Being frugal and making friends with beekeepers have benefits for the experimental meadmaker, and occasionally I get to play with crystallized comb or rinsings from extruded frames. But because the sugar concentration of washed honey is unknown, the density (specific gravity) of diluted honey has to be checked. Today we enjoy access to the wonderful glass hydrometer, but hydrometers have a habit of rolling off the counter when brew day rolls around. In the absence of a hydrometer, brewers in the past often used an egg. Really.
How the Floating Egg Test Works
The floating egg technique works thanks to the internal anatomy of an egg, which includes an air sac at the rounded end for the bird embryo to breathe. A fresh egg has a relatively small air sac, but egg shells are slightly porous, and the air sac grows as the contents of the egg slowly evaporate and dry out. A very fresh egg sinks in water, but an old egg with a large air sac will bob up and float. This float test is still used in modern times to test whether or not an egg is fit to eat before cracking it.
Specific gravity is a dimensionless quantity that compares the density of a substance to that of water. By definition, the specific gravity of water is 1. Anything with a specific gravity greater than 1, like wort or must, is denser than water, and anything with a specific gravity less than 1, like ethanol (alcohol), is less dense than water. When salts or sugars (like honey) are dissolved into water, the extra particles change the density of the solution.
A fresh egg has a density between 1.03-1.1 g/mL, which means it will float in a solution that has a density greater than or equal to 1.03-1.1 g/mL.1 (The density of pure water is an even 1.0 g/mL.) Thus, an egg placed in must or wort having a specific gravity greater than approximately 1.030 to 1.100 will float. The denser the liquid, the higher the egg will bob in the solution.
Must Density Matters
Throughout most of history, every last bit of honey would have been used, not just the honey that’s easy to extract. The centrifugal honey extruder is a modern convenience that allows for high yield with minimal processing, but before this useful invention, honey would have been extracted by hand, first by leaking and breaking up the combs, and then by washing the broken combs in warm water to dissolve the remaining and any crystallized honey.
The resulting mixture of honey and water would be of unknown strength and would have needed to be checked before brewing: insufficient fermentable sugars could result in an easily spoiled brew, and too much sugar can inhibit yeast growth and stall fermentation. Master brewers likely could eyeball or taste and have a perfect brew each time, but the less experienced benefited from a visual aide like a floating egg. By the end of the 17th century, a hen’s egg was specified for this purpose.2
Historic brewing recipes are uncertain as to whether the must should be heated or not, and they often recommend testing the strength before boiling,3 apparently not realizing this evaporates water and increases density. The brewers are also not quite able to make up their mind if the must should be cold, blood-warm, or boiling, which could indicate they did not understand how temperature affects specific gravity either.
The 1597 Dutch beekeeping manual Van de Byen by Theodorus Clutius says, “and let it cook / until an Egg can float in the liquid / then set it off the fire,” which could have yielded a nicely boiled egg if it were not removed quickly enough. A 1616 Danish cookbook–the oldest such known–advises one to “put an egg or two into this lukewarm brew so that there is a part of egg as big as a 2 shilling over the water then it is sweet and fat enough,” which probably is the most accurate measurement.
Several 17th-century brewing recipes associated coins, especially twopence and the groat, with the floating egg. The diameter of the coin would be used as a size measurement of the diameter of the bit of shell sticking above the water surface, and this measurement averages a ratio of 3:1 to 4:1 of water to honey dilution,4 which are appropriate ratios for mead must.
How to Float an Egg
The diameter of an ideal groat is approximately 23-24 mm, and the diameter of a US quarter is 24.26 mm (0.955 inches), which makes a quarter the perfect stand-in for the old groat.
The size of the air sac changes as an egg ages–interfering with the results of our density test–so it is very important to use a fresh egg that has not yet had time to evaporate. It is also important to check the supposedly fresh egg, as eggs sold in the supermarket are not always as fresh as you might assume. To do this, calibrate the egg in plain water before every density test to make sure it sinks flat to the bottom, with both butt and tip level (see figure below).
Differences in breed, health, age, and diet can also cause the size and shape of the egg to differ quite significantly. For the best results, compare several eggs and pick out the most average one.
The table below correlates egg position–that portion of the egg visible above the surface of the liquid–with specific gravity, giving us an idea of what to aim for. Egg readings are given for ale yeasts with alcohol tolerances of 10% and 12% by volume.5
|Mead Style||Starting Gravity (10% ABV tolerant yeast)||Diameter of Egg Visible above Surface||Starting Gravity (12% ABV tolerant yeast)||Diameter of Egg Visible above Surface|
|Medium||1.095||18 mm||1.110||26 mm|
|Sweet||1.100||20 mm||1.120||30 mm|
|Dessert||1.100+||>20 mm||1.120+||>30 mm|
To make sure there is enough sugar for the yeast to feed on, the egg should float. But if it starts to tip over and not reliably point up anymore, the solution has become too strong, with too much honey sugar for the yeast to properly work, and fermentation will likely stall. The average range of 1.08 to 1.12 g/mL at which the average round, fresh-laid egg floats and points upward is also the ideal range of sugar content for starting a successful brew using standard yeast.6
So, after all this, where do you start? Start with a fresh egg no more than two days old, of the roundest kind, weighing less than or about 2 ounces (57 g). Calibrate the egg in room-temperature water to make sure it sinks flat. Make your honey must, heat and evaporate as needed, let cool down to blood temperature, and add your egg.
If the egg sinks, the must is too weak. If it floats close to tipping or tips, the must is too strong. If it is too weak, add more honey, stir well, and try again. If it’s too strong, add a little water, stir well, and try again. As you can imagine, it is easier to start with too strong a solution and dilute it, than to start with a weak solution and try to incrementally dissolve more honey. If your must does need some strengthening, make a strong honey or sugar solution first, and incrementally add that to your must, not straight up undiluted honey.
It has been my experience that if the egg tip touches but does not break the surface of the must, it will ferment to a nice, dry wine-like mead. If you prefer something sweeter, aim to have the egg break the surface and show an area about the size of a quarter, or a bit less. If at first you don’t feel at ease trusting your fresh-laid egg, use a modern hydrometer the first few times, in conjunction with your egg, to get the hang of this technique. Before you know it you’ll reach for an egg, conveniently located in your kitchen.
As a homesteader and medieval reenactor, Susan Verberg enjoys researching alternative ingredients and old fashioned techniques. Verberg primarily brews mead and keeps bees when possible, but also occasionally dabbles in historic–especially herbed–beers. For more on Verberg’s mead brewing, check out her collection of medieval recipes and techniques.
-  http://homesteadlaboratory.blogspot.com/2014/02/historical-lye-making-part-2.html
-  Digby, Kenelme. The Closet of the Eminently Learned Sir Kenelme Digby Knight Opened, 1669
- 2005 https://www.gutenberg.org/ebooks/16441
-  Digby, To Make Metheglin
-  To Bear An Egge, Making mead with medieval hydrometers. SCUM 16, p.21-28.
-  Sibly, Belinda. The Egg Test for Period Brewers and Mead Makers, 2004, p.20-29.
-  Sibley