Should You Cover Your Wort When Boiling and Cooling?

Boiling and then chilling the wort to bring the temperature down for yeast pitching is an essential process for the homebrewer. But should you cover your container during the boil?

In order to make sure sulfur compounds are removed from the wort, the cover should be left off during the boil. If covered, these sulfur compounds can impart an undesirable cooked cabbage or corn flavor. Once the wort begins to cool, the cover should be in place to prevent airborne bacteria from entering the sterilized wort and spoiling the brew.

Read on to learn when, how, and why to cover your wort when boiling and cooling.

Do you need to cover your wort when boiling?

Let’s start with the reason why we boil the wort in the first place.

Boiling the wort serves four critical purposes in the homebrew process:

  1. Sterilization
  2. Stopping the starch to sugar conversion in the mash
  3. Extracting flavor from hop
  4. Reducing wort to final volume (typically 5 gallons)

As the heat rises, the mixture will stabilize into a solution. Part of this process is the expulsion of sulfur compounds into the atmosphere. 

If the expulsion of sulfur compounds is physically blocked by a lid, they will condense and fall back into the mixture. These compounds then form dimethyl sulfides (DMS) and, in large enough amounts, they will give the brew a cooked cabbage or corn flavor.

The biggest benefit of leaving the kettle uncovered is to allow these compounds to leave the wort.

Another benefit is that the brewer can easily watch out for potential boilovers. A boilover occurs when the boil becomes so violent it starts to spill over the sides of the container. This is not the way to achieve wort reduction and the heat should be adjusted to keep this from happening.

If there is a negative to leaving the kettle uncovered, it would be that it can make it more difficult for the brewer to achieve a boil.

If you’ve ever boiled water in the past, you probably know that it comes to a boil more quickly when the pot is covered with a lid. This is especially true if you have a lot of water in a large pot.

Depending on the homebrew recipe, you might end up with 7 gallons of wort prior to boil. Other than a large enough container, you will also need enough heat to reduce it to 5 gallons in the recommended time (usually 1-2 hours).

Because of these requirements – and the limitations of their equipment (typically a stovetop), some homebrewers opt to boil their wort with a lid. If they can, they will leave an opening for the sulfide compounds to escape.

Boiling terminology can also throw off a lot of new homebrewers. The difference between a vigorous boil and a rolling boil might be hard to visualize for some. 

Check out this video for a visual reference on what your boil should look like:

The video creator did a good job describing the characteristics associated with the term and even shows a boil over.

One other thing to note about boiling wort: altitude matters. It is a little easier to boil liquid at higher altitudes than it is at sea level (less atmospheric pressure) and your recipe may need tweaking.

Do you cover the wort when cooling?

Now that you’ve finished boiling the wort, you will need to bring it down to yeast-pitching temperature (about 75℉) in a relatively short amount of time. For a homebrewer, this generally means an ice and water bath and constant monitoring of wort temperature.

Unless you are doing this in a sterile environment (like commercial brewers), it will be impossible for you to make sure bacteria or wild yeast doesn’t contaminate your wort – unless you use a lid or some type of covering. 

Depending on your cooling setup, you might also need to stir the wort in order to make sure that there are no hotspots, leading to incorrect temperature readings. 

Some brewers opt to swirl the mixture but this might not be possible if your 5-gallon container is sitting in a kitchen sink.

The only real downside to using a cover is that you will need to remove it several times in order to check the temperature. 

It is vital that any implements you use – to include the lid itself – must be sterilized! You should also take extreme care as to how you set the lid and implements aside to make sure you don’t introduce bacteria back into the wort when using them again.

Finally, if you are pouring the wort into a fermenter, make sure you completely dry off the outside of the kettle. You don’t want any of the unsanitized cooling water to drip into your fermentation system.

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