By Dolores A. Shaw and Beth Ann Browne, The Dow Chemical Company; Tony Rook, The Sherwin-Williams Company; Phil Geis, Advanced Testing Laboratory and Vidya Ananth, The Clorox Company
For the Consumer Specialty Products Association Microbiology Preservative Subcommittee
The formulation of household products, including laundry detergents, fabric softeners, stain removers, pre-spotters, hard surface cleaners and air fresheners, requires effective preservation technologies to guard against the potential for microbial spoilage. Unpreserved formulations may be susceptible to microbial contamination if the chemical composition of the finished product provides an environment suitable for microbial growth. Likewise, the improper selection of a preservation package may continue to leave the product at risk for microbial spoilage.
The Microbiology Preservative Subcommittee (MPS) of the Consumer Specialty Products Association (CSPA) is committed to developing best practices and standards to guide the industry in manufacturing consumer and industrial products of acceptable microbial quality. This article provides an overview of the critical elements a product manufacturer should consider during the formulation of water-based household products.
One of the key areas for selecting an effective preservation package includes understanding the impact of a formula’s components on the microbial susceptibility of the product. The formulator should assess the compatibility and effectiveness of a preservative during the development of a product. The inclusion of effective microbial control practices for maintaining adequate industrial hygiene should be considered. Balancing these considerations enables manufacturers to ensure the adequate shelf life of formulations, which may inherently possess a higher level of microbial susceptibility.
Water is an essential requirement for microbial growth, and it is a major ingredient in many consumer products. Several key ingredients used to formulate household products provide essential nutrients required for microorganism growth; these include surfactants, dispersants, rheology modifiers, enzymes and fragrances. The product’s pH is another important factor that affects the ability of microorganisms to thrive. The typical pH range for bacterial growth is pH 4 to 9, while fungal growth generally occurs within a range of pH 3 to 10. Microorganisms present in manufacturing environments are often acclimated to thrive beyond these limits of pH. In fact, environmentally isolated microbes have been known to persist within a pH range of 2.5 to 10.5. Figure 1 displays the pH range for bacteria and fungi (which includes yeast and mold), as well as acclimated microorganisms in the manufacturing environment.1 Also displayed are the typical pH ranges for household products requiring protection from bacterial and fungal contamination.
Many water-based household products provide conditions that are favorable for the rapid growth of microorganisms when improperly preserved. As manufacturers work toward formulating products that are more pH neutral, the risk of microbial contamination tends to increase within these products. The rigorous process of ensuring the microbiological quality of finished formulations requires a multi-faceted approach. An effective industrial microbial control program consists of four essential aspects:
- Use of effective and product-compatible preservatives;
- Microbiological evaluation of preservation systems;
- Routine microbiological monitoring of the manufacturing environment, finished product and raw materials (ingredients); and
- Regularly scheduled cleaning and sanitization programs in the manufacturing facilities.
Preservatives typically represent only a minor portion of a household product’s total formulation. In fact, preservatives are often included as less than 0.1% of the final formula but can have significant benefits to the product. Microbial contamination can destroy a formulation’s stability and performance. For example, acidic by-products generated during microbial growth can greatly affect the pH of formulations. Within cleaning products, this can decrease the cleaning efficiency and formulation homogeneity of the product. Microbial growth can also result in odor and/or color formation affecting the overall aesthetics of the product.2 Exposing any consumers to microbial contamination should be avoided, but this is especially important for immunosuppressed individuals. In fact, the Consumer Product Safety Commission has announced recalls of contaminated products based on this risk.3
A list of commonly used preservatives (biocides) which have been registered by the United States Environmental Protection Agency (EPA) for use in household products is provided in Table 1 (see p. 84). These “traditional” or synthetic preservatives are organized within this table by chemical classification, which also includes information regarding the microbial spectrum of activity, optimal pH conditions for activity (performance), and EPA registered minimum and maximum use rates for these active ingredients.4
The described preservatives’ active ingredients and their blends are thoroughly evaluated by the EPA on a routine basis to ensure they meet federal safety standards to protect human health and the environment. Preservatives (pesticides, biocides, microbicides) are required by federal law to be registered with the EPA under authority of the Federal, Insecticide, Fungicide, and Rodenticide Act (FIFRA) enacted in 1947 and amended in 1996 (7 U.S. C §136 et seq, (1996)). The EPA classifies preservatives as antimicrobial pesticides. FIFRA “requires special tests to ensure efficacy of public health pesticides when the pests are invisible disease-causing microbes rather than insects or rodents that may be harboring disease organisms.”5
Before selling or distributing an antimicrobial pesticide, a registration must be obtained from the EPA. The EPA requires the applicant to conduct numerous scientific studies including exposure assessments, toxicological studies, environmental fate determinations, product chemistry and in-use microbicidal efficacy. These evaluations allow the EPA to ensure that the antimicrobial pesticide, when used according to label directions, can be used with a reasonable certainty of no harm to human health and without posing unreasonable risks to the environment.
The EPA completes both human health and ecological risk assessments based on the applicant’s submitted data. There are several programs used by the EPA to ensure the periodic review of registered antimicrobial pesticides. The agency continues to expand its data requirements for registration based on use and applications. The current time to register a new preservative is typically 18 months with an investment of several million dollars.6
Unlike conventional preservatives that are EPA registered, the majority of more “natural” alternatives, such as organic acids, essential oils, and plant extracts are not EPA registered and may lack safety, toxicological, microbicidal efficacy and environmental data. Additionally, the agricultural supply of natural alternatives is likely to be variable, which may impact the chemical composition and/or concentration of the active ingredients.7
Consumers place significant, and sometimes, contradictory demands on product manufacturers. The demand for increased use of environmentally acceptable ingredients must be coupled with the expectation of protecting the final product against the potential for microbial spoilage throughout the product’s use life.
