Preservative Efficacy Test (PET) Explained: Complete Guide for Microbiology & Pharma Students

Table of Contents

• Introduction
• Principles of Preservative Efficacy Testing
• Regulatory Guidelines and Standards
• Test Microorganisms
• Materials and Equipment
• Test Procedure
• Methods of Microbial Enumeration
• Interpretation of Results
• Factors Affecting Test Results
• Applications of Preservative Efficacy Test
• Advantages of Preservative Efficacy Test
• Limitations of Preservative Efficacy Test
• Recent Advances in PET
• Conclusion
• References

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Introduction to Preservative Efficacy Testing (PET)

• Preservative Efficacy Testing (PET) is a quality control method used to evaluate the ability of preservatives to inhibit or eliminate microbial contamination.
• Also known as a challenge test, it involves intentional inoculation of microorganisms into a product to assess preservative performance.
• PET is essential for ensuring microbiological safety in:
• Pharmaceuticals
• Cosmetics
• Food products
• The primary objective of PET is to:
• Prevent microbial spoilage
• Protect against pathogenic microorganisms
• Ensure consumer safety during storage and use
• Antimicrobial preservatives are added to formulations to:
• Extend shelf life
• Maintain product stability and integrity
• Prevent contamination during repeated use
• Types of preservatives include:
• Synthetic preservatives (e.g., parabens, isothiazolinones)
• Natural preservatives (e.g., plant extracts, essential oils)
• Products containing water (aqueous systems) are highly susceptible to:
• Microbial growth
• Contamination during handling
• PET is particularly critical for:
• Multi-dose pharmaceutical preparations
• Water-based cosmetics (creams, lotions, shampoos)
• Processed and preserved food products
• Regulatory bodies such as:
• United States Pharmacopeia
• European Pharmacopoeia
• International Organization for Standardization
  have established standardized PET protocols to ensure product safety before market release.
• Growing consumer demand is driving:
• Shift toward safer and natural preservatives
• Research into low-toxicity, high-efficacy alternatives
• Overall, PET acts as a critical safeguard to ensure that products remain:
• Safe
• Stable
• Effective throughout their shelf life

Principles of Preservative Efficacy Testing (PET)

• Preservative Efficacy Testing (PET) is based on microbial challenge testing, where a finished product is deliberately inoculated with selected microorganisms to evaluate whether the preservative system can control contamination over time.
• The test simulates real-life contamination conditions during manufacturing, storage, and consumer use, ensuring that the product remains microbiologically safe throughout its shelf life.
• Standard challenge organisms include:
• Staphylococcus aureus
• Pseudomonas aeruginosa
• Candida albicans
• Aspergillus brasiliensis
• A defined microbial load (typically 10⁵–10⁶ CFU/mL) is introduced, and microbial survival is monitored over a fixed period (commonly 28 days) to determine log reduction and preservative effectiveness.
• Preservatives act through multiple antimicrobial mechanisms, including:
• Disruption of cell membranes leading to leakage of cellular contents
• Inhibition of metabolic processes and depletion of cellular energy (ATP)
• Inactivation of enzymes and proteins, sometimes through metal chelation
• Induction of oxidative stress and damage to microbial DNA
• The effectiveness of a preservative system is not fixed and depends on formulation-specific factors such as:
• pH, which influences preservative activity and ionization
• Water activity, affecting microbial growth and preservative mobility
• Product composition, where fats, proteins, or surfactants may reduce preservative availability
• Temperature and storage conditions, which alter both microbial growth and preservative stability
• Packaging and usage, especially in multi-use products where repeated exposure increases contamination risk
• Due to these variables, PET must be tailored to each formulation, making it an essential tool for validating product-specific preservation systems.
• Overall, the principle of PET lies in challenging the product under controlled conditions and verifying that the preservative system can consistently maintain microbiological safety.

