Updated: Nov 10, 2019
I have had many people ask me the questions “If I add milk to my soap, won’t it cause it to spoil?” and “How do I know that there won’t be bacteria and mold in my soap if I use yogurt at the end?”. These are both excellent questions! When I first began experimenting in soap making, I had the exact same concerns.
A minimum cure time of 3-6 weeks in a well-ventilated and dry area is necessary to produce a harder and more structurally stable bar of hot process soap. (Learn more about curing hot process soaps in this article). Dairy products must be kept cold in a refrigerator or freezer and used before an expiration date. Signs that dairy products have spoiled include a bad smell, discoloration, curdling, bitter taste, and an increase in bacteria and mold growth.
So, if dairy products must be used before their expiration date and kept cold in order to prevent spoiling, how is it that we can use milk and yogurt in our soaps that cure for weeks and sit on shelves without refrigeration? To answer this question, it is important to take a deeper look into what exactly causes dairy products to spoil, the process we use to make our soap and the final chemical composition of the bar.
Milk spoils when bacteria convert lactose into glucose and galactose, which results in the production of lactic acid. Lactic acid produces casein and then forms a curd that can quickly cause the rest of the contents to curdle within 24 hours. The bacteria in milk include Micrococci, Bacilli, Staphyloccoci, Lactobacilli, and Pseudomonas. The most common bacteria in yogurt is Lactobacillus bulgaricus. Yogurt is different than milk because it already contains a small amount of lactic acid; however, this does not increase the rate of spoiling as it does in milk because of the how the yogurt is processed and its overall pH.
The most important factor that prevents spoilage in soaps made with dairy products is low water activity or low water availability. Water activity refers to the total amount of available water that can be used by microbes for growth and reproduction. In order for water to be available, it must not be chemically-bound or must be free water. All microbes require water for growth and reproduction; without it, they become inactive and unable to cause microbial rancidity or spread pathogenic diseases. In bar soap, a large concentration of the water evaporates during the curing period and the final resulting bar has an average water concentration of 10-15% of the total bar weight. The water that remains is chemically bound to the hydrogen and glycerin within the bar. Without available water, microbes simply aren’t able to grow or reproduce.
Another factor that helps prevent microbial growth in soap is the alkaline pH. The average pH of milk is between 6.4 and 6.8 and the average pH of yogurt is 4.4. In hot process soap, the saponification reaction rate is increased by heat and mechanical mixing so that the lye solution and fatty acids are neutralized during the process resulting in a recipe that is pH safe after the cook.
In The Ultimate Guide to Hot Process Soap, we learned that the pH of finished soap is between 8.5-10.0. Most bacteria can’t survive in pH environments that are not neutral and can only reproduce in a pH of 6.5-7.0, although there are certainly some bacteria that thrive in environments like this, one of the many reasons why a preservative is recommended for liquid soap. The average survivability of the most common bacteria in milk is between 6.1-7.2. The highest pH survivability of lactobacillus bulgaricus (yogurt) is between 6.7-7.1. Because the pH of soap is between 8.0-10.0, the alkaline environment does not support the life or reproduction of the most common bacteria found in dairy products. This means that any bacteria in the dairy product that may have caused spoiling will not be able to survive in your soap recipe. The alkaline environment of the soap also means that most bacteria will not be able to survive on your soap in between uses and in storage.
The alkaline pH prevents the growth and reproduction of most bacteria, but there is also another mechanism that fights bacteria and mold reproduction during hot process soap making: temperature. Most bacteria, including those in milk and yogurt, can’t reproduce over 102F and can’t survive over 120F. The temperature of LTHP averages between 130F-160F, maybe even higher depending on your preference and heating container. The temperature of HTFHP begins at 215F and the yogurt is added at temperatures between 160F-180F. In both LTHP and HTFHP the temperature is too high to support the life of bacteria found in dairy products.
Now that we have covered bacteria, what about mold? Most molds can’t survive at temperatures over 140F or at pH levels that are over 7.3.. Most organisms on the planet can't survive over pH levels of 8 or temperatures over 100F (although there are some, including bacteria groups called alkaphiles and halophiles). Again, in LTHP and HTFHP, the pH and temperature are not conducive to survivability or reproduction of mold. These three factors- low water availability, alkalinity, and high production temperatures- prevent the growth and reproduction of microbes in our soap. The combination of the three to prevent microbial growth is called hurdle technology, or the use of multiple applications to better preserve our products.
If you are planning on using milk, yogurt, or other dairy products in your soap, make sure that you thoroughly mix them into your recipe either before, or after the cook. If you create an informed recipe and follow safe soaping practices, you can rest easy that the same smell and appearance of spoiled dairy, won’t happen to your beautiful soaps. I absolutely love to use yogurt and fresh goat milk in my recipes. In HTFHP, I have found that the addition of a small amount of yogurt can create a much more fluid consistency and use it regularly in many of my recipes.
Looking for a goat milk HTFHP recipe? Check out the newly added Creamy Goat Milk HTFHP Recipe in the Student Recipe Book! For more information about soap science, recipe formulating, hot process soap and fluid hot process soap, be sure to get your copy of The Ultimate Guide to Hot Process Soap today by visiting our bookstore now!
Do you like to use dairy in your soaps? Have a favorite recipe? Share your thoughts below!