What Causes Soap To Mold?

Soap mold is a common occurrence caused by excess moisture, improper pH, lack of preservatives, and improper storage conditions. Mold spores are present everywhere in the air and can begin growing when the conditions are right. The main factors that contribute to mold growth in soap include high water content, neutral pH, lack of preservatives, air exposure, and contamination from unclean molds or utensils. Visible signs include white, blue, green, or black fuzz or spots. While mild mold is harmless on skin, it can cause quality and appearance issues. Proper formulation, storing, and handling can help prevent soap from molding.

Ingredients

The main ingredients in handmade soap are oils, butters, and fats combined with an alkali, usually sodium hydroxide (lye). Common oils used in soapmaking include coconut, olive, palm, sunflower, safflower, shea butter, cocoa butter, and various nut oils like almond and jojoba. Each oil contributes different properties to the soap – coconut oil brings fluffy lather, while olive oil provides moisturizing qualities.

Animal fats like tallow and lard are also popular soapmaking oils, valued for their creamy and hardening properties. More exotic oils like mango, avocado, and rice bran add nourishing vitamins and fatty acids. Ultimately there are many oil combinations that can create a great bar of soap.

Water Content

The amount of water in handmade soap plays a significant role in mold growth. Soaps contain both water and oil components. The water is added when mixing the lye solution and provides important functions in soapmaking. However, leftover water content makes the soap vulnerable to mold.

According to the Soap Making Essentials website, most soap recipes call for around 38% water as a percentage of oils [1]. Higher water content produces a softer bar of soap right away. However, excess water also takes longer to evaporate, leaving the soap wet and prone to mold. As the article explains, leftover water allows mold spores an environment to thrive.

To reduce mold growth, soap makers often use a “water discount,” lowering the water amount below the full 38% [2]. This makes a firmer bar but removes excess moisture. Finding the right balance means enough water for saponification without leftover water for molds.

pH Level

The pH level of soap can impact how prone it is to growing mold. Handmade soaps typically have a pH between 10-8, which is alkaline and provides good cleansing ability. However, a high pH above 11 can be too harsh for skin (https://artizsoap.com/soap-ph-indications/).

Studies show that most bar soaps have a pH of 9-10, while shampoos are lower at 6-7 pH (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4171909/). Mold generally grows best in environments between 4-6 pH. Therefore, the alkaline pH of most soaps actually inhibits mold growth.

However, over time on the shelf, some soaps may drop in pH as the alkalinity decreases. This can allow mold to start growing if the pH falls into the 4-6 range. Checking and maintaining proper pH when formulating soaps is important to prevent lowering pH over time.

Preservatives

Preservatives play a critical role in preventing mold growth in soap. Mold needs moisture, food and proper pH levels to thrive, so controlling these factors with preservatives helps stop mold before it starts.

Most artisanal and handmade soaps use natural preservatives like rosemary extract, vitamin E oil, and grapefruit seed extract. These plant-derived ingredients are broad spectrum antimicrobials that inhibit bacterial and fungal growth.

The right preservative and amount is key. Research shows 0.5-1% of soap’s weight in preservative is optimal. Too little, and mold can still grow. Too much can be drying or irritating.

Preservatives that prevent mold in soap include: rosemary oleoresin, vitamin E, neem oil, grapefruit seed extract, benzoic acid, and sodium lactate. When used properly, these natural options can extend a soap’s shelf life dramatically.

Storage

Proper storage is key to preventing soap from developing mold. Ideal storage conditions for bar soap include a cool, dark, and dry environment. According to Kowalski Mountain, soap should be stored in a place with a consistent mild temperature between 60-75°F. Drastic temperature fluctuations can lead to sweating and moisture buildup on the soap. Direct light exposure should also be avoided, as UV rays can accelerate the chemical breakdown of oils in the soap over time. Storing soap in a closed cabinet, box, or opaque soap dish is recommended. Excess humidity is problematic for soap, so storage in a bathroom with frequent steam from showering is not ideal. Bars of soap should be allowed to dry thoroughly between uses. A soap dish with holes for drainage or a wire rack works best. Proper air circulation around the soap is important to prevent mold growth.

