I’m approaching the topic of spoilage and preservatives with a little trepidation as many things can be argued. It has been an interest and study of mine for five plus years so perhaps I can shed a little informed light on the subject.
Shaving soap has a ph range of 10-12 and this is a very difficult environment for bacteria, mold and yeast to start and thrive.
Formulated properly a soft shaving soap is no more susceptible to spoilage than a hard soap. By ‘formulating properly’ I mean the percentage use of the alkaline solution to achieve the soft consistency of the shaving soap. An example of formulating improperly would be using water or other liquid(s) to soften the product.
So, now let’s take a look at what we know…
The greatest risk of any sort of spoilage of shaving soap is rancidity which is caused by an excess of oils/fats/butters added to the soap for moisturizing properties. Care must be taken to use oils/fats/butters low in linoleic acid which have a short shelf life.
Suttocide A (Sodium Hydroxymethylglycinate) is the best and arguably only broad spectrum preservative available that is effective against both gram positive and negative bacteria in cosmetic products with a high ph range of 10-12 such as shaving soap. Suttocide A is derived from the naturally occurring amino acid Glycine. Is it safe? Probably yes at the low concentration of use, .5% of total batch weight. However you can read both pros and cons as with most anything with the greatest concern being that it might break down into formaldehyde. The real problem from a practical standpoint is that the efficacy of this and most other preservatives lies in the fact that we are almost always adding water to our shaving soaps/creams when using them. This drastically lowers the effectiveness of most any preservative and can render them useless.
The real benefit of using a preservative in a shaving soap/cream is long term storage BEFORE opening. This helps a vendor with many soaps/creams in stock sleep better at night without wondering if they are sitting on a stockpile of product spoiling on their shelves.
I have considered using Suttocide A as a broad spectrum preservative in our shaving soaps but so far I have not found it necessary. We have only had one report of spoilage (to us) in our (CRSW) shaving soaps. I will venture to say other cases are most likely due to improper handling and or storage conditions. As an example, a single whisker introduced into a jar of shaving soap can be the cause of a bacterial growth.
I have and continue to test Suttocide A extensively and have not had any skin, eye or mucus membrane irritation in test batches. I also have run controlled scenarios using extreme humidity and other mold/bacteria causing situations and find both preserved and non preserved soaps hold up equally well in these stress test.
Unless I see data otherwise in a large sample size I have determined that for our (CRSW) shaving soaps, a broad spectrum preservative is not needed.
I can't speak for other artisan or commercial shaving soap manufacturers but as for me personally these are the steps I take to insure as best I can that the risk of spoilage with CRSW is low.
1) Clean, sanitized manufacture environment
2) Low liquid content in shaving soap
3) Careful selection of moisturizing ingredients (low in linoleic acid)
4) Use shea butter, and cocoa butter that retard rancidity as moisturizers
5) Use glycerin and in some products lanolin as a natural moisturizer
6) Use Toceprheral (Vitamin E) to help retard rancidity in free oils
7) Make products in small batches and do not wholesale for full control
8) Always suggest proper storage of products
My rule of thumb and general suggestions are to use shaving soap/cream within a year of purchase, allow product to dry before storage and store in a cool, dark, dry place.
To summarize, it is my opinion that in general, using a non-preserved artisan or commercial shaving soap poses no greater risk of spoilage and possibly less risk health wise than a shaving soap prepared with preservatives.