Polyunsaturated Fats: Oxidation and Rancidity in Oil and Soap

Updated: Aug 11, 2019


The Ultimate Guide to Hot Process Soap- Polyunsaturated Fats- Oxidation and Rancidity in Oil and Soap

The Ultimate Guide to Hot Process Soap goes into great detail about the chemical composition of fats and oils. In the book, you will learn that fats and oils are made up of triglycerides, which are comprised of one glycerol molecule and three fatty acids (“tri”-three). The fatty acids are made up of chains of carbon atoms bonded to other carbon atoms and can be classified as saturated or unsaturated. Saturated fats are made up of carbon chains with single carbon bonds (C-C-C-C-) and unsaturated fats are made up of carbon chains that have both single and double carbon bonds (C-C-C=C-C).


Saturated fatty acids include lauric, stearic, myristic and palmitic. These fatty acids lack double bonds and do not have any "kinks" in their chemical structure. Unsaturated fatty acids include oleic, linoleic, linolenic and ricinoleic. These fatty acids have at least one or more double bonds, which create a “kink” or bend in their chemical structure. A monounsaturated fat is an unsaturated fat that only has one double bond, and the rest are single carbon bonds (“mono” is the suffix meaning “single” or “one”). A polyunsaturated fat is an unsaturated fat with two or more double bonds (“poly” meaning “multiple”).

Review the image below of the saturated and unsaturated fatty acids used in soap making. Notice the bends or kinks in the structure? Each "=" represents a double carbon bond. The saturated fats don’t have any double bonds, but oleic has one, linoleic has two and linolenic has three.


The Ultimate Guide to Hot Process Soap- Oxidation and Rancidity in Soap & Oils

Every double bond creates a weak spot in the chain and acts as a sort of "corner" that increases the likelihood of “attack” by other molecules it encounters. Imagine that the fatty acid is a rope and that each of the double carbon bonds is a weak spot. The more double bonds a fat has, the more kinks and weak spots it has. The more weak spots, the more susceptible the fat is to an “attack” or "damage."


If an oxygen molecule comes along and bumps into the double carbon bond, it “attacks” the carbon to carbon (C=C) bond and creates an oxygen-carbon bond in a process called oxidation. The chemicals that are most likely to attack are atoms, molecules or ions with an unpaired electron, called free radicals. The most common free radicals we need to worry about include O2 (oxygen, but can be found alone in O form), H2O2 (hydrogen peroxide), and OH- (hydroxide). Water is also comprised of oxygen (H2O), and if it encounters a double carbon bond, it too will insert itself into the carbon-carbon bond in a process called hydration. Oxidation caused by this process is called hydrolytic rancidity. Notice the common denominator? The free radicals that cause oxidation or oxidative rancidity all contain oxygen, which is where the name oxidation comes from.

Now that we know more about double bonds and their role in oxidation, let’s take another look at the unsaturated fatty acids used in soap making. Review the image from the previous page and examine each of the fatty acids, noticing their double bonds. Oleic and Ricinoleic acid both only have one double carbon bond, or one "weak spot." Linoleic has two double carbon bonds or two "weak spots" and linolenic has three. We can use this information to predict which oils will be more susceptible to oxidation and rancidity by reviewing the individual oil’s fatty acid compositions from the provided chart in the previous pages, by reviewing the oils in a soap calculator or by requesting information from your oil supplier.

Oils that are high in linolenic and linoleic polyunsaturated fatty acids will often have a reduced shelf life and are more prone to rancidity and DOS (dreaded orange spots) than other oils. Oils that are primarily comprised of monounsaturated fatty acids have less risk, but they still have a single double carbon bond that increases the risk of oxidation more than oils and fats made up of primarily saturated fatty acids. If you use a high concentration of polyunsaturated fatty acids in your recipe, it will have a higher risk of complications associated with rancidity. If you purchase large quantities of oils that are high in polyunsaturated fats or purchase smaller quantities and do not use them within a specified amount of time, they too will exhibit rancidity and may turn different shades of orange and/or develop an unpleasant taste and/or odor.



