Can someone explain the difference in hop bitterness between alpha acids in my wort vs. beta acids bittering during aging? And is it predictable enough to plan for in recipe formulation?
This is a nice technical question involving some organic chemistry I do not comprehend. I'll begin with what I do know about bittering contributions from alpha acid and beta acids.
These acids are components of the hop cone and contribute to bitterness in slightly different ways. The more familiar one is probably alpha acid since most hop bags are labeled with the percent of AA. This number represents the percentage of the hop, by weight, that is composed of alpha acids.
Alpha acids do not actually contribute to bitterness until boiled in wort (or water even). At this time the acids change form, lining up to become isomerized alpha acid. The process takes time, which is why bittering hops are boiled for at least 60 minutes. iso-alpha acid is the main component of the bitter flavor in beer.
Beta acid, like alpha, almost immediately dissolves into solution when added to boiling wort. However, unlike alpha acid, it does not isomerize and passes into the finished product unchanged. In this form they do not contribute to beer's bitterness, but do lend aroma.
During storage and aging (and in fermentation) the beer is exposed, to some degree, to oxygen. Under these circumstances iso-alpha acid degrades and loses bitterness. When beta acid oxidizes it transforms into a bitter-tasting compound. On-balance the beer looses bitterness.
There is little research into how this transformation affects the flavor stability of beer. What I have read is that hop varieties with at least twice as much alpha as beta acids generally keep their bitterness over time. I know of no software or formula that takes the ratio and stability into account when calculating IBUs.
Further listening and reading:
Yes, hops contain two major organic acids generally refereed to as alpha acids and beta acids. When hops are added to boiling wort about 40% of the alpha acids undergo a thermal isomerization to form isoalpha acids. Iso-alpha acids are the actual bitter compound found in beer. When people talk about IBU they are talking about the concentration of isoalpha acids in beer - at the ppm level. Note: Alpha acids are only slightly bitter, perhaps 1/6 that of isoalpha acids. Beta acids are not bitter. During fermentation virtually all the beta acids and most of the alpha acids and some of the isoalpha acids absorb onto the yeast. As a results most beers (that have not been dried hopped) contain no beta acids, and less than 1 ppm alpha acids. Most home brewers use 30% to estimate the alpha acids to isoalpha acids utilizations when boiling hops for 60 minutes. This is correct since about 20% of the isoalpha acids formed during boiling absorb onto the yeast. Don't waste your time trying to factor in any bitterness from beta acids or alpha they are most likely no present in your beer and even if they are they're at such low levels they're insignificant. Prost !
I'll get you more details on this later (when I have my Designing Great Beers in hand). But, I'm pretty sure the levels of beta acids are negligible on a homebrew system. More to follow.
Processed hop pellets sold in vacuumed packages will last up to one years time, hop cones are much more unstable and typically do not keep longer than 9 months. As hops age the level of α-acids decreases and level of β-acids increases, if you are strictly concerned with bittering capacity you want to focus on the α-acid amounts. 30-50% of the α-acid in the hop will isomerise during the boil and this will contribute to your beers bittering units. β-acids will be destroyed during the wort boil and will not contribute to the bitterness of your beer unless you are cask conditioning. Hops added after boil are traditionally used solely for their aromatic properties unless they are pre-isomerised and being used to produce a light stable product intended to be sold in a clear or green bottle.