Rancidity eventually results when fatty acids and superfat in soap oxidize (break down) from exposure to heat, light, water, bacteria, or various contaminants. As this oxidation process progresses, smelly chemicals called ketones and aldehydes eventually form. These chemicals are what create the distinctive odor of rancidity.
Rancidity in soap is not a rare issue. Anyone who loftily claims they have never had rancid soap and gets "judgy" toward soapers who have had rancid soap will most likely have to eat their words someday. Rancidity happens to nearly all soap makers at all levels of experience, competence, and ability. It happens to the largest of commercial soap manufacturers and to the smallest of hobby soap makers.
Rancid soap, whether liquid or bar, will smell "off" or musty. Rancid bars may have a wet-feeling, slick surface. The bar may be entirely orange or rust colored or may have just scattered rusty orange blotches and spots. These spots are often called "DOS" or "Dreaded Orange Spots" by soapers.
Ingredients that slow rancidity
There are two groups of additives that are used to retard rancidity in soap --
Chelators. Metallic contamination is the single most important trigger for rancidity in soap. It is nearly impossible to eliminate all metal contamination, no matter how careful you are. It comes from the machinery needed for harvesting and processing natural ingredients, tap water, your utensils, dust and pollution from the air, contamination on your hands, ingredients that naturally contain metallic compounds, and so on. Adding a chelator will immobilize these unavoidable trace metals and prevent them from triggering rancidity. Two common chelators used in small scale soap making are EDTA and citrate.
Antioxidants. Antioxidants are chemicals that oxidize more easily than fats and fatty acids, so they "sacrifice" themselves by oxidizing first so the fats can remain intact. For best results, I recommend adding antioxidants to fats right after purchase, rather than later when you make soap. Fats with unusually short shelf lives (hemp, grapeseed, macadamia, etc.) will especially benefit from this (as well as from refrigeration). An antioxidant can be added to any fat, however. For example, I make a point to add rosemary oleoresin (ROE) to my home-rendered lard and tallow. Another antioxidant used by small scale soap makers is BHT.
Which one would I use, if I could use only one?
If I had to use either a chelator or an antioxidant for soap, I would definitely choose the chelator. My personal experience and my studies of soap chemistry agree -- for most soaps most of the time, trace metals cause rancidity to develop quickly. Those orange freckles of DOS that are the soapers' bane are proof -- each freckle is centered on a tiny, often invisible spot of contamination.
Both additives are more effective than just a chelator, however. When an entire bar turns color and smells stinky, this overall rancidity is most likely not from trace metallic contamination. An antioxidant will be more effective at controlling this type of rancidity. According to experiments described by Kevin Dunn in his book Scientific Soapmaking, EDTA pairs well with the antioxidant rosemary oleoresin (ROE). Citrate is effective when combined with the antioxidant BHT.
Ingredients that accelerate rancidity
Certain ingredients may make soap more prone to becoming rancid. If you have one batch of soap that gets DOS but other similar batches are fine, evaluate what might be the cause. Here are a couple of examples from my and others' experience --
Fats high in free fatty acids (FFAs). Fats gradually and naturally are prone to break down in a process called oxidation. Fats that are older or fats stored under less-than-idea conditions will oxidize more quickly than fats stored in cool, dark conditions. As fats break down, they release free fatty acids which react very quickly with alkali.
Fats high in unsaturated fatty acids. Poly-unsaturated fats are more prone to oxidize than fats that are saturated or fats that are mono-unsaturated.
To reduce the chance of rancidity, minimize the percentage of polyunsaturated fat in your recipes. A general rule of thumb is to keep the total amount of linoleic and linolenic acid at or below 15%. There is not a strong scientific basis for this rule of thumb, but it does seem to work. Another way people control this is to monitor the INS and iodine numbers for their recipes.
Oxidized essential oils (EOs). Lavender EO that has oxidized due to age can definitely trigger overall rancidity. I suspect oxidized mint EO can also do likewise. To prevent this problem, refrigerate EOs if you can, and use EOs within a reasonable time. Use the last drops of older EOs in potpourri, not in soap.
Ingredients high in copper. Powdered spirulina, a seaweed, is sometimes added to soap to achive a green color. One soaper shared photos of a severe case of rancidity in soap that he had colored deep green with spirulina. Spirulina is high in copper (5), and copper is a known trigger for soap rancidity. Other foods high in copper include spinach and swiss chard.
Do you have other examples to add to this list? Contact DeeAnna with your suggestions....
