Whole eggs, egg yolks, and especially egg whites have been added to soap for at least 100 years.
Egg white is the "secret" in the venerable Swedish soap Lanolin-Agg-Tval. (1) These tiny (50 gram, 1.75 ounce) bars have long been touted by their maker as the Swedish secret for a lovely complexion.
In 1908, authors W.H. Simmons and H.A. Appleton were less enthusiastic:
"...The use of albumen [egg white]... in soap has been persistently advocated in this country [the United States] during the past few years. The claims attributed to albumen are, that it neutralises free alkali, causes the soap to yield a more copious lather, and helps to bind it more closely... Experiments made by the authors did not appear to justify any enthusiasm on the subject, and the use of albumen for soap-making in this country appears to be very slight, however popular it may be on the Continent...." --Simmonds and Appleton, Handbook of Soap Manufacture, 1908.
Egg yolk supposedly adds richness or thickness to the lather, possibly due to the small amount of lecithin in the yolk. Lecithin is an emulsifier and surfactant that can bind oil, water, and egg into the creamy emulsion we call mayonnaise. It seems reasonable to think egg yolk might affect the lathering properties of soap, although in my experience the effect is small.
Some soapmakers add pure lecithin at 1-3 teaspoons (roughly 5-15 grams) per 16 ounces (500 grams) of fats to slow how fast their soap batter reaches trace. (4) There is only about 1 gram of lecithin per egg yolk, however, so yolks will probably not be very helpful if your main goal is to slow trace.
So far, I have used only whole chicken eggs in my soap, but you can use just yolks or whites as you please and you can use eggs from other poultry. The egg soap batches I have made seem to saponify and harden in the mold about the same as non-egg batches. I do not see any big differences in the hardness, lather, and other qualities of the finished, cured soap.
Like Simmonds and Appleton, I do not think egg is a miracle additive, but it sure is fun to include and adds label appeal.
How much to use
Note: All of the following information is based on chicken eggs. If you want to use other poultry eggs, check the USDA FoodData Central database (2) for information about the fat and water in the eggs you want to use.
I suggest 1 whole egg or 1-2 egg yolks or 1 egg white per pound (or per 500 grams) of fats. Rounding to whole grams, egg contains the following water, fat, and protein amounts --
1 large whole egg, 50 g total, supplies 5 grams fat, 38 g water, and 7 g protein
1 large egg yolk, 17 g total, supplies 5 g fat, 9 g of water, and 3 g protein
1 large egg white, 34 g total, supplies no fat, 29 g water, and 4 g protein (2)
Adjust for the water in egg
Egg yolk will add 4% to 8% additional water to a batch of soap if you add 1 yolk per pound or per 500 grams of fats (9 grams water per yolk).
Whole egg or egg white will add considerably more water. One whole egg per pound or per 500 grams of fats will supply 15% to 30% of the total water needed to make the soap (38 grams water per whole egg). The white alone will add almost that much (29 grams water per egg white).
To account for this added water, use your usual lye concentration (or water:lye ratio) to calculate the total water for the soap batch.
Next, calculate the water added by the egg.
Finally, subtract the water in the egg from the total water for the soap batch to get the additional water needed.
Here is an example --
My latest egg soap recipe has a total of 1600 grams of fat
Total water = 510 grams based on a 33% lye concentration (2.0 water:lye ratio)
I added 3 whole eggs to the batch, so the water added by the egg = 3 x 38 = 114 grams
Additional water needed = 510 - 114 = 396 grams
Adjust for the fat in egg
A whole egg or egg yolk per pound (or 500 grams) of fats adds only a small amount of fat -- about 1% -- to the recipe. Whether you ignore this added fat or not is up to you.
If you want to include the egg fat in the soap recipe calculations, here is one way to do that --
The saponification value of egg fat is about the same as for canola oil -- about 0.133 -- so use canola oil as a stand-in for the egg fat.
Since each egg yolk contains 5 grams of egg fat, enter 5 grams of canola oil for every egg yolk you will be adding. This will trick the calculator into calculating the correct weight of NaOH for your recipe.
IMPORTANT -- Be sure to enter only the weight of egg fat, not the total weight of egg.
Canola oil does not have the same fatty acid profile as egg fat, so the fatty acid profile of your recipe will be slightly off. The fatty acids in egg fat are roughly comparable to emu oil or rice bran oil.
