Circulating Polyphenols Extraction System with High-Voltage

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Circulating Polyphenols Extraction System with High Voltage Electrical Discharge: Design and Performance Evaluation Yong Deng, Ting Ju, and Jun Xi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03827 • Publication Date (Web): 13 Sep 2018 Downloaded from http://pubs.acs.org on September 19, 2018

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ACS Sustainable Chemistry & Engineering

Title: Circulating Polyphenols Extraction System with High Voltage Electrical Discharge: Design and Performance Evaluation

Authors: Yong Deng, Ting Ju, and Jun Xi*

Address: School of Chemical Engineering, Sichuan University, 24 south Section 1, Ring Road No.1, Chengdu, Sichuan 610065, China

*Corresponding author: Jun Xi

Address: School of Chemical Engineering, Sichuan University, Chengdu 610065, China Tel: +86 28 65292503; Fax: +86 28 65292503 E-mail address: [email protected] (J Xi)

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ABSTRACT: Spent coffee grounds (SCG) were an abundant industrial waste material containing rich polyphenols. In this study, a novel high voltage electric discharge (HVED) circulating extraction system with a “needle to ring electrode” treatment chamber was for the first time designed to extract polyphenols from SCG.

This new system consisted of high

voltage pulse generator, treatment chamber, extracting tank, and transport unit. The optimized conditions for circulating polyphenols extraction by single factor analysis were: 20 min extraction time, 50 mL/g liquid to solid ratio, 200 mL/min flow rate, 24% ethanol concentration and 11 kV discharge voltage. Accordingly maximum yield of polyphenols was 59.83 ± 1.53 mg/g, which was higher than solvent extraction (53.52 ± 1.41 mg/g) by 20.03 % with 150 min extraction time and “converged electric field type” method (49.84 ± 1.16 mg/g) by 11.78 % with 91.6 min extraction time. Energy consumption of circulating extraction was 1962.1 kJ/kg, which was far lower than those of “converged electric field type” method (6015.9 kJ/kg) and solvent extraction (5423.5 kJ/kg). Therefore circulating extraction could reach a highest yield with lowest extraction time and energy consumption. Results indicated that HVED circulating extraction system was most efficient among three methods. KEYWORDS: high voltage electrical discharge, circulating extraction, needle to ring electrodes, spent coffee grounds, polyphenols.

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INTRODUCTION Coffee is the second largest traded commodity just behind the petroleum in the world. With the great market demand of coffee, a large amount of solid residues known as spent coffee grounds (SCG, Figure 1) which contain abundant compounds such as polyphenols, caffeine and polysaccharide etc., are generated after brewing in the coffee industry.1, 2 During the production process, only 0.33 - 0.45 kg of instant coffee is gotten from 1 kg of green coffee beans.3 That’s to say, SCG account for up to about 60% of the total mass of green beans. According to the data of United States Department of Agriculture, the world coffee production for 2016/2017 is about 159 million bags (60 kilograms per bag), and world consumption keeps increasing rapidly year by year. 4 However, SCG are typically subjected to incineration and landfill disposal as industry waste material, which is unfortunately improper due to the ecotoxicity of caffeine, tannins and phenolic compounds contained in SCG.5 Recently reuses of SCG have been extensively explored such as biofuels production, recovery of bioactive compounds, adsorbents, which offer some environment-friendly ways to dispose the residues and present the potential of applications of SCG.6-9 Natural polyphenols from food and herbal plants have attracted much attention owing to the benefits for health.10 Polyphenols are good antioxidants for human. Studies exhibit that polyphenols can lower the risk of various diseases (cancer, heart diseases, etc.).11,12 Based on the analyses of chemical composition of SCG, SCG contain rich polyphenols including chlorogenic acid and hydroxyhydroquinone.13,14 Hence, SCG are a renewable valuable source of polyphenols. To obtain the polyphenols and meanwhile ameliorate the ecotoxicological and environmental impacts of SCG, it is very meaningful to extract polyphenols from SCG. 3



