His question: "I'm brewing a light-colored beer (a Kolsch). I want to be sure it doesn't come out too dark. Should I limit my boil to only 20 minutes?"
The gut reaction when you hear this question is to say "No. Boil for the full 60-90 minutes." After all, boiling is important to breaking down proteins and making sure that you get the full bitterness from your hops pellets. If you're doing an all-grain recipe for a light-colored beer like this, you probably want to do a full-size, full-length boil to ensure you get a good result.
An extract-based recipe changes the answer a little. Malt extract doesn't generally need a full-length boil. It's basically just a concentrated, already boiled wort. You want it to boil for about 15 minutes to ensure that any bacteria or other nasties that might ruin your beer get killed, but boiling for 60 or 90 minutes isn't strictly necessary. In fact, boiling a pale malt extract for too long can cause a Maillard reaction (sometimes referred to as caramelization) that can alter the flavor and color of the beer. A 60-minute boil on an all-extract Kolsch might result in a light-brown beer instead of the familiar yellow or gold color. It might also be annoyingly sweet for the style. For a light-color, all-extract boil, I might very well not boil the extract for the full length.
For a light-colored recipe that includes some specialty grains, I might take a slightly different approach. I'd steep the specialty grains as directed by the recipe. Then I'd bring it to a boil and include my hops pellets. If there are other non-extract components to the recipe, I'd add those as directed as well. Then, during the final 15 minutes of the boil, I'd temporarily remove the kettle from the heat, stir in my malt extract until it's dissolved, and bring the wort back to a boil with my wort chiller inside. That would kill off anything that might be in the malt extract (or on my wort chiller) that could ruin my beer. As soon as the boil was over, I'd get the kettle off the heat, then use the wort chiller to bring the temperatures down as fast as I could to minimize any Maillard reaction.
But a 20-minute boil can be the right answer. If you look in homebrew supply catalogs, you'll see 20-minute boil kits. These usually include malt extract with few other ingredients. These are usually lighter-colored all-extract (or nearly all-extract) recipes. If you want to adjust from a 60-minute boil down to a 20-minute boil for an existing recipe, you'll need to keep an eye on the bittering.
Let's say you wanted to use the following simple recipe:
- 6.5 pounds Golden Light DME
- 1 ounce Cascade hops pellets (5.4% AA) for 60 minutes
- Wyeast 1056
The Beer Tools Pro software says that if we boil the DME and hops for 60 minutes, we'll end up with a bitterness of approximately 20.6 IBUs, a color of 5.2 SRM, an original gravity of 1.054, and a terminal gravity of 1.013. Realistically, there's a chance that if we boil that light colored DME for 60 minutes we'll brown it up a bit and miss our desired color.
What happens if I dial that back to a 20-minute boil? Beer Tools Pro tells us that we will now end up with a bitterness of only 11.5 IBUs. If our goal was 20.6, we're going to need to add hops pellets to get the recipe back on track. How much more? Beer Tools says we'll need to increase from 1 ounce to 1.79 ounces to maintain the same bitterness level. Changing the boil time down to 20 minutes also means that more of the aroma compounds of the hops will survive the boil, perhaps altering the aroma of the beer slightly.
So the answer to the question of whether a 20-minute boil is the right choice or not depends on the style of beer you're making, the color of the finished product, and the amount of extract in the mix. A 20-minute boil probably won't work well for an all-grain stout or porter, but it might be ideal for an almost-all-extract wheat beer, Kolsch, or light beer.