Updated: Aug 27, 2019
Excerpt from the book The Ultimate Guide to Liquid Soap
www.ultimateliquidsoap.com One of the most commonly asked questions that I see and receive about liquid soap making is “Do I need to add a preservative?" If you have wondered about this question yourself or have tried to do your own research about the subject, you will find many varied answers. Different soap making blogs and resources that are held to as the “standard” for quality soap making information also have mixed answers; some believe a preservative is not necessary, while others believe a preservative should be used in every batch. If you ask the question “Do I need a preservative in liquid soap?” in a soap making forum or in a soap making Facebook group, you will typically find these four answers (although they have been made shorter… and politer/pleasant): · “I won’t use anything with chemicals in my soap because it doesn’t need it.” · “No, it has too high of a pH so nothing will survive.”
· “Sometimes, if it has lots of ingredients, or if you use a lot of water.”
· “Yes, every time.”
With such varied answers, it becomes difficult for the new liquid soap maker to make an informed decision about whether or not to include a preservative in their soap. When the question is asked in a group, the discussion becomes very heated, very quickly, with two very different sides.
“To preserve… or not to preserve? That is the question. Whether tis nobler in the mind to suffer (about the answer to this question) or to take arms against a sea of (Facebook users), and by opposing end them?” Ok, so maybe it’s not Shakespeare-level dramatic, but it certainly can be a confusing and loaded question. Let’s take a more in-depth look into the world of preservatives so that you can make your own informed decision- to use, or not use.
What is a preservative?
A preservative is a chemical additive that prevents the growth of microorganisms. In cosmetics, this serves two purposes:
1. A preservative prevents the spread of disease (by preventing microbial growth and reproduction)
2. A preservative prevents microbial rancidity and spoilage of the product (by preventing microbial growth and reproduction)
What is a microbe?
A microbe is an organism that is too small to be seen with the naked eye. Microbes are living entities of microscopic (prefix micro-very small and suffix scopic-observable) size and include bacteria, viruses, fungi, algae, and protozoans. Microbes are incredibly adaptive organisms and exist everywhere on the planet. There is thought to be more than 1 trillion different species on the planet, with 99.9% of them unidentified. They are present in and on humans, animals, and plants. They can live in soil, water, and the air. They can also survive in volatile conditions like extreme cold, heat, acidity, alkalinity and even outside of the earth’s atmosphere. Microbes can be free-living in the environment and find or make their own food, or they can be dependent on other organisms for their living. They can be classified as commensals (existing with others), symbionts (surviving with another organism) or pathogens (consuming host and spreading disease).
(Fun Fact: There is thought to be over 5,000,000,000,000,000,000,000,000,000,000 bacteria living on the planet at this very minute. This is five million trillion trillion or 5 x 10 to the 30th power.)
In the previous chapters, we talked about oxidative rancidity and hydrolytic rancidity, but microbes can cause a new form of rancidity- microbial rancidity. Microbial rancidity refers to a process in which microorganisms use their enzymes, like lipases, to break down fats into fatty acids, which results in oxidation. Signs of microbial rancidity are like that of oxidative or hydrolytic rancidity, which includes foul odors and discoloration, often called “spoilage.” For example, “spoiled milk” can due to a type of microbial rancidity or a lotion or body butter that develops mold. It should be noted that for microbial rancidity to occur, microbes must be able to survive in their environment.
Microbial Water Availability
Microbes, like all organisms, require water to survive. Not all microbes use water in the same way, and water can be utilized in different ways such as a solvent, temperature buffer, metabolite and/or as a living environment. A virus is a microscopic pathogen that consists primarily of proteins, DNA and RNA that is not capable of surviving without a host, but even viruses require water for survival. Without "interstitial" water, a virus would not be viable, and it can only exist in a wet external environment. Even viruses that can be transmitted through the air have to exist inside of tiny droplet of water. Without water, microbes, including the most adaptive and pathogenic microbes like many types of viruses, cannot survive.