In industry, solid-liquid extraction is a widespread application in recovery of polyphenols and other bioactive compounds. However, the traditional polyphenols extraction methods (such as solvent extraction) have lots of disadvantages, for instance long extraction time, environmental pollution, waste of energy and solvent.15 Some emerging technologies, such as pulsed electric field,16 microwave,17 ultrasound,18 and high voltage electrical discharge (HVED),19 have been devoted to recovery of polyphenols from SCG, manifesting a tremendous potential for polyphenols extraction and residues reuse.20-22 In particular, HVED is a non-thermal treatment technology, with which the strong discharges and strong local influences are initiated through underwater electrode couple to bring about chemical reactions and physical processes involving in shock wave, formation of hydroxyl radicals, UV light, cavitation bubble and strong turbulence.23 When these strong enough influences acting upon the cell tissue in the discharge process, cell disruptions are initiated to intensify the mass transfer especially the release of intracellular compounds.24, 25 Lots of studies and applications with HVED extraction have shown its obvious efficiency and advantages in extracting bioactive compounds.19, 24, 26-30 According to previous investigations, the extraction operation mode can be categorized into two groups: batch operation and continuous operation. The significant difference is that the former is performed with relatively static fluid in a pool-like treatment chamber while the latter is done with the flowing fluid. The pool-like structure of batch operation is extensively used but the diffusion time is usually long.19 Recently, our research group has designed a continuous apparatus (Figure 2) with a “converged electric field type” treatment chamber. As shown in Figure 2, the continuous apparatus mainly consists of peristaltic pump, flow meter, cooling system, “converged electric field type” treatment chamber, high-voltage pulse 4


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generator, and two beakers for untreated and treated products respectively. Peristaltic pump provides power for flowing and high-voltage pulse generator produces discharge for treatment. The structure of treatment chamber is also provided in Figure 2, which shows the insulating plate with a hole (1mm of the diameter) and the central treatment region between two electrodes. The inlet and outlet of fluid are at the bottom and top, respectively. By applying to phenolic extraction from pomegranate peel, it has shown a promising prospect of continuous operation. Regrettably, the diameter of the hole in the insulating plate is just 1mm. It is so small that the flow rate is only 12 mL/min, and the materials must be smashed to fine particles (particles passing through a 100 mesh sieve) to prevent clogging.31 So it’s desired for our group to develop a system with larger flux to solve these problems. The objective of this study was to develop an efficient circulating extraction system with HVED and new electrode couple (needle to ring electrodes) to extract polyphenols from SCG and evaluate the performance of the system. It is the first time that this new circulating extraction system was applied to extraction active ingredient from industrial waste material. Five extraction parameters (extraction time, liquid to solid ratio, flow rate, ethanol concentration and discharge voltage) were investigated and optimized by single factor analysis. The performances (polyphenols yield, energy consumption, etc.) of three methods, i.e., including circulating extraction, “converged electric field type” method and solvent extraction, were also compared.

MATERIALS AND METHODS Raw Material and Chemicals. SCG were obtained from an instant coffee manufactory in Shanghai of China (coffee beans are Arabica produced in Yunnan, China). The moisture 5



