Assessing the Effects of Sunlight on the Photooxidation of Tropical Oils with Experimental and Computational Approaches

Abstract

When vegetable oils are released from the manufacturers or oil producers, they customarily meet the standard and regulations to ensure that they do not pose any future threat to consumers or users. However, their inappropriate handling and storage at the marketplace during commercialisation could introduce new compounds that could threaten consumers or reduce the oil’s nutritional value. Hence, this study aims to investigate the influence of sunlight radiation on the photooxidation of tropical oils (TO) stored in the marketplace. Coconut oil (CNO), palm oil (PO), and palm kernel oil (PKO) were chosen for determining the indicators of photooxidation when exposed to and in the absence of sunlight for seven weeks. The results showed a significant (P < 0.05) increase in free fatty acid (FFA) levels and peroxide value (PV) when the TO were exposed to sunlight. The iodine value and colour content decreased significantly (P < 0.05) due to the decomposition of unsaturated FFA owing to the breaking down of the π-bonds and the degradation of colour pigments during photooxidation. Fourier transform infrared spectroscopy (FTIR) analysis showed strong vibrational absorptions at 1721 and 3505 cm−1, 1720 and 3560 cm−1, and 1721 and 3554 cm−1 for the CNO, PO, and PKO samples exposed to sunlight, respectively. These bands can be attributed to secondary oxidation products, which were absent in the TO that were not exposed to sunlight. A simulation was performed to support the FTIR results, indicating peaks from the secondary oxidation products at 1744 and 3660 cm−1. The study also revealed that the photooxidation rate was different for each TO. The rate of oxidation followed the order PO > PKO > CNO. In contrast, no notable changes in the TO kept away from sunlight were observed. These results suggest that exposing TO to sunlight influences their oxidation stability and quality.