Regulatory Guidelines for Preservative Efficacy Testing (PET)

• Preservative efficacy testing is regulated by international standards to ensure that products are microbiologically safe before market release.
• Major regulatory frameworks include:
• United States Pharmacopeia (USP <51>)
• European Pharmacopoeia (EP 5.1.3)
• British Pharmacopoeia
• International Organization for Standardization (ISO 11930 for cosmetics)
• All guidelines are based on microbial challenge testing, where products are inoculated with standard microorganisms and monitored for reduction in microbial count over time.
• Common test organisms include:
• Staphylococcus aureus
• Pseudomonas aeruginosa
• Escherichia coli
• Candida albicans
• Aspergillus brasiliensis
• Typical procedure involves:
• Inoculation with ~10⁴–10⁶ CFU/mL
• Incubation under controlled conditions
• Sampling at defined intervals (e.g., Day 0, 7, 14, 28)
• Evaluation based on log reduction criteria
• USP <51>:
• Applies mainly to pharmaceutical products
• Requires specific log reductions in bacteria and fungi within defined time points
• Considered a baseline standard
• EP 5.1.3:
• Covers a wide range of dosage forms
• Often more stringent than USP, especially for fungi and later time points
• Uses detailed category-based acceptance criteria
• BP Guidelines:
• Closely aligned with EP
• In some cases more rigorous than USP, especially for sterile preparations
• ISO 11930:
• Specifically designed for cosmetic products
• Follows an EP-style challenge test approach
• Includes criteria A and B for acceptance
• Differences between these standards include:
• Inoculum size and test conditions
• Acceptance criteria (log reductions required)
• Product categories and applications
• In practice, manufacturers often design preservative systems to meet the most stringent standard (usually EP or ISO) to ensure global regulatory compliance.
• Overall, these guidelines provide a standardized framework to evaluate preservative performance and ensure product safety, quality, and regulatory acceptance.

Standard Test Microorganisms in Preservative Efficacy Testing

Preservative (challenge) tests use a small panel of “indicator” microbes chosen for their clinical relevance and known resistance in formulations. These organisms simulate real-world contamination and provide a standardized basis for comparing preservative systems across products and laboratories.

Bacterial Strains

Rationale for the Standard Bacteria

• Staphylococcus aureus
  A major skin and mucosal pathogen and frequent cosmetic contaminant in creams and personal care products. Used in EN ISO 11930 and pharmacopoeial tests due to its relevance to skin infections and its ability to survive in nutrient-rich emulsions.
• Escherichia coli
  An indicator of fecal contamination and an important foodborne pathogen (e.g., O157:H7). Commonly included in pharmacopeial panels and widely used in evaluating preservative systems and biosensors.
• Pseudomonas aeruginosa
  A highly preservative-tolerant Gram-negative bacterium and a frequent contaminant of cosmetic and household products. Known for its resilience, biofilm formation, and association with product recalls.

Representative Roles in Testing

Species	Typical Role in PET	Key Properties	Citations
S. aureus	Skin/cream contaminant model	Gram+, toxin-producing, common in cosmetics	(Charnock & Otterholt, 2012; Almoughrabie et al., 2020; Alshehrei, 2025)
E. coli	Fecal/enteric contamination model	Gram−, foodborne pathogen, indicator organism	(Pato et al., 2021; Pedreira et al., 2023; Chueca et al., 2016; Zhao et al., 1993)
P. aeruginosa	Industrial/cosmetic spoilage model	Gram−, preservative-tolerant, biofilm-former	(Charnock & Otterholt, 2012; Rushton et al., 2021; Weiser et al., 2019; Weiser, 2015)

Table 1: Roles of core bacterial strains in preservative testing.

Fungal Strains

• Candida albicans
  A yeast associated with human mucosa and opportunistic infections. It can persist in aqueous and semi-solid products and is used as the standard yeast in EP/ISO challenge tests.
• Aspergillus brasiliensis
  A filamentous mold widely used in preservative testing due to its environmental ubiquity and ability to grow in diverse formulations. It is part of the official challenge panel for both cosmetics and pharmaceuticals.

Materials and Equipment for Preservative Efficacy Testing (PET)

Preservative efficacy (challenge) testing requires appropriate media, calibrated equipment, sterile tools, and validated neutralizing systems to ensure accurate results. Any deficiency in these components can lead to false estimation of preservative effectiveness.