content was 13.31%, and SCG was oven dried at 40°C for 15h to control the moisture content at a low level to prevent microbial destruction during reserve. SCG were stored in a hermetic brown glass bottle at 4°C in the dark place until use. The Sinopharm Chemical Reagent Co., Ltd (Beijing, China) provided gallic acid (3, 4, 5-trihydroxybenzoic acid, with purity than 99.99%), Folin-Ciocalteu’s phenol reagent, and Na2CO3. Ethanol used in the experiments was analytical reagent grade chemicals (Beijing Chemical Reagents Company, Beijing, China). Other reagents were of analytical grade and purchased from Chengdu Chemical Industry (Chengdu, China). All solutions were prepared with analytical chemicals and deionized water was used for preparing aqueous solutions. The UV-Vis spectrophotometer (751-GW) was from Shanghai Analytical Instrument Overall Factory (Shanghai, China). Design of Circulating Extraction System with HVED. The setup of circulating extraction system was described in Figure 3(A, B), which mainly consisted of four parts: high voltage pulse generator, treatment chamber, extracting tank, and transport unit, to make the extraction dynamic. The high voltage pulse generator (Shanghai Xuji Electric Co., Ltd, Shanghai, China) was used to produce repetitive high voltage pulses with duration of few microseconds and peak voltage of 0-40 kV adjustable. The capacitance of capacitor of the discharge equipment was 200 nF. High voltage was detected by voltage measurement with an oscilloscope and a voltage sensor (Shenzhen Zhi Yong Electronics Co., Ltd, Shenzhen, China). The treatment chamber (42 mL of interior volume, Figure 4(A, B)) was mainly made up of insulating wall (polycarbonate) and stainless steel “needle to ring” electrodes. Insulating wall was necessary and important for not only insulating high voltage safely but also 6


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producing a cavity for flow. A venthole set on the top of cavity kept pressure the same inside and outside wall thus averting the pressure and air gathered at the earliest from the inlet tube. Withal, the new electrode couple was used in extraction for the first time. Figure 4C showed the structure of “needle to ring” electrodes, and the point of needle was in the very center of circle and between the upper and lower surface of ring electrode. The needle (0.67 mm of diameter of a pinpoint and 37 mm of length) was well-shaped and easy to be replaced through a clamping arrangement. The bore diameter of ring electrode was 4 mm with a thickness of 0.25 mm and thus allowed large particles through. For convenience, a well-sealed stainless steel stick was used to connect with ring electrode as an auxiliary electrode attached to ground. The inlet of treatment chamber fastened to pipeline was punched on the side wall and the outlet was at the bottom of treatment chamber. And the space close to the center of ring and needle point was the main discharge region where the local electric field is highly intensified. With needle electrode attached to high voltage terminal and ring electrode grounded, discharges were initiated with very strong bright light which presented a good morphology, as Figure 4(B, D) shown. The extracting tank including a tank (1100mL, covering the interior volume of pump) and agitator (200 rpm, Jintan Hongke Instrument Factory, Jintan, China) was used to perform diffusion. The tank was cone-shaped beneficial to motion of solid thus avoiding SCG accumulation in corner and the agitator made sure that solution was mixed homogeneously. The transport unit including a pump and pipeline provided the power and route for circulating flow. Energy for flowing was supported by screw pump with a variable-frequency drive motor (Shanghai Nuoni Light Industrial Machinery Co., Ltd, Shanghai, China) for speed adjusting. The max circulating flow rate was 300 mL/min at 50 Hz and frequency could be 7



adjusted in the range of 0 to 50 Hz, thus permitting to control the flow rate of solution accurately. Besides, Valve 1 was conveniently set for dismantling and cleaning the interior of equipment and valve 2 was set for outlet. The system had a material inlet and an outlet. Generally, the inlet and outlet were closed, and just an inner circulation was existed for extraction. Liquid level of solution should be spatially higher than the ring electrode to ensure that the “needle to ring” electrode was immersed. Solution close to center of electrodes was treated by discharge and transported into tank for diffusion with agitation. At the same time, solution of the same volume was supplemented into treatment chamber through pipeline by screw pump, thus keeping liquid level stable. Therefore, the system was stably circulating and processed substantial feedstock through a local treatment in a small treatment chamber. With such setup, the efficiency could be elevated which was in accordance with the results. The system was well-sealed and no leakage of solution. Circulating Extraction with HVED. The corresponding mass of SCG according to the liquid to solid ratio was dissolved into 1100mL of aqueous ethanol and the mixture was stirred well. The prepared solution of SCG gradually was added into the tank from inlet of apparatus until drowning the ring electrodes with an appropriate liquid level, and then the inlet was sealed off and solution was stirred by agitator at 200 rpm. Needle electrode was allocated to high voltage terminal and the ring electrode (the bore diameter of 4mm) was grounded. Discharge frequency is 3 Hz. The circulating extraction was performed for given conditions in each experiment. The initial temperature of all samples was room temperature, and finally the elevation temperature was below 5 °C. The single-factor experiments were used for studying the influences of extraction time 8


