Whenever a product contains water or will come in contact with water, a broad spectrum preservative is needed. Examples, using hydrosols or floral water, aloe vera, and scrub used with wet fingers
Even for anhydrous (no water) products, if water may be introduced to the product or the product will be used in a humid bathroom, then a preservative is advisable.
If skincare products are not well-preserved, the potential of microbial contamination will be introduced, causing nasty infections. Natural products will not stay fresh for long and are more susceptible to microbial growth. It is hard to find “all-natural” products as the application of natural preservatives are often discouraged due to their loss of activity in dilutions, pH-dependency, volatility and inadequate/undesirable for some products.
Even plant extracts and essential oils, i.e. tea tree essential oil, grapefruit seed extract, rosemary extract, and vitamin E, are added to cosmetics are well-recognised for their antioxidants and antimicrobial properties, but they are NOT PRESERVATIVES. These antioxidants slow down the rancidity, mainly for oils and butters, do not prevent bacteria, yeast, or mould from spoiling the products. The case of Herbivore’s mouldy face cream is a good example of the inadequacy of using the preservative system.
It is not just about adding preservatives to the products, and there are other factors that will also affect the effectiveness of the preservatives.
The selection of the primary packaging of your product presents a significant influence on the potential for microbial contamination. For example, jars and bottles are more likely to cause microbial contamination, and you will introduce microorganisms into your product every time you put your hand in it (no matter how clean your hands are!). Airless pumps are less accessible to contamination.
2. Water Activity
Water is the most ideal growth factor for microorganisms. Reducing the amount of infiltrating water in your formulation will help to inhibit microbial growth. To solve this problem, certain substances can reduce the water activity such as salts, polyols (sorbitol, glycerol, ethoxydiglycol, etc.), protein hydrolysates, amino acids, and hydrocolloids (xanthan gum, guar gum, etc.), glyceryl polyacrylate gel, sodium polyacrylate, and sodium chloride.
3. pH Level
The optimum pH for microorganism’s growth in cosmetic products is between 5 to 8, meaning that any pH outside this range induces unfavorable conditions, thus decreasing their growth rate. The acidic pH of products containing salicylic acid and aluminum compounds such as antiperspirants (pH ranging from 3.5 to 4.5) and liquid soaps having an alkaline pH (pH 9.5 to 10.5) can inhibit the growth of microorganisms. Generally speaking, microorganisms cannot proliferate or survive in a cosmetic formulation with a pH of less than 4 or greater than 10. However, products of such pH are not suitable for the skin on daily use for long term.
4. Multifunctional Ingredients (also known as Preservative Boosters)
Each ingredient is added to the cosmetic formulation for a well-defined function, but it can contribute to another effect such as antimicrobial activity, thus acting as a multifunctional ingredient. Chemical composition of a cosmetic formulation such as surfactants, additives, etc., can cause some preservatives to be inactivated due to the antagonistic effect of such cosmetic ingredient.
The influence of some cosmetic constituents on preservation: –
Multifunctional ingredients are known for their self-preservation, can be used in combination of traditional preservatives to enhance the antimicrobial efficacy of the preservatives. Chelating agents, surfactants, humectants, and phenolic compounds are examples of multifunctional ingredients.
Chelating agents (e.g., EDTA ‘ethylenediaminetetraacetic acid’, GLDA ‘glutamic acid, lactic acid, citric acid, and phytic acid) block the iron required for metabolism and microbial growth, thus enhance the antimicrobial efficacy of the used preservatives.
The use of humectants, such as glycerin, sorbitol, and xylitol, at sufficient levels, increases the strength of the formula. However, using a high concentration of glycerin will also increase the stickiness of the product. Other glycols, such as propanediol, caprylyl glycol, pentylene glycol may also be used but can affect viscosity, stability and affect skin feel.