Culture Media

• General growth media such as Tryptic Soy Agar/Broth (TSA/TSB), Nutrient Agar, and Soybean–Casein Digest Agar are used for bacterial recovery.
• Fungal media such as Sabouraud Dextrose Agar/Broth are used for yeasts and molds.
• Culture media must support recovery of stressed or injured microorganisms, not just actively growing cells, to avoid underestimation of surviving organisms.

Laboratory Equipment

• Essential equipment includes:
• Incubators (20–25 °C for fungi; 30–35 °C for bacteria)
• Biosafety cabinet (BSC) for aseptic handling
• Colony counters, pipettes, vortex mixers
• Membrane filtration units
• Stomacher/blenders for complex samples
• Advanced systems:
• Plate readers and rapid microbiological methods (e.g., impedance or metabolic activity systems) may be used as alternatives to traditional PET.

Core Materials and Their Roles

Item Type	Examples	Purpose in PET	Citations
Media	TSA, SDA	Recovery of surviving microorganisms	(Alshehrei, 2024; Halla et al., 2018; Cui et al., 2025)
Equipment	Incubators, BSC, filters	Controlled testing and enumeration	(Halla et al., 2018; Cremieux et al., 2005; Watanabe et al., 2025)
Tools/Containers	Sterile tubes, plates	Aseptic sampling and handling	(Halla et al., 2018; 2006)
Neutralizers	Polysorbate 80, lecithin, Letheen broth	Inactivation of residual preservatives	(Katerji et al., 2023; 2006; Eldemerdash et al., 2025)

Table 2: Key materials required for preservative challenge tests.

Sterile Tools and Containers

• Use of sterile Petri dishes, tubes, flasks, pipette tips, syringes, and membrane filters is essential to prevent external contamination.
• Single-use, pre-sterilized consumables are preferred for reliability.
• Containers must be:
• Compatible with the product
• Non-reactive with preservatives and neutralizers
• Free from adsorption effects

Neutralizing Agents

• Neutralization is critical to ensure that residual preservatives do not continue killing microorganisms after sampling, which could lead to false results.
• Methods of neutralization include:
• Chemical neutralizers such as polysorbate 80, lecithin, sodium thiosulfate, D/E broth, and Letheen broth
• Dilution or membrane filtration with rinsing, often combined with surfactants
• Neutralizers must be:
• Validated for each product
• Non-toxic to microorganisms
• Effective in completely inactivating the preservative system

Test Procedure of Preservative Efficacy Testing (PET)

• The test procedure involves preparation of microbial inoculum, product inoculation, incubation, and periodic microbial enumeration to assess preservative performance.

Preparation of Microbial Inoculum

• Reference strains such as:
• Staphylococcus aureus
• Escherichia coli
• Pseudomonas aeruginosa
• Candida albicans
• Aspergillus brasiliensis
  are revived from lyophilized or frozen stocks.
• Cultures are grown on suitable media:
• 24–48 hours for bacteria
• 48–72 hours for yeasts and molds
• Colonies are suspended in sterile saline or buffer and adjusted to ~10⁷–10⁸ CFU/mL using McFarland standards or spectrophotometry.
• The suspension is further diluted to obtain a final inoculum of ~10⁵–10⁶ CFU/mL in the test product.

Sample Preparation

• Product samples (20–50 mL or g) are dispensed into sterile containers under aseptic conditions.
• Viscous or oil-based formulations are mixed (e.g., vortexing or stirring) to ensure uniform consistency.
• Neutralizing diluents and recovery media are pre-validated to ensure effective inactivation of preservatives during testing.

Inoculation of Test Product

• Each container is inoculated with a specific microorganism separately.
• Final microbial concentration is adjusted to ~10⁵–10⁶ CFU/mL (or g).
• Inoculum volume is kept ≤1% of product volume to avoid altering formulation properties.
• Samples are mixed thoroughly to ensure uniform microbial distribution.
• Day 0 samples are taken immediately after inoculation to determine the initial microbial count.

Incubation Conditions

• Inoculated samples are stored at controlled conditions (20–25 °C) reflecting normal product storage.
• Containers are kept closed except during sampling to prevent contamination.
• Samples are generally not agitated, unless required to simulate real-use conditions.

