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.