The topic is short and compact
L-ascorbic acid only penetrates the skin significantly at a pH value below 3.5; the acidic environment also improves stability, but does not protect against air and light.
Derivatives such as phosphates or 3-O-ethyl ascorbic acid are more stable, but their conversion in the skin must be proven for each raw material and limits the efficacy claims.
Yellow or brown discoloration indicates oxidation to dehydroascorbic acid; therefore, the active ingredient content is tested, not just the color.
With a shelf life of up to 30 months, a best-before date is mandatory, which limits batch size and storage duration.
Stability must be tested in the final sales container; changing packaging materials after testing requires new data.
So läuft die Entwicklung bei Labtree
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Konzeptionierung
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Bemusterung – Standardmuster in 24 h
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Individualisierung
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Prototyping
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Produktion
White Label 2–3 Monate · Private Label 3–6 Monate · Produktion ab 1.000 Stück
Which form of vitamin C is right for which product concept?
The choice of the vitamin C form determines the pH range, solvent system, packaging, and the potential claim. It is therefore the first decision in the briefing, not a detail of formulation adjustment.
L-Ascorbic Acid
L-ascorbic acid is the most biologically active and well-studied form, yet hydrophilic and unstable. According to a review in the Journal of Clinical and Aesthetic Dermatology, it only penetrates the skin significantly at a pH below 3.5 because the molecule is uncharged at this point; commercially available serums range from 10 to 20 percent, and higher concentrations provide no additional benefit according to the same study, but can cause irritation. The review in the Indian Dermatology Online Journal likewise describes the effect as concentration-dependent only up to 20 percent. For the formulation, this means: a strongly acidic, aqueous or glycolic system that may tingle sensorially and whose skin compatibility must be evaluated in the safety report.
Phosphate Derivatives (Sodium and Magnesium Ascorbyl Phosphate)
The phosphate group makes the molecule more stable; it is typically formulated within a pH range closer to the skin than L-ascorbic acid. According to a review in the journal Nutrients, however, these derivatives are only slowly converted to ascorbic acid in the skin and are poorly absorbed. Those working with phosphate derivatives gain stability and skin compatibility, but must phrase the efficacy claim more cautiously accordingly.
3-O-Ethyl Ascorbic Acid
In this derivative, an ethyl group is attached to the third carbon atom. As a characterization study in the International Journal of Pharmaceutics: X shows, this protects the 3-OH group from ionization and thus the molecule from oxidation. With a log P of −1.07 compared to −1.85 for ascorbic acid, it is less hydrophilic, yet not fat-soluble, and is therefore typically processed in aqueous-glycolic systems. The same study points out that forms esterified at C6, such as ascorbyl palmitate, showed no improved stability in aqueous solutions and formulations.
Lipophilic Esters
Oil-soluble esters such as tetrahexyldecyl ascorbate are suitable for anhydrous serums and oils. The review in Nutrients notes for derivatives with lipophilic groups that they are not necessarily more stable and their conversion in the skin is questionable. Whether and to what extent an ester becomes ascorbic acid must therefore be substantiated for each raw material. The use concentration is based on the raw material supplier's data and the safety assessment, not on a blanket range.
Why does a vitamin C serum change color?
A vitamin C serum discolors because ascorbic acid oxidizes into dehydroascorbic acid, which causes a yellow color; the mentioned review in the Indian Dermatology Online Journal describes this for serums exposed to light. According to the study on 3-O-ethyl ascorbic acid, the cause lies in the electron-rich enediol group in the ring, which makes the molecule vulnerable under both aerobic and anaerobic conditions. Yellow to brown discoloration is therefore the visible sign of active ingredient loss.
Typical formulation problems
pH drift: If the pH shifts upward, L-ascorbic acid generally degrades faster. The pH is therefore tracked throughout stability testing, not just during manufacturing.
Oxygen in the product: Air introduced during stirring or filling is sufficient for measurable oxidation. Typically, stirring is performed with low air intake, and the headspace in the packaging is kept small.
Metal ions: Traces of iron or copper from raw materials or equipment components catalyze oxidation. Chelating agents and testing raw material specifications are therefore part of the formulation.
Viscosity at low pH: Many common thickeners, such as neutralized carbomers, build hardly any viscosity below pH 4. In acidic serums, acid-stable thickeners are generally used, which alters texture and sensory profile.