Sampling and Microbial Enumeration

• Samples are collected at Day 0, 7, 14, and 28 under aseptic conditions.
• Each sample is:
• Transferred into neutralizing diluent
• Serially diluted
• Plated using pour plate or spread plate methods
• Incubation conditions:
• Bacteria: 30–35 °C for 48–72 hours
• Fungi: 20–25 °C for up to 5–7 days
• Microbial counts are recorded as CFU/mL or CFU/g.
• Results are compared with Day 0 values to calculate log reduction and determine compliance with acceptance criteria.

Microbial Enumeration Methods in Preservative Efficacy Testing

• Microbial enumeration methods are used to estimate the number of viable microorganisms in a sample, usually expressed as colony-forming units (CFU/mL or CFU/g) or statistical counts.
• These methods are essential in PET to measure microbial survival over time and determine the effectiveness of preservatives.
• The three main culture-based methods are plate count, membrane filtration, and most probable number (MPN), each selected based on sample type and microbial load.

Plate Count Method

• Microorganisms are spread on or within solid agar, where each viable cell forms a visible colony.
• Considered a standard and widely used method for foods, pharmaceuticals, and surfaces.
• Suitable for moderate to high microbial loads and routine analysis.
• Limitations include:
• Underestimation due to cell clumping
• Inability to detect viable but non-culturable (VBNC) cells
• Modern adaptations (e.g., rapid plate systems) improve efficiency.

Membrane Filtration Method

• A known volume of sample is passed through a 0.2–0.45 µm membrane filter, which retains microorganisms.
• The membrane is then placed on agar for growth and enumeration.
• Highly effective for:
• Low microbial counts
• Large-volume liquid samples
• Commonly used in:
• Water quality testing
• Pharmaceutical and cosmetic products
• Advantages include:
• Concentration of microorganisms
• Faster detection and improved sensitivity

Probable Number (MPN) Method

• A statistical method based on growth/no-growth patterns in serial dilutions of liquid media.
• Results are interpreted using MPN tables or calculations.
• Suitable for:
• Low-density microorganisms
• Turbid or particulate samples
• Situations where colony formation is difficult
• Detection may be based on:
• Gas production
• Color change
• Substrate utilization
• Less precise than plate count but useful for complex samples.

Interpretation of Preservative Efficacy Test (PET) Results

• Interpretation of PET results is based on log₁₀ reduction in microbial counts over time, which indicates the effectiveness of the preservative system.
• Log reduction is calculated as the difference between the initial microbial count and the count at a given time (e.g., reduction from 10⁵ to 10² CFU/mL equals a 3-log reduction).
• Microbial counts are measured at specific intervals (commonly Day 7, 14, and 28) and compared with standard acceptance criteria.
• A critical requirement is “No Increase (NI)”, meaning microbial counts should not rise significantly after reduction (typically ≤0.5 log increase due to normal variation).
• Results are interpreted according to regulatory standards such as:
• United States Pharmacopeia (USP <51>)
• European Pharmacopoeia (EP 5.1.3)
• In general, USP criteria require a gradual reduction in bacterial counts over time and no increase in fungal counts, and are considered a baseline standard.
• EP criteria are usually more stringent, requiring faster and greater microbial reduction, with stricter conditions (Criteria A) and more flexible options (Criteria B).
• To determine results:
• Convert microbial counts to log₁₀ values
• Calculate log reduction relative to initial count (Day 0)
• Compare with the required standards for the specific product
• A product passes if all tested microorganisms meet the required log reductions and show no increase at specified time points.
• If any microorganism fails to meet the criteria at any time, the product is considered an overall failure.
• Overall, PET interpretation links quantitative microbial reduction with regulatory standards, ensuring that preservative systems provide adequate protection throughout the product’s shelf life.