For most microbes, water is required to be able to grow and reproduce. Microbes like bacteria, mold, and yeast take up water by moving it across their cell membrane in a process called osmosis. Osmosis occurs when molecules pass through a membrane from a less concentrated solution to a more concentrated solution, creating an equal balance of solution on both sides of the membrane. A simple example of osmosis would be hydrating a piece of dried fruit. If you placed a raisin in a glass of water, you could visibly see the signs of osmosis as the water passes through the fruit’s membranes, and the raisin becomes plump with water and returns to its near original round shape. The movement of water from the microbe’s environment to the inside of the microbe’s cell depends on a water activity (aw) gradient. Water activity refers to water that is not chemically bound, often called free-water or unbound water. Free-water is water that is not saturated and is available for use by microbes.
For water to move into the organism’s cell through osmosis, there must be a high-water activity environment outside the cell in order to balance the lower water activity environment within the cell. When the water outside the cell becomes too low, because there isn’t enough available water, it causes what is called osmotic stress. Osmotic stress is when the cell can’t take in any more water, because it’s not available, which causes it to become dormant. The microorganisms are not eliminated; they simply become unable to grow. Microorganisms that are unable to grow, are unable to cause problems.
Let’s look at a common example of osmotic stress. Sugar is an excellent source of nutrition for many microbes and provides the necessary nutrients that many microbes need to grow and reproduce. Honey is comprised of both sugar and water. If something provides microbes with both food and nutrition, it must be the perfect environment for them to live in, right? Unfortunately for the microbe, the water in honey is chemically bound to the sugar and there is no water available. This causes osmotic stress and prevents the microbes from growing. This is the reason why if we put bacteria on the honey, we would not see any growth or reproduction. Microbes that are not able to grow and reproduce are not able to cause unwanted changes to our liquid soap and are not able to cause infection. Different organisms cope with osmotic stress in different ways. That’s why there are different growth limits for each organism. Some types of molds and yeasts have adapted to withstand very low water activity levels. View the table below for the average water activity levels of most microbes.
Water Activity in Soap
Bar soaps are comprised of a mixture of solid and liquid soap crystals. Bar soaps must be cured after production, which allows water to evaporate from the bar and undergo chemical changes that make the bar harder through crystallization. The water evaporates until it reaches a point of equilibrium with the water in the environment around it. After the soap has cured, although a large percentage of the water evaporated, there is still an average of 12-18% water that remains. This is because the water is chemically bound to the glycerin and hydrogen within the bar. A report released by Decagon Devices Inc. showed that bar soap made with glycerin (the natural glycerin was not extracted) had a water activity of 0.659-0.759. As you can see from the water activity chart on the previous page, there is not enough available water for bacterial growth, although there is enough water for some select species of osmophilic yeasts.
Liquid soap is different than bar soap because it exists in a phase of soap dissolved in water. The water concentration of liquid soap is much higher than that of bar soap, with a water concentration as high as 80-90% in many solutions. By increasing the available water (water that is not chemically bound and can be used by microbes), we increase the water activity and open ourselves up to bacteria who thrive in a water activity of 0.87-0.99. Bacteria are of special concern because they can be pathogenic or disease-causing. Common bacteria that grow in these levels of water activity include Clostridium botulinum (salmonella), Escherichia coli (E. coli) and Pseudomonas, which can cause skin and eye infections, toxic shock, strep throat, and even food poisoning if transmitted by hand.
