I first learned it by reading Theresa Loe’s blog at livinghomegrown.com: the Jarden Company, who make lids for Ball canning jars, advise new procedures for using canning lids.
It has to do with the red gasket on the inside of the lid. As of 2013, lids are made without BPA and are labeled “Made in USA.” 2014 boxes are also labeled “BPA Free.”
The red gaskets used to be made of rubber, and the proper procedure was to simmer the lids in warm water, to ensure they would be pliable enough to form a good seal. That’s not how to do it now!
The red gasket is now made of plastisol, and if it gets too warm when not in contact with the jar, it’ll spread out thinly and won’t seal properly. The new 2014 boxes show these new instructions – but if you’re an experienced canner, you might not have caught the new procedure. So here it is — go ahead now, read the box.
That’s right – don’t boil the lids, and don’t simmer them in warm water. Just wash them before use and set them aside at room temperature, ready to go. This is true both for waterbath canning and pressure canning. The plastisol will work well even at the higher temperature of the pressure canner because it’s held properly against the jar rim.
And here’s the Ball Corporation’s word on the matter: look at the bottom of this page which provides detailed instructions for water bath canning. With sincere thanks to Theresa Loe for paying attention, and getting the word out!