 

Factors Affecting Preservative Efficacy Test (PET) Results

• The outcome of preservative efficacy testing is strongly influenced by formulation characteristics and storage conditions, which can significantly alter antimicrobial performance.
• pH plays a critical role by affecting the ionization, stability, and activity of preservatives; many preservatives are only effective within a specific pH range, and their activity may decrease or they may degrade at extreme pH levels.
• Water activity (aᵥ) determines the ability of microorganisms to grow; reducing water activity through humectants (e.g., glycerin, ethanol) can limit microbial growth and enhance preservative effectiveness, sometimes even reducing the need for strong preservatives.
• Preservative concentration directly impacts antimicrobial activity; higher concentrations generally increase effectiveness, but the relationship is often non-linear and formulation-dependent.
• Combining preservatives (e.g., organic acids with alcohols) can produce synergistic effects, making preservative systems more effective than single agents.
• The product matrix (presence of fats, proteins, surfactants, or emulsions) can reduce preservative availability by binding or partitioning the preservative, leading to decreased antimicrobial activity.
• Packaging materials influence preservative performance by:
• Absorbing or binding preservatives
• Causing migration of preservatives into packaging or non-aqueous phases
• In some cases, contributing antimicrobial effects (e.g., active packaging systems)
• Storage conditions, especially temperature, affect both:
• Microbial survival and growth
• Stability and activity of preservatives
• Lower temperatures may slow microbial growth but can also allow psychrotolerant organisms to persist, while higher temperatures may degrade certain preservatives.
• The duration of storage and testing determines whether required log reductions are achieved, and borderline preservative systems may pass less stringent standards but fail stricter ones.
• Overall, preservative efficacy is highly product-specific, and PET results depend on the combined influence of formulation, preservative system, packaging, and storage conditions.

Applications of Preservative Efficacy Testing (PET)

• Preservative efficacy (challenge) testing is a critical tool for ensuring microbiological safety and shelf life of products in pharmaceuticals, cosmetics, and food industries.
• In pharmaceutical products, PET is used to:
• Verify that multi-dose sterile and non-sterile formulations (e.g., parenterals, ophthalmics, oral and topical liquids) meet antimicrobial effectiveness requirements.
• Support formulation development by selecting suitable preservatives and their optimal concentrations.
• Confirm preservative stability during shelf-life and storage conditions.
• Evaluate performance under different temperatures, as some products may pass basic standards but fail stricter conditions.
• In cosmetic and personal care products, PET is:
• A regulatory requirement under standards such as International Organization for Standardization and pharmacopeial guidelines.
• Used to ensure the product can resist microbial contamination during consumer use.
• Applied to optimize preservative systems, including traditional and self-preserving formulations.
• Important for assessing the effectiveness of packaging in preventing contamination.
• Required for safety documentation before product release.
• Supported by emerging rapid testing methods for faster screening of multiple formulations.

Example Uses Across Consumer Products

Sector	Main Purpose of PET	Typical Stage	Citations
OTC drugs, Rx	Meet pharmacopeial AET; choose preservative	Development & stability	(Eck, 2016; Ghahfarrokhi et al., 2025; Rdzok et al., 1955; Hodges, 2020)
Cosmetics	Validate system vs. in-use contamination; packaging check	Development; regulatory dossier	(Russell, 2003; Halla et al., 2018; Fiorentino et al., 2011; Alshehrei, 2024)
Detergents/other HPC	Rapid screening of many formulas	Development	(Watanabe et al., 2025; Connolly et al., 1994)

Table 3: Applications of preservative tests by product sector

• In the food industry, similar principles are applied:
• Preservatives such as sorbic acid, benzoic acid, nisin, and natamycin are used to prevent spoilage and microbial growth.
• Microbiological challenge studies are conducted to evaluate preservative effectiveness in real food systems.
• Rapid and analytical methods are increasingly used to monitor quality and safety during storage.
• Although formal pharmacopeial PET is less standardized in foods, challenge testing remains essential for validating preservation strategies.
• Overall, PET plays a vital role in:
• Product development and optimization
• Regulatory compliance
• Packaging evaluation
• Ensuring long-term microbiological safety across multiple industries

Advantages of Preservative Efficacy Testing (PET)

• Ensures real product safety by testing preservatives directly in the final formulation under realistic conditions
• Confirms protection against bacteria, yeasts, and molds during storage and actual use
• Supports compliance with regulatory standards such as United States Pharmacopeia, European Pharmacopoeia, and International Organization for Standardization
• Provides a standardized and reliable method for evaluating preservative performance
• Helps in selection and optimization of preservative systems (type and concentration)
• Allows comparison between different preservatives and formulations
• Verifies that preservative effectiveness is maintained during shelf life and storage conditions
• Assesses the impact of packaging and product design on microbial safety
• Serves as a basis for quality control and safety documentation
• Supports development of safer and more efficient preservative systems
• Enables validation of rapid alternative microbiological testing methods
• Applicable across multiple industries including pharmaceuticals, cosmetics, and food products

