Photostability Studies of Retinyl Palmitate Entrapped in Policosanol Oleogel Matrice

Retinyl palmitate (RP) can easily lose its biological activity when exposed to ultraviolet radiation due to the UV-mediated degradation. There is a demand to explore new approaches to protect RP in an easy, economical and efficient way. Oleogels show a great potential to improve photostability of RP. The objective of this research was to determine the kinetics of RP photodegradation in policosanol oleogels (PCO) upon UVA exposure and the photoprotective mechanism of RP taking place in PCOs. Photostability of RP (0.04%, 0.1% and 1%, w/w) was studied in PCO matrices (7%, 10%, and 12%, w/w, policosanol in soybean oil) after UVA irradiation. PCOs efficiently protected RP from UVA-mediated degradation. Over 75% RP-activity remained in PCOs after 4 days of UVA irradiation, while only 12% RP-activity remained in soybean oil. HPLC analysis showed that cis-RP was formed in liquid soybean oil after 2 days of UVA irradiation while it was absent in PCOs matrices. For all samples, RP photodegradation followed a 2nd order reaction. From the reaction kinetics, it would be possible to predict the RP photodegradation rate in PCO matrices. PCOs were shown to be a promising matrix to efficiently protect RP from photodegradation. PCOs can efficiently bock UVA energy absorption and further dampen the UVA mediated degradation. PCOs prepared at higher cooling rates had smaller crystal particle area sizes and provided better RP protection due to the molecular immobilization. Microscopies and matrix mobility results suggested that PCOs efficiently immobilized RP in the network and improved RP photostability by reducing molecular collisions. PV and p-A.V. results indicated that PCOs can improve oil oxidative stability and further hinder the progress of free radical-mediated reaction of RP.