Integrated rice bran processing by supercritical CO2 extraction and subcritical water hydrolysis to obtain oil, fermentable sugars, and platform chemicals
Graphical Abstract
Introduction
The increase in pollutant emissions into the atmosphere and the search for alternative sources of renewable energy have promoted initiatives by several countries for the wide use of these sources. Agroindustrial waste may be a promising source of energy and an option to reduce negative environmental impacts [1]. The production of biofuels from waste biomass generated after industrial, agricultural, and forestry activities has become an important area of research [2].
Rice (Oryza sativa L.) is the third most cultivated grain in the world, only behind corn and wheat. In the 2018 harvest, 513 million tons of rice were produced, which resulted in the generation of a large amount of rice bran from the rice polishing process for the production of parboiled rice [3]. Vietnam, China, India, Indonesia, and Bangladesh use rice bran as animal feed or for burning as fuel. Rice bran contains 18–22 wt% of oil [4], [5]. Rice bran oil can be used in the production of bakery foods as it improves the quality of cooking [5]. Rice bran extracts are used in cosmetics, in the treatment of skin-related disorders (e.g. melanin-related disorders), and for minimizing wrinkles due to their high antioxidant potential [6].
Two main techniques can be highlighted to extract rice bran oil: the conventional technique, which uses organic solvents; and supercritical fluid extraction with carbon dioxide (SFE) [6]. The conventional technology performs the extraction with an organic solvent, such as n-hexane. This type of extraction has the drawback of needing an additional oil refining step before use. Therefore, SFE could be a potential alternative [7]. The supercritical fluid technology can also be used in the reaction step for the production of biodiesel [8]. For example, rapeseed oil and bioethanol were used in a supercritical fluid pilot plant to produce up to 144 L/day of biodiesel [9]. Therefore, supercritical fluid technology can be used both in the oil extraction process and in the reaction stage for the production of biodiesel, which implies a higher chance of making this technology feasible.
Rice bran also contains a significant amount of cellulose and hemicelluloses. The characterization of rice bran by recent studies [10], [11], [12], [13] shows cellulose and hemicelluloses contents of 20–35 and 15–28 wt%, respectively. Lignin is also found in rice bran (16–32 wt%) and it is a recalcitrant component for some reaction processes. Cellulose and hemicelluloses found in agroindustrial residues can be hydrolyzed into platform chemicals and fermentable sugars, which are used in the fermentation process to produce second-generation ethanol [14].
Many challenges to producing second-generation ethanol and platform chemicals at competitive costs are seen, including the determination of the best process to obtain monosaccharides. Two main hydrolysis processes can be highlighted: acid and enzymatic processes. The acid route undesirably produces a large amount of molecules that inhibit fermentation for ethanol production, while the enzymatic route has the disadvantage of being high cost and demanding a long time for the process [15]. Thus, sustainable processing routes, which improve the conversion efficiency of lignocellulosic biomass, have been researched.
An alternative is the use of subcritical water hydrolysis (SWH) because water is an environmentally friendly solvent and its thermodynamic variables are easily manipulated [12]. Several lignocellulosic biomass, such as soybean straw and husk [16], rice straw and husk [17], [18], and sugarcane bagasse [19] were subjected to SWH.
A few small companies, such as Tyton Biosciences located in Virginia (USA) and Rematec Corp. Sakai SC (Japan) use subcritical water technology in industrial applications. Tyton Biosciences developed new methods based on subcritical water technologies (4 MT/day biomass throughput) that were successfully tested to obtain lignocellulosic sugars or many other applications that require partial hydrolysis. Subcritical water technology for lignocellulosic biomass processing is widely used by the Rematec Corp plant, with a high processing capacity (70 tons per day) [20], [21].
In this context, the integrated processing of rice bran was evaluated within the concept of biorefinery, aiming at maximum use of this raw material. Therefore, the objective of this study was to evaluate the use of rice bran (RB) as a raw material in SFE using CO2 and the sequential use of defatted rice bran (DRB) for SWH of the cellulose and hemicelluloses found in its structure to obtain oil, fermentable sugars, and platform chemicals.
Section snippets
Materials
RB was acquired from the rice manufacturing agribusiness located in Rio Grande do Sul - Brazil. The RB particles had an average diameter of 0.348 mm. It was packaged without milling in plastic films and stored at a temperature of − 5 °C until the assays. The n-hexane was supplied by Sigma-Aldrich (Brazil).
Rice bran extraction by SFE
The experimental assays were performed in the same extraction unit used in other studies of the research group [22]. The extractor used is made of 316 L stainless steel, with an internal
Raw material characterization
The moisture, oil content (n-hexane solvent extraction), extractives (ethanol and water extraction solvent), protein, and ash content in fresh rice bran were 8.52 ± 0.80 wt%, 20.30 ± 1.00 wt%, 21.77 ± 1.34 wt%, 13.12 ± 1.38 wt%, and 9.92 ± 0.30 wt%, respectively. Having the biomass composition is important because the moisture content and the ash content were used in the calculation basis for the determination of cellulose, hemicellulose, and lignin contents. The extractives are composed
Conclusion
RB has been subjected to SFE and a high oil yield (17.47 wt%) was found when using 20 MPa and 40 °C. DRB presented high YFS (6.53 g sugars/100 g DRB) at 260 °C / S/F-100. The results obtained agree with the scientific literature in terms of the dissociation of the lignocellulosic structure at different temperatures. In this context, supercritical fluid extraction and subcritical water hydrolysis might be opening a new perspective on the future of the agricultural waste recycling business.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors would like to thank the Coordination for the Improvement of Higher Education Personnel (CAPES), National Council of Technological and Scientific Development (CNPq: 407105/2016-6), and the Research Support Foundation of the State of Rio Grande do Sul (FAPERGS: 19/2551-0001261-6). F. Castilhos, M. V. Tres and G. L. Zabot (308067/2021-5) thank CNPq for the productivity grants.
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