Boil-Off Calculation: Does It Matter?

A recent discussion on the AHA Forum brought up a useful thought about boil-off rate calculations for brewing. Several brewing recipe calculators provide the ability to calculate your kettle boil-off rate as either a percentage of the starting volume or as a rate such as gallons per hour. Since a percentage calculation is more difficult to use, I’ve typically used the gallon (or liters) per hour calculation since you can quickly correlate that loss into what your final post-boil wort volume should be.

The problem is that we still need to look at what the total water boil-off is with respect to the starting volume in order to assess how much we are concentrating the wort and the minerals in the water. For my typical 6 to 6.5 gallon starting volume, I end up boiling off about 17 to 23 percent of the water since I boil at about 1 gal/hr. Now, if I were to reduce my batch size to 2 gallons and start the boil with 3 gallons of wort, my percentage of water loss jumps to 33 percent. This happened even though I still boiled off at 1 gal/hr. As you can see, I’m going to end up with much higher gravity and I’m going to have more concentrated mineral content in that boiled wort than if I had made my normal 5 gallon batch.

So, my advice to brewers is to consider both calculations when brewing. Yes, you can use the more convenient gallons or liters per hour boil-off rate, but do check what the total percentage of water loss you are actually creating with that boil. If your water loss is more than about 20 to 25 percent, you may need to add post-boil water to dilute the boiled wort to a desired concentration or you may need to reduce the total mineral additions to avoid over-concentrating them. If your percentage loss is much lower than that, you may need to boost your mineral additions to produce your intended effect in the beer.

Enjoy!