SAP of used oil?

[Kat Soap]

Hi everyone~
Getting ready to make my second batch of soap. I haven't been able to find the answer to a question of mine anywhere on the forums, so please forgive me if I'm beating a dead horse here, but...
Does the SAP change depending on whether the oil has been used for cooking? For instance, I make some doughnuts, filter the oil, and want to use that oil for soap--can I? I noticed that soapcalc doesn't ask if the oil is new or used, so I'm assuming it doesn't matter, but I've learned from experience what happens when I assume!
Thanks from a newbie!

 

[chandler Trev]

Don't know for certain. Folks who use tallow and meat drippings are using "used" oil. I don't see why short of burning the oil the fatty acids would change. Perhaps if sugar were introduced into the oil the soap might be a bit different. Make a small (8oz) batch from used and another from "fresh" and see what happens. Run it through SoapCalc for the lye/water amount. Anybody else? Keep us posted

 

[Bubbles Galore]

I'm not certain either but I have never read anything that says that the sap value changes. It would depend on what has been cooked too, wouldn't it? Meat with fat that joined the oil would change the sap value. Did you really mean donuts or were you using them as an example?

Has the oil only been used once? To cook what? What temp? I actually wouldn't like to use oil that has been used for high temperature cooking for anything.

 

[chandler Trev]

starts a campfire nicely :shock:

 

[Bubbles Galore]

starts a campfire nicely :shock:

Except that.

 

[carebear]

Assuming you weren't cooking something like large portions of bacon in another fat, the SAP wouldn't change.

This said, heating degrades the oil so it will likely be more DOS prone - plus any contaminants from the food (even if you filter it) or the utensils will add to that likelihood.

Won't hurt anyone, as long as you know there are no cleaning chemicals, etc in it but I don't know that I'd gift any soap made from used oil. but that's just me.

 

[knicelyr]

I use used oil to make biodiesel. I also make soap.

I make sure that my liquid soaps are fully saponified so I pay close attention to the pH and how much lye I am using. I can say that for canola and soybean oil the SAP does not seem to change if the oil has been used for cooking.

When we make biodiesel we test for fatty acid content by titrating the oil. We do this because when the oil is used for cooking some of the oil is broken down into fatty acids, di glycerides and mono glycerides.

What this means is that used oil may contain a high percentage of free fatty acids. These fatty acids easily turn to soaps, that is one reason our glycerin is so good as a soap ingredient -- It contains all the soaps that came from those free fatty acids and all the glycerin from all the oil being used for Biodiesel.

How do the free fatty acids affects us soapers?
- The free fatty acids turn to soap very quickly and that might cause a quicker trace than you are used to. Very similar to adding stearic acid to your soap recipe.

 

[blue hill]

I have no experience , but i follow the blog of a spanish soaper and one of her recipes is for Laundry soap. She says she uses "Used oil" (doesnt specify which kind) and ive wondered whether it makes a difference(guess not). Heres the link, its translated into english just scroll down till you find laundry soap. Good luck and let us know!

http://jabon-soap.blogspot.com/search/label/Recetas de jabón / Soap recipes

 

[carebear]

fine for "industrial" uses IMO - like cleaning stuff

 

[Kat Soap]

I used the oil to fry up some cannoli shells, at about 375 F. The only "fat" used in the shells is also vegetable oil, which minimizes some of the error I could foresee.
I'm going to try this out, but being so new I'm not sure we'll be able to chalk up the failure to the recipe and not the cook.
Thanks for the help!

 

[Bubbles Galore]

Good luck Kat. :wink:

 

[tamarajane]

I'll be interested in results too! ( and of course pics!!!)

 

[Legal Eagle]

G'day Kat;
To answer your question; yes the SAP will be different as when using used cooking oils the FFA content increases over oil that has not been used before.

A simple titration can tell you the level of FFA's in any given oil; this is something we do in biodiesel production all the time, so as to determine the FFA level so we know how much caustic to use in order to neutralise the acid value and hense get full conversion of the esters part of the oil.

If you kown the acid number of your unused oil and then titrate it to determine the FFA level you can then adjust the SAP accordingly.

That or you can hold off a little bit; I have a couple chemists working on a universal SAP calculator, including the use of biodiesel glycerine for soap making. It will work with any acid though, not just the biodiesel glycerine.
When it's done, which should be very soon (couple days or so), I will upload it to my website; http://www.blackcrownsoap.com.

This calculator will also be featured in our new magazine (PDF format) about to be released called Lather! for those making soap from the biodiesel glycerine layer.

I am the author of "Biodiesel Glycerine Soap - The Guide" which is a guide book read in 5 continents on how to make soap from the biodiesel glycerine layer, albeit taking a "purist" position, meaning using the absolutely least amount of chemicals necessary in order to saponify the acid, and we do not use synthetics or artificial colorants or fragrances of any kind.

The 5th edition of The Guide is about to be released also, it is now in proof reading and should be available by Monday Jan 11/10 if interested at the website above. I can also be emailed through there as well should you have any further questions.

Hope this was helpful.

Luc

 

[Kat Soap]

Luc, yes that was helpful. Thanks very much.

 

[ChrissyB]

How did this turn out?
And I just wanted to tell you I love your blog, your sense of humour cracks me up Kat.