One of the hose clamps for the cooling spiral malfunctioned, so my cooled wort got contaminated by roughly 10 liters of tap water (in a 55 L batch). This is a lager beer which will be stored for five months before drinking, so ignoring the contamination is out of the question.

The wort is being re-boiled and re-hopped, but is there any consequences of re-boiling (apart from losing hop aroma)?

  • How large is your batch? 10 liters may be a lot or a little, and influences whether you should even bother trying to save the batch or just brew again.
    – mdma
    Commented Feb 27, 2016 at 22:06
  • 2
    What contamination? What the heck is in your tap water that could contaminate???
    – Robert
    Commented Feb 28, 2016 at 0:44
  • 1
    Unless your tap water is terrible for some reason, I would have just added some DME to compensate for the dilution and left it at that, without re-boiling. I don't any more, but I used to top up carboys with plain un-boiled tap water, and I don't think any harm came from it. Come to think of it, I used to make batches from wort-in-a-bag kits mixed with plain old tap water. They were more than 50% tap water by volume, and they were great.
    – Jeff Roe
    Commented Feb 28, 2016 at 6:59
  • Our tap-water is clean enough for drinking and brewing, sure, and had this been a drink-straight-out-of-the-bucket kind of ale, i would have just ignored it and started fermentation. This is a lager-beer, however, so I'm a bit more neurotic than usual.
    – FredrikH-R
    Commented Feb 29, 2016 at 9:20

6 Answers 6


Reboiling will increase bitterness of all the hops that went in 'late' in the kettle. Obviously, as you said you'll lose your aroma charge will decrease in proportion to the length of the reboil.

You may likely increase the maillard profile of the malt character depending on how long you boil. The complexity of the original grist will dictate the extent of that impact on flavor.

And depending on how much of the 10L you plan to boil off, your mineral content will certainly change as well. Again, depends of the mineral profile of your base water as to the total effect on the beer.

Depending on how long you plan to reboil you may expect to re-dissolve some protein into the beer. (Some of the early Lambic and Belgian brewers would conduct really long boils that drove some portion of the protein in the hot break to break down further allowing it to no longer be break but be soluble peptides in the beer, changing the mouthfeel). This again is base beer dependent along with length of boil.


When you say it got contaminated you mean that some tap water went in contact (mixed) with your wort, right? I wouldn't say that is contamination. IMHO, contamination is that some bacteria has started to grow and eat the nutrients of your wort thus producing some compounds and off-flavours. In this case I wouldn't boil since these compounds are already in your wort and cannot be removed by boiling. Otherwise, I would boil and maybe add DME (dry malt extract) to compensate the gravity points you lose with tap water.

  • I assumed that there was enough microorganisms mixed in, which would be able to compete significantly with the yeast for the nutrients.
    – FredrikH-R
    Commented Mar 1, 2016 at 12:31
  • I don't think tap water has that many microorganisms due to chlorines and other additives used to "purify" the water we drink. Anyway, it won't make any harm to boil the mixture. In fact, boiling may help get rid of the chlorines which may affect flavor. Just relax and have a homebrew ;-)
    – rutex
    Commented Mar 2, 2016 at 15:18

Unless your tap water is bad, I wouldn't call dilution "contamination". If this happened to me, I would do a small boil with DME or LME (just enough to get everybody dissolved), cool it down and dump it in. If you don't want to do that, you can ferment it, bottle/keg it and send it to me. I'll be more than happy to take care of it for you.

Seriously, relax. It's just beer. Don't make it into some sort of rocket surgery.


Sounds like you know of and are dealing with the alpha acid issues.

The tap water may impact flavor if it was chlorinated, regardless of boil time.

Reboiling or longer boiling helps with DMS removal.

I would boil off the 10 liters to keep your gravities on point, or accept the dilution if you're only doing a short reboil for sanitation.


You say this is a lager, so I'm going to assume a high percentage of pilsner. In that case, I would not recommend re-boiling anything. Some extract brewers tap up from the faucet on every brew (I used to). If you re-heat the wort, you are going to create DMS, which will be a much bigger problem than any potential contamination from unboiled tap water, never mind the difficulty in retaining the desired flavor profile. You could potentially start the boil over, adding more water to account for a full boil (to allow the new DMS to be boiled off), but the hop utilization and maillard reactions will be different from intended. I think a safer alternative would be to rack to two vessels (to prevent the fermentation from overflowing) and just ferment as normal. You'll produce a thinner, less ABV beer than intended, but otherwise, should be fine.


Better to dump it than to take the chance of bad fermentation and contaminated beer..

  • 1
    Why would water contaminate? Worst case the beer is lighter than expected. It's totally beyond me why you would want to dump a beer just because it has less alcohol than expected.
    – Robert
    Commented Feb 28, 2016 at 0:46
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    That's zany, man! Dump it because it might be contaminated!? He's already done most of the work. He can dump it after fermentation if it seems bad, but advising him to dump it now is horrible advise!
    – Jeff Roe
    Commented Feb 28, 2016 at 6:55

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