Limitations of Preservative Efficacy Testing (PET)

• May not fully reflect real-life conditions, as tests use controlled laboratory environments and standardized microorganisms
• Uses single (monoculture) organisms, whereas real contamination often involves mixed microbial populations
• Standard test strains may be less resistant than real-world or clinical isolates, leading to overestimation of preservative effectiveness
• Does not adequately represent biofilm-forming microorganisms, which are more resistant to preservatives
• Plate count methods may miss damaged or stressed cells, underestimating surviving microorganisms
• Limited sampling intervals can miss actual microbial survival patterns, especially slow or resistant populations
• Results can differ between in vitro (lab) and in vivo (real use) conditions due to interactions with skin, tissues, or microbiota
• Highly influenced by formulation factors such as pH, excipients, and water activity, requiring separate testing for each product
• Requires effective neutralization of preservatives, and incomplete neutralization can give inaccurate results
• Does not always simulate extreme storage or usage conditions, especially for diluted or frequently used products
• Time-consuming process (often up to 28 days) compared to newer rapid methods
• May require product-specific modifications, reducing standardization across different formulations

Recent Advances in Preservative Efficacy Testing (PET)

• Development of rapid microbiological methods that significantly reduce testing time compared to traditional 28-day PET
• Use of metabolic activity-based assays (e.g., ATP detection) to quickly assess microbial viability instead of waiting for colony growth
• Application of impedance microbiology to measure microbial growth and death rates within a few days
• Introduction of high-content imaging and confocal microscopy for fast and detailed analysis of microbial inactivation
• Implementation of automated detection systems for high-throughput and reproducible testing
• Use of ATP bioluminescence devices for rapid estimation of microbial contamination
• Adoption of molecular techniques such as PCR and qPCR for highly sensitive detection of microorganisms
• Ability to detect viable but non-culturable (VBNC) and stressed cells that are often missed by plate count methods
• Integration of advanced techniques (LAMP, ddPCR, next-generation sequencing) for deeper microbial analysis and resistance profiling
• Improved speed, sensitivity, and automation of preservative testing workflows
• Despite advancements, classical PET remains the regulatory reference standard for validation

Conclusion

• Preservative Efficacy Testing (PET) is a critical method to ensure microbiological safety, product stability, and extended shelf life
• It evaluates the real performance of preservatives under simulated contamination conditions
• PET is essential for pharmaceuticals, cosmetics, and food products, especially those containing water or used multiple times
• Standardized guidelines (USP, EP, ISO) provide a reliable framework for consistent testing and regulatory compliance
• The effectiveness of preservatives depends on formulation factors such as pH, water activity, composition, packaging, and storage conditions
• PET supports formulation development by helping select optimal preservative systems and concentrations
• It ensures that products remain protected against bacteria, yeasts, and molds throughout their lifecycle
• Despite some limitations, PET remains the gold standard for validating antimicrobial preservation
• Recent advances are improving speed, sensitivity, and automation, but do not replace classical PET
• Overall, PET plays a vital role in quality control, consumer safety, and regulatory approval of modern products

 

