Why Uncovered Boil?

I am curious to the reason wort is always uncovered when boiling. I understand the need to watch for boil overs but I also see two advantages of being covered. One is less water needed to compensate for evaporation and the other is a covered pot maintains a boil at a much smaller burner output. What am I missing? How about partially covered?

The biggest thing is to allow DMS (It produces that cooked corn aroma in beer.) to be driven off. The second is that that extra evaporation means that all grain brewers have more water to use in order to rinse extra sugars out of the grain.

There are chemicals given off during the boil that you want driven off in the steam, that if it is trapped and falls back down can cause off flavors. One of them is called DMS which can leave a cooked corn taste to the beer.

If you need to use the lid to get it to boiling temp, fine, but take it of for the majority of the boil.

A covered pot also boils over faster/more often. It'd have to watched even more.

And you know, a watched pot never boils.

Just a few of the benefits of a vigorous, uncovered boil:

-Extracts, isomerizes and dissolves the hop α-acids
-Stops enzymatic activity
-Kills bacteria, fungi, and wild yeast
-Coagulates undesired proteins and polyphenols in the hot break
-Evaporates undesirable harsh hop oils, sulfur compounds, ketones, and esters.
-Promotes the formation of melanoidins and caramelizes some of the wort sugars (although not desirable in all styles)
-Evaporates water vapor, condensing the wort to the proper volume and gravity (this is not a primary reason, it's a side effect of the process)
-Drive off DMS.
-Prevents boilovers.
-Concentrates wort.
-Bind hop compounds to polypeptides, forming colloids that remain in the beer and help form a good stable head
-Lowers pH in order for proper cold break to form and fermentation to proceed normally.

Great information, thanks. I knew there must be a reason or everyone would be doing it.

I wish they would have offered beer making 101 back in my high school days as an elective class. Something I could use would have been nice but instead I know how to sew on buttons.

I used to use a half covered pot during most of the boil to keep the rolling boil going, which was necessary on the stove top. I never had a problem with any of the off flavors associated with a covered boil, as they were driven off in the steam escaping from the uncovered side of the pot. I always made sure to take the lid off occasionally throughout the boil and tilt the lid down and away from the boil, which allowed the condensation to run off of the lid and onto the counter, keeping it out of my wort.

WhiteDog87 said:

I used to use a half covered pot during most of the boil to keep the rolling boil going, which was necessary on the stove top.

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Are you boiling 8-10 gallons on an electric range? You could try straddling two burners.

I've never had a problem keeping a rolling boil of 5-6 gallons on my relatively weak gas range.

I was boiling 8-10 gallons on a gas range straddling two burners. Used to take forever to get my boil going, pretty much impossible without having the kettle half covered.

I recently bought a camp chef propane burner so i no longer have to deal with the stove and waiting forever to get a proper boil. It boils 10 gallons of 60F water in less than an hour, and gets my wort boiling in 20-30 minutes. Shaved a ton of time off my brew day.

I brew 4-6 gallon batches...I could see how 8-10 gallons would be a problem indoors.

The only stages where I use a tight fitting lid are 1) to bring the water to mashout temps, and 2) during flameout/whirlpool.