OXIDATIVE STABILITY OF COSMETIC EMULSIONS WITH PLANT EXTRACTS 191 METHODS FOR ASSESSING THE OXIDATIVE STABILITY OF COSMETIC EMULSIONS Scientists have developed various methods to determine primary and secondary oxi- dation process products of lipid- and oil-based formulations (Figure 1). Primary oxi- dation products include lipid hydroperoxides and conjugated diene hydroperoxides, which may be determined spectrophotometrically. Spectropohotometric measurement of lipid hydroperoxide concentration is based on the fact that iron catalyzes the formation and decomposition of lipid hydroperoxides (ROOH) into highly reactive peroxyl (ROO•) and alkoxyl (RO•) radicals. These radicals further interact with other unsaturated lipid components in the formulation, leading to the formation of additional radicals (16–23). The peroxide value (PV) is used for quantifi cation of hydroperoxides (21). Lipid hydroperoxides are measured at higher wavelengths than conjugated diene hydro- peroxides (17,19). Generally viewed, interaction between molecular oxygen and un- saturated fatty acids in the presence of heat, light, or chemical initiators leads to the production of hydroperoxides. Secondary oxidation products involve aldehydes, ketones, alcohols, hydrocarbons, organic acids, and epoxy compounds, which are formed after decomposition of intermediate products (alkoxyl radicals). Volatile secondary products may affect the odor of cosmetic emulsion (19,21). Volatile compounds detected in stability studies include propanal, 2-butenal, 2-pentenaol, and 2,4-hexadienal which resulted from the oxidation of n–3 polyunsaturated fatty acids (PUFAs), as well as pentane, hexanal, 2-heptenal, and 2-octenal, formed by the oxidation of n–6 PUFAs (16). The validated qualitative method for evaluating volatile secondary oxidation products in emulsions is gas chromatography tandem mass spectrometry, while pentanal and hexanal are used as standards (24). Monitoring hexanal concentration in oil-in-water emulsions is possible by using gas chromatography (25). Figure 1. Methods for assessing the oxidative stability of cosmetic emulsions.

JOURNAL OF COSMETIC SCIENCE 192 The volatile oxidation products (dicarboxylic acids) can be evaluated by using the Rancimat assay, whereas monitoring of lipid hydroperoxide intermediate formation can be followed by using the ferric–xylenol orange technique (24). Anisidine values (AVs) are indicators of the formed aldehydes during oxidation of fats or oils and can be determined spectrophoto- metrically by a technique approved by the American Oil Chemists’ Society (24,26). In ad- dition, the PV is an indicator of primary oxidation products (21). The total oxidation value (defi ned as 2PV + AV) is an indicator of production of both primary and secondary oxida- tion products and represents the overall stability of formulation (21). Secondary oxidation products can be determined by thiobarbituric acid measurements. This method is probably the most used method for measuring lipid peroxidation, in which malondialdehyde reacts with thiobarbituric acid to give a pink-colored species that absorbs at 532 nm (21,27). IMPACT OF ANTIOXIDANTS ON OXIDATIVE STABILITY OF COSMETIC EMULSIONS: THE ROLE OF PLANT EXTRACTS The addition of antioxidants in O/W formulations has been proposed as one of the effi cient methods to improve stability (28). Synthetic antioxidants that have been used include butylated hydroxyanisole, butylated hydroxytoluene (BHT), and tertiary butyl hydroquinone (TBHQ) (29). The potential of antioxidants to suppress oxidation processes strongly depends on their concentration, partitioning between the water phase and the interface, polarity, etc. (10). Hydrophilic antioxidants may be incorpo- rated into the water phase, whereas lipophilic antioxidants in the oil phase (30). The possibility to scavenge the radicals directly is increased if antioxidants are present in the oil phase, thus indicating greater effectiveness of nonpolar compounds (10). The current challenge is the assessment of the most suitable antioxidant for combating lipid oxidation in emulsions. Antioxidants can either protect the target lipids from oxidation initiators or disturb the propagation phase (29). Components able to inactivate or chelate metal ions as initiators of lipid oxidations are frequently added to cosmetic emulsions (10). There are growing demands for replacement of synthetic ingredients with natural products in the cosmetic industry (31). In that sense, manufacturers of cosmetic emul- sions are increasingly interested in using plant-derived products because of multiple benefi ts. Plant species are a valuable source of antioxidants which may act protectively on skin and aging-related changes on skin (21). Phenolic compounds are one of the most signifi cant classes of plants’ secondary metabolites, which possess remarkable an- tioxidant capacity (29). Apart from attenuation of skin’s oxidative damage, plant-based products are able to enhance the stability of cosmetic emulsions (21). It has been known that extraction solvent strongly affects the extract composition (32). A safety profi le is an important factor to be considered while choosing the solvent for plant extraction in cosmetology. So far, natural products extracted from nuts, fruits, herbs, and algae have been found to possess the potential to improve oxidative stability of O/W emulsions (Figure 2). The plant species described in this article include Sargassum muticum, Ulva lactuca, Fucus vesiculosus, Castanea sativa, Malpighia punicifolia, Rosa canina, and Salix alba. The choice of the plant species was based on their valuable chemical composition and prominent antioxidant and antimicrobial activities, which enables their applicability in