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(min), liquid to solid ratio (mL/g), flow rate (mL/min), ethanol concentration (%) and discharge voltage (kV) on polyphenols extraction from SCG, and the parameters were concretely scheduled in Table 1. When analyzing a factor, the other factors were kept constant. All experiments were repeated three times. Continuous Extraction using “Converged Electric Field Type” Method. As discussed, the “converged electric field type” method had the disadvantages of low flux and easily clogging. For comparison, the control experiments of polyphenols extraction from SCG were conducted using the device shown as Figure 2 reported by our group 31 and optimal conditions made by previous study was 12 mL/min flow rate, 2 mm electrodes gap distance, 50 mL/g liquid to solid ratio, 24% ethanol concentration and 9 kV discharge voltage. 28 22 g sieved fine SCG powder (100 mesh sieve) and 1100mL aqueous ethanol were used for extractions under 9 kV discharge voltage according to the optimal conditions. Discharge frequency is 3 Hz. The rotational speed of agitator ran at 200 rpm. The flow was driven by a peristaltic pump (WG600S, Baoding Lead Fluid Technology Co., Ltd, Baoding, China). The polyphenols yield was compared with that obtained with HVED circulating extraction system. The experiments were performed three times. Conventional Solvent Extraction. Conventional solvent extraction (maceration) of polyphenols was carried out independently in the water bath as a control experiment according to conditions (47 °C temperature, 150 min extraction time, 58% ethanol concentration and 48 mL/g liquid to solid ratio ).32 22 g SCG was dissolved into aqueous ethanol and the mixture was stirred well. The prepared solution was added into round bottom flask and suffered water bath. Extraction was performed with 200 rpm of agitation. Extraction yield was compared to the yield obtained with HVED circulating extraction system. 9



Experiments were performed three times. Determination of the Polyphenol Contents. The content of polyphenols was determined spectrophotometrically by the typical means of Folin-Ciocalteu method.33 0.2 mL of diluted extract was used to mix with 1 mL of ten-times diluted Folin-Ciocalteu reagent in a test tube, and then 0.8 mL of Na2CO3 (75 g/L) was added into the mixed solution. Afterwards, the tube with mixture was transferred into incubator and incubated at 50°C for 10min, and then cooled at room temperature. An identical mixture with 0.2 mL of distilled water rather than diluted extract was used as a control. The specific absorbance was measured at 750 nm by the UV-Vis spectrophotometer (Shanghai Analytical Instrument Overall Factory, Shanghai, China). Results were expressed as mg GAE/g dry matter (DM) through the calibration curve plotted by using gallic acid standard solutions. Each analysis was repeated three times. The yield of polyphenols Y was calculated by equation (1):34 Y=

×

(1)

Where C was the average concentration (mg/mL) of polyphenols of three repeatable tests, obtained by Folin–Ciocalteu method; V (mL) was the volume used in experiment, fixed at 1100mL; M (g) was the medial mass used for extraction of three tests; Y was the yield of polyphenols, mg/g. Calculation of Energy Consumption. The energy consumption usually used as a key token to evaluate the systemic efficiency. The low energy consumption was usually expected to mark an excellent performance of extraction system, which meant relative low cost and high energy utilization. The energy consumption of three methods (circulating extraction, “converged electric field type” method and solvent extraction) was respectively calculated in order to compare the performance of extraction systems. The unit of energy consumption was 10


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kJ/kg. For HVED methods, total energy consumption in the extraction process was estimated by equation (2):30, 35 WH=

(2)

Where C is capacity (F), U is the discharging voltage (V), n is the pulse numbers in the treatment which could be known with discharge frequency and extraction time and m is material mass (kg). For solvent extraction, total energy consumption for treatment was given by equation (3):36 W S=

∆

(3)

Where (about 1100g roughly) and m are the mass of liquid and material, Cp is heat capacity (estimated as 4.2×103 J/kg, K) and ∆ is the temperature increasing from room temperature 20 °C (∆=27 °C).

