What happens to hop aroma during boiling (if anything)?

[RichardS]

Let's say you have 100g of aroma hops. Does it make any difference when you add the hops to the boil on the total amount of aroma in the beer? In other words, do you get the same amount of aroma whether you add them at the beginning of the boil, or during a post-boil hop stand? BTW, I know that the bitterness is affected but I'm just asking about the aroma side of the equation.
If early boil hop aroma disappears, then why wouldn't we use a purely bittering, high alpha hop early in the boil, saving all the aroma hops for later?

 

[Agentgonzo]

Yes, aroma reduces the longer you boil.

It's not that early hop aroma disappears completely, it's just greatly reduced.

 

[Jim Brewster]

The aroma molecules are more "volatile" and during the boil they evaporate. The longer and hotter the more are lost. This is the reason for adding flavour and aroma hops right towards the end of the boil and things like a post boil hop stand, hop back and dry hopping


The alpha acids that create the bitterness aren't as volatile and stay in solution and become bitter during the boil (some kind of chemical reaction happens which is called isomerising)

why wouldn't we use a purely bittering, high alpha hop early in the boil, saving all the aroma hops for later?

You can absolutely do that. A lot of people use for example Magnum as a bittering hop at the start, and add the aroma hops at the end of the boil

 

[MashBag]

If early boil hop aroma disappears, then why wouldn't we use a purely bittering, high alpha hop early in the boil, saving all the aroma hops for later?

You do just that, bittering hops first, aroma hops later. Hop stands after boil at say 85c and even dry hopping.

And... you can just buy dual purpose hops. Add them for bittering and then again for aroma. Cascade being one example.

I often think hopping often comes second place to mashing for a lot of people and really deserves more attention.

 

[The Baron]

If you wants lots of aroma and flavour you will soon get into whirlpool hops and dry hopping this also cuts down the bitterness from the hops too unless you want a highly bittered beer which some brewers do.
The aroma and flavour will drift with time especially dry hopped beers so drink them reasonably young to get them at their best