This time of year, lots of people are making and canning strawberry jam. Though it’s a universally loved preserve, I find it to be one of the trickier jams to get right, particularly for beginning jam makers. One of the reasons that people struggle with strawberries is that the finished jam has the tendency to separate into two layers* once it has cooled in the jars.
If you are one of the ones who have struggled with this two layer jam, worry not. It’s not a sign of danger or even that you did something wrong. It’s simply a sign that there is still some air trapped in the strawberries. They are lighter than the syrup and so rise to the tops of the jars.
I find that this jam separation happens primary in recipes that call for relatively short cooking times or very large pieces of fruit that have not been given a long maceration period.
You can work to prevent this two layer effect by chopping the fruit into smaller pieces, macerating it with the sugar overnight, mashing it with a potato masher during cooking (this action is best if you’re noticing big hunks of fruit bobbing around towards the end of cooking), and even extending the cooking period a bit.
If you’ve taken these actions and you’re still noticing that your jam is separating during the cooling stage, you can gently shake the cooling jars to reintegrate the fruit and the syrup.
My preferred method of dealing with this separation is simply to tell people that I meant it to be that way and that if you want a more integrated preserve, that they should stir the fruit into the now-set jelly when they open the jars.
*This can also happen with other varieties of fruit as well, but is simply most common with strawberries.