Vegetable oils form a good part of human diet, providing concentrated sources of energy and essential nutrients (Min, 1983). They are often displayed under direct sunlight in open markets and under fluorescent light in the supermarkets. Such exposures can result in the production of off-flavours, colour defects as well as loss of nutritional quality of the oils (Ihekoronye and Ngoddy, 1988; Sanders, 1983; Mecollum, 1956). In the process, the essential fatty acids and vitamins present in the oils are oxidized and their nutritive values reduced.
When oils are exposed to sunlight their π-bond contents become photoxidized
(Fekarurhobo et al., 2005; Ullah et al., 2003). In the process,
oxygen reacts with the double bonds of fatty acids to form peroxides and/or
free radicals in the presence of light (Przybylski, 2005) (Fig.
1). Indeed, the oxidation levels of vegetable oils are important quality
criteria in food chemistry because oxidation increases their toxicity by the
formation of products such as hydroperoxides, adlehydes, ketones, etc (Muik
et al., 2005; Harold and Kirk, 1981).
The rate of photoxidation is affected by some factors such as the presence
of oxygen, intensity of radiation, degree of unsaturation and temperature. In
general, the higher the unsaturation, the more the oil is susceptible to oxidative
rancidity. However, the susceptibility of vegetable oils to photoxidation is
not only due to high concentration of the unsaturated fatty acids. The oils
also, contain pigments (as chlorophyll) and their decomposition products, which
can act as photosensitizers to generate singlet Oxygen (1O2)
in the presence of light and atmospheric triplet oxygen (Barr et al.,
2002; Nawar, 1972). 1O2, being a strong electrophile,
reacts with the π bonds of unsaturated fatty acids to form the initial
hydroperoxides, which can break down and form free-radicals to initiate autoxidation
||Fatty-acid-type compositions of the oils
Natural antioxidants found in many fruits and vegetables, which include β-carotene,
ascorbic acids, tocopherol, which can act as 1O2 quenchers,
help to reduce the rate of photoxidation in the oils.
Soybean oil, groundnut oil, palm kernel oil and palm oil are popularly used
in food preparations. Their fatty acid type compositions vary therefore, their
degrees of susceptibility to sunlight are expected to be different. The fatty
acid compositions of the vegetable oils are shown in Table 1
(USDA, 1979). The extent of deterioration of the oils reflects the molecular
changes effected by their exposure to sunlight. The degree of the molecular
alterations can be studied by monitoring changes in physico-chemical parameters
such as peroxide value, iodine value, free fatty acids, refractive index, relative
density and viscosity.
||The free radicals in the presence of light
MATERIALS AND METHODS
Oil extraction: Seeds were purchased from mile 3 market in Port Harcourt.
They were cleaned and all except the palm fruits were milled. The milled groundnuts
and palm kernels were Soxhlet extracted with petroleum spirit (60-80), while
the milled soybean oil was extracted with straight-run gasoline (40-60). Palm
oil was traditionally extracted. The oils were dried in the oven at 105°C
for 2 h.
Irradiation: One hundred milliliter each of the 4 oil samples were irradiated under natural sunlight in transparent Pyrex glass beakers in the months of December to July. Non-irradiated oil samples were kept in opaque bottles in a dark chamber to serve as control.
Measurement of quality parameters: The quality parameters studied were peroxide value, free fatty acid, refractive index, iodine value, viscosity and relative density. The parameters were measured before irradiation and at 5th day intervals for both the irradiated and unirradiated samples.
RESULTS AND DISCUSSION
The results obtained are shown in Table 2 and Fig.
The physical changes observed in the oils are shown in Table 2. They include the separation of liquid and solid phases in palm oil sample after exposure for 5 days, the appearance of a thin crust on the surface of the palm oil after exposure for 5-10 days, the disappearance of this crust from the surface of the exposed oil after 10 days of exposure, the formation of a sludge at the bottom of the oil and increase in thickness of the oil samples with exposure duration. The observations may be attributed to photolysis/photoxidation. Such physical changes observed in the exposure of crude oil to sunlight were similarly explained (Fekarurhobo et al., 2005).