Jackie Thompson, the author of the book Liquid Soapmaking, had her liquid soaps sent to a lab and tested for water activity. Her test results were as follows: soap paste- 0.866, liquid soap made with a 65% water and 35% soap concentration- 0.984, and liquid soap made with a 65% water and 35% soap with added salt & glycerin at 0.941. From her test results, the only sample that was near the water activity needed to no longer support bacterial growth was her anhydrate (not hydrated) soap paste. We can see from her test results how water that is chemically bound in soap, like the water in soap paste, reduces the total available water for microbial growth. Furthermore, the addition of salt and glycerin also reduce the total available water for microbial growth. Although we don’t have the exact concentration of salt and glycerin that was used in Jackie’s tests, we can see that salt and glycerin both can act as a natural preservative because they bind to the water in the recipe and reduce the available water for microbial growth (it should be noted that although the concentration of salt and glycerin reduced the water availability, it was not high enough reduce the aw below the limit for bacterial growth). With the results from her tests and the information that we have available to us regarding water activity and water availability, we can see that the amount of water in liquid soap provides a suitable environment for bacteria to thrive in. Based on this information, liquid soap provides a water activity level that is conducive to the growth of microbes, including pathogenic bacteria.
The total water available for microbial growth is one aspect of liquid soap making, but another factor that affects microbial growth is the pH. Water activity and pH are two of the most important intrinsic factors that determine if a product supports the growth and reproduction of pathogenic and spoilage microorganisms. We already know by now that pH is a measure of the degree of acidity or alkalinity of a solution. We know that acidic solutions have a pH value between 0-7, alkaline values have a pH between 7-14, and a solution is neutral if it has a pH of 7.
Microorganisms have limits to the pH level that they can grow in, just like they have limits with water availability. Almost all microorganisms prefer a neutral pH for optimum growth. Most microbes stop growing at an acidic pH of 5.0. and an alkaline pH of 8.5. From this, we can see that most microbes will not be able to continue growing in our liquid soap, which has an alkalic pH level of 9-10.5. Although most bacteria are not capable of growing in an alkaline environment, there are groups of bacteria that can survive in highly alkaline surroundings called alkaphiles and other bacteria that can survive in highly alkaline salt environments called halophiles. Vibrio parahaemolyticus can grow in a pH up to 11 and causes an intestinal infection. Vibrio vulnificus which causes cholerae can grow in a pH up to a 10.5. Many of the microorganisms that can survive in highly alkaline environments above a pH of 8 are from very specific environments like the ocean and near volcanoes, and although contamination is not likely, it is still possible. (The last reported case of cholerae in the United States was in 2015 and there was only one report). View the chart on the below for some common pathogenic microorganisms and their pH growth levels.
The effects of water activity and pH can be combined through hurdle technology to control microorganisms more effectively. In the case of water activity and pH, the combined effect of both hurdles is greater than the effects each hurdle alone. This means you can have effective microbial control at levels that would typically be considered unsafe for either pH or for water activity independently.
How does a preservative work?
Preservatives are effective at preventing growth and reproduction of microbes by producing a biochemical disruption that can cause dormancy or death. This includes disrupting pH, breaking of the cell walls, cellular leaking and more. After a preservative has worked to destroy the microbe, there is no longer any microbe-fighting power left. This is the reason why it is very important to start with a clean and sterilized working environment, including disinfected and clean equipment, properly stored and preserved ingredients, the use of distilled water, and protective packaging (tests have shown that pump tops are more effective at preventing microbial growth than open caps). During the bottling process, it is imperative to use a clean and sterile packaging environment, in addition to the use of sterilized bottling materials, in order to prevent the risk of microbial product contamination.
The preservative used should be broad-spectrum, which means that it prevents the growth of multiple classes of microbes. It should prevent the growth of bacteria, mold, yeast, and fungi. It must also be active in an alkaline soap solution, be dermatologically tested and shown to be non-irritating at recommended levels, and it should be affordable. Most preservatives available for purchase are called synergistic preservatives, which include combinations of preservatives intended to eliminate multiple contaminants that we could see in our products.
Liquid Soap Preservatives
There are quite a few different preservatives that can be used, and you can do your research on which one you prefer, but two of the most common preservatives used in liquid soap making include Liquid Germall Plus & Suttocide. Liquid Germall Plus is comprised of Propylene Glycol, Diazolidinyl Urea, and Iodopropynyl Butylcarbamate. Suttocide is comprised of hydroxymethylglycinate, a type of formaldehyde-releaser that works by slowly releasing formaldehyde into the product and kills a broad spectrum of microbes.
