The Challenge of "Preservation Systems" in Makeup Removers: Antimicrobial Strategies Under High Water Activity

I. Makeup Removers Are Among the Highest Microbiological Risk Products in Cosmetics

A face cream has an oil phase, a serum has high-concentration active ingredients, a sunscreen has an emulsion system—these factors can all restrict microbial growth to some extent. But what about makeup removers?

The formulation base of makeup removers typically contains over 85% water, with low surfactant concentrations, extremely low or zero oil phase content, limited humectant usage, and a pH close to the neutral range. When all these characteristics are combined, the Water Activity (Aw) is almost equivalent to pure water, usually reaching as high as 0.98–0.99.

Water activity is a key variable in predicting whether microorganisms can grow. The minimum growth thresholds for common pathogens include: Gram-negative bacilli such as Pseudomonas aeruginosa can reproduce normally at a water activity of 0.97.

This means that makeup removers with water activity close to 0.99 provide theoretically the most favorable growth environment for the vast majority of bacteria, yeasts, and molds. Meanwhile, every time consumers open the bottle cap and contact the product with their fingers or cotton pads, they are introducing new microbial sources.

Data from FDA enforcement reports shows that among 142 cosmetics recalled due to microbial contamination between 2005–2025, Pseudomonas spp. was the most frequently detected contaminating microorganism, accounting for 56.34% of all microbiologically contaminated samples.

This is not a statistic to be taken lightly. For formulation engineers, the design of the preservation system for makeup removers is one of the most technically challenging aspects of the entire formulation development process.

II. The Essence of High Water Activity: Why Traditional Preservation Logic Fails Here

To understand the preservation challenges of makeup removers, one must first clarify the two adverse effects that "high water activity" imposes on the preservation system:

Effect 1: The Effective Concentration of Preservatives is "Diluted"

The mechanism of action for most traditional preservatives (phenoxyethanol, benzoic acid, sorbic acid, etc.) is to penetrate the microbial cell membrane in their undissociated molecular form and interfere with their metabolism. In high water activity systems, large amounts of water molecules are present, the osmotic pressure environment of microbial cells is close to normal physiological conditions, cell membrane integrity is higher, and the penetration ability of preservative molecules is correspondingly weakened. A simple analogy: in concentrated salt water, cells lose water and shrivel, making it easier for external preservatives to invade; while in an environment close to pure water, cells are in their "healthiest" state with the strongest resistance.

Effect 2: Reduced pH Buffering Capacity, Narrowed Effective Window for Preservation

As discussed in the previous article, the efficacy of many preservatives is highly dependent on pH. High water activity makeup remover systems typically have low buffer capacity, and pH is more prone to drift due to external disturbances (CO₂ dissolution, repeated opening, consumer contact). Once the pH deviates from the effective working range of the preservatives, the risk of preservation failure rises sharply.

III. Characteristics and Limitations of Common Preservatives in Makeup Removers

Phenoxyethanol: Most Widely Used, but with Coverage Blind Spots When Used Alone

Phenoxyethanol is currently one of the most widely used cosmetic preservatives globally. Research data shows that in surveys of preservatives for baby and sensitive skin products, phenoxyethanol ranks first in usage frequency among traditional preservatives, far exceeding other categories.

Its advantages include: phenoxyethanol remains stable across a broad pH range of 3–10, has broad-spectrum antimicrobial activity (covering Gram-positive bacteria, Gram-negative bacteria, and yeasts), and has good compatibility with common cosmetic ingredients.

However, phenoxyethanol used alone has obvious coverage blind spots: insufficient inhibition of molds and certain yeasts. In high water activity makeup remover systems, this weakness is further amplified—consumers use the product in humid environments such as bathrooms, where the probability of mold spore contact is significantly higher than in other product types.

Recommended dosage in makeup removers: 0.5–0.8% (slightly lower than the commonly used 1% in face creams), used in combination with synergistic preservative ingredients.

Benzoic Acid / Sodium Benzoate: Strong pH Dependency, Must Be Linked with Buffer Systems

The effective bacteriostatic form of benzoic acid is the undissociated benzoic acid molecule (pKa = 4.19). Organic acid preservatives like benzoic acid significantly lose their bacteriostatic activity when the pH exceeds 6 due to increased ionization.

For makeup removers targeting pH 6.5–7.0, the effective activity of the benzoic acid system is already near the critical state. If the pH drifts above 7.0, the risk of preservation failure rises sharply. This makes the actual preservation capability of benzoic acid in makeup removers without pH buffer system protection far lower than what its labeled concentration implies.

Polyol Synergistic Preservatives: The "Second Line of Defense" for High Water Activity Systems

Alkanediol (Polyol) ingredients have become the core breakthrough direction in makeup remover preservation system design in recent years. Their value lies in: reducing water activity while directly exerting bacteriostatic effects by disrupting microbial cell membrane structures, and being classified as "multifunctional ingredients" rather than "preservatives" in regulatory terms, aligning with Clean Label market positioning.

The primary mechanism by which alkanediol ingredients exert antimicrobial effects is membrane disruption. Alkanediols are amphiphilic structures that can embed into microbial cell membranes, disrupting the integrity of the membrane bilayer. The closer the distance between hydroxyl groups and the longer the fatty chain, the stronger the hydrophilic-lipophilic duality, making it easier to penetrate the membrane bilayer. For example, Caprylyl Glycol has been proven to have stronger bacteriostatic efficacy than Pentylene Glycol. Ethylhexylglycerin, as a multifunctional ingredient, is also widely used in cosmetics for its antimicrobial properties.

