ALL-TRANS-RETINOL IN CREAM 195 35O o • 300 ii ii • 250 o '" 200 '" 150 100 ' • ' ' ' 10 20 30 40 Weight percentage of total oils in cream(%) Figure 3. Weight percentage of total oils and percent increase of 13-cis-retinol in 400 ppm of retinol in oil-in-water cream after two months storage at 50øC. Next, we checked the influence of the possibility of contact of water with all-trans- retinol by controlling the quantity of water or adding a surface-active agent in retinol- ethanolic solution. Figure 5 shows the relation between the percent of water in the retinol-ethanolic solutions, with or without a surface-active agent, and the percent remaining of all-trans-retinol in the solutions after ten days storage at 50øC. All-trans- retinol in ethanolic solutions decreased as the contact between all-trans-retinol and water was enhanced by an increase of water and surface-active agent. As all-trans-retinol was decreasing, one main product was found on the HPLC chromatogram. Figure 6 shows the UV spectra of this product in retinol ethanolic solution with a surface-active agent, after ten days storage at 50øC. This product was identified as anhydrovitamin A by using experimentally obtained standard anhydrovitamin A (7). This result corresponds with the findings of Anmo eta/., who reported that anhydrovitamin A was recognized as the product of purified vitamin A alcohol when the stability of vitamin A alcohol ethanolic solutions was investigated (8). The contact between all-trans-retinol and water acceler- ated the increase of dehydration and decreased all-trans-retinol without increasing 13- cis-retinol. We supposed the contact between 13-cis-retinol and water also promoted the decrease of 13-cis-retinol, and we could not observe an increase of 13-cis-retinol. The structural formulas of all-trans-retinol, 13-cis-retinol, and anhydrovitamin A are shown in Figure 7. EFFECT OF ANTIOXIDANTS The effect of BHT, BHA, vitamin E, and vitamin C as antioxidants against oxidation

196 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS O96 ß - 94 ! (• 92 (•90 0 E 84 , I • I • 40 50 60 70 Weight percentage of water in cream(%) Figure 4. Weight percentage of water and percent remaining of all-trans-retinol in 400 ppm of retinol in oil-in-water cream after two months storage at 50øC (with argon gas blanket). was evaluated in retinol-ethanol solution (Table II). These antioxidants had a high effect against oxidation of all-trans-retinol. Since oil-soluble antioxidants also have an effect against thermal isomerization and decomposition by lipid peroxide (9), use of antioxi- dants in cream seems to be invaluable. DISCUSSION Besides thermal isomerization, decomposition by contact with water, and oxidation, we have already known that peroxide in oil or surface-active agent aggravated the instability of all-trans-retinol (4,7). From the above results, we could construct a concept that if ultraviolet light is completely shielded, the decrease of all-trans-retinol in creams is more or less caused by these factors: thermal isomerization by temperature, dehydration by water, and decomposition by oxygen and lipid peroxide. And for stabilizing all-trans- retinol in creams, it is important 1) for prevention of thermal isomerization, to pay attention to the total amount of oil phase in cream and to avoid high temperatures during the manufacturing process of creams and not to preserve creams at high tem- perature 2) for prevention of decomposition by water, to lower the possibility of contact between retinol and water by controlling the concentration of water and surface-active agents in creams 3) for prevention of oxidation, to blow and bubble an inert gas through the ingredients of creams 4) for prevention of decomposition of lipid peroxide, not to use oxidized or easily oxidizable oils and surface-active agents in creams and 5) not only for prevention of oxidation but also for prevention of thermal isomerization and decom- position by lipid peroxide, to use oil-soluble antioxidants like BHT and BHA.