Harsh off flavor, muted maltiness, but only in some bottles

I'm not certain whether this is the best forum to post this in, but I'm fairly sure it's a bottling-related problem, so I'll start here...

On my last few batches, I've had an odd off flavor that turns up in only a few bottles. I've not been tracking the incidence rate, but I'd estimate it's somewhere between 1/5 and 1/3 of the bottles that have the problem.

The off flavor doesn't really match any of the descriptions I've seen on the standard beer diagnosis charts, at least not that I remember seeing or that I found in Palmer and a couple other random ones I looked up this evening. It's a bit hard to describe, but it's sort of two things. First is a harsh flavor, maybe a bit bitter or astringent, but not strongly either of those things. (It's certainly not like drinking overbrewed black tea, and I'm very familiar with that.) The second part is a muting of the flavor of the underlying beer. It may just be that it is hidden, but the harshness isn't overpowering, so it's almost like it has just faded. If I really concentrate, I can find some of the flavors, but they don't have any pop.

It's hit a dry stout, an ESB, and a beer based on Yooper's Dead Guy clone. In all cases, aside from low carb in the ESB, the good beers have been really good. The stout had a very strong chocolate finish, the ESB had a pretty nice maltiness, and the Dead Guy clone has a strong raisiny caramelly maltiness to it. Certainly neither the stout nor the Dead Guy clone could be called "subtle" in their malt tones. In the harsh cases, though, it's just not there.

All the beers have tasted very good at bottling---enough that I happily drank the uncarbonated gravity samples and the post-bottling dregs. They carbonated for about 3 weeks untouched, and the ESB and Dead Guy clone went another couple weeks before they were well carbonated, so maybe 5-6 weeks in bottles. I don't recall exactly when the off flavors become evident, ie, whether it comes up before that time or not, and the incidence is rare enough that it'd be hard to be certain. The ESB has now been bottled about 3 months, and still most are good but occasional bottles are bad.

So I'm not sure what to do about this, because it's a bit hard to place. It only strikes some bottles, so I think it has to be bottling related. A few notes on my process:

* Bottles are rescued commercial bottles, most of which have been used a couple times already. They're cleaned by soaking in PBW or Oxyclean (and possibly bleach solution on the stout, I don't recall when I switched over). The soak is either overnight or for an hour or so, and they've all started well-rinsed shortly after they were emptied on the previous use.

* The bottles have been sanitized by using a squirt bottle to spritz a few squirts of saniclean into the bottles. I start with one squirt deep into the bottle, another pulled halfway out to hit the sides of the neck, and another an inch or two above to hit the outside lip and the inside top edge. Each bottle sits 1-2 minutes or more to provide contact time, then is emptied out and filled.

* Bottle caps are squirted with sanitizer as well, and sit at least long enough to fill 6 bottles.

* Filling is done using a spring-loaded bottling wand attached to a bottling spigot. I fill until the beer meniscus is touching the top of the bottle, and leave the headspace created by pulling out the bottling wand. The headspace is indistinguishable from commercially filled bottles.

* Bottling equipment, including siphon hoses, etc, is cleaned after each use by running hot (135-140°F) tap water through everything and sanitizing immediately after use. The spigot and all the hose connections are disconnected or disassembled before cleaning. Before assembling them for use, I sanitize all the individual parts, and then run sanitizer through every hose, siphon, or spigot to be sure.

* I take reasonable care to avoid splashing during siphoning. I boil the priming sugar for 10 minutes or so, then cool slightly and add it to the bucket. Then I siphon on top and let the swirling mix it up. Once filled, I set a lid on top and move the bucket as gently as possible about 50 feet from the garage to the kitchen counter. I do not have any CO2 to do active blanketing, etc.

Anyway, that's a whole lot of info, just trying to give some idea of how I'm doing things. Any ideas about what's going on? I'm sort of assuming by default that it's either some kind of infection or oxidation, but it doesn't have any of the classic characteristics of either. There's no odor at all, and no taste reminiscent of cardboard...

