I use jars to freeze fruit, sauces, etc. You must make sure not to fill the jar to the rim. Allow at least an inch or more top space. Many times, after filling the jar, I cover it with a couple of layers of plastic wrap and then a double layer of foil pressed tightly over the top of the jar. I have been doing this for years and have never had a problem. You can make freezer jam with little or no sugar depending on the sweetness of the fruit. Just cook the fruit with a little lemon juice (orange or lime can also be used)and a bit of water or fruit juice ( not much). Add the amount of sugar, honey, sweetener you want -- I like a nice tart jam so use very little or no sugar. Cook the fruit until it thickens a little ( doesn't take long -- maybe 10 minutes). Cool and spoon into jars or other containers. Stand the jars in your freezer -- don't let them bump against each other or they may crack. Good Luck!