Clean pata well and put in a bowl with the baking soda and enough water to cover.
Let to stand for a few minutes, rinse well to remove soda and drain.
Return to the bowl and add the marinade. Let to stand for at least 2 hours.
Put pork and the marinade in a casserole. Add about 1 1/2 cups water.
Bring to a boil and then simmer till pork is tender for about 2 hours.
Correct seasoning and add castaňas. Simmer for another 5 minutes . Serve hot.