RESULTS AND DISCUSSION Effect of Extraction Time. The optimal extraction time is a key factor to put up efficiency. Excessive extraction time will cause needless energy input, while too little time will cause an insufficient extraction of bio-compounds. In this study, we investigated 10, 15, 20, 25 min of extraction time impacting on polyphenols yield. The experiments have been performed at 50 mL/g liquid to solid ratio, 24% ethanol concentration, 200 mL/min flow rate and 11 kV discharge voltage. As shown in Figure 5A, it’s obvious that extraction efficiency ascends rapidly before 20min and the maximum yield of polyphenols is 59.83 ± 1.53 mg GAE/g DM 11



(dry matter) at 20 min. This influence may be related to several factors, such as quantity of discharges and diffusion time. Obviously, the more extraction time is, the more discharges and diffusion time the solution receives. Previous studies demonstrate that the physical processes (shock wave, bubbles cavitation, etc.) caused by discharge would initiate cell disruption thus facilitating compounds releasing, 24,

26-29, 37, 38

and meanwhile the releasing of compounds

from matrices to solution needs time, so more discharges and more diffusion time are synergistically favorable to releasing. Thus the higher extraction yield can be obtained. For example, the yield increases from 36.93 ± 1.22 mg/g to 59.83 ± 1.53 mg/g when the extraction time increases from 10 to 20min. However, a threshold is existed when equilibrium solute concentration has achieved in spite of more contact time between solution and particles.22 Yet in the extraction process, a slight drop of polyphenols yield from 20min to 25min is observed. It implies that there exists an appropriate extraction time, and discharge treatment under long time condition may have a negative effect on polyphenols yield. Results suggest a suited time for extraction and 20 min is chosen for experiments. Effect of Liquid to Solid Ratio. The liquid to solid ratio is a very important variable impacting the yield of polyphenols in the extraction. The equilibrium concentration of solute is correlated to liquid to solid ratio and an optimal liquid to solid ratio is necessary for extraction.19 The experiments on the effect of liquid to solid ratio (30, 40, 50, and 60 mL/g) have been done on the condition of 20 min extraction time, 24% ethanol concentration, 200 mL/min flow rate and 11 kV discharge voltage. The polyphenols yield with different liquid to solid ratio is presented in Figure 5B. The yield gradually rises from liquid to solid ratio of 30 to 50 mL/g because the feedstock is superabundant at 30 mL/g, and the yield reaches the maximum at 50 mL/g which means the appropriate amount of SCG at 50 mL/g for the 12


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experimental conditions. It is reported that the driving force for extraction process is higher to facilitate extraction with a higher liquid to solid ratio according to the mass transfer principle.39 While it exceeds 50 mL/g, the polyphenols yield reduces. This may be resulted by the relative reduction of energy acting upon SCG. That’s to say, as increasing of liquid to solid ratio, increasing of the proportion of solvent may result in solvent absorbing more energy thus accordingly weakening the disruption effect and decreasing the yield acting on SCG.31 Therefore, 50 mL/g of liquid to solid ratio is reckoned as the optimal ratio. Effect of Flow Rate. Flow rate is an indispensable parameter in continuous and circulating extraction. The flow of solution will facilitate the mass transfer inside and outside plant cell according to the mass transfer principle. Film theory is used for qualitative analysis which tells that at steady-state diffusion mass transfer coefficient in aqueous phase is related to diffusion coefficient and thickness of the Nernst diffusion film ; exactly, = /.40, 41 Hence, with various flow rate the mass transfer coefficient is different and also the extraction efficiency is different. To study the effects of flow rate, four levels as 67, 133, 200 and 267 mL/min are set with the condition of 20 min extraction time, 50 mL/g liquid to solid ratio, 24% ethanol concentration and 11 kV discharge voltage. As shown in Figure 5C, polyphenols yield is raised from 50.14 ± 1.80 mg/g to 59.83 ± 1.53 mg/g when the flow rate varies from 67 to 200 mL/min. According to the film theory, the thickness of the boundary layer decreases as the flow rate increasing, thus lowering resistance of mass transfer and enhancing extraction31, 40-42 and elevating the polyphenols yield. It finally reaches the maximum at 50mL/g. However, the extraction yield reduces after 200 mL/min, which may be resulted by the difficulty to establish extraction equilibrium between intracellular and extracellular solutions caused by 13



