Effects on Peroxide Value
PV is an important indicator that provides the initial evidence of
primary oxidation in TOs. As shown in Table 2, the evolution of PV was
monitored for seven consecutive weeks at 95% CL. The TOs that were not
exposed to sunlight showed the lowest increase in PV as a function of
time. The PV (meqO2 / kg) in the TOs increased slightly
from 2.03\(\pm 0.01\) to 2.14\(\pm 0.02\) (CNO), 6.13\(\pm 0.01\) to
6.23\(\pm 0.04\) (PKO), and 3.09\(\pm 0.01\) to 3.22\(\pm 0.04\) (PO)
for the oils. The increase in PV (meqO2 / kg) despite
the protection from light can be attributed to the presence of dissolved
oxygen in the TOs (Johnson and Decker, 2015). Dissolved oxygen can
induce oxidation of unsaturated FFAs in the absence of light (Johnson
and Decker, 2015). In addition, the results in Figure 1 show that the
increase in PV was more pronounced in the sunlight-exposed TOs. The TOs
exposed to light showed a significant increase of the PV from
2.03\(\pm 0.01\) to 4.8\(\pm 0.03\) (CNO), 6.13\(\pm 0.01\) to
9.33\(\pm\)0.03 (PKO), and 3.09\(\pm\)0.01 to 8.43\(\pm\)0.05 (PO). This
shows that the effect of sunlight on PV is significantly different
(p < 0.05) for the control and exposed TOs. The rapid
increase in PV can be ascribed to the chemical changes that occur in the
TOs when exposed to sunlight. Therefore, prolonged exposure of TOs to
relatively high light intensities caused significant initiation of
photooxidation in the oils. These findings are in agreement with the
work of Rukmini and Raharjo (2010), who observed a similar pattern. They
reported an increase in the PV of VCO upon exposure to fluorescent light
at higher storage times. Such an increase in PV has also been studied by
Almeida et al. (2019), who stored refined PO (RPO) in an enclosed dark
area (20–25 °C) and at ambient temperature with exposure to natural
light (26–32 °C). After 12 months of storage with exposure to light,
the PV (meq O2/ kg) for the RPO increased drastically from 00.52 to
85.29. The RPO stored away from light increased slightly from 00.59 to
14.64 meq O2/kg.
However, different types of oils can either undergo the same or
different changes during the initial stages of oxidation. This is
evidenced by the variations in the levels of PV for each TOs shown in
figure 2. The maximum PV was recorded for PO on the
7th day (Figure 2), followed by PKO and CNO. At all
stages, the increase in the PV of PO was more rapid than in CNO and PKO,
which can be attributed to the increase in PV with the increase the
degree of unsaturation in the TOs. PO consists of higher levels of
unsaturated FFAs (C18:1) compared to CNO (C18:1) and PKO (C18:1), owing
to which, PO showed a rapid increase in PV when exposed to sunlight than
the PKO and CNO. Therefore, the observed increase in PV agrees well with
the degree of unsaturation in the TOs. Suryani et al. (2020) also
reported that CNO should theoretically exhibit a low rate of oxidation
due to its low unsaturated FFA content. Thus, unsaturated FFAs readily
form peroxides (or hydroperoxides) compared to the saturated congeners,
which causes the high PV. A higher PV reflects lower chemical stability
of the oil and renders the oil less safe for food and non-food uses
(Frankel and Huang, 1994). Therefore, the chemical stability of TOs
needs to be enhanced by protecting the oils from direct exposure to
sunlight. This can be achieved by the appropriate revision and
improvement of the packaging and storage conditions of these oils.
Attention to such handling of TOs will improve their acceptability.