Protect against rancidity with proper handling and storage
Chelators and antioxidants can only do so much to protect against rancidity. Preventing unnecessary contamination is also a wise idea. Kenna from Modern Soapmaking evaluated 25 different bars of soap one year after they were made. She noted many of the bars had developed some DOS, and she drew this conclusion --
"...One of the things that this round of testing reinforced is the importance of using distilled water in soapmaking, keeping soap in its packaging until use, and not exposing [the soap] to metal...." (1)
Here are some suggestions for storing and handling soap and fats. These ideas are based on my studies about rancidity in fats and soap and on my and others' personal experience --
Make soap with distilled, deionized, or reverse osmosis water. Avoid tap, spring, or drinking water, because they may contain trace metals that can trigger rancidity.
Wash silicone and other "no line" soap molds carefully to eliminate residues
Keep soap covered or packaged whenever practical to protect against dust
Use freshly washed hands or wear gloves when handling soap to minimize contaminants from your skin
Use plastic equipment as much as possible when making soap. If you must use metal utensils (such as a stick blender), choose stainless steel.
Avoid equipment made from copper or brass, bronze, and other copper-containing alloys. Copper is a surefire trigger for rancidity.
Never cure or store soap directly touching any type of metal. If you must use metal shelving or screens, cover it with cloth or plastic to separate the soap from the metal. A layer of waxed or parchment paper is not enough -- metal ions can migrate through thin, permeable materials like these.
Store fats in plastic or glass containers, not metal.
Exclude oxygen as much as possible from containers of fats and oils for long term storage. Ideas --
Compress the sides of a plastic container to squeeze out excess air before sealing tightly.
Pour small amounts of fat into smaller containers.
Fill the air space with an inert gas, such as nitrogen. Look for aerosol sprays sold for preserving wine.
Store fats and soap in a cool, dark environment to reduce exposure to light and heat. Fats can also be refrigerated or frozen.
Dealing with rancidity when it happens
There is no way to fix rancidity after it develops -- the best cure is prevention. When the inevitable happens, here are some ways to deal with rancid soap or fat --
The best tests for rancidity is your nose followed by your eyes. Soap will often smell slightly musty or "off" well before it develops spots or turns orange. Some people say rancid soap smells like old crayons.
If bar soap develops a few rancid spots (DOS) here and there, the spots can be cut out and the remaining soap kept for your personal use.
In case someone is wondering -- No, don't even think about selling it or giving rancid soap away.
And to answer another common question -- No, rebatching the soap is not a cure for rancidity.
Rancidity on bar soap is not exactly "contagious," but contamination that may trigger rancidity can be spread from bar to bar. It is best to store and handle rancid bars separately from good bars. This will prevent any surface contamination from being transferred.
If soap becomes rancid over most or all of the bar, it should be discarded. I doubt it will hurt anyone to bathe with it, but the rancid odor is unpleasant and the odor can linger on the skin.
If liquid soap smells rancid, it is best to discard the soap. Unlike bar soap, you cannot cut out the bad spots.
If fat becomes rancid, either discard it or wash it. Do not soap with smelly fat. Rancid-smelling fat makes rancid smelling soap.
Rancid fat can be washed with hot, salty water to remove the smelly ketones and aldehydes. Based on my tests, I cannot promise this washing will remove absolutely every hint of rancid odor, but it is worth a try if you are interested. Here are two methods --
Method 1. "...Wash your oil with a brine made of 1 part salt to 10 parts of 180-degree water. You should have equal parts of water to oil. Stir vigorously until it looks cloudy, then allow it to separate. Pour off the oil with a gravy separator and throw away the brine....." (2,4)
Method 2. Make a 10% brine solution (1 part salt to 9 parts water). Mix one part of this brine to about three parts oil. Heat to to 160F / 70C. Stir the mixture at 60 revolutions per minute for 1 hour, let the mixture sit quietly for 6 hours, and then separate the brine from the fat. (3)
This washing step can be repeated several times if needed. If the odor is still objectionable after two or three washings, the fat is probably not salvageable.
Never discard fat (nor soap batter for that matter) down the drain. Put it in the regular trash or find an alternate use for it.
1. Kenna. Updated: Lather Lovers Additive Tests, One Year Later! Modern Soapmaking. Undated version viewed 26 September 2020. https://www.modernsoapmaking.com/updated-lather-lovers-additive-testing/
2. Heide Braley. How to use rancid oil for soap. HomeSteady. Version dated 21 July 2017. https://homesteady.com/12363410/how-to-use-rancid-oil-for-soap
3. A. Y. Girgis. Production of high quality castile soap from high rancid olive oil. Grasas y Aceites, 54:3 (2003), pg 226-233.
4. Grayceworks. My procedure for salting out. Soap Making Forum. Post #15 dated 11 December 2013. https://www.soapmakingforum.com/threads/my-procedure-for-salting-out.32717/#post-381830
5. Self Nutrition Data. Seaweed, spirulina, dried. https://nutritiondata.self.com/facts/vegetables-and-vegetable-products/2765/2
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