Antioxidants as part of the system
Vitamin C is frequently combined with vitamin E and ferulic acid. The review by Al-Niaimi and Chiang describes that ferulic acid in such a combination contributes both to stabilization and to adjusting the pH below 3.5; the review in the Indian Dermatology Online Journal cites 15 percent vitamin C, 1 percent vitamin E, and 0.5 percent ferulic acid as the studied combination. Before adopting such a combination, a patent review is recommended. Alternatively, anhydrous systems or suspensions are chosen in which L-ascorbic acid has less contact with water; however, they require more effort regarding dispersion and skin feel.
What stability tests does a vitamin C serum need?
According to Annex I Part A No. 2 of Regulation (EC) No 1223/2009, the safety report must demonstrate the stability of the cosmetic product under reasonably foreseeable storage conditions. The Commission Guidelines on Annex I (Implementing Decision 2013/674/EU) add that the minimum durability and period-after-opening are derived from these data, and that the composition tested must correspond to the product placed on the market. According to the guidelines, physical stability tests are carried out using inert containers and the packaging intended to be used on the market.
Test protocol for a vitamin C serum
Accelerated storage: The Cosmetics Europe stability guideline lists 37, 40, or 45 °C over one, two, or three months as common conditions, depending on the product type. As a control, it mentions product in glass containers, for example; for vitamin C, a refrigerated reference sample is usually also included.
Active ingredient content: The ascorbic acid or derivative content is determined analytically at each test point, usually by HPLC. Color alone is not a sufficient indicator.
Color, odor, pH, and viscosity: These parameters show early on whether the system is destabilizing and belong in every testing phase.
Light stability: According to Cosmetics Europe, products whose packaging allows light to pass through should undergo light testing. This applies to clear glass and transparent plastic bottles.
Smallest packaging size: If there are multiple sizes, the guideline recommends testing in the smallest container; if there are multiple packaging material types, each type should be tested.
Accelerated results are a prediction. Cosmetics Europe recommends confirming them on production batches in the final packaging, for example using retain samples at room temperature. The article Stability and Safety Testing for Cosmetics describes how stability and challenge tests are planned overall.
Which packaging material protects vitamin C?
For a vitamin C serum, the packaging material is part of the stabilization. The guidelines on Annex I state: If a formulation is sensitive to light or air and would change in a way that impairs safety or efficacy, suitable packaging must be used. At the same time, Annex I Part A No. 4 of the Regulation requires information on the relevant characteristics of the packaging material, in particular purity and stability; the guidelines mention interactions with the formulation, barrier properties, and migration as influencing factors.
Packaging material | Protection | Limitation |
|---|---|---|
Airless dispenser | No air backflow, virtually no headspace during use | Check residual emptying and dosage depending on viscosity; avoid metal springs in the product path where possible |
Dropper bottle made of amber or violet glass | Light protection | Each dispensing introduces air; better suited for more stable derivatives |
Aluminum tube with internal coating | Light and oxygen barrier | Coating compatibility with highly acidic formulations must be proven |
Single doses or ampoules | Each dose remains sealed until application | Higher packaging and filling effort per milliliter |
The choice of packaging affects filling and quantity: airless components and specialty glasses have their own minimum order quantities from suppliers and sometimes longer lead times. It should therefore be finalized before accelerated stability testing, as a subsequent change requires retesting in the final container. MOQ in cosmetics explains what factors determine minimum quantities in detail.
Best-before date or shelf life after opening?
Art. 19(1)(c) of Regulation (EC) No 1223/2009 requires a date of minimum durability if the minimum durability is 30 months or less. If it exceeds 30 months, the jar symbol according to Annex VII No. 2 is used instead to indicate how long the product can be safely used after opening.
For serums with L-ascorbic acid, stability testing may result in a shelf life of 30 months or less; in this case, the date of minimum durability must be indicated. This has practical consequences:
The date is applied to packaging materials or folding boxes per batch, which determines printing or labelling during filling.
Storage time in one's own warehouse and at retail counts against the shelf life. Large initial batches with slow sell-through represent a cost risk with unstable active ingredients.
If stability depends on storage conditions, such as refrigeration, these conditions must be communicated along the supply chain according to the guidelines and, where relevant to consumers, indicated on the label.
The period after opening is justified by data, not estimated. For dropper bottles, it is often shorter than for airless dispensers due to repeated contact with air.
Which advertising claims can be substantiated?
Advertising claims must meet the common criteria of Regulation (EU) No 655/2013. For vitamin C serums, two points in the Annex are particularly relevant: claims about the properties of an ingredient must not imply that the finished product has the same properties if it does not (point 2, paragraph 2); and anyone attributing the properties of an ingredient to the finished product requires sufficient and verifiable evidence, such as an effective concentration in the product (point 3, paragraph 6).
