I am new to homebrewing and am getting help from an experienced brewer. So far I think everything has gone OK. I am doing all-grain brewing on a stovetop. I have done 2 brews. I am doing the mash and boil in a 15 gallon kettle. As far as the mash, I have had no problem hitting my target saccharification rest temp and can hold it there (within about +- 3°) for 60 minutes. However when attempting the boil I have run into somewhat of a problem. I can't quite reach a rolling boil. I can only hit about 210° with the lid off. And over the length of the boil, the temp slowly decreases to about 205°. If I leave the lid on, I can hit a nice rolling boil. For future brews, I plan on having a smaller amount of liquid in the kettle and will leave the lid on. Once I noticed I can get a much better boil with the lid it was too late to brew that way because I had about 6.8 gallons in my kettle and needed some of it to evaporate so I can get down to about 5.5 gallons.

So my question is this: What are the chemical, taste, etc. consequences of doing a "weak" or "light" boil at about 210° ? Will I taste a difference? Would an experienced brewer taste a difference? Was it not 100% sterilized?

  • Your best path forward is to brew with more BTU's. Propane burners are the easiest way to go, if you have a place outdoors where you can brew. I brew in my garage with the door open.
    – jalynn2
    Commented Feb 18, 2014 at 18:52
  • The general consensus is that I need to increase to a rolling strong boil while keeping the lid open. I think I'm going to build a heat stick. Commented Feb 18, 2014 at 20:57

4 Answers 4


There's a few consequences, none of them are game over's.

First off, don't leave your lid on the kettle when you boil. This will effect your evaporation rate and you'll likely not hit your Original Gravity as you would expect. Also, Dimethyl sulfide (DMS) is a sulfur compound which leaves a taste of cooked or cream corn in your beer (a signature off flavor found in lagers like Rolling Rock). DMS will evaporate off, therefor if your lid is on, it will condense and fall back into the solution with the rest of what can't evaporate.

From homebrewtalk.com's wiki:

Kettle boiling hydrolizes SMM to DMS which is removed during evaporation. The half life or time needed to remove half of the DMS is 40 minutes so that three-fourths is removed in 90 minutes. Narssis recommends a 100 minute boil to reduce the level of SMM and DMS to acceptable levels in most beers.

Therefor you may need to under-boil for a bit longer (before starting your hop additions) in order to evaporate off adequate amounts of DMS.

Aside from DMS, and evaporation, the other consequence is alpha acid isomerization efficiency. Your hop additions will suffer when it comes to measuring your IBUs. I don't have an exact measurement as I don't think this has been adequately analyzed in a controlled environment yet, isomerization of the alpha acids occurs around the 175°F - 180°F, but only barely at that temperature. It quickly drops off once you go below boiling, so you may find that if you can't hit 212°F, you may want to increase your hop additions to compensate. Then again, the big thing is consistency. If you can consistently have the same isomerization efficiency, you can adjust all your recipes accordingly.

Also, I can recommend: Allied Precision Ind 742G Bucket Heater With Guard, 1000-Watts

It isn't consistent heat, as it turns itself on and off at regular intervals (likely to prevent it from burning up), but it should help boosting your temperature, and help with quickly increasing the heat. You also might consider wrapping your boil kettle in Reflectix insulation to help keep the heat contained to the kettle and prevent heat loss from the sides.

Long story short, try not to cover your boil kettle, you'll suffer in alpha acid isomerization, and evaporation rates will decrease, requiring a longer boil time.

  • Very helpful answer. Thank you. I'll make sure I keep the lid off and probably kick my "boil" time up to at least 100 minutes. And I'll absolutely invest in a bucket heater and probably wrap the kettle in Reflectix. Commented Feb 18, 2014 at 17:15
  • Regarding evaporation and target OG: depends on how you formulate the recipe. If you know your system and mostly boil with the lid on, you can adjust the amount of water accordingly.
    – Robert
    Commented Feb 18, 2014 at 17:33
  • The important thing to bear in mind regarding that though is that boiling with the lid on is generally regarded as bad practice, therefor any attempt at using a kit or someone's recipe would need to be re-configured to undo their calculated loss due to evaporation (if listed as part of the steps or anticipated volumes), and incorporate your own loss. That said, yes, you can compensate by reducing water volume.
    – Scott
    Commented Feb 18, 2014 at 17:57
  • @Scott: At this point I am more worried about the DMS not evaporating. I'd very much prefer to not get that kind of a flavor in my beer. Commented Feb 18, 2014 at 18:05

One thing that no one has mentioned yet is that a stronger boil promotes beer clarity. The proteins in the wort are forced to "clump together" more. Increasing the size of the proteins like that causes them to fall out more easily.

  • I'll keep this in mind. Although, my first brew is almost done bottle conditioning. And I have to say it cleared very nicely. Could be an issue for lighter beers as it was an Imperial IPA. Commented Feb 18, 2014 at 17:13
  • That's what I've seen too: I keep much more hop trub in the kettle in rolling boils.
    – Robert
    Commented Feb 18, 2014 at 17:31
  • Very true, I was going to write the same.
    – mdma
    Commented Feb 20, 2014 at 17:31

I believe alpha-acid isomerization occurs well short of boiling (like 170-180°F), so you should be fine on hop additions and bitterness.

You'll get less boiloff/evaporation, especially with the lid on, so you might need to adjust your original gravity expectations accordingly.

There might be a reduction in any kettle caramelization or darkening you would see; for some light styles, this might be desirable.

The biggest issue is going to be the retention of DMS precursors that we use the boil to eliminate.

  • 2
    Kettle caramelization can't happen. You can't reach the temps needed in a kettle of liquid like that. You can, however, form Maillard reaction components that influence color and flavor.
    – Denny Conn
    Commented Feb 18, 2014 at 16:46

Further to Denny Conn's remark about protein clumping: this is usually called the 'protein break,' and it takes a vigorous rolling boil to make it happen. Apparently the physical action assists the process, as do real (i.e. not pelletized) hops.

Back when I brewed beer from grain, I had to do the boil on an electric range, which limited the amount of heat I could pump into it. I could tell when I got a good protein break by scooping some hot wort into a glass mug (because it has a handle; saves fingers) and having a close squint at it. Between the swirling, floaty bits & pieces, the liquid itself is crystal clear not murky. This is a GOOD THING, though if it doesn't happen the beer should still be drinkable.

Re: whether or not to keep the lid on, that's not a worry. You either leave it off. or deal with a nasty, messy cleanup job. I always adjusted the SG as req'd once it was safely in the primary.

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