Citric Acid: A Multifunctional Ingredient in Modern Oral Hygiene

Quick Summary

• Citric acid is a weak organic acid (multiple pK_a) widely used as a pH regulator and buffer and a chelating agent (citrate formation).
• In toothpastes, it helps to stabilize pH, optimize the performance of the surfactant system, and improve the compatibility of certain active ingredients (e.g., tin, peroxides in low-concentration whiteners, flavors).
• The cleaning effect is indirect: by controlling pH and complexing ions, it promotes homogeneous cleaning and foam maintenance, without replacing abrasives or mechanical brushing.
• Its safety is widely supported in food and cosmetics; in oral hygiene, it must be used in controlled concentrations and in non-erosive formulas.
• Erosion depends on pH, contact time, and concentration. Quality toothpastes are formulated with a neutral or near-neutral pH and limit effective acidity.
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What is Citric Acid and Why is it Used in Toothpastes?

Citric acid is naturally present in citrus fruits (lemon, orange, lime) and acts as an intermediate in the Krebs cycle. Industrially, it is mostly obtained by fermentation of sugary substrates using Aspergillus niger, an efficient, standardized process with a low impurity footprint if properly controlled.

In the context of toothpastes, citric acid does not whiten on its own or remove plaque through direct chemical reaction; its primary role is technological: to adjust pH, buffer variations during storage and use, and sequester ions (Ca²⁺, Mg²⁺) that affect foam and the stability of some active salts. All of this results in a more stable toothpaste with consistent foam and a uniform brushing experience.

Properties and Benefits in Toothpastes

1) pH Regulation and Buffering

Controlled pH is key for: (a) the stability of active ingredients (e.g., some tin salts or peroxides in low-concentration whiteners), (b) oral mucosa comfort, and (c) preservation (moderate acidity helps limit microbial growth, although the main protection comes from approved preservatives).

Citric acid, in equilibrium with its conjugate base (citrate, usually as sodium/potassium), forms a buffer system that maintains the pH within a target range (usually near neutral in conventional toothpastes). This buffering ensures that the product's pH does not "change" due to small variations in formulation, temperature, or during brushing.

Key idea: toothpastes with a very acidic pH and prolonged exposure could increase the risk of erosion. Therefore, for daily hygiene, the best formulas are designed to be non-erosive, with controlled pH and short contact times.

2) Calcium Chelation and Hardness Control

Citrate (derived from citric acid) is a chelating agent: it reversibly binds to ions such as Ca²⁺ and Mg²⁺. Why is this important in toothpaste? (i) because calcium ions present in water or saliva can destabilize foam and affect surfactant performance, and (ii) because certain active ingredients benefit from an environment with lower hardness to remain effective.

In "tartar control" formulas, the anti-calculus strategy usually relies primarily on pyrophosphates and/or zinc citrates. Citric acid—and its citrate pair—contribute to adjusting the environment, but do not replace these specific agents; their role is one of technological support.

3) Support for Cleaning and Foam

Surfactants (e.g., SCI, SLS, SLES, amphoterics) clean by reducing surface tension and forming micelles that help detach biofilm. Citric acid, by optimizing pH and chelating cations that harden water, can improve the stability and creaminess of the foam. It does not "clean" on its own, but it improves the performance of the surfactant system, especially when the water is hard.

In "whitening" toothpastes with peroxide (low concentrations), citric acid can act as a stabilizer/pH adjuster. In these cases, special vigilance of the final pH is required because excessively low levels increase the risk of erosion; responsible formulation uses buffers and clear usage times/indications to prevent this.

4) Formula Stability and Compatibilities

The stability of a toothpaste depends on many factors: pH, abrasive dispersion, surfactant-flavor compatibility, fluoride interaction, storage temperature, etc. Citric acid:

• Reduces pH fluctuations (citrate buffer systems), stabilizing flavor and sensory perception over time.
• Improves the compatibility of some salts sensitive to water/saliva hardness.
• Can support the stability of peroxides in low-concentration whiteners when the target pH is correctly defined and buffered.

Again, balance is key: the goal is a toothpaste that cleans well, is pleasant, and remains stable without compromising enamel or gums.

Health Impact and Erosion Risk

Citric acid is widely accepted in food and cosmetics. In oral hygiene products, the critical parameters are the toothpaste's final pH and contact time. Exposure is brief (2–3 minutes) and the salivary buffer rapidly neutralizes acids; therefore, a well-designed toothpaste with a pH close to neutrality does not increase the risk of erosion.

