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.