Teresa, I have used 1% and 3% EDTA in a batch of soap. The mixture of EDTA with NaOH solution turns into a thick pasty gravy-like stuff that's worse the more EDTA you use. It's not obvious if you put the EDTA solution into the fats instead and then add the lye, but that's probably just disguising the problem rather than preventing it. In any case, the finished soap with higher EDTA seems okay as far as I can tell. I've since went back to 0.5% since I don't see the higher rates doing any more than the 0.5% as far as reducing soap scum. And certainly you could use much less if you only want to guard against rancidity.
Soapmakers in the 1800s and early 1900s were generally of the opinion that superfat made the soap more liable to rancidity. They preferred the boiled method of soapmaking for that reason, because one can positively ensure there is no superfat in the soap with the boiled method, unlike the cold process method. I have to say, unlike hobby soap makers like us, these fellows probably had the experience to know what they were talking about. In addition to minimizing superfat, some older manuals suggested making no more soap than could be sold reasonably quickly, not only for economic reasons but also because some types of soap deteriorated in storage due to rancidity, shrinkage and warpage due to high water content, and the effects of some of the fillers used in the cheaper types of soap.
Before about 1920, as far as I can tell, soapmakers did not have effective chelators or antioxidants to use. Modern commercial soaps also have a low superfat, but they also often contain chelators and sometimes antioxidants.
