3.4.4. Oryzanol contents
Rice bran is one of the most abundant sources of oryzanol. RBO contains about 0.9-2.9% oryzanol which can vary greatly according to the origin of the rice bran (Arab, Alemzadeh & Maghsoudi, 2011). Oryzanol is a mixture of esters of ferulic acid with sterols and triterpene alcohols, and has similar antioxidant activity as vitamin E (Lloyd, Siebenmorgen & Beers, 2007). In Table 3, the content of oryzanol in AEEO was significantly higher (p < 0.05) than in SEO, which can be attributed to the degradation of oryzanol caused by the increase of temperature and the prolongation of SE treatment time.
3.5 Thermal behavior of AEEO and SEO
DSC is to determine the thermal behavior of oils during heating or cooling and it provides another method for studying the chemical composition of oils. The crystallization and melting curves of the AEEO and SEO were shown in Fig. 2 and their corresponding transition temperatures were also given.
In Fig. 2a, the crystallization curves of RBO extracted by Alcalase 2.4L and n-hexane were represented. The cooling curve of AEEO and SEO exhibited three exothermic peaks, with a major peak (a2, b2) and two small shoulder peaks (a1, b1) and (a3, b3). It could be clearly seen that the points of crystallization peaks of the two extraction methods were obviously different. The initial crystallization peaks of AEEO and SEO were 30.93 ℃ and 33.70 ℃, respectively. Compared with the SEO, the crystallization point of AEEO was lower. The various of crystallization peaks might be due to the different extraction methods, which could influence the degree of saturation in RBO (Tan, Chong, Hamzah & Ghazali, 2018). Generally, each kind of oil has unique characteristic of fatty acids and triacylglycerol (TAG) profiles and the thermal transition of lipids is affected by the compositional changes of the degree of saturation and fatty acid chain length. The crystallization peaks of oils with high levels of saturated fatty acids and tri-saturated TAGs would be at a relatively high temperature (Mohammadpour, Sadrameli, Eslami & Asoodeh, 2019).
The melting curves of RBO extracted by Alcalase 2.4L and n-hexane were represented in Fig. 2b. It was found that SEO displayed one peak (b1) at low-melting region (< 0 ℃) and the other peak (b2) at high-melting region (> 0 ℃). However, this phenomenon was not observed in the melting curves of AEEO and all the melting peaks of AEEO were located in the low-melting region. Results showed that the melting behavior of RBO was significantly related to the extraction method. As discussed, the possible reason of these differences might be the variations in the proportional distributions of the identified fatty acid and TAG components. It might be due to the saturated compounds of SEO were slightly higher than AEEO. Therefore, higher temperatures were needed to melt SEO completely.