3.4.2 Sterols contents
The content and composition of phytosterols in RBO extracted using various methods were listed in Table 3. It was found that the levels of phytosterols in RBO obtained by AEE was much higher compared with that of SEO. It was in agreement with Konopka, Roszkowska, Czaplicki & Tańska (2016) who reported that higher phytosterols content in the enzyme-extracted oil. While, it was different from the results of Nyam et al. (2009) that the Kalahari melon seed oil extracted by organic solvents contained higher sterols contents. As could be seen in Table 3, the predominant sterols in RBO were β-sitosterol (50% of total sterols), followed by campesterol, stigmastanol and stigmasterol. It indicated that enzymatic treatment can release more antioxidant compounds from rice bran due to the hydrolysis of cell walls. In addition, enzymatic hydrolysis can reduce the degree of interaction between antioxidants and proteins, polysaccharides and pectins in the bran and promote their release into oils (Fang, Du, Luo & Jin, 2015). The lower content of unsaponifiable compounds in SEO might be due to the low polarity of n-hexane.
3.4.3Squalene contents
Squalene is a highly unsaturated aliphatic hydrocarbon (C30H50), which is widely found in olive oil, soybean oil and peanut oil (Chanioti & Tzia, 2019). Previous studies have shown that squalene has natural antioxidant activity, which makes it widely used in cosmetics. However, phenols and tocopherols are the main antioxidants in oils, which makes the antioxidant activity of squalene less obvious (Psomiadou & Tsimidou, 1999). Comparison of AEEO and SEO showed significant (p < 0.05) differences of squalene contents which were 2962.56 mg/kg and 2479.56 mg/kg, respectively. It indicated that AEE promoted the release of squalene compared with SE. The contents of squalene obtained by extraction were much less compared with that by cold pressing (3189 mg/kg) (Pokkanta, Sookwong, Tanang, Setchaiyan, Boontakham & Mahatheeranont, 2019). It illustrated that the content of squalene in crude oils depended on the different extraction methods.