☞ TESTER TIP: If you don’t have a candy or instant-read thermometer, alternatively, you can test for doneness by slipping a couple of small plates in the freezer while the jam cooks and then spoon a little of the cooked jam onto a chilled plate and return the plate to the freezer for a couple of minutes to cool quickly. The jam should firm to a soft jelly consistency. If it has not thickened enough, cook for a few minutes longer, then test again on another chilled plate.