Loss of Bitterness from OG to FG

[foam_top]

I brewed this batch a few weeks back, the recipe calculated the IBUs to be about 45 like I intended. When I drank the hydrometer sample on brew day I would have guessed that number to be very accurate, a decent bite to the flavor. However, I bottled my beer yesterday and again drank the sample and a lot of that bitterness was gone. I would compare it to 20-25 IBUs. I realize a homebrewer shouldn't obsess about the IBU number itself, but there is definitely a noticeable difference. I've never had this happen before so I'm at a complete loss. The beer itself tasted pretty good, no off-flavors, but its intended bitterness just plummeted. Any ideas on what's happening? Thanks in advance.

 

[foam_top]

Well, I've been thinking on this and I had a couple thoughts...
I used orange rind at flameout. Is it possible that I was confusing the zest/citrus notes for hop bitterness and those flavors have faded since brew day?
The only other explanation that I came up with is oxidation, which I think I remember hearing can affect hop bitterness. Although, I don't know when or where that would have happened.

...

 

[Oginme]

The impression of hop bitterness does drop over time. Isomerized hop acids are partially removed by the yeast during fermentation, as the cell walls of yeast will absorb them. When you read a description of a strain of yeast and it claims to be malty, or malt forward in nature, that usually means that that strain of yeast particularly will strip more hop alpha acids from the wort during fermentation.

Hop presence and bitterness also decreases over time after fermentation. This is why a lot of the very highly hopped DIPAs don't store well and those such as Pliney state to drink when fresh.