Consumers often make purchasing decisions that are fully or partially based on ecolabeling. Within the consumer and industrial products industry, some relevant ecolabeling organizations include the EPA’s Design for the Environment (DfE), Natural Products Association and Green Seal, to name a few. The EPA’s Design for the Environment (DfE) symbol, which is a predominant ecolabel for household products in the US, is highlighted as an example within the remainder of this article.
Manufacturers want to meet and exceed customer expectations by certifying their products with DfE while simultaneously maintaining their current standards of product quality and effectiveness. This balance between adequate preservation and ecolabel certification can be challenging. DfE classifies preservatives (and other chemicals) by one of four categories per the Safer Chemical Ingredients list, and these include:
- Low concern based on experimental and environmental data as designated by a green circle;
- Expected to be of low concern but additional data is necessary to improve the confidence of the assessment as designated by a green half circle;
- Considered an area for safer chemistry innovation due to the chemical’s hazard profile as designated by a yellow triangle; and
- Not acceptable for use in products that are candidates for the DfE label as designated by a gray square.8
The current status of some preservatives in DfE-certified products is shown in Figure 2. The microbial efficacy and pH compatibility are shown and color-coded for organic acids and the isothiazolinone-containing preservatives. Areas of preservative performance overlap with the pH ranges of the household products to be preserved. The illustration in Figure 3 moves one step further to depict a scenario in which all preservatives in the third DfE category (considered an area for safety innovation) are no longer permitted.
For example, if organic acids are the only preservative options for household product manufacturers, there will be considerable gaps in efficacy depending on a product’s pH. Lower pH formulations (pH 2 to 6) will have some gaps in protection against bacteria. More significant is the situation for formulations with a pH > 6. There will be gaps in protection against both bacteria and fungi. These gaps may limit a manufacturer’s flexibility in formulating products with the desired quality, aesthetics and performance. There is a need to balance the functional requirements of product preservation with the environmentally favorable expectations of product formulation. A holistic approach to understanding product preservation requirements may potentially allow for a structured framework for assigning environmentally acceptable status to “traditional” or synthetic microbicidal active ingredients.
During the development of household product formulations, preservatives are often added to those products determined to be susceptible to microbial contamination. Manufacturers perform preservative efficacy testing, commonly known as challenge testing, by successively insulting the preserved product with a microbial challenge. This testing provides an indication of the ability of the preservative to protect the product from microbial spoilage during the manufacturing process as well as throughout the consumer-use life of the product. As an additional benefit, this testing allows the formulator to more efficiently preserve household products by avoiding the addition of excessive preservative beyond the determined effective level.9
Minimizing or reducing the overall bio-burden introduced during the manufacturing process contributes to the overall microbiological quality of household products and can result in more efficient use of preservatives or preservative systems. Routine/periodic microbiological monitoring of raw materials (ingredients), finished product formulations, and the plant environment provides data that allow for identification of potential sources of bio-burden or areas of concern.
This information is a trigger for cleaning and sanitization activities that reduce bioburden, improve the efficiency of the preservative use and ensure the microbiological quality of household products. When there is a situation of microbial contamination in manufacturing equipment, raw materials, or a final product before it is released from the facility, a rapid-acting biocide may be used for decontamination and/or recovery. Typically, chemistries such as 2,2-dibromo-3-nitrilopropionamide (DBNPA) and quaternary ammonium compounds are used for decontamination. Sodium hypochlorite (commonly referred to as bleach) can also be used for equipment cleaning, but incompatibilities prevent its use with raw materials or final products. These chemistries are not listed in Table 1 because they are typically not considered long-term preservatives; rather, they fill a unique and very important niche for product manufacturers as rapid-acting chemistries.
The authors are unaware of a non-conventional biocide that fits in the rapid decontamination category. In a survey conducted by CSPA, it was determined that manufacturers of household products understand the importance of minimizing bioburden using the approach described herein.10
In general, household products that have not been adequately preserved provide favorable conditions for microbial growth by bacteria and fungi (including both yeast and mold), depending on the product’s pH. The practice of microbiological monitoring of the manufacturing environment, raw materials, and finished household products, along with effective cleaning and sanitization programs, help minimize the potential of microbial contamination. However, the use of preservatives is still an essential component of a microbial control program due to the high water content, moderate pH and availability of nutrients in many household products. Some of the most commonly used preservatives for household products are chemically well understood, efficiently used (just a minor component of the overall household formulation) and have been thoroughly tested to meet the requirements of the EPA to ensure protection of human health and the environment.
- Paulus, W. (1993). Microbicides for the Protection of Materials. A Handbook. p. 454. London, UK :Chapman and Hall.
- About cleaning products website. http://www.aboutcleaningproduct.com/ingredients/preservatives/
- Paulus, W. (2005). Directory of Microbicides for the Protection of Materials. A Handbook. Netherlands: Springer.
- Browne, B. A., Geis, P., Rook, T. (2012). Conventional vs. natural preservatives. Happi http://www.happi.com/issues/2012-05/view_features/conventional-vs-natural-preservatives/
- DfE’s Safer Chemical Ingredients List. http://www.epa.gov/dfe/saferingredients.htm#about
- Machtiger, N.A., et al. Determination of the Efficacy or Preservation on Non-Eye Area Water-Miscible Cosmetic and Toiletry Formulations: Collaborative Study. Journal of AOAC International 84:101-109, 2001.
- Geis, P., Rook, T. (2010). Microbiological Quality of Consumer Products. Happi http://www.happi.com/contents/view_features/2011-05-03/microbiological-quality-of-consumer-products/