too high velocity of solution.31, 42 So 200 mL/min is seen as an appropriate flow rate for circulating polyphenols extraction of SCG with new system in this study. Effect of Ethanol Concentration. Normally, the medium used for electrical discharge is water thanks to the low conductivity, easy accessibility and nontoxicity. But there have reports showed that polyphenols extraction will be significantly better with ethanol aqueous than that with water.16 In this study, before the extraction experiments of polyphenols carried, we have tested the discharging performance of “needle to ring electrode” in aqueous ethanol (concretely, 0, 20, 35, 50 and 100%). The results show that the discharge didn’t happen at higher ethanol concentration (50, 100%) under 11 kV in our experiment condition, while 0, 20 and 35% of ethanol concentration are easier to discharge. That’s to say, high ethanol concentration impedes the discharge. Hence, even though higher ethanol concentration means higher solubility of polyphenols, we have to choose proper ethanol concentration for extraction. In this study, several low ethanol concentrations have been investigated as set in Table 1. The experiment concentrations are determined at four levels (0, 12, 24, and 36%) and experiments are performed with 20 min extraction time, 50 mL/g liquid to solid ratio, 200 mL/min flow rate and 11 kV discharge voltage. It can be observed from Figure 5D that the polyphenols yield increases gradually with the increase of ethanol concentration from 0 to 24%, and the maximum yield reaches at 24% ethanol aqueous. This result is consistent with the fact, as discussed above, that solution with constant volume and higher ethanol concentration dissolves more polyphenols because the solubility of polyphenols in ethanol is larger than that in water. Also ethanol aqueous is apt to dissolving more polyphenols which is consistent with the results obtained by Boussetta et al..16, 32 However, the polyphenols yield 14


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then decreases with the increase of ethanol concentration from 24% to 36%, because the discharge weakened at higher ethanol concentration. With weakened discharge the physical processes would recede to influence the cell disruption so it’s relatively more difficult for compounds releasing and mass transfer.24,

26-29, 37, 38

Hence, extensively high ethanol

concentration influences the extraction of polyphenols and reduces the polyphenols yield. Moreover, since the positive effect that higher ethanol concentration dissolves more polyphenols and the negative effect that higher ethanol concentration weakens discharge intensity are existed simultaneously, it’s obvious to obtain from results that 24% is the threshold of the two effects. Before 24%, positive effect exceeds negative one while after 24% the negative surpasses the positive. Therefore, 24% ethanol aqueous was selected as the optimal ethanol concentration. Effect of Discharge Voltage. Discharge voltage is a crucial factor affecting extraction yields. Usually, the bio-compounds yields increase with the increasing of applied voltage, however, organic molecules may be degraded by discharges obtained by previous work, 19, 25, 29, 43, 44

which is at variance with our purpose. So it’s imperative to obtain a suitable applied

voltage. In this study, four levels of 8, 11, 14, and 17 kV have been applied to extract polyphenols. The experiments are performed with 20 min extraction time, 50 mL/g liquid to solid ratio, 200 mL/min flow rate and 24% ethanol concentration. The results are displayed in Figure 5E. It’s obvious that the maximum yields reaches at 11 kV on account of the stronger discharges. A bit decreasing of yield is observed from 11 to 17 kV. It can be explained that too high voltage leads to degradation of polyphenols. Hence, the experiments of other parameters are conducted at 11 kV discharge voltage. Discharge voltage also has a significant effect on the polyphenols yield. In fact, the 15



