Dear Mr. Wizard:

I just brewed an all-grain amber recipe from the May 1999 issue. This was my third all-grain attempt, and it raises a few questions. Since I started using a 100,000-BTU burner, I end up with a smaller volume than the recipes anticipate. I know this is due to a more aggressive boil, but I am concerned with having to top off my fermenter. Is this a problem? Should I start with more wort? Also, the recipe calls for the gravity to start at 1.064. My yield prior to the boil was 1.042. Does it change during the boil? Should I check it again after topping off to 5 gallons?

Rob Allen
Salt Lake City, Utah

Mr. Wizard replies:

Recipes printed in BYO give the "original gravity" of the wort. This number refers to the specific gravity of the wort prior to fermentation-in other words, after the boil. Wort gravity increases during boiling because the wort is concentrated as water is evaporated. Most commercial brewers want to evaporate at least 8 percent of the wort volume during boiling to ensure adequate removal of volatiles, such as dimethyl sulfide. Excessive evaporation can result in the formation of unwanted flavors. It also wastes energy. Energy is not a huge concern with homebrewing, but imagine a commercial brewkettle with a pre-boil volume of 800 barrels (24,800 gallons). A typical boil can evaporate 64 barrels (1,984 gallons) of water in 60 minutes! That requires a lot of energy.

For starters, to hit the target gravity and the target volume of a recipe you must know the efficiency of your brewing system. If you don't know your efficiency and strictly follow a recipe, you most likely will miss one of your targets. If your system is very efficient, you may hit the gravity target by over-shooting the volume target (by the way, adding water after the boil causes no problem as long as it is pre-boiled). Missing your target volume may not seem like a big deal, but it affects the bitterness of the final beer. More beer means a dilution of the hop bitterness. The bottom line is that getting a handle on your efficiency will help you. Your target was 1.064 post-boil and you had 1.042 pre-boil (which translates to about 1.050 post-boil); this indicates that your system may yield less from the malt than the recipe you followed.

Another technique you may want to adopt is tracking your evaporation. You can do this by calibrating your brew kettle and noting the pre- and post-boil volumes. Here's the formula to use:

Evaporation percent = 100 - (post-boil volume x 100 ÷ pre-boil volume).

For example: Say you collect 5.5 gallons of wort and boil it down to 5 gallons. This would be 100 - (500 ÷ 5.5), or 100 - 90.9, which equals an evaporation rate of 9.1 percent.

Most commercial brewers target between 8 and 10 percent evaporation during the boil. For homebrewers, a good evaporation rate is 6 to 8 percent per hour. A kettle that evaporates 6 percent per hour will evaporate 8 percent of the volume in 80 minutes. If another kettle evaporates 8 percent per hour, then a 60-minute boil will evaporate the same wort volume. As you collect more data, you can adjust your flame to fine-tune your evaporation rate. If you don't like topping off your batch with preboiled water, tweaking your boil is an easy way to eliminate this practice.

Mr. Wizard, BYO's resident expert, is a leading authority in homebrewing whose identity, like the identity of all superheroes, must be kept confidential. To see more of Mr. Wizard, check out the latest issue of Brew Your Own at better homebrew shops and newstand locations.