Re: Why do grape, black currant, and oak leaves keep pickles crisp?

You are indeed correct, it does have to do with tannin and pectin.

According to http://www.uga.edu/nchfp/questions/FAQ_pickle.html#6, Grape leaves contain a substance (tannin) that inhibits the enzymes that make pickles soft. Those enzymes are called pectinases, because they "chop up" the pectin molecules.

Research on this topic was done back in the 1950's and 1960's. To read the original research articles, go here: http://fsweb2.schaub.ncsu.edu/USDAars/html/Fflbiblio1.htm and search the page for the word "grape".

Any leaves that contain tannins might indeed have this effect.

Be careful about adding leaves to your pickles because some may contain toxins.

As noted in the first link above, removing the blossom ends of the cucumbers will make the addition of leaves unnecessary.

Also, be sure only to use trusted, USDA approved recipes http://www.uga.edu/nchfp/how/can6b_pickle.html to insure safety.