OXIDATIVE STABILITY OF COSMETIC EMULSIONS WITH PLANT EXTRACTS 195 bur hydrolysis extract (collected in Spain) was added to avocado cream to test its potential to improve oxidative stability of the product. The results have shown the ability of chest- nut extract to inhibit lipid peroxidation (determined by peroxide and anisidine values), which was in correlation with the phenolic content (21). MALPIGHIA PUNICIFOLIA (MALPIGHIACEAE) Acerola (Malpighia punicifolia L.) or Barbados cherry is a tropical plant whose fruit is consider to be a good source of ascorbic acid, phenolic compounds, fl avonoids, carot- enoids, anthocyanins, and phenylpropanoids. Acerola extracts or isolated compounds showed anticancer, antihyperglycemic, anti-infl ammatory, skin protective, and antioxi- dant properties (46,47). The protective effect of acerola fruit on human dermal fi bro- blasts is related to a high content of vitamin C, folic acid, and polyphenols (48). Oxidative stability of acerola fruit hydroglycolic extract, added to oil–water cosmetic emulsion in two different concentrations (1 and 5%), was determined spectrophoto- metrically following peroxide formation due to various storage conditions. Emulsion was made with wheat germ oil and stored at 5°C for 6 mo, 20°C for 6 mo, and 40°C for one month. Higher concentration of acerola extract showed better oxidative stabil- ity and slower rate of peroxide formation than the control sample (without antioxi- dants) and 1% of acerola extract emulsion (49). Findings of the earlier study indicated that lower concentration of antioxidants used in formulation can lead to pro-oxidant properties after long time of storage (50). Moreover, it was reported that antioxidants such as ascorbic acid used in lower concentration promote oxidation by increasing cat- alytic activity of iron and copper, whereas antioxidant properties manifest when high concentration of ascorbic acid is used. Additional property of ascorbic acid could be oxidative degradation of proteins and lipids through decomposition of lipid hydroperox- ide (51). Examination of stability of those preparations revealed that 5% acerola extract emulsion stored in cold or at 40°C showed the best antioxidant properties compared with extracts with lower concentration and extracts stored at temperatures of 5 and 20°C (49). Potential explanation for different oxidative stability on various temperatures could be due to different decomposition rates of hydroperoxides (52). ROSA CANINA (ROSACEA) Rose bud (Rosa canina L.) is a very important perennial herb which is used as a source of food and medicine in many cultures. The fruits and extract of R. canina are rich in fl avo- noids, phenolics, amino and unsaturated fatty acids, carotenoids, pectiones, and tannins. Investigations described antibacterial, antidiabetic, anti-infl ammatory, antitumor and antiobesity properties of this herb, mainly attributed to the presence of aforementioned ingredients. Nevertheless, the only clinically approved indication is osteoarthritis. Effects of R. canina components in cosmetic products are various: abrasive, astringent, skin conditioning, humectant, and exfoliant. To examine oxidative stability by measuring the rate of peroxide formation spectrophotometrically, a group of authors used differ- ent concentrations of rose glycolic extract (1 and 5%) in O/W cosmetic emulsion. Both concentrations of rose glycolic extract were effective against peroxide formation under different storage conditions (5°C for 6 mo, 20°C for 6 mo, and 40°C for a month).

JOURNAL OF COSMETIC SCIENCE 196 However, 5% concentration was more effective than 1% concentration in providing oxidative stability of cosmetic emulsion at 40°C. Prolonged peroxide formation in emulsion might be a consequence of secondary oxidation product (alkanes, alkenes, alde- hydes, ketones, alcohols, and esters) transformation. Rose extract (1–5%) can be applied in cosmetic emulsions by producers because the extract showed better antioxidant prop- erties than BHT at 5 and 20°C storage (49). It seems that very high protection from oxi- dative stress might be due to the level of carotenoids, as well as phenolic compounds and unsaturation of fatty acids (55). SALI X ALBA (SALICACEAE) Sali x alba L., known as willow bark, is a traditional and folk medicine which has been used for nearly 600 years. The extract of willow bark is composed of salicin-related con- tents, polyphenols, and fl avonoids which are most commonly used therapeutic compo- nents in conditions including pain, fever, infl ammation such as osteoarthritis, headache, tendonitis, and generalized pain (56,57). In cosmetic formulation, this extract has astrin- gent, tonic, and skin-conditioning properties (58). A group of Poland researchers inves- tigated effects of willow bark extracts on oxidative stability in O/W cosmetic emulsions containing wheat germ oil. After evaluation of antioxidant properties of willow bark extract in different concentrations (1 and 5%) exposed to various temperatures and durations of storage, they revealed that both cosmetic emulsions showed good oxidative stability. Actually, the rate of peroxide formation spectrophotometrically measured was slow, and willow bark extract showed more effective antioxidant property at higher concentration (5%) than at lower concentration (1%) at temperatures of 5, 20, and 40°C. There are assumptions that antioxidant capacity of this extract is associated with aforementioned ingredients such as salicin, phenols, and fl avonoids (49). Application of willow extract in cosmetic emulsions can be useful because it showed antioxidant properties and did not show prooxidant activity at both concentrations of 1 and 5%. CONCL USION It be comes the prime responsibility of the cosmetic product manufacturers to provide adequate oxidative stability for long-term storage and safe consumption. Plant extracts rich with antioxidants, especially phenolic compounds, are continuously gaining atten- tion as potential agents, which could protect cosmetic preparations against lipid peroxi- dation processes. This review summarizes all data about plant extracts which could be potentially used for their antioxidant properties in cosmetic emulsions. Investigations conducted so far do not provide complete understanding of the role of plant extracts in improving oxidative stability of formulation because they were mostly focused on moni- toring product stability only at room temperature. Stability testing in elevated tempera- ture storage conditions is certainly required in the future studies to enable wide application of the examined plant extracts in product formulation. Moreover, herbal extracts should be adjusted to a defi ned content of a constituent or a group of substances with known antioxidant activity. Plant extracts appear to be a safe, effi cient, and valuable source of numerous bioactive compounds that might be exploited by pharmaceutical and cos- metic industries.