Maximize Partial Boils

If you are an extract brewer, there is a good chance you will hear (if you haven’t already) that you need to be doing full-wort boils (boiling your entire 5-gallon/19-L batch of beer). The recommendation isn’t bad — a full-wort boil is ideal as it results in a higher hop utilization, ensures all of the ingredients are sterilized, and often results in a more predictable colored beer. However, if you don’t own or want to invest in an 8-gallon (30-L) brew pot just yet, very good beer can still be brewed with a partial boil. Plus, time is on your side with a partial boil as it takes about half the time to heat it up and later cool it down.

A partial boil is when you boil only a portion of the wort (all of the malt, hops, and adjuncts you normally would, but with about half the water) and then dilute it with water in the fermenter to reach full volume.
When doing a partial boil, you’ll want to boil from 2–3 gallons (7.5–11 L) of water for your wort (different recipes call for different amounts, but anywhere in this range will work). Let’s say you are going to do a partial boil on a 5-gallon (19-L) batch with 2.5 gallons (9.5 L) of water in the kettle. This is half of the intended resulting beer, so the specific gravity (SG) during your boil will be twice as high if all of your extract is added to the boil. The higher gravity will in turn result in lower hop utilization, therefore lowering IBUs. You have two options to compensate for this: Add more hops or lower the SG.

As a beginner, it may be fun to play with your hop additions and experiment with just how much more hops need to be added to achieve the bitterness you desire. But if you want an estimate of what you are losing in a partial boil, refer to Glen Tenseth’s hop utilization chart that estimates utilization as a function of time versus boil gravity. Using this chart, which is accessible through a quick Internet search, as long as you know your SG you can determine how much to tweak your hop additions so the IBUs of your finished beer is equal to that of a full-wort boil. The drawbacks of increasing the hop additions are added cost (by a dollar or two) and additional hop debris left behind, which decreases your wort yield.

If that doesn’t sound appealing to you, then there are two easy ways to lower the SG: Increase the amount of boiling water (which is only an option if your brew kettle is big enough) or wait to add a portion of the malt extract. Adding half of your malt extract at the start of a 2.5-gallon (9.5-L) boil will result in the same SG as adding all of your extract in a full-wort boil. Then with 10 to 15 minutes left in the boil add the rest of your extract, which will ensure that any potential contaminants in it will be killed off in the boil.

Holding off on adding some of your extract will also help with color. The reason the color in partial boils is often darker than desired is because non-enzymatic browning, also known as the Maillard reaction, is more pronounced during high gravity wort boiling. This isn’t a big problem if you are brewing a stout, but if you are going for a more delicately colored blond ale, you definitely don’t want your intended light golden hue to be closer to copper.

Issue: March-April 2015