Refined oils
First, a published general refining process was applied to both oils,
which involved degumming with citric acid (Diosady et al., 1982; Ghazani
et al., 2013), alkali refining with 2% sodium hydroxide solution
(Suliman et al., 2013), and then bleaching with Fuller’s earth (Soldo et
al., 2019). Table 4 shows the composition and oxidative stability of
refined silflower and sunflower oils. While chlorophyll in sunflower oil
was completely removed by this refining process, the chlorophyll level
in silflower oil was still high (725.5 mg/kg). TPC slightly decreased
while total phenolics did not change for both oils after the refining
process. Radical scavenging ability determined by ABTS assay of
silflower oil decreased after refining while that of sunflower was not
significantly changed (Tables 1 and 4).
Silflower oil lost a very slight amount of squalene. Nergiz and
Celikkale (2011) also observed a very slight loss (0.002-0.1%, w/w, in
oil) of squalene during a neutralization/physical refining followed by
bleaching. The FFA levels in both oils were significantly lowered after
the refining process, and it was not detected in refined sunflower oil.
It is interesting that α-tocopherol in silflower oil increased. In
general, tocopherols and other phytochemicals decompose during refining.
However, some studies (Pestana et al., 2008; Rossi et al., 2001; Van
Hoed et al., 2006) reported increased tocopherols after refining
processes including bleaching treatments with acid clays, degumming, and
deodorization.
OSI of silflower oil slightly lowered, and the thermal stability
measured by TGA also decreased after refining. Changes in silflower oil
by the refining process such as the loss of total phenolics might have
negatively affected the oxidative stability. OSI and thermal stability
of sunflower oil significantly decreased after refining, which might be
attributed to the decreased tocopherol level.
Oil color is also an important for sensory property for the
acceptability of edible oils (Rhazi et al., 2022). The yellowish color
of crude sunflower oil was removed after refining (Fig. 2). Refined
silflower oil still had a dark color reflecting the high level of
chlorophyll even though it was somewhat lightened. Table 5 shows the
color analysis results. Reflecting the lighter color of refined oils
shown in Fig. 2, lightness (L*) increased from 73.56 to 80.49 for
sunflower oil and 31.22 to 46.47 for silflower oils. Chlorophyll is the
major component responsible for green color. Table 4 showed that
a* value was higher for refined
silflower oil compared to crude oil reflecting the decreased chlorophyll
level. The greater negative a* value for refined sunflower oil indicates
that some compounds that are responsible for red color were removed
along with the small amount of chlorophyll. The positive b* values of
both oils before and after refining indicate that they all had yellowish
color. The low b* value of refined sunflower oil reflected its very
light color as shown in Figure 2.