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.