In the past, kidneys were often stronger in flavor and soaked to soften their taste. Modern breeding and husbandry methods mean that it is unnecessary to soak calf's or lamb's kidneys. Whether or not you soak the stronger-tasting pig's kidneys is a matter of personal choice. Many people prefer to soak ox kidneys in milk, generally for at least an hour, especially if they are a prominent ingredient in a recipe.
** Asian Recipes