I may have found a solution to the metallic taste in the grape juice.
I opened a quart yesterday to see if the flavor was a bad as I remembered. I noted a layer of grayish sediment in the bottom and I carefully decanted the juice to eliminate the sediment. Voila ! The metallic flavor was all but gone !
I have had tartrate crystals in my juice before and never had the metallic flavor of this batch. I do not 'think' that the metallic flavor was caused by tartrate crystals because I didn't see any crystals yesterday. I remember letting the juice settle in the refrigerator for at least a day to eliminate the crystals before canning.
I'm no chemist so I can't be sure of what was in that sediment. I will be carefully observing the remaining jars when I open them.
I am, however, very happy to have good, sweet grape juice.
NB: I used my new stainless steel steam juice extractor yesterday on the remainder of my apples. It worked slicker than sliced bread. Very little effort and good quality juice.