Polyunsaturates, saponification and DOS?

[saffy]

Any chemists out there?
Does the level of polyunsaturation (and cis or trans) have an impact on the saponification in terms of speed or completeness?

I got DOS in my 1st batch of CP soap made using 30% sunflower (no super fatting) - SF is the most unsaturated fat I have used.

No sign of them in later batches where I had mixed much more thoroughly - does molecular shape impact the nature of the emulsion and therefore particle size with knock on impact on saponification?

I like the mix with sunflower but don't want to risk loss of batches due to DOS!

 

[DeeAnna]

When all is said and done, you probably have some superfat even in a zero superfat soap, due to impurities in the NaOH (unless you're accounting for the actual analysis of your NaOH or are using reagent grade NaOH) and the use of averaged saponfication values for the oils (unless you're testing the specific oils you are using for the sap value).

I personally am not going to get too lathered up about the issue of specific molecular shape -- there are too many gross variables that are known or suspected triggers for DOS and I don't know of any research about saponification rates vs type of oil. There might be something in the industrial literature, but for hand crafted soap, it doesn't seem to be a real issue.

And regarding droplet size in the emulsion vs. DOS -- if the droplet size in the emulsion was critical, then hand stirred soap should be far more likely to get DOS than stick blended soap. I don't recall that happening with my grandmother's soap or hearing that in the discussions on this forum about soap that has minimal mixing or is hand mixed. I suppose this could be an issue ... but I'm not picking up much about that.

The common wisdom is to limit polyunsaturated fats to about 15% max of the total oil weight to prevent overly soft soap and to minimize the chance of DOS. I also use sunflower and follow that rule of thumb with good results.

 

[saffy]

Polyunsaturates, SAPs, NaOH and DOS

Thanks for the response re polyunsaturates etc – my interest in the CP chemistry is ultimately to give me tighter control over my own soapmaking.


I do account for the impurities in the NAOH and I calculate my own SAPs based on the Codex Alimentarius because I was not happy with the lack of literature support for SAPs used by most online lye calculators – the CA is not perfect but at least it is a source of recognised provenance.


Have even tried some stoichiometric calcs based on fatty acid profiles from various sources just to see how they compared with the CA values and other online calculators – was surprised at how close they were.


Some time ago I was given several CP soaps made with 20% more NaOH than they should have been – even assuming a 0% fat discount – I pH tested them with an analytical grade calibrated pH probe and they were absolutely fine – same pH range as commercial products, no sign of unreacted reagent – I also tested the calibration of the weighing scale which was also fine.



The water used was rain water stored in a large tank under the house – I guess the mineral content could have been high – but high enough to deal with that much supposed excess of NaOH?


Any ideas why this should be? I wondered if the historic SAPs typically used were maybe inappropriate for todays oils - given changes in genetics, farming practises and weather etc - but then I would have expected my stiochiometric calcs to be quite different to published SAPs - but as said above they were very similar.


Does anybody know of any publications comparing historic vs more recent SAPs? Better still has anyone done any SAP determinations of their own and how did they compare?

 

[saffy]

Polyunsaturates, water, trace and DOS

I have a friend who has been making CP soap very successfully for a long time using 33% sunflower oil (the cheapest she can buy) plus olive oil and coconut oil – and no sign of DOS – but she uses a high water content, a professional cooks grade paddle blender and mixes for a long time - I suspect her emulsion is very homogeneous and would love to do some particle sizing!

I have read and found that sunflower oil delays trace – possibly due to its wonky oil molecules? – as does higher water content – has my friend accidentally hit on the ideal physicochemical blend which permits a long mixing time resulting (in her case) in a very small particle emulsion before saponification really gets going – what she calls ‘trace’ would not count for anything to most soapmakers – yet her soaps do their thing without any DOS appearing.


As you say – many gross variables – lots of opportunity for research should anyone have the facilities and time – and I would be very interested to get answers to some of these issues.

 

[DeeAnna]

"...Any ideas why this should be?..."

Given enough time, soap pH will drift downward due to carbonate formation. There are many other possibilities, but not having made the soap myself, it is not productive to speculate.

"...publications comparing historic vs more recent SAPs..."

I suggest you research that yourself. Soap making publications from the late 1800s and early 1900s are readily available in the Internet Archive, Project Gutenberg, Google Books, et. al.

"...Better still has anyone done any SAP determinations of their own and how did they compare?..."

Not that I have read about, and I have not done so myself. It's not that hard, however; I just don't have the interest.

The methods are outlined in the industrial soapmaking references, both modern and old. Kevin Dunn has adapted a laboratory method for the purposes of home soap chemists in his book Scientific Soapmaking. Why not check it out and do SAP testing for yourself?