Air Exposure

Exposure to oxygen is one of the key factors that allows mold to grow on soap. Mold is a type of fungus that requires oxygen to survive and proliferate. When soap is exposed to air, the oxygen present can initiate and fuel mold growth under the right conditions.

According to experts, one way that air exposure encourages mold growth on soap is by causing soda ash formation. Soda ash (sodium carbonate) develops as a white film on the surface of handmade soaps through a chemical reaction with carbon dioxide in the air (Source: https://angelapalmer.com/why-does-my-soap-have-white-film-on-top/). This soda ash layer provides an ideal environment for mold by increasing surface pH and moisture retention.

Air exposure can also directly deposit mold spores onto the soap over time if storage conditions are damp. Oxygen enables these spores to germinate and grow into hyphae that spread across the soap. Proper drying and curing is important to prevent this, as dampness encourages mold growth even if the soap’s internal moisture content is low.

In summary, air exposure contributes to soap molding primarily by creating a damp, alkaline environment through soda ash formation. Oxygen also allows any deposited mold spores to thrive. Minimizing air exposure helps deter mold growth by limiting these factors.

Contaminants

Mold and other microorganisms need food and moisture to grow. Soap can provide both of these elements. Bacteria, yeasts, and mold spores are present in the air, water, and ingredients used to make soap (1). These contaminants can get into soap in different ways:

• Using tap water that contains bacteria
• Adding fresh botanicals that have mold spores on them
• Exposing the soap to air, which can deposit spores and bacteria on the surface
• Using equipment that is not properly sanitized when making the soap

Once contaminants like bacteria, yeasts, or mold spores get into the soap, they can multiply if the conditions are right. The main organisms that cause soap to mold are species of Aspergillus, Penicillium, Mucor, and Rhizopus. Their spores are very common in the environment (2). If given enough moisture and food source, these spores germinate, grow hyphae, and produce mold colonies.

References:

(1) https://www.lehman.edu/administration/environmental-health-safety/mold.php

(2) https://www.allure.com/story/mold-in-skin-care-products-causes

Visual Signs

Mold can appear on soap in a variety of colors and textures depending on the type of mold. Some common visual signs of mold growth on soap include:

– Greenish or black spots or patches on the surface of the soap. This is often caused by common molds like Aspergillus and Penicillium.

– White fuzz or cotton-like growth, which may be powdery mold such as Geotrichum candidum.

– Blue, gray, or other vibrantly colored fuzz or spots, potentially indicating mold species like Fusarium, Chaetomium, or Aureobasidium.

– Textured areas that look crumbly, fuzzy, slimy, or appear to have tendrils or roots growing just under the surface.

– Deformation of the soap shape or appearance of sunken/shriveled patches as mold eats away at the soap.

– Webbing between different mold growths as the mold spreads across the surface of the soap.

In general, any colorful, fuzzy, or irregular growth on soap that wasn’t originally part of the soap’s design can indicate mold contamination. Examining mold under a microscope can help identify the exact species present.

Prevention

There are several tips you can follow to prevent and remove mold from handmade soap:

• Use 0.5-1% of a natural preservative like rosemary oleoresin or vitamin E in your soap recipe. The antioxidants in natural preservatives inhibit mold growth (Source).
• Avoid adding dried botanicals directly to cold process soap batter. Add them at trace once the soap has started saponifying to allow the water to evaporate during cure (Source).
• Cure soap in a well-ventilated area out of humidity and direct sunlight to allow it to dry thoroughly.
• Wrap cured soap tightly in butcher paper or wax paper to prevent air exposure and moisture.
• Check regularly for any fuzz or discoloration which could indicate mold growth.
• If mold appears, trim off at least 1/4 inch around the affected area and re-wrap the soap.

Following proper soapmaking techniques and storage can help prevent mold growth and keep handmade bars fresh.