Effects on Iodine
Value
IV is a vital characteristic of oils and represents the degree of
unsaturated fatty acids (double bonds) present in the oil (Kumaret al. , 2012). This amount represents the unsaturated fatty acids
before and after the oxidation of the oil. Thus, IV indicates the
oxidative stability of oil and is a useful parameter for studying the
changes in the unsaturated fatty acid contents in TOs exposed to
sunlight. Table 3 presents the decrease in IVs of TOs stored under
different conditions at 95% CL. The TOs stored away from light
exhibited the lowest reduction of IV per week, unlike the oils exposed
to light. The IV (g of iodine/100 g of oil) in the unexposed TOs
decreased from 6.10\(\pm\)0.01 to 6.03\(\pm\)0.02 for CNO,
18.25\(\pm\)0.01 to 18.12\(\pm\)0.02 for PKO, and 49.35\(\pm\)0.01 to
49.19\(\pm\)0.03 for PO. The exposed TOs showed a significant decrease
in IV per week from 6.10\(\pm\)0.01 to 4.94\(\pm\)0.04 for CNO,
18.25\(\pm\)0.01 to 16.6\(\pm\)0.01 for PKO, and 49.35\(\pm 0.01\) to
44.12\(\pm\)0.05 for PO. This shows that the effect of sunlight on IV is
significantly different (p < 0.05) for the control and
exposed TOs. IV decreases in TOs because it is proportional to the
amount of iodine required to saturate the FFAs present in the TOs. The
different types of FFA in the TOs account for the varied IV values for
each oil. Approximately 51% of unsaturated fatty acids are found in PO
(oleic acid), 35% in PKO (myristic acid), and 15% in CNO (myristic
acid) (Gunstone, 2011). In addition, Figure 3 provides a comparison
between the decreasing rates of IVs of protected and unprotected TOs
using a bar chart, which shows that the decrease in IV is more
pronounced when the TOs are exposed to sunlight. Thus, when the TOs are
exposed to sunlight, a higher number of unsaturated FFA (C=C) in TOs are
likely decomposed into radicals, thereby reducing the degree of
unsaturation (C=C) in TOs. Dawodu et al. (2015) reported a similar trend
when PKO was exposed to different temperatures. The IV in the PKO,
initially at 5.00, decreased to 7.00 when the temperature was increased
to 300 °C. Fekarurhobo et al. (2009) also reported a decrease in IV from
20.0 to 18.5 and 50.0 to 40.1 in PKO and PO, respectively. In their
research, PO and PKO were exposed to sunlight for a shorter time (25
days) than that in the present study (49 days). However, PKO and PO
showed a decrease in IV after 25 days of sunlight exposure. They further
noted that the reduction of IV
could be ascribed to the release
of unsaturated FFAs as the π-bonds broke down during photooxidation. In
addition, the highest significant decrease of IV was found in PO,
followed closely by PKO and CNO. As previously mentioned, the degree of
unsaturation of oils accounts for the rate at which the IV decreases. PO
with the highest number of unsaturated FFA are likely to have more rapid
reduction of IV than PKO and CNO. PKO has a higher number of unsaturated
FFA, and therefore, its IV decreases faster than that of CNO. In
summary, IV is a useful diagnostic of TOs photooxidation and provides
useful information on the oxidative stability of TOs. In addition, it is
a straightforward and convenient method for evaluation of TOs in typical
laboratory settings and can be used by TOs producers to monitor the
changes in TOs exposed to sunlight.
Effects on Colour
Colour is an indirect measure of product quality or product condition.
The colours of the TOs were measured using a Lovibond Tintometer Pfx 880
series. The Lovibond colour scale was used to express the colour in the
red and yellow AOCS tintometer units. Table 4 depicts the colour changes
in the TOs samples during the seven day test period. The initial colours
of the TOs were 1.11R\(\pm 3.40\)Y, 2.07R\(\pm\)18.2Y, and
3.63R\(\pm\)9.40Y for CNO, PKO, and, PO, respectively, before exposure
to sunlight. After seven days of maintaining the samples in the absence
of sunlight, the colours of the TOs reduced to 1.04R\(\pm 3.33\)Y,
2.05R\(\pm\)9.36Y, and 3.57R\(\pm\)9.36Y for CNO, PKO, and PO,
respectively. These results indicate an insignificant change in the
colour of TOs stored in the absence of sunlight. However, the TOs
exposed to light showed a notable decrease in colour to
0.44R\(\pm\)2.51Y, 1.13R\(\pm\)16.8Y, and 1.80R\(\pm\)9.08Y for CNO,
PKO, and PO, respectively. This shows that the effect of sunlight on the
colour content is significantly different (p < 0.05)
for the control and exposed TOs. These results are supported by the
findings of Tonfack et al. (2019) (Tonfack Djikeng et al. , 2019)
who reported the decrease in colour from 1.00R\(\pm 6.00\) to
0.20R\(\pm\)0.80Y after 90 days of sunlight exposure. Almeida et al.
(2019) and Taluri et al. (2019) also reported that high-temperature
storage conditions could reduce the colour content of oils. They
reported that the degradation of colour pigments in oil during
photooxidation affects the colour of the oil. These changes are
pronounced in oils with a higher degree of unsaturation during prolonged
exposure to light. The colour changes of the exposed TOs can be
attributed to the damage of the colour pigments present in the oil.