Natural preservative options are formulated with a plant-derived compound called caprylyl. Preservative Cap-5 contains Caprylyl Glycol, Phenoxyethanol and Hexylene Glycol and is paraben-free, formaldehyde-releaser-free & vegan. Two other natural soap preservative options with caprylyl glycol include Microkill and Lincoserve HpH-2. Euxyl K 900 is a broad spectrum organic-compliant preservative, made with benzyl alcohol, ethylhexylglyceria and tocopherol that is suitable in pH levels up to 12.
We can improve the efficacy of our preservatives by including additives like alcohols, humectants, salts, and sugars, all which lower the water availability and provide additional beneficial properties to our soap. Additives like sodium chloride, cane sugar, vegetable glycerin, and sodium lactate chemically bind to water and reduce the total amount of water available for microbes to grow in. Using a chelating agent like sodium citrate or EDTA will also increase the potency of the preservative.
We know that there is a possibility of microbial growth in liquid soap and if contaminated, it could spread an infectious disease or cause microbial rancidity in our products. After a product is sold, you have no control over it. You can’t control who handles the product, how it is used, how it is stored, when it’s used (could be five years later), or any other aspect of the product. These things are all up to the end user.
We learned in the science of soap chapter that all things which exist in the world and take up space are made up of chemicals, and that nothing can be chemical-free if it comprised of matter. All chemicals can be harmful if you are exposed to a high enough level. Even the chemicals that are necessary for our survival can cause harm. If you are exposed to too much water, it could be fatal (this is called drowning!). We know that preservatives are designed to kill harmful microbes, this is what makes them effective. Unfortunately, this also makes them potentially hazardous because the chemicals are not able to discriminate between a good and a bad cell. Many people fear preservatives specifically “because they are a chemical” and have the potential to cause harm, but the reality is- pathogenic bacteria is “all-natural," not made from chemicals and can and will cause harm, including illness, disease, and death.
Thankfully, there have not been very many documented cases of bacterial outbreaks due to contaminated liquid soap, but there have been a few and their effects were devastating. It has also been researched and proven that the use of contaminated soap spreads bacteria. When a contaminated soap is used, it not only spreads bacteria to the surface of the skin, but also to other surfaces that are touched afterward.
(Above are sample images from a controlled study to determine the number of bacteria from contaminated hands transferred to an agar surface before (A and C) and after (B and D) hand washing with bacteria contaminated soap from American Society for Microbiology study called “Bacterial Hand Contamination and Transfer after Use of Contaminated Bulk-Soap-Refillable Dispensers”)
For those who make liquid soap for at-home and personal use, you can control all aspects of the pre- and post-soap making process. You can ensure that the soap is used in a timely fashion, stored in a cool and dry location, and used in a clean and responsible manner. I make small batches of liquid soap for personal use and formulate my recipes with a high concentration of water-binding additives like glycerin, salt and sodium lactate. Unless I include additives that provide additional microbial nutrition like milk or beer, I do not use a preservative in my small-batch soaps made for personal use at a lower water concentration and I feel comfortable making this informed decision.
If I was selling my liquid soap, as a responsible seller, I would want to reduce any possible risk of contamination. Although the likelihood that your liquid soap will be contaminated is minimal, and many soap makers elect not to use a preservative, including some large companies, I personally feel it is my responsibly as a seller to protect not only my family and my business, but also my product, my customer, their families and the people in their lives.
There are both safe and unsafe levels of synthetic chemicals, and it is important to weigh the risks and benefits of their use. It is up to the individual soap maker to use the information provided by UG2LS and other resources regarding microbial growth and preservatives to make an informed decision about including, or not including, a preservative in his or her product.
Be sure to get your copy of The Ultimate Guide to Liquid Soap today by visiting our Bookstore!