JOURNAL OF COSMETIC SCIENCE 84 from sunlight. Acid value of oil samples stored at 40°C remained stable for 6 months, and then consistently and almost linearly increased to reach 0.9 mg KOH/g after 12 months of storage. Despite this variation, after 1 year, the acid value of the evaluated samples crossed the low-margin 4 mg KOH/g limit imposed by the cosmetic industry. Change in acid value is a direct marker of the free acid amount formed in the oil as a function of time. Our results show that some amount of the triglycerides of cosmetic argan oil gets hydrolyzed during prolonged storage following a process that is favored by heating, insensitive to light, and whose kinetic is different from that of edible argan oil (7). There- fore, it is very likely that the cosmetic argan oil moisture level is the major factor that governs the observed fast triacylglyceride hydrolysis in cosmetic argan oil. Peroxide value is a primary oxidation marker. Peroxides are feared in the cosmetic domain since, in ad- dition to generating free radical species, they can also increase radical-induced lipid per- oxidation, leading to an accelerated rancidity and possibly the formation of off-fl avors inappropriate for cosmetic products (17). Peroxides can also alter other oxygen-sensitive components included in cosmetic formulations or react with cosmetic containers. Finally, they might also lead to the formation of mixtures of brownish oxidation compounds un- acceptable for cosmetics. For all these reasons, the cosmetic industry has restrictive rules regarding the argan oil peroxide value that must remain below 10 meq O2/kg (16), whereas the Moroccan norm accepts a peroxide value up to 15 meq O2/kg (15). The ini- tial peroxide value of cosmetic argan oil was 0.9 meq O2/kg (Figure 1). This value rapidly increased to reach 4.3 meq O2/kg after 1 month in oil samples stored at 40°C. In argan oil stored at 25°C and exposed to sunlight, this value reached 3.98 meq O2/kg after the same delay. When protected from sunlight, a moderate but signifi cant increase was also observed (3.35 meq O2/kg). Prolonged storage at 40°C led to a continuous and almost linear increase in peroxide value indicating a rapid peroxide formation, and in these con- ditions, the 10 meq O2/kg limit value was reached after 5 months of storage. At 40°C, 9 months was necessary to reach the 15 meq O2/kg limit value imposed by the Moroccan norm. At 25°C, sunlight protection moderately reduced peroxide formation and the 10 meq O2/kg limit value was reached after 7 months. Interestingly, for oil samples Figure 1. Acid (full line, left axis) and peroxide (dot line, right axis) value of cosmetic argan oil stored at various temperatures as a function of time.

OXIDATIVE STABILITY OF COSMETIC ARGAN OIL 85 stored at 25°C and sunlight protected, peroxide value reached a plateau after 10 months of storage. Such an event has as a consequence the 15 meq O2/kg limit value was never reached. It is also evidenced that after some months, the kinetic of FA peroxidation and of secondary oxidation product formation were similar. The peroxide value of sunlight- protected edible argan oil stored at 25°C has been shown to reach 10 meq O2/kg after 2 years (8). Therefore, our combined results clearly indicate cosmetic argan oil is much more sensitive to peroxide formation than edible argan oil. This not only emphasizes the critical importance of the Maillard’s compounds formed during roasting in argan oil overoxidation preservation but also demonstrates that to be introduced in cosmetics, cos- metic laboratories should not store argan oil more than 7 months at room temperature. In addition to a peroxide value determination, we also monitored primary oxidation product formation from extinction coeffi cient at 232 nm (E232) even though the cosmetic industry does not impose any rules regarding this analysis (Figure 2, full line). E232 under- went a particularly fast increase for cosmetic argan oil stored at 40°C, confi rming an impor- tant peroxide formation over a 1-year period. At 25°C, sunlight exposure favored primary oxidation product formation, although not as rapidly as at 40°C. Sunlight protection al- lowed to reduce the rate of formation of primary oxidation product, resulted in a plateau being reached after 8 months. We also determined the extinction coeffi cient at 270 nm (E270) to evaluate secondary peroxidation product formation (Figure 2, dot line). Because these oxidation products encompass highly chemically reactive compounds such as alde- hyde or ketones, a drastic limit of 0.2 is imposed by the cosmetic industry, whereas the limit of the Moroccan norm is 0.35. The 0.2 limit was reached after only 1 month for argan oil stored at 40°C. It was reached between April and June for samples stored at 25°C whether protected or not from sunlight. These measurements indicate that after 6 months of storage at 25°C, cosmetic argan oil is not suitable anymore to be introduced in cosmetic formula- tion according to industrial requirements, even though, in terms of Moroccan norm, the limit value is not reached after 1 year of storage at 25°C and using sunlight protection. Cosmetic argan oil properties result from its physicochemical properties and from its specifi c chemical composition (4). Main components, as unsaturated FA, and minor Figure 2. Extinction coeffi cient at 232 nm (full line, left axis) and 270 nm (dot line, right axis) of cosmetic argan oil stored at various temperatures as a function of time.