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In this answer it is mentioned that boiling with the lid on or off may make a difference in final gravity. Which do you recommend and why?

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You want to boil with your lid off.

Part of the process of boiling is to remove dimethyl sulfide (DMS), which is a sulfur compound off flavor that tastes like cooked corn. DMS is formed by heating the wort. If you leave the lid on the kettle DMS won't evaporate with the steam and you'll have more of the flavor in your beer.

You might also run the risk of boilover if you leave the lid on, but DMS is the larger concern. This is also one of the reasons you want a real, rolling boil as it allows more DMS to escape.

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  • Interesting, never knew that about DMS but it makes sense that it can escape with the steam. Does the DMS affect the gravity, or is it more of a micro-substance that just affects flavor?
    – G__
    Commented Nov 12, 2010 at 16:40
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    I don't know about the gravity. That's a good question. Most grain has DMS precursors already in it; heating it causes it to form, so you can't escape having it. Some grains have more DMS than others. For example, Pilsner malt tends to have more DMS which is why you'll often see recipes with Pilsner malt recommend a 90 minute boil. You can read more here: homebrewtalk.com/wiki/index.php/DMS
    – sgwill
    Commented Nov 12, 2010 at 16:44
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    What Greg said in the answer below. But to clarify. A typical pot geometry will lose 9-11% volume per hour. However, that is dependent on environmental humidity (and to some degree temperature too). Your evaporation rate will be lower when it is humid. Recipes are designed with this loss in mind and should have a higher starting volume. I.e. a ten gallon batch will start with about 11 gallons of extract at the start of a 60 minute boil.
    – danielshiplett
    Commented Nov 12, 2010 at 19:32
  • Yes, definitely keep the lid off; primarily for DMS elimination as sgwill said. @Greg, DMS won't affect the gravity; it's just a compound. Gravity is a measure of the dissolved sugar.
    – Morgan
    Commented Nov 13, 2010 at 0:33
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    @Morgan - I believe that technically gravity is a measure of everything in the mixture. If the DMS is of sufficient quantity and with significantly different mass than water, it would affect gravity. But I think it's probably in such small relative quantity that gravity effects are negligible.
    – G__
    Commented Nov 13, 2010 at 1:44
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I think the difference (regarding gravity) would just be water volume. Lid off = liquid lost to steam. You can compensate by adding more (or less) water after the boil.

The actual amount of liquid lost depends on the geometry of your kettle, but I think 1-1.5 gallons per hour is a rule of thumb. The easy way to know for sure is to measure before and after a boil.

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There's an interesting experiment into this here https://brulosophy.com/2016/10/31/the-boil-lid-on-vs-lid-off-exbeeriment-results/

It's worth reading the whole thing but the TL:DR version is that he did side by side lid on and lid off brews which were otherwise identical, and nobody could tell the difference with blind taste/smell testing - even him. The two beers were for all intents and purposes identical.

One interesting thing is that both brews lost the same amount of liquid during the boil, and one of his theories was that even with the lid on a lot of steam still escapes, and that maybe still carried the DMS off with it. He did say he was going to continue doing lid-off brews though.

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