Here's my take on preservatives, based on a lot of research and Q&A with reps of Dr. Bronners. Research what a preservative is and how it applies to liquid soap. That way you can make the best informed decision on the type you want to use in any product you create. As for preservatives in liquid soap, this is what I know, and is commonly agreed upon, especially by the folks of Dr. Bronners: the natural ph of liquid soap is too high to be conducive to microbial growth. Coupled with maintaining a high soap concentration of 30-40% soap, plus maintaining GMP- good manufacturing practices (sanitation of tools and work area), you should have no issue with 'nasties' taking over. If you're reusing containers, sanitize them, don't just clean them. Sanitize after you rinse them at the tap and store them with the santizer left in them. 70 or 91% alcohol is perfect for this as it will evaporate once the container is opened and you shake excess out. If you still choose to use a preservative, you have to make sure that it is effective in our pH range. As of now, there are only 2 preservatives available that remain effective in high ph environments; Succtocide and Opthiphen MIT Plus. Here is a current list of available preservatives, their application use plus ph restrictions. This list can be found again on the Liquid Soapmakers FB group in the files section.
https://drive.google.com/file/d/0B3e5_Ew1Rt6fdkJmckUtaldObG8/edit?usp=sharing
Here is a link to the group, if the above link does not work. Here you will find more info.
https://www.facebook.com/groups/liquid.soapmakers/763495690347394/
The incident that happened with Lindy was a first time for her. All it proved is that crap happens, things fail. Even preservatives can fail. Does it mean that method, or preservative is bad? No it doesn't. It just means that for that instance, something overtook. Could it happen again? Possibly. If the incident reoccurs frequently, then it's time to look into a new method, be it to start using a preservative, or to change preservation type. If it's once in a blue moon, chock it up as a failed batch, takes notes and move on.
Do i use preservatives? Nope. I haven't in the 2 years I've been liquid soaping and I have a 1 year old batch of diluted Castile soap chilling out in my closet right now with no problems thus far. Granted, i can't see what is happening on a microscopic level, however, IF something were going on, the soap would be toast right now. Plain and simple. Bronners also gives their soaps a MINIMUM shelf life of 3 years, if that tells you anything, since they don't use a broad spectrum chemical preservative either.
