Introduction
Rice bran, as a raw material for rice bran oil (RBO) extraction, is a by-product of rice processing, in which oil content can reach 12-22% (Amarasinghe, Kumarasiri & Gangodavilage, 2009; Pandey & Shrivastava, 2018). RBO is a new type of edible oil with high nutritional value. Its fatty acid composition is reasonable and contains more than 80% oleic acid, linoleic acid and linolenic acid. In addition, RBO contains significant amounts of γ-oryzanol, tocopherols, squalene, phytosterol and other important bioactive substances, so it is known as ”nutritional health oil” by nutritionists (Khoei & Chekin, 2016; Trevisani Juchen, Nolasco Araujo, Hamerski, Corazza & Pedersen Voll, 2019).
Current extraction methods of RBO used in oil industry mainly include cold pressing method, solvent extraction (SE) method and pre-pressed followed by SE. Cold pressing method can maintain the unique flavor of oils (Liu et al., 2011). However, this method has many problems, such as high residual oil rate, high cost, high labor intensity and power consumption, and easy to cause protein denaturation (Dun et al., 2019). SE method is often used in commercial RBO production, which is relatively efficient in oil production, and the residue of oil is low (Amarasinghe & Gangodavilage, 2004), but there are also some shortcomings such as its toxicity, no renewability and easy to cause environmental pollution (Hanmoungjai, Pyle & Niranjan, 2001). Pre-pressed treatment followed by SE method can obtain high oil extration yield. This process, however, has some problems in the protein structure leading to protein denaturation. Furthermore, n-hexane has been identified as an air pollutant that is emitted during extraction and recovery.
Compared with the other two methods, aqueous enzymatic extraction (AEE) process has the advantages of good oil quality, no chemical pollution, low energy consumption and good retention of protein, polysaccharide and other components. However, AEE has certain limitations. Due to the low solubility of oil in water, the efficiency of AEE is lower than that of SE. This disadvantage may be overcomed by using selected enzymes and optimizing process parameters (Mat Yusoff, Gordon, Ezeh & Niranjan, 2016). AEE has become a new oil extraction technology for simultaneous extraction of oil and protein, polysaccharide and other components with broad application prospects.
It was reported that Sengupta & Bhattacharyya (1996) used pectinase and cellulase for the first time to extract RBO by AEE combined with organic solvent. Hernandez, Rodriguez-Alegría, Gonzalez & Lopez-Munguia (2000) obtained glucose, protein hydrolysate, RBO in three steps by adding glucoamylase and protease before extracting RBO with solvent. After that, Hanmoungjai, Pyle & Niranjan (2002) focused on the extraction yield of RBO, protein and the level of reducing sugar in the enzymatic hydrolysis process, and finally indicated that Alcalase 2.4L could significantly improve the yield of oil and protein, while the use of carbohydrase could improve the level of reducing sugar in the enzymatic hydrolysate. Although these papers have published on the extraction of RBO by AEE, no systematic studies have been conducted on the analysis of quality, thermal behavior and extraction mechanism of the extracted oil.
The objective of this study was to investigate the extraction of oil from rice bran using the AEE with different enzymes (Celluclast 1.5L, Hemicellulase, Pectinex Ultra SP-L, Viscozyme L and Alcalase 2.4L) and selected appropriate enzymes for AEE. The physicochemical properties, fatty acid compositions, bioactive compounds and thermal behavior of AEE-extracted oil (AEEO) were evaluated and compared with those of Soxhlet-extracted oil (SEO). In addition, the microstructure of rice bran before and after different extraction processes were observed, and the mechanism of rice bran oil by enzyme extraction was clarified.