INTRODUCTION

With the increasing awareness about health and safety in modern societies, the usage of tropical oils (TOs) is declining. Typical TOs include coconut oil (CNO), palm oil (PO), and palm kernel oil (PKO), which predominantly comprise saturated fatty acids (Ramadan, 2019). The high degree of saturation in TOs (CNO, ≈ 93%; PKO, ≈ 82%; PO, ≈ 50%) (Dubois et al. , 2007) has raised several health concerns and debates on the safety and health risks of saturated fatty acid consumption (Cassiday, 2017)(Noordwijk, 2020). However, TOs are essential for food and other uses and are potential solutions for achieving food security in the future (Puah and Kochhar, 2019). To meet the requirements of the ever-increasing world population, improvement and reassessment of the existing food resources is critical.
TOs differ majorly in their chemical compositions and applications. CNO and PKO predominantly comprise lauric (C12) and myristic acids (C14) which contain a high proportion of medium-chain fatty acids (MCFAs) (Gunstone, 2011). PO also comprises predominant levels of long-chain fatty acids (LCFAs), such as palmitic acid (C16) and oleic acid (C18:1) (Kostik, Memeti and Bauer, 2013). In addition to the differences in their chemical compositions, some novel potential applications of TOs have been recently discovered. PO and PKOs are currently considered as substitutes for green fossil fuels despite inadequate clarity on the sustainability of such usage. Ana et al. reported that PO-based biofuels possess the potential for increasing greenhouse gas savings in the near future (Hu and Cheng, 2013). CNO is also emerging as a solution to various health problems and could play a critical role in the health sector (Woolley et al., 2020).
Notwithstanding the emerging applications and advantages, the quality of TOs has always been a concern. The poor handling and storage of TOs adversely affects their stability and quality, which depreciates their market, economic, and nutritional values. These issues can be widely attributed to the unconventional storage of TOs in the presence of sunlight. In Africa, one of the traditional refining practices by many TOs producers is the exposure of oils to sunlight (Tonfack Djikenget al. , 2019). This approach is cost-effective, convenient, and economical for evaporating the residual water in oils. A related approach adopted by the distributors and vendors of TOs involves exposing the oils to sunlight for marketing purposes (Mwanza and Ingari, 2015). This is due to the high viscosity, high melting points, and the saturated fatty acids present in TOs, owing to which, these oils solidify readily under ambient conditions within a short time. Marketers or vendors address this challenge by exposing the oils to sunlight for liquefying the solidified oils and maintaining the liquid or semi-solid forms (Ngono Ngane Annie, 2014), which increases the marketability and consumer acceptance. While these practices are suitable from the marketing and economics outlook, from a scientific perspective, they pose risks to the stability of the oils by oxidation or the related decomposition and are inappropriate (Maszewska et al., 2018 & Oh et al., 2014).
Like many other edible oils, TOs can undergo lipid oxidation upon prolonged exposure to sunlight. Light is a photochemical initiator capable of inducing photochemical reactions when exposed to food (Zebet al. , 2008). The mechanism of the photochemical oxidation of vegetable oils has been extensively studied (Choe & Min, 2006; Koutchma, 2019), and the oxidation of lipids under light has been termed photooxidation. Irradiation of edible oils reduces their oxidative stability and renders them prone to rancidity (Min and Boff, 2002). TOs are susceptible to oxidation due to their low unsaturated fatty acid contents and the presence of colour pigments (photosensitisers) in minor quantities. Moreover, investigations have revealed that antioxidants that prevent oxidation in oils can be destroyed when exposed to prolonged sunlight (Oh, Lee and Choe, 2014). Photooxidation leads to the formation of peroxides and other volatile and harmful products in oils. This renders the oil less stable, reduces its value (economic, nutrition, and market) and safety. Importantly, the oils also lose their flavour and sensory quality and become unattractive and unacceptable to consumers, which results in economic losses to both the food and non-food industries (Redondo-Cuevas et al. , 2018).
Therefore, the impact of sunlight on TOs is not well understood, and comprehensive studies that compare and evaluate the effects of sunlight on the photooxidation of TOs are sparse. Hence, this study aims to monitor the changes in three TOs after exposure to sunlight for seven consecutive weeks. Iodine value (IV), free fatty acid (FFA), colour content and peroxide value (PV) were measured as the indicators of the degree of photooxidation in the TOs. Fourier transform infrared spectroscopy (FTIR) was performed to study the photooxidation in TOs further. A simulation was performed to support the FTIR results and study the oxidation of TOs. Statistical analysis was performed to verify the validity and significance of these findings.