IV. Preservation System Design Framework for High Water Activity Makeup Removers

Strategy 1: "Primary Preservative + Polyol Synergy" Compound System

This is currently the most mainstream preservation solution for high water activity makeup removers in the industry.

Typical configuration schemes for polyol synergistic combinations include: compound examples replacing traditional preservatives—using Caprylyl Glycol (0.5–1%) + Phenoxyethanol (0.5–0.8%) to replace parabens; using Caprylyl Glycol (0.5–1%) + Pentylene Glycol (1–2%) to replace MIT/CMIT combinations. These combinations can pass challenge tests while avoiding ingredients that consumers are increasingly resistant to. Compared to traditional preservative combinations, costs are approximately 15–30% higher, but premium and natural-positioned brands are willing to bear this premium for a cleaner ingredient list.

Strategy 2: Reduce Water Activity — Fundamentally Weaken the Microbial Growth Advantage

Reducing the effective water activity of the formulation is a fundamental means of alleviating preservative pressure.

Research shows that reducing water activity by adding natural multifunctional ingredients in O/W emulsion systems can significantly enhance antimicrobial effects. Even adding just 0.5% Glyceryl Caprylate and Magnolia Bark Extract can further enhance the antimicrobial performance of the formulation. The evaluation of overall antimicrobial protection for the formulation is conducted with reference to the ISO 11930 standard, and testing must achieve Grade A or Grade B to be considered qualified.

Practical means to reduce water activity in makeup removers:

• Polyol Compounding: Compounding Glycerin (2%) + Propylene Glycol (3%) + Pentylene Glycol (2%) can achieve a total polyol concentration of 7% without causing stickiness, effectively reducing water activity.

• Betaine (1–3%): A natural humectant that also has a mild water activity-lowering effect while enhancing the product's skin feel smoothness.

• Trehalose (0.5–1%): A natural polysaccharide, Clean Label friendly, that reduces water activity while providing a moisturizing feel.

Strategy 3: Chelating Agent Synergy — Cutting Off the Mineral Supply for Microorganisms

Chelating Agents are often the most overlooked component in preservation systems, but their value is particularly prominent in high water activity systems.

EDTA (Disodium EDTA) is the most widely used chelating agent. Its mechanism of action is to chelate Ca²⁺ and Mg²⁺ ions in the formulation, disrupting the stability of the outer membrane of Gram-negative bacteria (such as Pseudomonas aeruginosa), making it easier for preservative molecules to penetrate the cell membrane and synergistically enhancing the bacteriostatic ability of the overall preservation system.

Recommended dosage: Disodium EDTA, 0.05–0.1%, used in combination with the primary preservative system in makeup remover formulations.

For products targeting the organic certification (COSMOS) market, Phytic Acid or Gluconic Acid can replace EDTA, but it should be noted that the synergistic ability is relatively weaker and must be verified through challenge testing.

V. Challenge Test: The Only Credible Verification Method for Preservation Systems

No matter how well-designed the preservation system is, before laboratory validation, it is merely a theoretical hypothesis.

The challenge test transforms the theoretical preservation design in the formulation (water activity, preservative compounding, packaging selection, pH, chelating agents) into concrete evidence, proving that the product will not become a breeding ground for microorganisms after leaving the factory. For EU compliance (CPSR & PIF), the challenge test is one of the indispensable supporting documents.

The core judgment indicators of the ISO 11930 standard are: bacteria must achieve a ≥3 log reduction at 7, 14, and 28 days (Standard A); yeasts and fungi must show no growth or achieve a ≥1 log reduction at specified time points. This test is primarily applicable to water-soluble or water-miscible cosmetics, which perfectly matches the formulation attributes of makeup removers. Its revised version (ISO/AWI 11930) is currently being drafted and will replace the 2019 version.

Typical scenarios requiring ISO 11930 testing include: high water content formulations (lotions, gels, toners) designed for long shelf life; products in wide-mouth jars or dropper packaging that consumers repeatedly contact; and any reformulation that adjusts pH, surfactant systems, or preservative dosages. Whenever the preservation system, raw material supplier, or packaging type changes, or every 3 years for compliance updates, it is recommended to re-conduct the challenge test.

Conclusion: The Preservation System is the Last Line of Defense for Product Safety

Makeup removers are high-frequency daily cleansing products. Repeated opening, cotton pad contact, and humid bathroom environments—every use puts pressure on the preservation system. A preservation scheme that has not been systematically designed and validated through challenge testing can typically only maintain safety for the first few months after leaving the factory under such usage scenarios, after which microbial risks will accumulate over time.

The claim on the label has to survive the stability test.

Brands specify an active for the story, then discover the story does not hold at month 24. We select the delivery system and the preservative compatibility together, so the claim is still true when the product is on shelf.

That is the gap between a marketing concept and a sellable formula — and it is closed in R&D, not in QC.

By collaborating with Explore our formulation and R&D capability you gain access to industry-leading expertise and innovative formulations that set your brand apart in the competitive global market. Tell us the claim and the target market; we will tell you what is achievable and what it costs.