Does anyone know how to thicken grocery-store fruit jam to use as cake filling? The jam bakeries use is thicker but more expensive. The grocery store stuff is so watery that it just soaks into the cake. Can I just add gelatin or something?

You can bring it to a boil and mix in 1 tablespoon cornstarch with 1/4 cup cold water, stir till it thickens to the consistancy you want.

I assume you refrigerate it to thicken before putting it on the cake. How long does it take to set?

Sorry it took so long to answer,
Ok. You actually just have to let it cool to room tempature as it thickens right away,and it does not need to be refridgerated. Hope that helps .

Oops! Forgot to tell you, Mix the cornstarch with cold water and then pour it slowly into jam while you stir it, when it comes to a boil it thickens right away.