Quenching mechanism and kinetics of ascorbyl palmitate for the reduction of the photosensitized oxidation of oils

Effects of 0, 500, 1000, and 1500 ppm (wt/vol) ascorbyl palmitate (AP) on the methylene-blue- and the chlorophyll-sensitized photooxidations of linoleic acid or soybean oil, either in methanol or in a solvent mixture (benzene/methanol, 4:1, vol/vol), were studied during storage under 3300 lux fluorescent light for 5 h. Steady-state kinetic approximation was used to determine a quenching mechanism and quenching rate constant of AP in the chlorophyll-sensitized photooxidation of methyl linoleate in a solvent mixture (benzene/methanol, 4:1, vol/vol). Both methylene blue and chlorophyll greatly increased the photooxidation of linoleic acid and soybean oil, as was expected. AP was extremely effective at minimizing both methylene-blue-and chlorophyll-sensitized photooxidations of linoleic acid and soybean oil, and its effectiveness was concentration-dependent. The addition of 500, 1000, and 1500 ppm AP resulted in 69.3, 83.6, and 94.6% inhibition of methyleneblue-sensitized photooxidation of linoleic acid, respectively, after 5-h storage under fluorescent light. AP showed significantly greater antiphotooxidative activity than α-tocopherol for the reduction of methylene blue-sensitized photooxidation of linoleic acid (P < 0.05). The steady-state kinetic studies indicated that AP quenched singlet oxygen only to minimize the chlorophyll-sensitized photooxidation of oils. The calculated total quenching rate of AP was 1.0 × 10⁸ M⁻¹s⁻¹. The present results clearly showed, for the first time, the effective singlet oxygen quenching ability of AP for the reduction of photosensitized oxidation of oils.