How does boil time directly affect ABV% & IBU?

Question

Dogfish Head makes IPAs called "60 Minute", "90 Minute", and "120 Minute". According to Wikipedia:

Their names refer to the length of the boil time of the wort in which the hops are continuously added.

There is also a limited edition "75 Minute" which is really a blend of the 60 & 90 varieties.

What I find odd however, is that these numbers also roughly correspond to the beers' ABV% & IBU.

Again, from Wikipedia:

• 60 Minute: 6.0% ABV, 60 IBU
• 90 Minute: 9.0% ABV, 90 IBU
• 120 Minute: 18% ABV, 120 IBU
• 75 Minute: 7.5% ABV, (no IBU listed)

Generally, the ABV% is about one-tenth the boil time in minutes, while the IBU is directly equivalent. Is this naturally due to the chemical reactions which occur during this part of the brewing process, or must Dogfish Head be doing something in particular which leads to an abnormally tight correlation here?

Answer 1

First, it's important to note that the boil time is not the only thing that increases with each of those beers. Dogfish Head continually hops during the boil, and the boil extracts the alpha acids from the hops, giving the beer bitterness, so a longer boil with more hops results in a wort that is more bitter.

Here is where the additional alcohol comes in: To counteract the extra bitterness, the beers that have boiled longer have both additional malt added to balance the bitterness, and are given additional fermentation and conditioning time to allow the yeast to convert all of those malt sugars into alcohol. In 120 Minute IPA's case, it spends an entire month in fermentation, resulting in an exceptionally high level of alcohol at the end.

Answer 2

There is no correlation between boil time and ABV.

The increased ABV stems from them adding more malt to balance out the extreme bitterness for the beers with longer boil times.

Answer 3

I disagree: boil time can affect ABV.

Assuming you're not trapping the steam, most of what boils off is water, meaning your final wort will have a higher O.G. In general, that will result in a smaller quantity of a higher ABV beer. (Of course, there are many variables, such as your yeast, your fermentation time, your ratio of fermentable:unfermentable sugars, and so on.)

Also, the starch->sugar conversion is generally terminated by your sparge (that being one of its purposes), so how fast you get to your boil doesn't have much of an impact.

Answer 4

As a note on the boil process and ABV. Boiling effectively stops the process of converting starches into sugars which the yeast can turn into alcohol by destroying enzymes in the malt. Therefore once you are boiling, this process has stopped. Boiling affects hops, by boiling off the aroma and extracting more of the organic acids that provide bitterness.

What does affect abv is the period of time the wort is kept warm before boiling. This time and temperature affects how much of the starches convert and therefore trades between body and abv. Boiling fast means you have a lot of body and low alcohol. Boiling later means you have more abv and a lot less body.