Originally Posted by apcoach
Also, I am adding centennial hops every 15 minutes. Should I keep them all in the kettle until I'm done or take out te old when I add the new?
Do not worry about how long it takes to get to a boil. The large quantity of DME just makes it harder to get the bioil started (raises the boiling point) As long as you can get it to boil and keep it boiling, you are OK.
Leave all the earlier hops additions in the kettle as you add more hops later in the boil. 60 minutes of boiling hops produces bitterness. Boiling hops for between 15 and 40 minutes adds a combination of bitterness, flavor and aroma. Any hops added with less than 15 minutes of boiling remaining are basically only for aroma.
Recipes with multiple hops additions like that are attempting to create a "more complex hops profile" where you have a combination of the correct level of bitterness, a pleasant hops aroma, and flavor.