Hydrodynamic study of a reactor for the electrochemical processing of Hibiscus sabdariffa l. extracts

 Corresponding author: Khady NDIAYE. Copyright © 2025 Author(s) retain the copyright of this article. This article is published under the terms of the Creative Commons Attribution License 4.0. Hydrodynamic study of a reactor for the electrochemical processing of Hibiscus sabdariffa l. extracts Khady NDIAYE 1, 2, 3, *, Serigne Fallou KA 2, 4, Cheikhou KANE 1, 2, 3, 4, Mouhamed NDOYE 1, 2, 3, Nicolas C. AYESSOU 1, 2, 3, 4 and Codou Mar DIOP 1, 3 1 Water, Energy, Environment and Industrial Processes Laboratory (LE3PI)/ESP-UCAD. 2 Higher Polytechnic School of Dakar (ESP)/SENEGAL. 3 Cheikh Anta Diop University (UCAD)/Dakar/SENEGAL. 4 Department of Chemical Engineering and Applied Biology/ESP/Dakar/SENEGAL. GSC Advanced Research and Reviews, 2025, 25(02), 206-222 Publication history: Received on 21 September 2025; revised on 13 November 2025; accepted on 15 November 2025 Article DOI: https://doi.org/10.30574/gscarr.2025.25.2.0347 Abstract Health requirements in recent years have made food products containing preservatives or treated at high temperatures less popular, explaining the new focus on innovative and interdisciplinary technologies. The calyxes of Hibiscus sabdariffa L. are rich in antioxidants such as anthocyanins but sensitive to degradation factors such as oxygen. Several technological alternatives have been proposed to reduce its action, such as bubbling with an inert gas or adding preservatives. This work proposes a new electrochemical approach consisting of cold reduction of dissolved oxygen in aqueous extracts of Hibiscus sabdariffa. An electrochemical treatment reactor consisting of an electrolysis cell designed and dimensioned (L=9cm ; l =3.3cm ; e =1cm) in the laboratory. Knowledge of the hydrodynamic parameters is essential for technology transfer on an industrial scale, so these were determined. For a useful volume of 29.7 cm³ and a flow rate of 0.28 mL/s, the transit time of the Hibiscus extract is approximately 107 s. The flow is laminar, with a Reynolds number of 71.57, hence the need for a turbulence promoter. The reactor is a piston type (Peclet number = 3). A non-significant difference between the electro-reduced extract and the initial extract was observed, confirming that this reduction does not dilute the product and that the Hibiscus extract does not contain a supporting electrolyte. Monitoring of the anthocyanins in the electro-reduced Hibiscus extract under these conditions showed better preservation of anthocyanins compared to heat treatments. Preservation was 35% in the first month of storage at 25°C and 30% after 15 days of storage at 37°C. This new electrochemical approach preserves the original quality of the extract at low temperatures without adding antioxidants, while maintaining the same organoleptic qualities as traditional methods. Keywords: Anthocyanins; Oxygen; Electrodes; Hibiscus; Stabilization 1. Introduction The stabilization of anthocyanins, pigments responsible for the red color of Hibiscus sabdariffa L calyxes [1], [2], [3], has been a major challenge for juice preservation in recent years. Traditional methods, such as heat treatment and the use of preservatives, have limitations, particularly in terms of nutrient degradation and potential health risks. Membrane technologies such as nanofiltration and tangential ultrafiltration are certainly alternatives to heat treatment [4] but have limitations with regard to dissolved oxygen. The latter significantly degrades the anthocyanins in Hibiscus extract during storage [5], [6], [7], [8], [9]. Innovative approaches, such as the electrochemical reduction of dissolved oxygen, offer promising alternatives [8]. GSC Advanced Research and Reviews, 2025, 25(02), 206-222 207 Recent advances have led to the introduction of electrochemical techniques to stabilize anthocyanins in hibiscus extracts. By applying a reduction current through a platinum electrode in a two-compartment electrolysis cell separated by a membrane, dissolved oxygen is removed without the addition of chemicals or heat. This method has been shown to significantly preserve anthocyanins during storage, outperforming traditional methods [10]. K NDIAYE and colleagues [8] set up a batch reactor for the electroreduction of Hibiscus extracts. Electroanalysis confirmed the degrading effect of dissolved oxygen, which is the only