Why boil adzuki beans for anko twice?
Please see the excerpt below from this  site . All of the recipes I've seen for anko recommend changing the water but this is the only explanation I've found.


  Amy Escobar MARCH 12, 2014, 3:13 PM
  
  Hey Nami, do you know why the boiling water is emptied and then refilled? Cooking With Dog uses the same method and I don’t know the reason.
  
  REPLY
  
  Nami MARCH 14, 2014, 12:52 AM
  Hi Amy! Thank you for asking the question. 
  
  Traditional method usually includes a process of emptying water for 1-2 times. The reason is to remove impurities (we call it “aku” – English translation is “scum” – not sure if that’s the right word) from the azuki beans. They taste bitter and you don’t want to cook with them, so we get rid of it by changing the water. Some people do once, some do twice, but you don’t want to lose too much of azuki flavor, so I believe one time is good. Hope this helps.