soap rancidity DOS
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Soap rancidity

When creating homemade soaps with the cold method we can run into a pretty common problem: soap rancidity. Unfortunately soap rancidity can happen to anyone even if we pay attention to the procedure or formulation. But what exactly is the phenomenon of soap rancidity? Can we avoid it? Can we somehow correct it once it has occurred in our soaps? To these questions, I wanted to give a clear and exhaustive answer to help you in the process of soap making with the cold method. Making soaps at home is not only about mixing ingredients but also being aware of chemical mechanisms that lead to structural changes in our ingredients.

Homemade soaps with the cold method | The chemistry behind it

Homemade soaps using the cold method are the result of a wonderful example of a hobby that combines creativity with the practical application of chemical concepts. I love this hobby precisely because through the chemical knowledge studied over the years, I can give a reason for certain phenomena observed during and after the creation of soaps.
Before looking at the rancidity phenomenon in detail, let’s take a small step back. Creating soaps with the cold (or hot) method means starting a chemical reaction, the so-called saponification reaction.

saponification reaction

If you’re interested in learning more and diving into the chemistry behind soap making, then I suggest you don’t miss my general course on creating homemade soaps using the cold method. In the course, you’ll be guided step by step through the entire cold process of soap making so you can finally create your own soaps according to your needs and with an understanding of the chemistry behind it. Check it out here!

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Alterations in homemade soap with the cold method

Soap is the result of the reaction between fatty acids contained in oils and butters and caustic soda dissolved in water. Their reaction gives origin to soap which chemically is made of the respective salts of fatty acids. These salts and any free fatty acids that give the soap more emollient characteristics, can however undergo structural changes. These alterations are easily recognized by a change in the color of the soap or by the alteration of its smell which will become more rancid. The change in color could be noticed especially in non colored soaps.

Soap rancidity

In this case, it is evident how the soap, by its nature white, shows a more yellowish color with the appearance of yellow orange spots. This phenomenon is known as D.O.S. which means “dreaded orange spots”. Their appearance is accompanied by a change in smell which becomes more pungent.

But why do we observe these alterations?

Homemade soap rancidity with the cold method

Rancidity is a chemical phenomenon also called oxidative rancidity.

All fatty acids can be subjected to this phenomenon with a higher or lower probability according to their chemical structure and the conditions of production and preservation of the oil itself.

Let’s take a closer look at these two points.

Causes of soap rancidity | Production

The first point to be analyzed is the production phase of the oil.

Let’s take olive oil as an example. If olives are harvested late or are subjected to extreme weather conditions such as frost or drought, the quality of the olives could be compromised. The fatty acids contained in the fruit could be subjected to the action of enzymes that would modify the structure thus making the oil obtained of poor quality. This results in a high value of peroxides present in vegetable oil; molecules that if decomposed would be transformed into ketones or aldehydes responsible for the bad smell typical of rancidity.

Causes of soap rancidity | Chemical structure

Vegetable oils are more prone to oxidative rancidity than oils of animal origin.

This depends on the chemical structure of the triglycerides or fatty acids they contain. Specifically, in vegetable oils are present more fatty acids with one or more double bonds, called mono- or polyunsaturated, such as oleic acid. In contrast, saturated fatty acids are found more in oils or butters of animal origin.

course excerpt “Soap Making 101 with cold method”

The process of oxidative rancidity occurs precisely in the proximity of double bonds, making the phenomenon more likely in oils rich in polyunsaturated fatty acids than in oils with a high percentage of saturated oils.

Let’s see why.

Oxidative rancidity | The chemical process

The process of oxidative rancidity can occur even in oils of excellent quality. This can take place because of excessive exposure to air, light or high temperatures.

The oil, in fact, if not kept in an optimal way, could change its structure. The triggering of a radical reaction could lead to the formation of a peroxidic radical just by contact with the oxygen contained in the air. Radical reactions, by their nature, continue to propagate quickly by raising significantly the values of peroxides present in the oil. Their propagation is greatly accelerated by the presence of oxygen, contact with solar radiation and temperature as well as metal ions dispersed in the mixture such as calcium and magnesium ions.

In the process of soap production, these ions can be introduced in the mixture unintentionally. For example, if non demineralized water is used or simply because the caustic soda we are using has impurities.
At the end of the propagation phase, the oxidized fatty acids will break down over time releasing smaller molecules. Those are characterized by a more pungent odor, typical of rancidity.

Phenomenon of rancidity in soap

We apply this newly learned chemical knowledge in the production of soap using the cold method.

In soap making, it is possible that rancidity can easily occur. This is because many more oils of vegetable origin are used than animal oils. The phenomenon could take place to the detriment of free fatty acids as well as newly produced soap molecules. This is because soap could also have double bonds.
Some factors that could trigger oxidative rancidity could be:

  • The use of expired oils
  • Improperly stored oils (in too hot places or in direct contact with light)
  • Use of non demineralized water
  • The use of caustic soda with impurities

If for the first three points we can try to reduce the probability of rancidity only by improving some conditions, for the last point it is not so.

Unfortunately we cannot control the degree of purity of caustic soda so we just have to use it and test its performances.

The phenomenon of soap rancidity and how to prevent it

In order to prevent the phenomenon of rancidity we will have to do some little tricks.

For example:

  1. take care to store our oils or butters in dry and dark places. In addition, we will have to check the expiration date. In this case, we will be sure to use fats in excellent condition and not already in an oxidative phase.
  2. Creating soaps with a high soda discount means having a lot of free fatty acids. Especially those in the surface layer of the soap, can oxidize more easily simply in contact with air. Working in a superfat regime between 2-6% could decrease the appearance of yellow spots (DOS) on soap.
  3. It is important to use only distilled water during the manufacturing process. Distilled water prevents the content of dissolved inorganic salts from catalyzing the rancidity process.
  4. Finally, using oils that contain a good amount of antioxidant substances, such as vitamin E or rosemary extract. Those are excellent natural remedies to prevent the rancidity process.

Can we correct it once it occurs in our soaps?

The rancidity process in soap is an irreversible phenomenon.

This means that once fatty acids are transformed into smaller molecules, there is no turning back. Soap that has yellow oxidative stains (DOS) has free fatty acids or in the worst case, compromised soap molecules inside. This makes the soap’s cleaning activity less effective or even non-existent. If the soap produced is for sale, let’s produce a new batch and avoid selling the rancid soap.

In case the soap is for your personal use then try to understand the state of the process. Is it only superficial or it affects the whole soap? If it is only superficial, you could try to remove the damaged layer and use the rest of the mass. If instead the process is much more serious, there is nothing left but to declare forfeit and produce new soap.

I hope this article has managed to answer your questions and clarify your doubts. For further info and questions ask me!

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Gabi