There is a longstanding debate in handcrafted soap making circles whether one should add a preservative to liquid soap to inhibit the growth of bacteria and fungi.
Most of the liquid soap makers I know of, including myself, do not add preservative to undiluted soap paste.
There is much less agreement about whether to add preservative to diluted liquid soap. Many soapers do not. Many do, including myself.
Of the ones who do not use preservative in liquid soap for personal use, some say they would add preservative if they sold the soap.
What preservative to use?
To further complicate this matter, very few of the many preservatives on the market are effective in the high pH of lye-based soap. I currently know of just three preservatives -- Suttocide A, Glydant Plus, and Liquid Germall Plus (LGP), although using LGP in liquid soap is an off-label use. (1)
What's my take on whether or not to add a preservative to diluted liquid soap?
Disclaimer -- I'm not a microbiologist. I do not use preservative in my undiluted soap paste, but I do use preservative in diluted liquid soap.
Have you ever had mold grow on jam? Even in the refrigerator and even with plenty of sugar in the jam for preservation, mold can and does grow, particularly around the edges of the jam where oxygen, light, and environmental conditions are more favorable for microbes to grow.
Microbes can and do grow on the surface of a soap bar, but they are regularly washed off by normal use. If the soap can dry between uses, that also discourages microbial growth. A soap bar is also essentially solid, which is a physical barrier that discourages microbes from penetrating into the soap structure.
With a liquid soap in a container, microbial contamination is not removed by normal use. The fluid nature of liquid soap also allows microbes to migrate around inside the container. These aspects encourage microbial growth. Microbial growth often starts in the thin film of product at the edges of the container, on the lid, and on or in the dip tube and pump mechanism. It can grow within the main body of the product as well if conditions are right.
The more dilute the soap is, the less the soap can act as its own preservative. Diluted liquid soap is very roughly 20% to 40% pure soap by weight. Bar soap (and liquid soap paste) is very roughly 70% to 90% pure soap by weight. That is a big difference.
By "pure soap," I mean the weights of the ingredients that actually turn into soap. That would be the fat plus the the NaOH or KOH. "Pure soap" does not include ingredients that do not turn into soap. This includes water, fragrance, color, and other additives.
What other practices can help liquid soap to stay sanitary?
A preservative cannot protect against all contamination. A broad spectrum preservative is insurance against the fungal and bacterial stragglers that sneak their way into the product and its container throughout the lifetime of the product.
Use good sanitation when making and packaging liquid soap. Research "best manufacturing practices" to learn more.
Choose packaging that reduces the chance of contamination by the user. For example, use pump bottles or squeeze bottles rather than open jars.
How to know if a product is well preserved?
Know your preservative system is suitable for the type of product you're making. Many preservatives are ineffective in the high pH of soap.
Learn how to manufacture and package in a sanitary environment -- those best manufacturing practices I mentioned earlier.
Do informal challenge testing on your own.
Send the finished product to a challenge-testing lab for an expert evaluation of your product.
What do I do personally?
I don't sell products that need preservation due to the greater responsibility, liability, and cost for ensuring consumer safety. For my own personal use, I --
Use a preservative in my diluted liquid soap (and lotion) per the manufacturer's recommendations
Use sanitary methods of production to ensure the packaged product starts out as clean as possible
Make small batches so I can use the product in a relatively short time
Refrigerate unused product to further lengthen its shelf life
Watch the product carefully for any changes in appearance, texture, and odor.
References
1. Barber, Jane. Preservatives: All you need to know! http://www.makingskincare.com/preservatives/
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