Just because an oil or fat is made up of unsaturated fatty acids does not mean that it will automatically create a recipe that will experience signs of rancidity. If an oil is liquid at room temperature, it will be largely comprised of unsaturated fats. Even fats that are solid at room temperature like palm or coconut oil have unsaturated fatty acids. This variation in fatty acids is nature's design and part of organic chemistry, there is no way around this. Instead of avoidance, we should learn how to work with it, instead of against it.



Unsaturated fats offer a lot of benefits to your recipe, including lowering the CMC of (critical micelle concentration, talked about more later) of soaps made with lauric acid for an amazing lather, and adding moisturizing and conditioning properties to the soap. Another benefit of oils that are high in unsaturated fats is that they are often very inexpensive, vegan, environmentally responsible and readily available. Affordable oil options include peanut oil, sunflower oil, grapeseed oil, olive oil, and soybean oil. These are also readily available in your local grocery stores, which can help you reduce product costs and eliminate shipping fees and waiting periods. Oils with unsaturated fats can (and should) be important in your recipe formulations and soap making process. I highly recommend using them when creating an informed soap recipe, and I personally use a balanced blend of unsaturated fatty acids from soft oils in every one of my recipes.

When formulating your recipe and deciding which oils to use, keep in mind that oils high in unsaturated and conditioning fatty acids like oleic and ricinoleic are less susceptible to oxidation than their counterparts linoleic and linolenic. This may mean that you formulate recipes with oils that are higher in these monounsaturated fats because they can be used in higher concentrations with less risk of rancidity.


So now that we have a little more information about oxidation and polyunsaturated fatty acids, how can we determine how much of them to include when formulating a recipe? How much is "too much" polyunsaturated fats and is there anything we can do to prevent rancidity?



There are lots of different ways to use mono- and unsaturated fats in your recipe and plenty of ways to handle the increased risk of oxidation. Some of your options include:


· Use a lower concentration of 15-20% polyunsaturated fats per recipe

· Use a moderate superfat with recipes that are high in polyunsaturated fatty acids. Unsaturated fats are slower to saponify and are more likely to remain as free fatty acids which increases the risk of oxidation. A moderate superfat ranges between 3-5% (we will go into more detail about superfatting in a later chapter)

· In hot process soap making, you can use higher concentrations of unsaturated fats during the cook (CSF) so that they saponify and then add a low oxidation risk oil or butter as your post cook superfat (PCSF). For example, you can use a low superfat during the cook (0-1%) to be sure that the included unsaturated fatty acids are saponified and then after the cook, you can add conditioning properties by using a more stable PCSF, one that will be more resistant to oxidation like cocoa butter, coconut oil or even olive oil.

· Chelating agents can reduce oxidation (and increase lather in hard water). More about that in this article.

· Use oils that have been processed to include more monounsaturated fatty acids. Examples include high-oleic sunflower oil, high-oleic canola oil and high-oleic safflower oil. Each of these examples have a significantly reduced polyunsaturated fatty acid content and have a high oleic acid content which increases the shelf-life and reduces the risk of oxidation.

· Minimize additives that will increase oxidation.

· Always use distilled water (Excellent article about this here)

· Add an antioxidant to your oils to prolong the shelf-life. May also mix in your PCSF. Antioxidants are molecules that fight oxidation by giving electrons to the free radicals and neutralizing them. Antioxidants include Vitamin E, Rosemary Oleoresin Extract (ROE) and Grapefruit Seed Extract (GSE). One of the most combinations that have been studied to reduce DOS is a combination of 1% Butylated Hydroxytoluene (BHT) and 1% sodium citrate.

· Store your oils and finished product at a cool temperature. We learned that temperature increases the rate of reaction in soap making, but this also applies to all other chemical reactions. Oxidation occurs faster at a higher temperature and slower at a lower temperature. A refrigerator is an excellent storage location for oils.

· Store your oils and finished product in a location out of direct light. Photooxidation and direct light increase the reaction rate. Storing these products in a dark location like a basement, closet or refrigerator will slow the rate of oxidation.



Use the information learned from this article and the resources available to you in The Ultimate Guide to Hot Process Soap to make an informed decision regarding the selection of your oils and additives used in your recipe. Take advantage of the soapy science available to you and make beautiful, creative recipes that are less prone to oxidation and signs of rancidity.



Happy Soaping!