References

• Akers, M., Boand, A., & Binkley, D. (1984). Preformulation method for parenteral preservative efficacy evaluation. Journal of Pharmaceutical Sciences, 73(7), 903–905. https://doi.org/10.1002/jps.2600730710
• Al-Rimawi, F., Amayreh, M., Rahhal, B., Sbeih, M., & Mudalal, S. (2024). Evaluation of the effectiveness of natural extract as a substituent for synthetic preservatives and antioxidants in pharmaceutical preparations. Saudi Pharmaceutical Journal, 32. https://doi.org/10.1016/j.jsps.2024.102014
• Al-Rimawi, F., Imtara, H., Abbadi, J., Sawalha, S., Atiya, M., Mudalal, S., & Mauriello, G. (2025). Evaluation of antimicrobial efficacy of Psidium guajava L. leaf extract in ketoconazole shampoo. Scientific Reports, 15. https://doi.org/10.1038/s41598-025-22965-5
• Alshehrei, F. (2024). Microbiological quality assessment of skin and body care cosmetics by using challenge test. Saudi Journal of Biological Sciences, 31. https://doi.org/10.1016/j.sjbs.2024.103965
• Alshehrei, F. (2025). Effect of preservatives on microorganisms isolated from contaminated cosmetics collected from Mecca region, Saudi Arabia. Journal of Pure and Applied Microbiology. https://doi.org/10.22207/jpam.19.3.06
• Angane, M., Swift, S., Huang, K., Butts, C., & Quek, S. (2022). Essential oils and their major components: An updated review on antimicrobial activities, mechanism of action and their potential application in the food industry. Foods, 11. https://doi.org/10.3390/foods11030464
• Bensid, A., Abed, N., Houicher, A., Regenstein, J., & Özoğul, F. (2020). Antioxidant and antimicrobial preservatives: Properties, mechanism of action and applications in food – A review. Critical Reviews in Food Science and Nutrition, 62, 2985–3001. https://doi.org/10.1080/10408398.2020.1862046
• Brul, S., & Coote, P. (1999). Preservative agents in foods: Mode of action and microbial resistance mechanisms. International Journal of Food Microbiology, 50(1–2), 1–17. https://doi.org/10.1016/S0168-1605(99)00072-0
• Buckley, H., Hart-Cooper, W., Kim, J., Faulkner, D., Cheng, L., Chan, K., Vulpe, C., Orts, W., Amrose, S., & Mulvihill, M. (2017). Design and testing of safer, more effective preservatives for consumer products. ACS Sustainable Chemistry & Engineering, 5, 4320–4331. https://doi.org/10.1021/acssuschemeng.7b00374
• Cedillo-Olivos, A., Juárez-Chairez, M., Cid-Gallegos, M., Sánchez-Chino, X., & Jiménez-Martínez, C. (2024). Natural preservatives used in foods: A review. Critical Reviews in Food Science and Nutrition, 65, 5049–5065. https://doi.org/10.1080/10408398.2024.2403000
• Charnock, C., & Otterholt, E. (2012). Evaluation of preservative efficacy in pharmaceutical products. Journal of Clinical Pharmacy and Therapeutics, 37. https://doi.org/10.1111/j.1365-2710.2012.01354.x
• Chen, T., & Chang, H. (2024). Deciphering trends in replacing preservatives in cosmetics intended for infants and sensitive population. Scientific Reports, 14. https://doi.org/10.1038/s41598-024-69624-9
• Cremieux, A., Cupferman, S., & Lens, C. (2005). Method for evaluation of the efficacy of antimicrobial preservatives in cosmetic wet wipes. International Journal of Cosmetic Science, 27. https://doi.org/10.1111/j.1467-2494.2005.00267.x
• Davison, A. (2017). Preservative efficacy testing of pharmaceuticals, cosmetics and toiletries and its limitations.
• De Oliveira Schmitt, P., Fischer, A., Silva, R., & Cruz, A. (2022). Compatibility and efficiency of preservatives in emulsive cosmetics containing high surfactant content. Brazilian Journal of Pharmaceutical Sciences. https://doi.org/10.1590/s2175-97902022e191088
• Dong, H., Xu, Y., Zhang, Q., Li, H., & Chen, L. (2024). Activity and safety evaluation of natural preservatives. Food Research International, 190, 114548. https://doi.org/10.1016/j.foodres.2024.114548
