Now the Veterinary Poisons Information Service has revealed evidence provided in a letter to the Journal of American Veterinary Medicine Association that suggests that the culprit may be tartaric acid. Tartaric acid is a crystalline organic acid that occurs naturally in many fruits as well as grapes, including bananas, tamarinds and citrus. Its salt, potassium bitartrate (cream of tartar), develops naturally during fermentation. When mixed with sodium bicarbonate, it is sold as baking powder and used as a leavening agent.
The authors of the letter noted that dogs showed similar signs when poisoned by cream of tartar as when poisoned by grapes or raisins. The amount of tartaric acid and potassium bitartrate in grapes varies by the type of grape, growing conditions and growth stage, but is still sufficient to cause renal failure in dogs. The variation in tartaric acid concentrations means that it is not possible to ascertain the toxic dose.
Further research is required but ingestion of grapes and raisins should always be a cause for alarm and a vet should be contacted immediately for advice.