 1 1/2 cups of sugar, but I reduced it (of course) and it still turned out fine and was plenty sweet. You don't want to play around too much as the gel to sugar ratio is important. It was so easy from start to finish maybe 30 minutes, are you kidding? I didn't have time to make my jam right after picking so I froze the berries. It had no adverse effects on the final product just thaw your fruit before using. Blackberries are in season now, so get picking and jamming.
I can imagine opening a jar on a dark winter morning and being reminded of that perfect summer night picking berries and my son's chubby, berry stained fingers and cheeks. It doesn't get much better than that.