Q: Sometimes I have too much fruit and too little time to make jam, so I freeze the fruit. Would freezing berries beforehand puncture the cell walls, making it easier to make jam? And would freezing concentrate the sugar in the fruit?
Freezing fruit until you have time to make jam is a great trick in the summer, when all the fruit seems to arrive at once.
I don't think that freezing the fruit first would actually make better jam. While the cell walls are punctured, making the fruit softer after it's thawed, that shouldn't make a big difference in cooking time. And freezing won't reduce the fruit's juice the way cooking does, so it shouldn't concentrate the natural sugar.
The softening action of the freezer actually makes it important to measure the fruit before you freeze it. Most fruits, particularly berries, collapse when thawed, so it can be difficult to get an accurate measurement for adding sugar and pectin.
You also can save time and simply use all that fruit to make freezer jam. Most packages of pectin include recipes for it, or you can find powdered pectin that's formulated specifically for making freezer jam with less sugar.
Kathleen Purvis answers cooking questions at www.charlotteobserver.com/food.