I believe that a strong suspect has been identified with regard to grape toxicity.
I have edited a long-ish article about it below
Tartaric acid and its salt, potassium bitartrate, are proposed to be the toxic principles in grapes leading to acute kidney failure in dogs. Tartaric acid levels in grapes vary in concentration with type, growing conditions, and degree of ripeness, which may explain why it has been so hard to pinpoint the toxic dose and predict the outcome for dogs that have eaten grapes based on the amount consumed.
Recently a group of vets in the USA noted a pattern of dogs exposed to potassium bitartrate (cream of tartar) exhibiting similar clinical signs to grape and raisin toxicosis (acute vomiting and kidney failure). In one dog that had eaten home-made play-dough (made with cream of tartar) and subsequently died, microscopic changes noted in the kidney were also similar to those previously found in grape and raisin poisonings. They also noted that the Animal Poison Control Centre had reports of severe vomiting and acute renal failure in dogs following large exposures to tamarinds, which are also uniquely high in tartaric acid.
