Preparing jars and lids for Jam
I always use glass jars for all my homemade jams. Each jar is washed thoroughly, rinsed and drained. All jars are then sterilized. Now most sources I have seen have suggested immersing the jars and lids in boiling water. My Mum taught me to heat the jars in the oven at a temperature of 100 degrees C for 1 hour. By sterilizing the jars in the oven, I do not have to worry about getting streaks on the inside of the jars as they dry. I still use boiling water on the lids, which then have to sit until they are dry. I usually sterilize the lids in a saucepan at least a day before I need them, place them out on trays or plates, and allow them to air dry.
Heat the jars again to 100 degrees C just before pouring the hot jam into them. Cold jars and hot jam DO NOT mix. If you pour hot jam into cold jars, you run the risk of the jars cracking or breaking with the dramatic change in temperature. While the jam and the jar are still hot, hold the jar with an oven mitt, pot holder or something else that protects your hand from the heat. This will also give you a good grip on the jar while you firmly screw the lids on. The lids will remain cool for long enough that you will not need protection for that hand.
Creating a vacuum seal
If you are using lids that have a raised circle in the middle of it, like a button, it is fairly easy to vacuum seal them. As the jam and the jar cool, if the lid has been put on tightly, a vacuum will be created and eventually the button on the lid will go down with a satisfying POP. many a nigh Mum and I have been sitting watching TV, listening to all the lids go POP. Sometimes they need a little help, so once the jars have cooled, just give the button a push with your finger, and it should go down with that POP if it had not already done so on it’s own. It is like popping bubble wrap. Fun!