Percentages: “15% Vitamin C” refers to the specific form used. If a derivative is claimed, it is advisable to indicate clearly that it is not 15 percent L-ascorbic acid. The content should be proven over the stated shelf life, not just at the time of manufacture.
Efficacy claims: Raw material supplier studies on radiance or pigmentation spots do not automatically prove the efficacy of the finished formulation. A product claim requires its own evidence or a substantiated extrapolation, such as using the same concentration in a comparable base.
“Stabilised”: This claim requires stability data in the final packaging showing a defined residual content over the shelf life.
Since the claim determines the scope of testing, it is defined in the briefing. The rules are covered in detail in the article Cosmetic advertising claims in the EU.
The duration depends primarily on whether an existing, tested formulation is adapted or a new one is developed. At Labtree, a white label product based on a market-ready, tested formulation with standard packaging is generally ready for delivery in 2 to 3 months; the standard formulation remains the property of Labtree. A private label new development according to brand specifications usually takes 3 to 6 months until market readiness; formulation rights or exclusivity can be contractually transferred to the brand. With vitamin C, stability data and, if applicable, efficacy tests determine the timeline more than laboratory work on texture.
When which model is suitable
White Label: when an established form of vitamin C, standard packaging, and modest claims are sufficient. If stability data is already available for the base, this shortens the path to the safety assessment, provided formulation and packaging remain unchanged.
Private Label: when an active ingredient combination, concentration, anhydrous system, or special packaging is intended to drive differentiation. In that case, new stability tests in the final container must be scheduled.
Labtree is a B2B cosmetics developer and cosmetics manufacturer based in Hamburg with its own laboratory and production capacities in Germany. Formulation development, sampling, feasibility testing, and cosmetics production fall under a single responsibility. Samples of pre-qualified formulations typically leave the sample warehouse within 24 hours, and standard sample shipping is free of charge. This allows the sensory profile and color of a base to be evaluated before stability testing begins. The minimum order quantity is generally 1,500 units per product, depending on product type, fill size, and packaging. The distinction between the models is explored in depth in White Label Kosmetik: Vorteile und Grenzen.
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FAQ
What concentration of vitamin C is effective in a serum?
For L-ascorbic acid, review articles cite a standard market range of 10 to 20 percent; according to them, higher concentrations provide no additional benefit, but can cause irritation. For derivatives, the practical concentration depends on the raw material data and safety assessment. A higher dose is not automatically more stable or more effective.
Is an anhydrous vitamin C serum more stable?
Generally yes, because L-ascorbic acid has less contact with dissolved oxygen and water in anhydrous systems. However, it is hardly soluble in oils and then exists as a suspension, which makes dispersion, skin feel, and filling consistency more challenging. Alternatively, oil-soluble esters are used, although their efficacy in the skin must be substantiated for each specific raw material.
Does a vitamin C serum need a preservative challenge test?
Annex I to Regulation (EC) No 1223/2009 requires the results of the preservative challenge test to be included in the safety report. According to the Commission Guidelines (Implementing Decision 2013/674/EU), products with a very low pH or those based on organic solvents may be considered microbiologically low-risk; in such cases, a scientific justification is sufficient instead of the test. The safety assessor determines whether an acidic or anhydrous serum falls into this category; otherwise, the test is generally conducted in accordance with ISO 11930.
Who owns the formulation of a vitamin C serum?
With white label, the tested standard formulation remains the property of the manufacturer, while the brand adapts it and sells it under its own name. With private label, the formulation is newly developed according to the brand's requirements, and rights or exclusivity are regulated contractually. How formulation ownership can be secured is covered in the Labtree article "Confidentiality with the contract manufacturer: NDA and formulation ownership".
Can vitamin C be combined with niacinamide or acids?
Technically, this is possible, but every combination shifts the pH value, skin compatibility, and stability, and must be tested in the final packaging. L-ascorbic acid requires a very low pH value, whereas many other active ingredients are formulated within a range closer to the skin's natural pH. It is often more practical to split the active ingredients across two products within a line.
What is the minimum order quantity for a vitamin C serum?
Labtree's minimum order quantity is generally 1,500 units per product; the exact quantity depends on the product type, fill size, and packaging. For vitamin C serums, minimum order quantities from packaging suppliers for airless dispensers or specialty glass are often added. Because the shelf life of unstable active ingredients is limited, the batch size should match expected sales.