Surfactants with antimicrobial properties are the 1,2-diols (from butanediol to octanediol, mainly caprylyl glycol) exhibit viscosity modulation properties that complement their antimicrobial properties.
Phenolic compounds such as propylic gallate, caffeic acid, coumaric acid, ferulic acid, citric acid, and tartaric acid have also demonstrated antimicrobial activity.
5. Solubility of the preservatives
To preserve an anhydrous product, oil soluble preservative will get locked in the oils so it will not reach any water. If water was introduced into the product, the preservative will stay in the oils and not move over to where the water is located to protect against bacteria and mold. Using water-soluble preservative in an anhydrous product will need to add an emulsifier to get that preservative mixed well with the oils.
6. Several other factors may also influence the stability of preservatives such as: –
– Temperature use during the addition of the preservative during use
– Use distilled, deionised or purified water, not tap/faucet or mineral water
– Minimise the use of “bug food,” examples include fruit, botanicals, tea, lecithin, mineral water, milk of any kind, honey, hydrosols, floral waters, aloe vera, extracts, etc. can be highly contaminated by microorganisms.
– Solid particles inorganic sunscreens, clays, pigments as the preservative may absorb onto the surface of the particles and so become unavailable to protect the water phase, and your preservative system will be compromised
– Water-in-oil (W/O) emulsions can minimise the risk of microbial contamination more than oil-in-water (O/W) emulsions. The size of the emulsions droplets can improve the cosmetics effectiveness, the decrease in the size of the emulsion droplets increases the antimicrobial activity.
Successful preservation depends on several factors that affect the antimicrobial efficacy and physicochemical stability of antimicrobial agents.
Overall, an ideal preservative should be stable, compatible, effective at low levels, non-toxic, consistent with cosmetic legislation, and non-expensive.
Here are some guidelines and tips: –
When it comes to cosmetics regulations, always take Europe as a reference as their cosmetic industry is highly regulated, and ingredients are “approved” based on their safety profile. When choosing preservatives, refer to the Annex V of the EU cosmetic legislation (regularly updated) Maximum allowable amounts for a single preservative are also listed.
A preservative booster (known as multifunctional ingredients) i.e. diols like caprylyl glycol or ethylhexylglycerin are the most used preservative boosters, can be added. The synergistic combination of preservatives and these multifunctional ingredients enhances the antimicrobial efficacy, reduce the amount of preservatives needed and improve the safety of formulations. If you’re looking for a natural booster, I suggest Propanediol (trade name Zemea).
Adding chelator i.e. EDTA, usually added at 0.2% to the formula, binds up the trace metal ion contaminants, starving them thus keeping the product microbe-free for longer, especially in your water-containing cosmetic. The EDTA complex is most common because of its broad effectiveness and compatibility with most cosmetic ingredients. Furthermore, it has been shown that chelating agents can have an additional boosting effect on preservative. An amazing natural alternative is Sodium Phytate (highly water-soluble as EDTA).
Check your pH before adding the preservatives to the water-phase (cool down phase) or at the end of the formula. Some preservatives only work within a specific pH range, so you want to make sure your product’s pH falls within the range. This information should be provided from the supplier and if the information is not available, ask your supplier!
Good Manufacturing Practices (GMP) should be strictly followed during the formulation of your products. The preparation of the products should follow a system of the “best practices” to prevent contamination of the products you made. This includes the cleaning and disinfecting the equipment, containers, working space, store and preserve your products hygienically and safely.
It is always good to combine two or more preservatives or a “preservative system” to obtain broad-spectrum protection.
Finally, you have to perform microbiological tests to validate the antimicrobial efficacy of the preservation system. You can prefer microbiology test using microbial test kit from Lotion Crafter or Formulator Sample shop.
Below is a list of commonly used preservatives.
It is advisable to look up the preservatives from organisations such as USDA Organic, Wholefoods, NPA, COSMOS etc. who have analysed and rated ingredients along several parameters, including the origin, sustainability and processing.