[Soap Maker]

When making soap using certain polyunsaturated fats, why does the resulting soap oxidize (become rancid)? 

I've been making soap for about 10 years and have found that soaps made with some fats and oils become rancid (we call it DOS--Dreaded Orange Spots.)  The time frame varies, but the soap gets orange or brown spots and sometimes smells bad.

Having taken high school and college chemistry over 20 years ago, I'm pulling at straws. 

At first I thought perhaps the saturated fats do not lead to oxidation, because the hydrogen atoms were somehow making the resulting soap more stable, less likely to oxidize.  BUT, soap made with 100% olive oil (polyunsaturated fat) rarely oxidizes.  Soaps made with soybean oil and corn oil, oxides quickly.

Any help would be appreciated.

Thank you.

[Borek]

Double bond present in unsaturated fats reacts relatively easily with atmospheric oxygen.

[Soap Maker]

Thank you for your reply.  However, I believe I need to clarify my question.

When making soap with olive oil, an unsaturated oil, the resulting soap does not spoil, become rancid, oxidize.  However, all other soaps I've made using unsaturated fats and oils become oxidized.  Olive oil seems to be behaving like a saturated fat or oil.  I'm trying to find out why.

I'm also curious about the saponification process.  When using a mixture of fats and oil, some saturated, some unsaturated, does the hydrolysis first, or more readily, take place between the NaOH and saturated fats?  I'm wondering if the NaOH has a preference and will break the single chemical bonds of the saturated fatty acid chains before choosing to break the double chemical bonds of the unsaturated fatty acid chains.  Not that the preference would be compared to choosing a mate, but comparable to ionic attraction.

If the NaOH is attracted to the single bonds of the saturated fats, will these fats be saponified first or more extensively, leaving unsaponified unsaturated fats in the resulting soap?

[Jd1828]

Not sure about the different oils going rancid.  I'd have to look into that more.  The saponification will have no preference between the different fats.  In theory you should be able to control the saponification by adjusting the temperature and concentration.  In practice it's is more of a force the reaction to completion using huge excess amounts of base situation.

[Soap Maker]

In small scale soap making, we generally want excess fats or oils to remain after the saponification process is complete.  When formulating I always use a lye solution that will not saponify all the fats/oils.  This is due to several factors.  First, I don't have true saponification values for the fats and oils I use.  I don't test my fats/oils and most suppliers do not have that information.  An average saponification value is used for each.  Therefore, I err on the side of caution and use less NaOH than I believe will saponify all the fats/oils.  As a soap maker, I do not want anyone who uses my soap to be burned by excess lye.  Secondly, having unsaponified oils results in better skin feel after using the soap.  I formulate to leave approximately 8% to 10% of the oils unsaponified.

The saponification will have no preference between the different fats.

Really?  The double chemical bonds found in the unsaturated oils, compared to the single chemical bonds of the saturated oils don't make a difference in hydrolysis? 

[billnotgatez]

How do you test for the chance you might have lye still in the soap in your home operation?

Is the length of the carbon chains related to your rancid issue?

[Soap Maker]

There are a couple of ways to test... dissolve some of the soap in water and test pH.  Or as some soap makers do, er, touch the soap to the tongue.  If you get zapped like touching your tongue to a 9V battery, there is lye present in the soap.  I use the former method.

I'm not sure about the answer to your second question.  I'll have to do more research.

[macman104]

The saponification will have no preference between the different fats.

Really?  The double chemical bonds found in the unsaturated oils, compared to the single chemical bonds of the saturated oils don't make a difference in hydrolysis?  

The ester functionality of the oils is the reactive piece in this process, and the unsaturated or saturated portion is not going to change during the hydrolysis.  So, if the double bond (in an unsaturated oil) is far removed from the ester functionality (which in most fats I believe it is) I don't think it will impact the reactivity of the esters.  So saturated or not, I imagine they will have roughly the same rate constant.

As for the olive oil, it looks like it contains some phenolic compounds as well that could be retarding the oxidation/spoiling process.

[Soap Maker]

Thank you for your reply.  I love that this forum doesn't mind answering questions from novices.

[ooosh]

There are often some polyphenols in crude olive oil,which act as anti oxygen reagent to protect the soap to be oxidized.So,I think you can add some anti oxygen reagent when you use other polyunsaturated fats,for example BHT(Butylated hydroxytoluene or  butylhydroxytoluene).You can buy it as a food addition agent.Actually,BHT often contained in some oils we eat or some petroleum products to protect them to be oxidized.