I. Types and characteristics of antioxidants
1. Natural antioxidants
Vitamin E (tocopherol)
A fat-soluble vitamin, the chemical structure contains two major categories of tocopherols and trienyl tocopherols, of which α-tocopherol is the most widely distributed and active form in nature. Its strong antioxidant capacity stems from the hydroxyl group and double bond structure, which can stabilise free radicals and delay oxidative damage to cells.
Characteristics: fat-soluble, heat-resistant (not decomposed at 220°C), but sensitive to light and oxygen, easily discoloured.
Mechanism of action: As a chain-terminating antioxidant, it provides hydrogen atoms to neutralise free radicals and inhibit lipid peroxidation.
Application: Widely used in anti-aging creams and eye creams, often compounded with vitamin C to enhance the effect.
Vitamin C (ascorbic acid)
Chemical name: L-ascorbic acid (L-ascorbic acid)
Physical Properties:White crystalline powder, easily soluble in water, acidic taste; sensitive to heat, light, oxygen, high temperature or long time exposure to oxidation failure.
Stability:Stable in acidic environment (e.g. lemon juice), easy to decompose in alkaline or neutral environment; the loss rate can reach 30%-50% when cooking (e.g. boiling vegetables).
Characteristics: Water-soluble, poor stability (easy to oxidise when exposed to light and heat), need derivatives (e.g. ascorbyl palmitate) to improve stability.
Mechanism of action: scavenging free radicals, promoting collagen synthesis, inhibiting melanin production.
Application: whitening essence, sunscreen products, need to avoid light packaging.
Tea Polyphenols (Green Tea Extract)
Tea polyphenol is the core active ingredient of green tea, accounting for about 30% of the dry matter content, mainly extracted from tea and its by-products (e.g., tea powder, pruning leaves), its molecular structure contains hydroxyl and double bonds, giving it a strong antioxidant capacity, scavenging free radicals is 18 times more effective than Vitamin E, and synergistic effect with Vitamin C.
Characteristics: Polyphenols, antioxidant capacity is 18 times that of vitamin E, easy to change colour when encountering metal ions.
Mechanism of action: through the phenolic hydroxyl group for hydrogen termination of free radical chain reaction, inhibit tyrosinase activity.
Application: Anti-allergy, anti-inflammatory skin care products, commonly found in serums and masks.
Coenzyme Q10 (ubiquinone)
A fat-soluble benzoquinone compound, the molecular structure contains 10 isoprene units, widely found in yeast, plant leaves and animal organs, is a key component of cellular energy metabolism, as the only renewable fat-soluble antioxidant in the human body, its content decreases with age, peaks at 20 years old, Participate in mitochondrial oxidative phosphorylation process, promote adenosine triphosphate (ATP) production, maintain cell viability.
Characteristics: Fat-soluble, endogenous antioxidant, decreases with age.
Mechanism of action: Participate in mitochondrial energy metabolism, neutralise free radicals, repair cell membranes.
Astaxanthin
Astaxanthin is a ketocarotenoid, chemically known as 3,3′-dihydroxy-4,4′-diketo-β,β′-carotene, is a red solid powder, fat-soluble and insoluble in water, the natural sources of red algae, salmon, krill and other aquatic organisms. Its antioxidant capacity far exceeds that of vitamin E, lutein, etc. It is one of the strongest known natural antioxidants, and will not be converted to vitamin A, avoiding the risk of pro-oxidation.
Mechanism of action: scavenging single-line oxygen, blocking free radical chain reaction; inhibit tyrosinase activity, reduce melanin production and accelerate the metabolism; enhance the skin’s ability to lock water, to assist in the protection against UV damage.
Applications: high-end anti-aging products, sunscreen.
2. Synthetic antioxidants
Butylhydroxytoluene (BHT)
It is a synthetic antioxidant in the form of white crystals or powder.It is resistant to high temperatures and metal ions, and is widely used in cosmetics to prevent lipid oxidation. Its molecular structure provides hydrogen atoms to neutralise free radicals, blocking oxidative chain reactions and effectively protecting product ingredients from oxidative degradation. The European Union’s Scientific Committee on Consumer Safety (CSSC) recommends limiting the concentration in cosmetic products (≤0.1% for resident products and ≤0.8% for rinse-off products) to avoid potential endocrine disruption risks. Under the concept of ‘pure skincare’, some brands have included it in the banned list, giving preference to milder stabilisation solutions and natural antioxidants.
Characteristics: Good thermal stability, odourless, no discolouration when exposed to metals, but potentially carcinogenic and controversial.
Application: Oil-based cosmetics (e.g. make-up remover oils, creams), the amount added is usually 0.01%~0.1%.
Tert-butylhydroquinone (TBHQ)
Chemical name is tert-butylhydroquinone (C₁₀H₁₄O₂), a white crystalline powder, melting point 126.5-128.5°C, boiling point 300°C, easily soluble in ethanol, ether, and grease, insoluble in water, and heat-stable, but it may take on a pinkish colour in the presence of light or alkaline conditions. It protects the stability of ingredients by providing hydrogen atoms to neutralise free radicals and block oxidative chain reactions. Concentration limits to be observed in cosmetic products are ≤ 0.8 per cent for drench products and ≤ 0.1 per cent for resident products.
Characteristics: Highly efficient antioxidant, high temperature resistant, suitable for high unsaturated fatty acid products.
Applications: emulsions and essences with high stability requirements.
Ethylenediaminetetraacetic acid (EDTA)
It is an organic polyacid chelating agent with chemical formula C₁₀H₁₆N₂O₈, white powder at room temperature, insoluble in water, but its sodium salt form (such as EDTA-2Na) is soluble in water, widely used in cosmetics. It protects the stability of product ingredients by forming stable complexes with metal ions (e.g. Ca²⁺, Mg²⁺, Fe³⁺, etc.) and blocking oxidation or precipitation reactions involving metal ions.
Characteristics: Metal chelating agent, preventive antioxidant, reduce metal ion catalysed oxidation.
Application: Used with other antioxidants, commonly used in cleaning products.
Mechanism
1. Chain termination: directly provide hydrogen atoms or electrons to neutralise free radicals (e.g. vitamin E, tea polyphenols).
2. Preventive type:
Chelate metal ions (such as EDTA, citric acid), blocking oxidation catalysis.
Decompose peroxides (e.g. thiodipropionic acid) to reduce free radical generation.
3. Enzymes: such as superoxide dismutase (SOD), catalysing the conversion of superoxide radicals into harmless substances.
III. Application
Compounding strategy:
Water-soluble with oil-soluble (e.g. vitamin C + vitamin E).
Chain-terminating and preventive synergistic (e.g. BHT + citric acid).
Product adaptation:
Sunscreen: astaxanthin + vitamin E for enhanced photoprotection.
Serum: Coenzyme Q10 + peptides for anti-aging and repair.
Sensitive skin products: tea polyphenols + liquorice extract, anti-inflammatory and antioxidant.