What temperature are you storing the bottles at and how much sediment is there in the bottom?

In 3 months you would surely notice haze, thinness or increased carbonation in an infected bottle. You seem very thorough, so I doubt you'd skim over a detail like that.

Oxidized is a difficult off flavor because it doesn't have a specific taste. It's most likely what you're noticing. Why it varies from bottle to bottle I can't say. Perhaps minor variations in cell counts or filling technique.

The other thing, which seems less likely, could be if you are leaving too much sani residue in your bottles. I upend mine and let them dry for a while to be safe.

Storing at room temp, which is pretty steady at 68-70°F. I haven't been monitoring them, but a batch of mead in a similar closet sits at 68°F +/- less than a degree most of the time.

There's very little sediment in these. I'm not certain about the stout, as it's hard to see, but I don't think I've seen even a trace of sediment in the ESB. The DG clone has a few little clumps, but leaving those behind only requires leaving maybe 1/8 inch of beer in the bottle.

The ESB has a bit of haze in general, can't say I've noticed a difference. Obviously it's hard to say with the stout.

The DG clone is quite clear. The bad one I had earlier today might have been a little bit cloudier than the second one I had later (for comparison! It's in the name of science!), but I was very conservative on the second not to let any sediment out. I may have let a little sediment into the glass the first time around. In any case, it was at that level where it's hard to even say it wasn't just condensation on the glass.

As for thinness, I don't think there's much difference, although it's hard to remember the mouthfeel. The flavor was certainly thinner, but I don't think it felt thinner. I'll have to pay closer attention if I find another one.

One difference I noticed is that the carbonation was better on the second one, which tasted fine. I let that one warm up for 20 minutes or so out of the fridge, versus 3-5 minutes for the first one, which may have had an effect. Both had a similar small persistent foam, but the later one foamed up quite a bit more initially. It seems most likely to me that it's the difference between pouring a cold beer and a warm one, but I'm just reporting the facts.

In the stout and the ESB, there were some variations in carbonation. I recall noting that one of the first bad ones I came across was somewhat more carbonated than most. However, I think there have been some that were similarly carbed and tasted ok. Certainly many or most of the bad ones are indistinguishable in carbonation.

It could be oxidation, but if so, I'm puzzled by how it's happening. Obviously I have a fairly simple setup, so I could do a lot more to prevent O2 contact. However, I think I'm pretty careful, and it seems to me that if significant oxidation were this easy to pick up, it'd be a much better known problem. I don't think I'm doing very extreme aging on these things! Plus, I have some much older beers from prior batches that were treated essentially identically and haven't had any signs of this.

In that sense, a minor infection seems likely to me, but I'm really not sure. So I guess I'd be appreciative of any suggestions as to how to try to isolate the cause.

Regarding the sanitizer, that's an interesting suggestion. I've gone back and forth a bit between bottling above the open dishwasher and simply setting up on the counter. When I use the dishwasher, I store the waiting bottles upside down on the rack. When I don't use the dishwasher, I tip out the sanitizer, set them down right side up to wait, and then tip them out again to be sure. That probably does leave a bit more in the bottle, plus it means they're more vulnerable to dust falling in. This seems a bit unlikely to be the culprit, as I'm using a pretty small amount of saniclean, which I'm very precise to dilute to the recommended concentration. But this might be a process difference between the batches with and without the problem. (Unfortunately, I don't keep notes on this so I don't know for sure which batches were which, but I'm pretty sure the three affected were done without the dishwasher.)

Anyway, thanks for the reply.

Following up on this for the sake of anyone who searches in the future, I am beginning to think it is a green beer effect. It doesn't strike me as the usual description of green beer, but after a few more weeks I haven't found any of these bad ones recently. Also, the ESB which was totally flat has finally reached a decent carb level---it gives a 3/4" head if I pour it straight into the bottom of a glass. So clearly it's continuing to evolve.

So, uh, I guess just be patient. It's easy to say, but hard to believe sometimes.