I just mentioned it to my brother. He is really into dogs. He suggested Vinegar or some feminine products that contain it. I am sure you know what I mean. I guess you need something acidic. Maybe along the line of Scott's suggestion some citrus juices might work. Lemon, Grapefruit etc. That is probably why people mention Tomato. It would be a little acidic too. Lemon and Grapefruit would seem to me as most acidic of citruses, to me.