• Fiorentino, F., Chorilli, M., & Salgado, H. (2011). The use of the challenge test to analyse preservative efficiency. Analytical Methods, 3, 790–798. https://doi.org/10.1039/c0ay00597e
• Ghahfarrokhi, S., Zarei, Z., Hamidian, K., Nikrou, S., Seidi, M., Jamalifar, H., & Samadi, N. (2025). Preservative efficacy testing of refrigerated pharmaceuticals. PDA Journal of Pharmaceutical Science and Technology, 79, 285–294. https://doi.org/10.5731/pdajpst.2024-003021.1
• Glavač, K., & Lunder, M. (2018). Preservative efficacy of selected antimicrobials of natural origin in a cosmetic emulsion. International Journal of Cosmetic Science, 40. https://doi.org/10.1111/ics.12461
• Głaz, P., Rosińska, A., Woźniak, S., Boguszewska-Czubara, A., Biernasiuk, A., & Matosiuk, D. (2023). Effect of commonly used cosmetic preservatives on healthy human skin cells. Cells, 12. https://doi.org/10.3390/cells12071076
• Gumudavelli, S., Srinija, G., & Palem, C. (2025). A comprehensive review on microbiological testing in pharmaceutical, nutraceutical and cosmetics industries. International Journal of Current Microbiology and Applied Sciences. https://doi.org/10.20546/ijcmas.2025.1405.009
• Halla, N., Fernandes, I., Heleno, S., Costa, P., Boucherit-Otmani, Z., Boucherit, K., Rodrigues, A., Ferreira, I., & Barreiro, M. (2018). Cosmetics preservation: A review on present strategies. Molecules, 23. https://doi.org/10.3390/molecules23071571
• Hodges, N. (2020). Assessment of preservative activity during stability studies.
• Katerji, A., Trefi, S., Bitar, Y., & Ibrahim, A. (2023). Evaluation of new formulations for neutralizing antimicrobial preservatives. Heliyon, 9. https://doi.org/10.1016/j.heliyon.2023.e14555
• Lather, A., Sharma, S., & Khatkar, A. (2021). Aesculin-based glucosamine-6-phosphate synthase inhibitors as novel preservatives. BMC Chemistry, 15. https://doi.org/10.1186/s13065-021-00769-8
• Lynn, L., Scholes, R., Kim, J., Wilson-Welder, J., Orts, W., & Hart-Cooper, W. (2024). Antimicrobial, preservative, and hazard assessments from eight chemical classes. ACS Omega, 9, 17869–17877. https://doi.org/10.1021/acsomega.3c08672
• Pang, D., Ni, Y., Li, H., Zhou, G., Li, Y., & Xu, B. (2025). Antimicrobial mechanism of microbial preservatives. Food Microbiology, 132, 104833. https://doi.org/10.1016/j.fm.2025.104833
• Perry, B. (2018). Preservation efficacy testing in the cosmetics and toiletries industries.
• Pinto, L., & Ayala-Zavala, J. (2024). Application of plant antimicrobials in the food sector. Foods, 13. https://doi.org/10.3390/foods13142222
• Pinto, L., Tapia-Rodríguez, M., Baruzzi, F., & Ayala-Zavala, J. (2023). Plant antimicrobials for food quality and safety. Foods, 12. https://doi.org/10.3390/foods12122315
• Russell, A. D. (2003). Challenge testing: Principles and practice. International Journal of Cosmetic Science, 25, 147–153. https://doi.org/10.1046/j.1467-2494.2003.00179.x
• Sajjadi, S., Shafizadeh, F., & Hallaj-Nezhadi, S. (2025). Comparative study of preservative efficacy tests across pharmacopeias. Journal of Medical Microbiology and Infectious Diseases. https://doi.org/10.61882/jommid.13.2.78
• Senthilkumar, K., et al. (2024). Preparation of self-preserving personal care cosmetic products. Scientific Reports, 14. https://doi.org/10.1038/s41598-024-57782-9
• Silvério, L., et al. (2025). Coffee and rosemary extracts as sustainable preservatives. Cosmetics. https://doi.org/10.3390/cosmetics12040147
• Watanabe, M., Mitani, A., Sato, J., Sonoda, T., & Shigemune, N. (2025). Alternative to preservation efficacy tests using metabolic activity. Journal of Microorganism Control, 30, 71–79. https://doi.org/10.4265/jmc.30.3_71
• Wu, Y., Jiang, L., Ran, W., Zhong, K., Zhao, Y., & Gao, H. (2024). Antimicrobial activities of natural flavonoids. Food Chemistry, 460, 140476. https://doi.org/10.1016/j.foodchem.2024.140476