Thus, prolonged exposure to sunlight can deteriorate the colour pigments
in oils. Choe et al. (2014) reported that colour changes could also be
affected by the type of packaging material used for the oils.
Transparent PET bottles used in the packaging of oils allow the
transmission of light through oils, thereby destroying the pigments in
oils. Hence, the colour content is reduced, changing the appearance of
the oil; this change could indicate a problem during storage or the
exposure of the oil to adverse conditions. The bar chart (Figure 4)
shows that the decrease in colour was faster in PO than in PKO and CNO.
As previously discussed, the degree of unsaturation is closely related
to the variation of colour in oils. The higher the degree of
unsaturation, the more likely are the colour pigments degraded during
photooxidation. Therefore, the colour of TOs is a significant indicator
of oil stability.
FTIR Studies on Photooxidation
ATR-FTIR analysis was performed to study the photooxidation of the TOs.
Computational simulation was used to support the findings of the
ATR-FTIR. Figures 5–7 show the experimental and simulated FTIR spectra
for the exposed and unexposed (control) TOs. The principal absorption
bands relevant for studying photooxidation were located at 3505
cm-3, 3560 cm-3, and 3554
cm-3, corresponding to CNO, PO, and PKO, respectively.
The absorption bands were ascribed to the presence of secondary
oxidation products, such as hydroperoxides or alcohol-related
compounds(Navarra et al. , 2011). The bands indicated the
stretching vibration of the O‒H bonds during oxidation. The stretching
vibration was observed to be weak in the unexposed TOs due to the low
degree of oxidation. A weak vibration was also observed for the exposed
CNO due to the high degree of saturation, which renders it more stable
against oxidation. These results comply with the those from the
simulated spectra demonstrated in Figure 7, producing a strong
absorption band at 3640 cm-3 for oxidised fatty acids
and a less intense band for the unoxidised fatty acids (Figure 6). A
similar result was reported by Poiana et al. (2015) (Poiana et
al. , 2015), who used FTIR spectroscopy to evaluate the oxidation of
edible oils after heating and frying. The band located at 3008
cm-3 in the spectrum of PO was assigned to the
symmetric stretching vibration of the C‒H bonds in the cis-double bonds
(Araújo et al. , 2011). This absorption was absent in the spectra
of CNO and PKO because of the low amounts of unsaturated double bonds
present in the oils. The simulated FTIR spectra from figure 6 &7 also
showed located band spectra for the absorption band at 3009‒3006
cm-3. The absorption band at 2850‒2920
cm-3 in the spectra of all the TOs can be assigned to
C‒H bonds for the symmetric and asymmetric stretching vibrations of the
aliphatic CH2 and CH3 groups (Karabacaket al. , 2012). Another significant absorption band relevant for
studying photooxidation was located near 1746 cm-3 in
the spectra of the unexposed TOs. This band was assigned to the ester
carbonyl functional group, which was more dominant in the less oxidised
oils, as seen in the unexposed TOs (Poiana et al. , 2015).
However, the vibrational frequencies of the absorption band decreased
for the exposed TOs. Absorption bands were detected in the spectra of
the oils at 1721 and 3505 cm-3 for the exposed CNO,
1720 and 3560 cm-3 for the exposed PO, and 1721 and
3554 cm-3 for the exposed PKO, and the reduction in
these frequencies is attributed to the transformation of secondary
oxidation products, such as aldehydes and other carbonyl
group-containing compounds of a portion of the esters, during the
oxidation process (Poiana et al. , 2015). These transformations
caused the lowering of the vibrational frequencies of the ester groups
with increasing absorbance. The simulated FTIR spectra also indicated a
decrease in the absorption bands of the oxidised fatty acids. The
simulated unoxidised lauric, palmitic, and oleic fatty acids exhibited a
strong band at 1749 cm-3, which decreased negligibly
to 1741 cm-3 after oxidation. Another vital absorption
band observed at 888–990 cm-3 was the bending
vibration of the trans group of disubstituted olefins (Poiana et
al. , 2015). This band provides related oxidation information about the
bands at 3009 and 3550 cm-3. The absorption band is
extremely weak, with a low intensity compared with the other bands.
Additional absorption bands and shifts in wavenumbers and their
comparison to the simulated FTIR are summarised in Table 5. The results
of the FTIR spectra from this study were comparable to those from the
reported FTIR studies on edible oils (Guillén and Goicoechea, 2007). The
results also agreed well with the simulated FTIR spectra shown in Figure
6 & 7.