electroactive element in Hibiscus sabdariffa L. extract. An electrochemical treatment intensity/time combination of -6mA/30min with electrodes immersed 4cm in the solution resulted in 10% anthocyanin preservation at 37°C compared to the extract in which the oxygen was not electrically reduced. In another study, K. NDIAYE et al [10] monitored anthocyanins in roselle extracts that were electrically reduced in a batch reactor and then stored at 4°C and 37°C. The results showed better anthocyanin stability in the first month and sixth month of storage at both 4°C and 37°C. The reduction of dissolved oxygen in the extract resulted in a 10% difference in anthocyanins at 37°C during the first month of storage, unlike the untreated control. At 4°C, a significant constant difference of 5% between the treated extract and the non-electro-reduced extract was noted between the first and sixth months of storage. Dissolved oxygen has long been removed from solutions using an inert gas [11], [12], [13], [14], [15]. However, for large volumes, the cost can be enormous. Unlike electrochemical treatments, methods such as nitrogen gas bubbling require prolonged periods (e.g., 2 hours) to achieve comparable results [8]. In addition, enzymatic systems involving oxidases, catalases, and superoxide dismutases have been explored for oxygen removal [16], [17]. However, these solutions often require the introduction of additional substances into the food matrix, which is not always desirable. This article presents the results of a hydrodynamic study of a continuous treatment reactor for Hibiscus sabdariffa L extracts using an electrochemical method. The study determined the residence time, average flow rate, Peclet number, and Reynolds number. The effect of electrochemical treatment was also monitored prior to a possible transfer of technology to an industrial scale. The electrochemical reduction of dissolved oxygen is an innovative, effective, and chemical-free method for preserving anthocyanins in extracts of Hibiscus sabdariffa L. This approach overcomes the limitations of traditional preservation methods and offers potential for wider applications in the food industry. 2. Materials and Methods 2.1. Materials 2.1.1. Plant material After cleaning, the roselle calyxes were macerated for 3 hours with a calyx/water ratio of 1:5. The extract obtained is the plant material. 2.1.2. Electrochemical equipment Filter press-type electrochemical reactors are compartmentalized electrolysers. They consist of a stack of membranes and flat electrodes between which the electrolytes circulate. Rapid circulation of the electrolytes (several tens of centimeters per second) ensures good agitation and efficient removal of thermal energy. A plastic mesh generally prevents contact between the membranes and electrodes while promoting turbulence. This type of reactor was first used in the MONSANTO process [18], [19] (synthesis of adiponitrile: an intermediate in manufacturing). The electrolysis cell (reactor) is designed and sized in the laboratory. It consists of two compartments separated by a Nafion membrane. The anionic part contains the electrolyte, a 0.1 N hydrochloric acid solution, in which the stainless steel auxiliary electrode is immersed, and the cathodic part contains the Hibiscus sabdariffa L extract and the platinum working electrode (Figure 1). GSC Advanced Research and Reviews, 2025, 25(02), 206-222 208 Figure 1 Continuous reactor with two compartments separated by a Nafion-type cationic membrane : a) cathode section with platinum working electrode b) anode section with stainless steel auxiliary electrode The electrochemical cell designed is inserted into the following device (Figure 2). The roselle extract feeds the cathode, while a hydrochloric acid solution circulates in the anion section. The whole system is connected to a PGZ100 potentiostat linked to a computer equipped with voltammetry software. The processing time and current density (or potential) are determined by cyclic voltammetry. Figure 2 Experimental setup for the continuous electroreduction of Hibiscus sabdariffa L extract a b GSC Advanced Research and Reviews, 2025, 25(02), 206-222 209 2.2. Methods 2.2.1. Electrochemical methods Cyclic voltammetry The electroanalysis of the hibiscus extract was performed by cyclic voltammetry, applying a potential ramp between - 500 and 1500 mV with a constant scan rate of 100 m.s-1. Chronoamperometry The oxygen