Subcritical dimethyl ether extraction as a simple method to extract nutraceuticals from byproducts from rice bran oil manufacture

Rice bran oil byproducts, including defatted rice bran (DFRB), rice bran wax (RBW), filter cake (FC), and acid oil, are rich sources of valuable fat-soluble nutraceuticals such as γ-oryzanol, tocopherol, tocotrienol, phytosterol, and policosanols (PCs). These nutraceuticals possess various bioactivities, including antioxidant, anticancer, anti-inflammatory, cholesterol-lowering, and cardiovascular disease protective effects. PCs, in particular, are a mixture of high-molecular-mass primary alcohols with cholesterol-lowering properties, making them commercially valuable as dietary supplements and ingredients in functional foods and pharmaceuticals. Traditional extraction methods often involve hazardous organic solvents, raising concerns about environmental impact and residual solvent contamination. Supercritical fluid extraction (SUPFE) offers a greener alternative, but it demands high pressure and temperature, making it less economically viable for large-scale applications. Subcritical fluid extraction (SUBFE), using solvents like dimethyl ether (DME), provides a more cost-effective and environmentally friendly approach. DME's low boiling point, solubility properties, and safety profile make it an attractive solvent. However, the application of SUBFDME for extracting γ-oryzanol, phytosterol, and PCs from rice bran oil byproducts and improving PC purity remains unexplored. This study aimed to evaluate the efficacy of SUBFDME in extracting these nutraceuticals from rice bran oil byproducts and determine its potential in enhancing policosanol purity.

Previous research has established the presence of significant amounts of nutraceuticals in rice bran oil byproducts generated from both solvent and cold-pressed extraction processes. The bioactivities of γ-oryzanol, tocotrienol, tocopherol, phytosterol, and policosanol have been extensively documented. Policosanols, specifically, have been shown to lower blood cholesterol levels, primarily through the inhibition of HMG-CoA reductase. Commercial policosanol is currently sourced from sugarcane wax or rice bran wax. Growing concerns about environmental sustainability and consumer preference for solvent-free products have driven the development of eco-friendly extraction techniques. Supercritical fluid extraction is a well-established green technology, but it is often expensive and energy-intensive. Subcritical fluid extraction offers a more sustainable and economical alternative by utilizing lower pressures and temperatures. Dimethyl ether has emerged as a promising solvent for SUBFE due to its low boiling point, low water solubility, excellent hydrophobic compound solubility, and approval by the European Food Safety Authority for food applications. However, studies on the efficacy of SUBFDME for the extraction of the specific nutraceuticals of interest from rice bran oil byproducts were lacking prior to this research.

This research used byproducts from commercial rice bran oil production, including DFRB-S and RBW from solvent extraction and DFRB-C and FC from cold-pressed extraction. These samples were dried, sieved, and stored before analysis. Standard γ-oryzanol, policosanol (docosanol, tetracosanol, hexacosanol, octacosanol, and triacontanol), and phytosterol (campesterol, stigmasterol, β-sitosterol, sitostanol, and 5α-cholestane) were used for quantification. A lab-scale SUBFDME apparatus with a capacity of 10 g was employed for extraction. Samples were placed in a cellulose thimble, DME was introduced, and extraction was performed under controlled pressure (<1 MPa), temperature, time (30 min), and stirring speed. For RBW, a transesterification (TE) method was used prior to SUBFDME extraction because of the freezing of RBW at lower temperatures. A comparative TE method was also used for extraction, involving the reaction of samples with a solution of NaOH in ethanol, followed by extraction with isooctane and ethanol. To enhance policosanol purity, transesterified RBW and FC underwent additional SUBFDME or solvent extraction (SE) using toluene. Residual solvent analysis in the final policosanol extracts was conducted using headspace gas chromatography-mass spectrometry (GC-MS). γ-oryzanol content was determined using LC-MS, phytosterols using GC-MS, and policosanols using GC-MS. Data analysis involved ANOVA and Duncan's test using SPSS 19.

SUBFDME showed a higher extraction yield and nutraceutical content (γ-oryzanol and phytosterol) compared to TE. DFRB-C yielded higher extraction yields (9.71%) than DFRB-S (3.60%) by SUBFDME. SUBFDME produced the highest γ-oryzanol content (924.51 mg/100 g) from DFRB-S and (829.88 mg/100 g) from FC. The highest phytosterol content was 367.54 mg/100 g. TE provided the highest policosanol yield (43.71%), with a policosanol content of 30,787 mg/100 g from RBW. Notably, combining TE and SUBFDME significantly increased policosanol content in transesterified RBW and FC to 84,913.14 mg/100 g. The residual solvent levels in the final policosanol extracts were below the ICH guidelines and other established limits, indicating the safety of the process. The predominant phytosterols were stigmasterol and β-sitosterol, consistent with previous research.

The findings demonstrate that SUBFDME is a superior method for extracting γ-oryzanol and phytosterol from rice bran oil byproducts, offering significant improvements in their content compared to traditional chemical extraction methods. The combination of transesterification and SUBFDME emerges as a highly efficient and simple two-step process for enhancing policosanol purity and recovery. The high policosanol content achieved using SUBFDME surpasses results obtained using other extraction methods reported in the literature. The low residual solvent levels in the final extracts confirm the environmental friendliness and safety of the proposed method. These results highlight the potential of SUBFDME as a valuable tool in the production of nutraceutical-rich dietary supplements, functional foods, and pharmaceutical products. The success of SUBFDME can be attributed to its ability to liberate fat-soluble bioactive compounds from the byproducts' matrix, thanks to DME's solubility characteristics and the principles of subcritical fluid extraction.

This study successfully demonstrated the efficacy of SUBFDME as a green technology for the extraction of γ-oryzanol and phytosterol from rice bran oil byproducts. The two-step process of transesterification followed by SUBFDME significantly enhanced policosanol purity and recovery. This method offers a promising, sustainable, and economically viable alternative to traditional extraction methods for the production of nutraceuticals. Further research could explore the optimization of SUBFDME parameters for even higher extraction yields and investigate the scalability and economic feasibility of this process for industrial applications.

This study focused on a specific set of rice bran oil byproducts and may not be directly generalizable to other sources or types of byproducts. The study was conducted using a lab-scale SUBFDME apparatus; larger-scale optimization may be needed before industrial application. Furthermore, while residual solvent analysis was conducted, a more comprehensive study of potential environmental impacts could be undertaken.