So I cracked open a nearly year-old bottle of oatmeal porter last night and it tasted a bit "yeasty". In fact, much more yeasty than I remember from a few months ago. I was fairly careful in my pouring so I don't think I drank much in the way of sediment. Is this "yeast-bite" or something else? What exactly is yeast-bite?
What you are tasting may be related to the carbonation level of the beer. If the beer is over carbonated it can develop a bite from the carbonic acid and can create a "twang" in the aftertaste. Also when opening the bottle the release of CO2 for solution can stir sediment up from the bottom of the bottle which could account for the yeasty taste.
In my opinion this is what many people used to describe as that "homebrew or extract twang." This along with oxidation and possibly off flavors from poor fermentation also contributing. I do still run into this on occasion while judging but is much less common these days.
This is just a theory and without a sample in front of me it would be hard to pinpoint exactly what you are tasting.
I think you're tasting oxidation. The little bit of oxygen that gets compressed into the bottle when you bottle a beer will age it just like a wine ages. Unfortunately, most beers lack the acidity, alcohol and tannin to benefit from micro-oxidation like wine does. So many beers just... spoil.. with age. Exceptions tend to be Belgian-inspired beers that are aged in wine barrels and pick up some brett in the barrel which affects the pH and acid profile, along with high alcohol beers like stouts that are built to be barrel aged. An oatmeal porter, if low in alcohol (under 8% or so) and generously hopped, will change a lot (possibly for the worst) after bottle aging for a year.
Basically, you need alcohol, acid and tannin to age a beer. And the kinds of tannins that come from grain husks aren't sufficient.
I was of the understanding that 'yeast-bite' was a consequence of poor temperature control during fermentation - particularly, too high a temperature. It gives earthy or broth-like flavours. There's a UK supplier called Murphy & son that mentions yeast-bite. Chris White's book 'Yeast' recounts a story of a Texas brewer who was getting earthy off flavours and could not work out why. Turns out that the top of the fermenting beer was above the top of the cooling jacket. The yeast was 'cooking' at the top of the FV and adding autolysis flavours.
Otherwise, it might be that your off-flavour is a consequence of autolysis in the bottle. If the sediment is thicker than a dusting across the bottom of the bottle then there was probably an excess of yeast during the conditioning, and now that some time has passed those flavours are making themselves known.
Or, it may be something else, maybe I could try some too... :-D
[EDIT] In response to @Denny down voting. I have done a cursory Google on "Yeast bite cause" and I would refer you to the top two results (UK Google).
1) BYO website Quote:
Length of fermentation and fermentation temperature are also important yeast-bite considerations. A fast, warm fermentation racked immediately into bottles will leave lots of yeast in suspension in the bottles and will also result in a thicker layer of sediment as the beer matures. Keep your fermentation temperatures well within the range for the style you are brewing, allow the yeast to settle for several days after primary fermentation, and rack the beer into another container, if possible.
Dead Yeast: Autolysis At the end of fermentation, when most of the yeast has settled into a hard pack of sediment, some of the cells will begin to die and undergo a process called autolysis. Autolysis is probably the single most important contributor to yeast bite. “Lysis,” the dissolution or disintegration of the cell, coupled with “auto,” self, basically means “self-destruction.” Simply put, when yeast cells die at the end of fermentation, the yeast’s own enzymes begin to digest the yeast cell itself. This process releases all kinds of nasty compounds into the beer, including fatty acids and sulfur- and nitrogen-containing compounds that cause the aromas and flavors described above as meaty, rubbery, and sour.
2)Drayman's Brewery and Distillery Quote:
Yeast autolysis or self-lysis is the breaking open or rupturing of the yeast cell and the transfer (leaking out) of undesirable substances and off-flavours to the beer. The flavour is described as yeast-bite, broth-like, meaty, sulphury and dirty diaper.