dissolved in the bissap extract was electrically reduced by imposing the reduction peak potential, obtained by cyclic voltammetry, as a function of time. Chronopotentiometry Chronopotentiometry is also a method of reducing the electroactive element. The intensity of the reduction peak was used to reduce the dissolved oxygen as a function of time. 2.2.2. Anthocyanin assay The assay was performed on extracts of Hibiscus sabdariffa L. The principle is based on the change in color of anthocyanins depending on pH (pH differential method) [20]. After diluting the calyx extract in two buffer solutions at pH = 1 and pH = 4.5, the absorbance is measured at wavelengths of 510 nm and 700 nm, allowing the anthocyanin concentration to be calculated using the following formula: Ca=(P_m*Fd*A*1000)/ε Equation 1 Ca: Anthocyanin concentrations in mg.L1 Pm: Molecular weight of anthocyanin; in this case, Ca is expressed in relation to delphinidin sambibioside, which is the main anthocyanin in the calyxes of Hibiscus sabdariffa L. Its molecular weight is 597 g.mol-1 • Fd: Dilution factor • ɛ: Molecular extinction coefficient equal to 26,000 mol.L-1.cm-1 • A: Absorbance calculated using the formula: A = (A1 – A2) – (A3 – A4) • A1: Absorbance measured at pH 1 at 510 nm • A2: Absorbance measured at pH 1 at 700 nm • A3: Absorbance measured at pH 4.5 at 510 nm • A4: Absorbance measured at pH 4.5 at 700 nm 3. Results and discussion 3.1. Determination of residence time The distribution of residence times was determined between the inlet and outlet of the reactor. Figure 3 shows the distribution of residence times obtained for a flow rate of 0.28 mL.s-1. This flow rate was obtained by considering the volume and processing time of the batch reactor [8]. GSC Advanced Research and Reviews, 2025, 25(02), 206-222 210 Figure 3 Distribution of dwell times as a function of time [21] The trace of the tracer fraction that remained in the reactor between t and t+dt as a function of time allows the curve to be compared with the response of a dispersion piston reactor. The average residence time tm in the reactor is 193 seconds, and the transit time, i.e., the ratio of the volume of the solution in the reactor to the flow rate, is 107 seconds. 3.2. Reactor dimensions The dimensions of the designed electrolysis cell are shown in Table 1 below. Table 1 Dimensions of the continuous reactor Dimension Valeurs Unités Longueur 9 cm Largeur 3.3 cm Epaisseur 1 cm Distance inter-électrodes 1 cm Volume total 59.4 cm3 Volume utile 29.7 cm3 Surface spécifique 1 m2.m-3 Densité du courant 6 mA.cm-2 To improve the efficiency of dissolved oxygen reduction or reduce secondary reactions such as solvent oxidation, which increases energy consumption, three parameters are adjusted (current density, specific surface area, and interelectrode distance). These parameters can significantly influence the efficiency of electroreduction [22] [23]. Similarly, the distance between electrodes crossed by the electric current increases electrical resistance, which can alter current density and cause secondary reactions [22]. To improve the reactor's production capacity, the surface area of the electrodes and, consequently, the volume of the reactor can be increased [24], [25]. However, a very large electrode surface area can lead to uneven current density at the electrodes and may cause secondary reactions. GSC Advanced Research and Reviews, 2025, 25(02), 206-222 211 3.3. Dimensionless numbers (Reynolds and Peclet) Table 2 below shows the dimensionless values determined during the hydrodynamic study. Table 2 Reynolds number and Peclet number values Reynolds 71.57 Peclet 3 We used the Reynolds number to determine whether or not we needed to put a turbulence promoter in the reactor, as we have a fairly low flow rate. The value found clearly shows laminar flow, so a turbulence promoter is needed to allow the dissolved oxygen to migrate from the surface layers to the working electrode. The Peclet number is needed to determine the type of flow in the reactor. According to the DTS curve, we have a pistontype flow with the presence of a short circuit or preferential path. Perhaps this short circuit could explain the low value of the Peclet number, which tends more towards the flow of a perfectly stirred reactor[26]. 