OXIDATIVE STABILITY OF COSMETIC ARGAN OIL 83 whereas the other half was in brown glass bottles and stored in a cupboard. Head-space volume for each bottle was 3.5 ml. Analyses were performed just after pressing, then after 1 month of storage, and then every 2 months over 1 year. ANALYSIS OF OILS Physicochemical parameters were determined using procedures that we have already re- peatedly described (7,8,18,19). In brief, acid and peroxide values were determined using International Standard Organization methods ISO-660 (20) and ISO-3960 (21), respec- tively. Results are expressed in mg KOH/g and meq O2/kg, respectively. Iodine index was determined using the International Standard Organization method ISO 3961 (22) or calculated using Carreras’ method (23). Results are expressed in gI2/g. Ultraviolet ab- sorption was determined at l 232 and 270 nm using the International Standard Organiza- tion method ISO 3656 (24). Fatty acid (FA) composition was determined using the International Standard Organization method ISO 5508 (25). A gas chromatograph (Var- ian CP-3800, Varian Inc., Middelburg, The Netherlands) equipped with a FID and a CP- Wax 52CB column (30 m × 0.25 mm i.d. Varian Inc.) were used. The carrier gas was helium, and the total gas fl ow rate was 1 ml/min. The initial and fi nal column tempera- ture was 170° and 230°C, respectively. Temperature was increased by steps of 4°C/min. The injector and detector temperature was 230°C. Data were processed using Varian Star Workstation v 6.30 (Varian Inc., Walnut Creek, CA). Results are expressed as the relative percentage of each individual FA present in the sample. Sterol composition was deter- mined using the International Standard Organization method ISO 6799 (26) using a Varian 3800 instrument equipped with a VF-1 ms column (30 m and 0.25 mm i.d.) and using helium (fl ow rate 1.6 ml/min) as carrier gas. Column temperature was isothermal at 270°C, and injector and detector temperature was 300°C. Injected quantity was 1 μL for each analysis. Data were processed using Varian Star Workstation v 6.30 (Varian Inc.). STATISTICAL ANALYSIS Values reported in tables and fi gures are the means ± S.E. of three replications. The sig- nifi cance level was set at p = 0.05. Separation of means was performed by Tukey’s test at the 0.05 signifi cance level. RESULTS AND DISCUSSION CHANGES IN ACID AND PEROXIDE VALUES AS A FUNCTION OF TIME AND STORAGE CONDITION Variations in acid and peroxide value as a function of time for the three evaluated storage conditions are presented in Figure 1. Cosmetic industry requires an acid value below 4 mg KOH/g (16). Such value corresponds to the “extra virgin” or “fi ne virgin” labels of argan oil and excludes the “ordinary virgin” and “lampant” quality according to the of- fi cial norm (15). Cosmetic argan oil presented an initial acid value of 0.5 mg KOH/g, a low value fully in accordance with our previous results (18). Acid value of cosmetic argan oil stored at 25°C did not signifi cantly change over 12 months whether or not protected

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.