Someone did an experiment several months ago in this same forum with Pau'Darco, GSE, Garlic and others. The idea was to culture some yogurt batches, adding each ingredient to a different batch.
Pau D'Arco and Garlic inhibited the yogurt to ferment. Grapefruit-Seed-Extract didn't. I repeated the sme experiment with Grapefruit-Seed-Extract with the same results. I don't remember the others.