The colour lightening observed in the oils may be as a result of loss of colour pigments. All natural fats and oils contain colour pigments (e.g., chlorophyll and carotenoids), which are highly oxidizable because of their π bond contents. Such changes have been reported (Jeorge, 1997; Daling, 1988; Nawar, 1972).
Figure 2-7 show that all the parameters
(except iodine value, which decreased) increased with exposure duration.
Relative density and viscosity: Relative density values increased at
the end of the exposures by 0.033, 0.025, 0.017 and 0.016 for soybean, groundnut,
palm and palm kernel oils, respectively. The viscosity values also, increased
on exposure by 86.41, 58.12, 24.10 and 21.86 for soybean, groundnut, palm and
palm kernel oils, respectively. The increase in the values of these parameters
may be as a result of photo-oxidation, which results in the formation of higher
molecular weight compounds. Such increases have been reported by Carlson et
al. (1976) and Hoffman (1989). The increases, which are in the order, soybean
>groundnut>palm>palm kernel oils, may be a reflection of the degree
of unsaturation in the oil samples (Table 1).
||Plot of relative density vs. exposure duration
||Plot of viscosity vs. exposure duration
Peroxide value: The peroxide value (meq kg-1) increased on
exposure by 125.39, 101.82, 59.99 and 48.22 for soybean, groundnut, palm and
palm kernel oils, respectively.
||Plot of free fatty acid vs. exposure duration
||Plot of iodine value vs. exposure duration
The increases in peroxide values of the oils indicate their deterioration during
exposure to sunlight. Such increases have been reported (Jeorge, 1997; Zeb and
Ahmad, 2004) and were ascribed to the degree of unsaturation of the oils. Thus,
the order of increase in the peroxide values observed in the current exposure
agrees with the degree of unsaturation in the oils (Table 1).
Iodine value (Wijs): Iodine values of the oil samples decreased
by 60.22, 36.60, 13.05 and 4.67 for exposed soya bean, groundnut, palm and palm
kernel oils, respectively. The decrease in iodine value may be as a result of
loss of unsaturation as the π-bonds become oxidized with exposure of the
oils. The order of decrease in the values while, it agrees with the order of
unsaturation in the oils, also corresponds to earlier findings by Zeb and Ahmad
||Plot of refractive index vs. exposure duration
||Plot of peroxide value vs exposure duration
Refractive index: The refractive indices of the four oil samples changed
only slightly throughout the exposures as can be shown in Fig. 6. The values
increased by 0.014, 0.0092, 0.0019 and 0.0018 for soybean, groundnut, palm and
palm kernel oils, respectively. The results confirm earlier observations that
irradiation does not have much effect on the refractive indices of the oils
(Sattar and deMan, 1976; Zeb and Ahmad, 2004). The trend in the increases is
the same as that observed in the other parameters, presumably reflecting the
degree of unsaturation in the oils.
Free Fatty Acids (FFA%): The results of the percentage free fatty acids
of the oil samples (Fig. 4) show that the values increased by 2.72, 2.13, 1.79
and 1.40 for soybean, groundnut, palm and palm kernel oils, respectively. The
fact that the trend in the increases is the same as the order of unsaturation
in the oils suggests that functional group interconversion in the triglycerides
alone cannot explain the result.
The observation that changes also occurred in the control samples suggests that a more plausible explanation is that the carbon-carbon π-bonds oxidize to give the free fatty acids. Thus, other factors such as temperature may also have contributed to the formation of the free fatty acids.
Exposure of soybean, palm, palm kernel and groundnut oils to sunlight accelerated their oxidation. The increasing change in the parameters measured indicates that the oils deteriorated on exposure to sunlight. Further studies however, are needed to identify the specific products formed and to determine their possible health implications.