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I've always soaked my bottle caps in sanitizer before using, but a friend of mine suggested boiling (throw in the same pot with priming sugar) instead because the cooties can theoretically hide from the sanitizer solution but not from the heat. This makes intuitive sense, although one could debate whether the extra effectiveness has any practical measure.

My question is what the downsides are (if any) of boiling the caps. Could a slight metallic taste be imparted? Weaker seal? Something else?

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    I think the lesson here is "TRUST YOUR SANITIZER". We cannot achieve sterilization but sanitizers are always better than boiling. Residual sanitizer keeps protecting. Boiled items are just as susceptible to infection from airborne sources as others. Trust your sanitizer. If you mix it right and keep it fresh, it won't let you down.
    – TinCoyote
    Commented Jan 12, 2011 at 19:22

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If you're using normal caps, you can boil or sanitize. I wouldn't follow the advice of boiling them with the priming solution though, if only because the caps would get sugary and sticky.

If you're uncertain about whether to boil or sanitize, why not handle them the same way that you handled your bottles. After all, the beer is going to be in greater contact with the bottle than with the caps. What's good for the goose is good for the gander.

However, if you're using oxygen-absorbing caps (like these), then you shouldn't be getting them wet at all. Moisture renders the oxygen-absorbing liner useless. So do not sanitize o2-absorbing caps. Updated, see below.

Update: Northern Brewer Chris has provided some more information on the caps I linked to, so I'm going to update this answer. I found these directions on sanitizing o2-absorption caps from a company that sells them. Their directions boil down to:

  • Pro-brewers don't bother to sanitize caps, so you can probably skip it.
  • If you're not comfortable skipping it, then do not boil the o2-absorption caps.
  • If you get an o2-absorption cap wet but don't use it right away, it's now useless.
  • Don't use hydrogen peroxide sanitizers (One-Step, B-Bright) on o2-absorption caps.
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    You information on the O2 absorbing caps is wrong. You should always sanitize your caps but with these in particular you don't want to sanitize them with one step, which is an oxygen based sanitizer and will saturate the cap with O2 making it ineffective. You do also need to get them wet in order to activate the O2 absorbing substance on the cap whether it is with the beer or an acid based sanitizer like Star San.
    – Northern Brewer Chris
    Commented Jan 12, 2011 at 15:44
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    Thanks, I've updated my answer. I'd love it if information about sanitizing these caps ended up on the Northern Brewer site. I've always wondered and there's a ton of mis-information out there.
    – Hopwise
    Commented Jan 12, 2011 at 16:49
  • Also, getting O2 absorbing caps wet doesn't make them useless! They are still usable as caps!!!
    – brewchez
    Commented Jan 13, 2011 at 14:44
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    Who says pro brewers don't sanitize caps? Doesn't sound right to me. Maybe breweries that pasteurize don't sanitize caps. I'd take that piece with a grain of salt.
    – brewchez
    Commented Jan 20, 2011 at 17:49
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    Regarding the pro-brewers, do they possibly use pre-sterilized/sanitized caps?
    – STW
    Commented Jan 20, 2011 at 20:19
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if you're mixing your sanitizer correctly, you shouldn't need the heat. the only way any bad things could "hide" from sanitizer is if your caps weren't clean, and if you're using dirty caps, you've got other, more serious problems.

downsides (i'm just guessing):

  • difficulty in handling to get the caps to the bottles. i touch the caps with my fingers, and would expect that getting burns whenever i bottle would make me less likely to bottle.
  • thermoplasticity of the glue holding the seal to the metal cap. at what temperature will the seal disconnect itself from the cap?

if you're just microbially paranoid, then i can see using heat, but if you're going to go to that extent, why not use a pressure cooker and actually sterilize the caps?

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    Agreed. I also think you don't need to be as paranoid about a cap infection, because even if it happened it will only affect a FEW beers, not ruin your entire batch. You don't boil your racking cane and siphoning tube do you? How about your bottling bucket?
    – Room3
    Commented Jan 12, 2011 at 14:04
  • Room3 makes an excellent point.
    – TinCoyote
    Commented Jan 12, 2011 at 19:25
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I've never sanitized my bottle caps in any way, and have never had any infection problems.

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    Likewise. Some bottle caps rust when the metal part (not the plasticized inner part) is exposed to water or sanitizer.
    – Nick
    Commented Jan 12, 2011 at 20:30
  • How are your caps packaged when you purchase them? Mine are bought out of a bulk-bin where you reach in and grab fistfuls... I don't trust their sanitation because of that. One sneeze near the bin and you might be bottling a batch of piggy-flu along with your ale
    – STW
    Commented Jan 20, 2011 at 20:18
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I've always boiled or at least simmered my bottle caps. In my case, I suspect its a hold-over from home canning, where you have a two-part seal, and the inner lid with the rubber gasket gets boiled (or at least strongly heated. Anyway, my usual process is to rinse the things thoroughly to get any dust or whatever surface crud off, and then lightly simmer for a few minutes. I usually let them cool enough to handle, in which case you needn't be concerned with any extra strain on the bottles.

I wouldn't put it in with the priming sugar, just in case. I doubt you'd get any kind of effect in the form of off-tastes, but why risk it?

I'll freely admit that my process is almost certainly overkill, and there's the very slight possibility of overheating the caps enough so that the inner sear could separate from the metal, causing a seal failure. I've never had it happen, though.

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A warm cap would contract when cooling theoretically providing a tighter seal, but also possibly causing damage to the bottle.

If you'r worried about taste sterilise separately.

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