Dear Mr. Wizard,

I am an all grain brewer and a full wort boiler. Every book, magazine and recipe I read says that the gravity of wort will increase during the boil as a result of the water evaporating and the wort concentrating. I have never seen this occur in my brewing process. I have taken readings with both a hydrometer and a refractometer, so I doubt it is a case of displaced idiocy. My raw pre-boil gravity reading is always within 0.002 of the post boil, post cool down reading. I just finished brewing the Sour Cherry Ale recipe found in the July-August issue. My pre-boil gravity was right on at 1.056. However, after boil and cool down, my gravity reading was the same 1.056 rather than the 1.062–1.065 that is called for. Knowing this, do I need to adjust my recipes to fit with my brewing process?

Doug Elrod
Des Moines, Iowa

Mr. Wizard replies:

I have made measurements on wort and ended up with seemingly impossible data. The great thing about brewing is that, even though magic sometimes appears to occur, the basic laws of science always apply! When wort boils and loses water through evaporation, the specific gravity must increase. Specific gravity is a term expressing the weight of a volume of liquid. Although the units can be anything (pounds per gallon, ounces per cubic meter, etc.), the most conventional unit is kilograms per liter.

When wort boils, the only thing that is lost is water and a relatively small amount of aroma compounds. The carbohydrates and proteins that contribute to extract (or solids) are not volatile and remain in the wort. This means that the solids content remains constant while the volume of the liquid decreases. Mathematically this means that the specific gravity must increase. You already know this, but are having doubts because of your data.

My advice on many problems begins with doubting the validity of numbers. In this case, I don’t believe your data. There are two things that could cause erroneous gravity readings taken before and after the boil. The first is temperature. Hydrometers are affected by temperature and the temperature of the two samples needs to be the same, or you need to correct the indicated reading to compensate for the difference. As temperature rises, specific gravity drops. Since the temperature after the boil is obviously higher, your post-boil readings may not look right due to the temperature variance. In your question you state that the final gravity is taken after cooling, so this point may not apply to you, but you still need to make sure the wort samples are measured at the same temperature or corrected.

The other problem, which I would guess you are facing, relates to sampling. I have seen this problem myself and know that wort layers in the kettle. If you have a sample valve on the bottom of your kettle or use some sort of sampling device like a turkey baster to grab a sample, you may inadvertently take a high gravity sample of the wort before the boil. What happens when you collect wort from the mash is that the first high gravity worts are gently filled into the brew kettle, usually from the top, followed by lower gravity worts. This method of filling the kettle does not mix the wort and the specific gravity in the kettle is not homogeneous. If you grab a sample from the bottom of the kettle you will measure a higher gravity than if you take a sample from the top of the kettle.

I have actually played around with this and found that aggressive stirring is required to make the wort gravity homogeneous throughout the kettle. The easiest thing to do is to actually wait until the wort starts to boil and take a sample at this point for the initial “pre-boil” sample since boiling is a very effective way of mixing. The post-boil sample is not something to question because the wort is stirred during the boil. However, if you add water to cool the wort or “top up” your kettle to 5 gallons, you can experience the same problem with the wort layering. The last beer you brewed with a lower than expected gravity is another question entirely. The gravity may have been low due to some sampling error or you could have simply gotten a different yield than the author of the recipe got. This is a common problem and the best way to combat it is to use recipes as a guide and to “tweak” them based upon what your particular system yields with respect to efficiency.

Dear Mr. Wizard,

I have read a lot of articles that refer to brewhouse efficiency, but I haven’t found anything showing how to calculate it. Have you?

Ron Fore
New Braunfels, Texas

Mr. Wizard replies:

The easiest way to calculate brewhouse efficiency is to go metric! You need four pieces of information to perform this straightforward calculation. The data you need is: post boil wort volume in liters (there are 3.785 liters per gallon), post boil specific gravity (for example, 1.056 kg/L), post boil wort density in ºPlato (to approximate, divide the number behind the decimal of the specific gravity by four — e.g. 56/4 = 14 ºPlato) and the weight of grains used in recipe (in kg).

Once you have this information the calculation is easy. The first thing that is calculated is the weight of extract in the wort. Extract = (volume) x (specific gravity) x (ºPlato — expressed in decimal form). For example, (20 liters) x (1.056 kg wort/liter wort) x (0.14 kg extract/kg wort) (Plato is a weight/weight measure) equals 2.96 kilograms of extract. This is how much stuff you extracted from the grain during mashing and lautering.

The efficiency number is determined by comparing what was extracted to what was used. For example, if 4.5 kilograms (9.9 pounds) of malt was used to produce 20 liters of 1.056 wort, the efficiency is 2.96 kg extract/4.5 kg malt or 0.658. This number can be multiplied by 100 and expressed as a percentage . . . like 66%.

Without going into the nitty gritty details of the “problem” with this number, I do want to point out that this number is pretty crude. The reason is that not all grains used in brewing have the same potential. In technical circles, brewers talk about laboratory or theoretical yields of different ingredients. Some ingredients like pale malt have a laboratory yield of around 78% and most specialty grains have laboratory yields ranging from 55-65%. This means that a pale beer without special malts has a better efficiency than beers made using special malts.

Since brewers, especially commercial brewers, want to get as much out of the grain as possible, it makes it difficult to examine efficiency. A low yield calculated the way I show above may be due to the type of beer being made or a problem in the brewing process. The solution to this dilemma is to compare the yield of a particular mash to its theoretical yield.

Malt specification sheets give the lab yield number and a theoretical yield can be estimated. If your brew contains 8.8 pounds (4 kg) pale malt with a lab yield of 78% and 1.1 pounds (0.5 kg) of crystal malt with a lab yield of 65%, you can estimate the combined lab yield of these grains using a weighted average. Estimated combined yield = (4 kg pale/4.5 kg total malt x 78%) + (0.5 kg crystal/4.5 kg total malt x 65%) = 76.5%.

This number can then be used as something to gauge the performance of your equipment against. If you got a yield of 66% and the lab yield is 76.5%, you can calculate what is know as the brewhouse yield. In this case, it is 66 dived by 76.5 or 86%. Most homebrewers do not calculate brewhouse yield because malt specification sheets are not always available. I hope I haven’t confused matters too much, but that’s how to run the yield calculations. Happy number crunching!