Prevent curdling and minimize nasty odors and weird colors
Egg is a good source of protein and the trace nutrients sulfur and iron. Curdling, odors, and discoloration can happen when these chemicals are heated or become alkaline.
Curdling happens when egg proteins clump together and form objectionable, firm particles. Ammonia odor can occur when egg protein breaks down and releases ammonia gas. Hydrogen sulfide (rotten egg odor) and green discoloration can occur when the sulfur and iron in egg react. (5)
Prevent curdling and reduce the chance of odor and discoloration by following these tips --
Start with room temperature or lukewarm ingredients
Dilute egg with other liquids (3) before adding lye solution or heat
Let the soap warm up gradually during saponification
Cold process (CP) soap making works fine, even if the soap eventually gets warm enough to gel. If you use egg whites, a white CP soap should remain white. In my experience yolks or whole eggs will tint white soap to a pastel yellow. I have never seen any greenish color change in my egg soap batches. I do smell an ammonia or rotten egg odor right after I mix the lye into the egg and fats, but the smell does not last long.
I have not personally tried hot process (HP), so I cannot say how that works. I have seen a photo of one HP soap that contained egg yolk. This soap was a light tan rather than yellow, indicating the yolk color may have darkened during the HP cook.
Practical steps for adding eggs to soap
Make sure all ingredients are at room temperature to lukewarm (below 105 F or 40 C).
Crack the egg(s) into a small bowl. If you want just the yolks or whites in your soap, separate the egg and reserve the unwanted part for another use.
Stick blend the egg until smooth. There is no need to pick out the chalaza (the white ropy bit on one side of the yolk) or any other membranes. The stick blender will buzz these parts into smoothness.
Check that the fats are cool enough.
Pour the blended egg into the fats. You can pour the egg through a strainer to catch any small bits the stick blender missed.
Stick blend a few seconds to bring the egg and fat to a consistent temperature. The mixture will begin to separate after you stop mixing; this is normal.
Make sure the lye solution is cool enough.
Add the lye to the fat and egg mixture. Stick blend or whisk the batter for a second or two immediately after adding the lye to ensure the egg, fat, and lye are evenly blended.
Right after adding the lye, the batter may darken and there may be an ammonia or "rotten eggs" odor for a short time. These changes are typical.
Make the soap as normal and allow it to saponify.
I do not insulate the mold nor add extra heat (CPOP, heating pad, etc.) I only lightly cover the mold to help the surface of the soap stay a bit warmer and keep the dust off.
Even without insulation or extra heat, all my batches of egg soap have gelled. Your results may differ from mine.
More info about eggs in soap
DeeAnna Weed. Farmhouse Soap with Whole Egg and Lanolin. Youtube video. https://youtu.be/PWzA1ffHT7Y
Watson, Anne L. Egg Soap (general information and recipes). http://www.annelwatson.com/soapmaking/creative/april/eggsoap.html
Nymeria. Egg Yolk Soap (recipe and tutorial). Craftster.org as archived on the Wayback Machine. https://web.archive.org/web/20191205050858/https://www.craftster.org/forum/index.php?topic=291241.0
1. Lanolin-Egg Soap. Victoria Soap. http://www.victoriasoap.com/products/lanolin-agg-tval/ Viewed 1 March 2021.
2. USDA Agricultural Research Service. FoodData Central database. 16 December 2019. https://ndb.nal.usda.gov/ Viewed 1 March 2021.
Egg yolk: https://ndb.nal.usda.gov/fdc-app.html#/food-details/748236/nutrients
Egg white: https://ndb.nal.usda.gov/fdc-app.html#/food-details/747997/nutrients
Egg whole: https://ndb.nal.usda.gov/fdc-app.html#/food-details/748967/nutrients
3. Cooks Illustrated. Do You Need to Temper Eggs? We wanted to know if tempering really is all about temperature. Undated. https://www.cooksillustrated.com/how_tos/10576-do-you-need-to-temper-eggs Viewed 1 March 2021.
5. Brunning, Andy. The chemistry of eggs and egg shells. Compound Interest. 16 March 2016. https://www.compoundchem.com/2016/03/26/eggs/ Viewed 1 March 2021.
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