My mother is of italian heritage and she says her mother always told her to add vinegar when you are boiling the meat and when the foam comes to the top to skim it off. Only at this time can you add the other ingredients. Though she can not tell me why this is done or even what the foam is. She has asked her mother and all her italian friends. They all do it and say it has been passed down to them to do it that way, but they have no idea why or what the foam is. I have tried to find the answer, and so far, this is the only recipe that says to "remove all the residue." So I was wondering, does anyone know why?