Different types of hops?

Hello, I understand why bittering hops are added toward the beginning and why aromatic hops are added at the end, but are the two interchangeable? One of my home brew books says that hops like willamette and cascade are aromatic. Then it says hops like chinook and northern brewer are bittering hops. Could you use an aromatic hop as a bittering hop or vice versa? What difference would this make in the finished beer?

You can use them whenever, but some hops just don't have great flavor or aroma. Also, hops with high cohumulone levels tend to have a harsher bitterness than one with lower levels.

As with many agricultural products, different cultivars are good for different purposes. Some potatoes suck for boiling but are great for baking or mashing, for instance. Some apples are way too acidic for eating, but they may be great for making cider.

In the case of hops, it's very inefficient to use aroma hops for bittering because they tend to have very low alpha acid content (the stuff that causes the bittering) - adding massive amounts of low-alpha hops for bittering is expensive and, due to the large amount of vegetal material, can result in unpleasant grassy flavors. Some bittering hops also have unpleasant flavors and aromas that get driven off during the boil, which is why you wouldn't want to add them late.

That said, feel free to experiment - some unusual combinations actually taste pretty good!

I'll add that bittering hops will have higher alpha acid levels, so it doesn't take much to achieve high IBU levels, and some of them are 'clean' so they aren't very useful for aroma/flavor. Aroma hops tend to have high oil levels.

However, any hop can be used at any stage of the process. Changing the bittering hop has less of an impact, but there is some.

Thank you all for clarifying that for me!

There is also the AAU levels that you see in recipies (oz x alpha acids). The higher alpha acids that are used for bittering, the less weight you have to use, and vice versa. Which is why I think it's suggested to use higher alpha hops for bittering. A money saver and will also help to reduce trub.

At the same time, I believe that there are some hops that have a better aroma and flavor than others, and that's the reason these are deemed to be good aromatic hops.

Experimenting with hops is a good idea, and you can determine through your own opinion which you enjoy as bittering hops and which you like for aroma and flavor. I believe you can mix and match, but others might have a different opinion.