Dental erosion caused by acids (including citric acid) has been demonstrated in in vitro and in situ studies when the pH is low and contact is prolonged (acidic drinks, citrus candies, prolonged acidic rinses). In toothpastes, modern formulation avoids these scenarios by balancing pH and limiting effective acidity. If you experience dentinal hypersensitivity or previous erosion, consult your dentist to select a toothpaste with an appropriate pH and low RDA.

Clinical summary: the risk is not determined by "an isolated ingredient," but by the entire formula (pH, abrasivity, surfactant) and how it is used (brushing technique, time, frequency).

Safety and Regulation

• FDA (21 CFR §184.1033): citric acid is affirmed as GRAS for direct use in food under Good Manufacturing Practices.
• CIR (Cosmetic Ingredient Review) 2014: assessment of citric acid and citrates concludes they are safe in current practices of use and concentration in cosmetics.
• CosIng (European Commission): official listing of citric acid (INCI: CITRIC ACID; CAS 77-92-9) with functions of pH adjuster, buffering, and chelating.
• Industrial production: mostly by fermentation with Aspergillus niger, standardized and widely documented.

Citric Acid vs. Other pH Regulators

In summary: citric acid/citrates offer a very practical balance between pH control, compatibility, and cost, provided it is buffered to maintain a non-erosive formula.

How we integrate it into ONAK®

• We use citrate/citric acid systems to buffer the pH within the functional range of our active ingredients and ensure oral mucosa comfort.
• We calibrate compatibility with mild surfactants (e.g., SCI/amphoterics) and with controlled cleaning abrasives (e.g., silica/CaCO₃), seeking an optimal cleaning–gentleness balance.
• We validate accelerated stability (thermal cycles), pH, rheology, and sensory attributes to ensure consistency throughout the product's shelf life.

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Frequently Asked Questions

Is citric acid harmful to teeth?

In adequate concentrations and with buffered pH, its use is safe and functional. Erosion appears with very low pH and prolonged exposures (which is not the scenario for a well-formulated toothpaste).

Does it always cause dental erosion?

No. Erosion depends on pH, concentration, and time. Modern toothpastes control these factors to ensure the formula is non-erosive.

Does it help whiten teeth?

It can indirectly contribute to a whiter appearance by facilitating the removal of extrinsic stains (via foam and cleaning). It is not an oxidizing whitener per se.

Why not just use baking soda?

Baking soda is a good buffer, but the citric/citrate pair offers a versatile buffering profile and helps with hardness (chelation) and compatibility with certain active ingredients.

Does it really come from citrus fruits?

Today, it is primarily produced by fermentation with Aspergillus niger. It is a safe and efficient process, widely used in food and cosmetic ingredients.

Curiosities

• Scientific history: isolated in 1784 by Carl W. Scheele from lemon juice.
• Metabolism: it is an intermediate in the Krebs cycle, present in virtually all living organisms.
• Modern fermentation: industrial production by microorganisms allows for a stable and sustainable supply.

Conclusion

Citric acid is a valuable multifunctional ingredient in toothpastes: it controls pH, aids in stability, and supports cleaning and foam. Properly buffered and within responsible formulation practices, it contributes to an effective and comfortable experience without compromising enamel. If you are looking for balanced toothpastes geared towards daily care, the citric/citrate system is a great technological ally.

References

1. Effect of citric acid on tooth enamel: an in vitro study. Journal of Applied Oral Science. (Study on erosive effect under laboratory conditions).
2. Fiume MM, et al. Safety Assessment of Citric Acid, Inorganic Citrate Salts, and Alkyl Citrate Esters as Used in Cosmetics. Int J Toxicol. 2014;33(2_suppl):16S–46S. Conclusion: safe in current practices of use and concentration.
3. FDA eCFR. 21 CFR §184.1033 – Citric acid. Citric acid affirmed as GRAS for direct use in food under GMP.
4. CosIng (European Commission). CITRIC ACID (CAS 77-92-9). Functions: pH adjuster, buffering, chelating (cosmetic ingredient database).
5. Kim JH, et al. Effects of a commercial whitening toothpaste containing hydrogen peroxide. BMC Oral Health. 2023. Note: some whiteners include citric acid as a stabilizer/pH adjuster; low pH increases erosion risk if not buffered.
6. de Souza BM, et al. Effect of different citrus sweets on the development of erosive tooth wear. 2020. Evidence of erosive tooth wear from prolonged acid exposure (foods/drinks), not generalizable to well-buffered toothpastes.
7. Show PL, et al. Overview of citric acid production from Aspergillus niger. 2015. Industrial review of the fermentative process.