Pressurized liquid extraction as an eco-friendly approach to recover anthocyanin from black rice bran

Black rice is becoming popular recently, being consumed as a functional food due to its health potential (Ito & Lacerda, 2019; Poonia & Pandey, 2022). However, from rice processing, about 20% of rice husk is produced, 10% of rice bran, and 14% of broken rice (Moraes et al., 2014; Nagrale, Hajare, & Modak, 2012; Van Hoed et al., 2006). Although these by-products have a high content of bioactive compounds, they are generally intended for animal nutrition (Das, Goud, & Das, 2018; Halee et al., 2020; Peanparkdee & Iwamoto, 2019) and one way to add value to the black rice by-product is extracting its bioactive compounds (Halee et al., 2020).

Among the mentioned by-products, black rice bran has a high amount of bioactive compounds, mainly anthocyanins, usually containing around 64–90% of cyanidin-3-O-glucoside (Huang & Lai, 2016). Anthocyanins are water-soluble natural colorants belonging to flavonoids, which are considered bioactive due to their antioxidant properties. They are related to health benefits, having anti-obesity, anti-diabetic, antimicrobial, anti-cancer, neuroprotective, and cardiovascular disease potential (Luzardo-Ocampo, Ramírez-Jiménez, Yañez, Mojica, & Luna-Vital, 2021; Tena, Martín, & Asuero, 2020).

The extraction of phenolic compounds, mainly anthocyanins, can be a viable alternative for reusing this material generated during the processing of black rice. However, using solvents to extract antioxidant compounds in a conventional process can be slow, expensive, and still present low efficiency (Halee et al., 2020). Therefore, innovative technologies can be an alternative to achieve better yields in the recovery of bioactive compounds. Among them for extracting anthocyanins from black rice, microwave-assisted extraction was the most used (Abdel-Aal, Akhtar, Rabalski, & Bryan, 2014; Halee et al., 2020; Jha, Das, & Deka, 2017; Moirangthem, Ramakrishna, Amer, & Tucker, 2021) followed by ultrasound-assisted extraction (Jha et al., 2017; Thakur, Gupta, Dhar, Deka, & Das, 2022).

One of the alternative methods with fast and efficient extraction is pressurized liquid extraction (PLE), which uses conditions of high temperature and pressure, ensuring that the solvent remains in a liquid state (above the boiling point but below its critical point) (Mustafa & Turner, 2011; Zielinski et al., 2021). High pressure improves solvent diffusion (high solvation power). Mass transfer increases due to the decrease in surface tension and viscosity under the applied conditions (Picot-Allain, Mahomoodally, Ak, & Zengin, 2021). Also, it results in high yields in less time and less solvent due to a facilitated rupture of solid matrix cells (Zielinski et al., 2021).

Furthermore, the extraction efficiency depends on the analyte to be extracted, the location of the analyte within the matrix, and the nature of the sample matrix (Mustafa & Turner, 2011). The main solvents used are ethanol and water and their mixtures, which result in high solubility of the pigments and meet environmental requirements, being considered GRAS (Generally Recognized as Safe) solvents (Viganó et al., 2022; Zielinski et al., 2021).

Using the PLE technique with water and/or ethanol as solvents is considered an eco-friendly approach widely used for obtaining anthocyanins from fruits, vegetables, and their by-products (Zielinski et al., 2021). Therefore, the technique used is essential to enhance the use of black rice bran, which, although it has a high nutritional value, is usually used as animal feed. To the best of our knowledge, this is the first study to optimize the recovery of anthocyanin-rich extract from black rice bran by PLE. The stability to temperature and fluorescent light of anthocyanin-rich extracts obtained by PLE was also evaluated, as well as the cytotoxicity and antitumoral activity of anthocyanin-rich extracts.

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