3.4. Monitoring of anthocyanins The electrochemical treatment was carried out in the continuous reactor (Figure 1). The current density and peak reduction potential were determined by cyclic voltammetry. Figure 4 below shows the results of the electroanalysis of the Hibiscus sabdariffa L. extract. Figure 4 Votammogram of Hibiscus sabdariffa L extract Electroanalysis of the bissap drink confirms the presence of dissolved oxygen at a current density of -6mA.cm-2 and a potential of -200mV. This result corroborates those obtained with the batch reactor [8], [10]. Figure 5 below shows the anthocyanin concentration of the electrically reduced extracts and the control, stored at 4°C, 25°C, and 37°C as a function of time. The flow rate is 0.28 mL.s-1. Chronopotentiometry, which consists of performing electrochemical treatment by fixing the current intensity, corresponds to the sample (-13mA). Chronoamperometry was used to perform the electroreduction with a potential of -200mV corresponding to the sample (-200mV). The control is the untreated Hibiscus sabdariffa extract, stored under the same conditions as the samples. GSC Advanced Research and Reviews, 2025, 25(02), 206-222 212 Figure 5 Concentrations of electro-reduced Hibiscus sabdariffa L extracts stored at 4°C, 25°C, and 37°C as a function of storage time GSC Advanced Research and Reviews, 2025, 25(02), 206-222 213 The monitoring of Hibiscus sabdariffa extracts, treated electrochemically (samples) and untreated (control) in a continuous reactor, after 2 months of storage at 4°C, 25°C, and 37°C (Figure 5) show the positive effect of dissolved oxygen reduction on the anthocyanins of Hibiscus sabdariffa L. The predominance of temperature over other factors in anthocyanin degradation was also observed at 37°C. The reduction of dissolved oxygen in Hibiscus sbdariffa L. extracts stored at 25°C shows a significant difference between the sample (18% loss) and the control (36% loss) from the second week of storage. This represents an 18% greater loss for the control compared to the sample whose oxygen was electrically reduced. In the fourth week of storage at 25°C, there was a 27% loss for the sample and a 58% loss for the control. This represents a significant difference of 36% loss in anthocyanin between the electrically reduced extract and the control. Similarly, in the eighth week, a 36% loss in anthocyanin was noted for the sample, compared to a 98% loss for the control. Electrochemical treatment of bissap extracts stored at 37°C also revealed the degrading effect of dissolved oxygen on the anthocyanins in Hibiscus sabdariffa L. In fact, by the second week of storage, the sample had lost 50% of its anthocyanins, while the control sample had lost nearly 84%. After one month of storage, the sample recorded an 86% loss, while the control sample lost 97% of its anthocyanins. Nevertheless, the differences are significant. After two months of storage, a 95% loss was noted for both the sample and the control sample: temperature predominates over the oxygen factor. However, at 4°C, losses were less than 1% in two weeks, approximately 2% in the first month, and 7% in the second month of storage. This result could be explained by the fact that the effect of dissolved oxygen on the anthocyanins in Hibiscus sabdariffa is catalyzed by a certain temperature [9]. Reducing the dissolved oxygen in Hibiscus sabdariffa L extracts (samples) significantly reduces anthocyanin losses, especially at room temperature (25°C), confirming the results obtained with the batch reactor [10], [27]. 3.5. Color monitoring The color of anthocyanins was also monitored over time. The a*, b*, and L* coordinates were measured using the CIElab system [28] [29] with a CR-5 colorimeter. Curves A, B, and C in Figure 6 below show the evolution of the red color, represented by the red/green component a*, over time. GSC Advanced Research and Reviews, 2025, 25(02), 206-222 214 Figure 6 Monitoring of the red/green color component of electrically reduced extracts (Samples) and untreated extracts (Control) over time at 4°C, 25°C, and 37°C. The results for color corroborate those for anthocyanins. At 4°C, 25°C, and 37°C, the red color of the electrically reduced samples was more intense than that of the control sample during the first four weeks of storage. Similarly, after two months of storage at 4°C and 25°C, significant differences were noted between the sample and the control. We have a* = 54 for the sample and a* = 53 for the control at 4°C at one month and two months. 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