mechanism of underwater discharge is involved in forming of plasma channel.24, 25, 45, 46 When electrical field is high enough under high voltage, electron avalanche happens in pre-existing underwater small bubbles or in bubbles produced by heating in liquid between two electrodes. As the avalanche expands and propagates towards opposite, electrode plasma channel forms. The plasma channel develops axially and transversely, and then discharge is initiated. During the discharge process, secondary phenomena (physical processes such as shock wave, UV light, etc. and chemical reactions such as forming of highly active species O3, OH and H2O2) are produced. Forming of plasma channel takes a part of energy and the remaining energy is deposited into solution in the discharge process. Hence, according to the mechanism of discharge under water, higher voltage can lead to higher energy input when other parameters keep the same. Thus it has a stronger effect on physical processes and chemical reactions resulting in a better cell disruption, which help elevate the extraction efficiency. Meanwhile, the electrical field between electrodes with a higher voltage is further intensified to make it easier to form plasma channel. Thus, the relatively more energy will be deposited into solution for cell disruption and compounds releasing to facilitate extraction.16, 36, 44 On the contrary less energy is used for cell disruption and extraction with lower voltage compared with the former. Considering the degradation of polyphenols in extensively high voltage, 11kV of discharge voltage is selected to increase extraction yield of polyphenols. Comparison of Circulating Extraction with Control Methods. As obtained above, the max polyphenols yield of HVED circulating extraction system reaches up to 59.83 ± 1.53 mg GAE/g DM (20 min extraction time, 50 mL/g liquid to solid ratio, 200 mL/min flow rate, 24% ethanol concentration and 11kV discharge voltage). Ramalakshmi et al. obtained the similar polyphenols yield at 21.9-63.2 mg/g.47 The control experiments include the extraction using 16


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“converged electric field type” method and conventional solvent extraction. Extraction of polyphenols from SCG using “converged electric field type” method affords a max polyphenols yield by 53.52 ± 1.41 mg GAE/g DM with 91.6 min extraction time, which is 11.78 % lower than that of circulating method. Obviously, the extraction time of the “converged electric field type” method (91.6min) is much longer than that of the circulating extraction time (20min). And the circulating flow rate (200mL/min) is over 16 times larger than the flow rate of “converged electric field type” method (12mL/min). Besides, the bore diameter of ring electrode of circulating method is 4mm, which is not clogged. Besides, the energy consumption of the circulating method is totally 1962.1 kJ/kg, accordingly the energy input is 43.6kJ within 20min, while the energy consumption of the “converged electric field type” method is 6015.9 kJ/kg, accordingly the energy input is 133.6 kJ within 91.6 min. Obviously, the energy consumption of the continuous method is over 3 times more than that of circulating method. It may be related to the operation mode of extraction process and the structure of electrodes and treatment chamber. For one thing, the circulating mode can make solution be treated repeatedly, and the energy can be utilized more efficiently. For another, the discharge area of the “converged electric field type” method is actually very limited, which lead to not only the solution volume directly treated by single discharge is very small, but also the insulating wall absorbs much energy because of shock wave and thus relatively less energy deposited into solution, that is, considerable energy has been wasted and wouldn’t be utilized validly. However, the circulating method does not have these drawbacks own to the larger discharge space. The flow section of circulating method is 16 times larger than that of “converged electric field type” method, which ensures that the more solution can be used for treatment and less energy is absorbed by the insulating wall 17



during the discharge process. Therefore, the circulating extraction method has lower energy consumption. As for conventional solvent extraction, the yield of polyphenols is 49.84 ± 1.16 mg GAE/g DM with extraction time of 150 min, which is lower by 20.03% than that of circulating method. In addition, the circulating system (ethanol concentration 24%) obviously saves amounts of ethanol in contrast to 58% ethanol concentration of solvent extraction. Up to 1100mL of solvent with higher ethanol concentration and 150 min extraction time are unacceptable in industrial application. The energy consumption of solvent extraction is 5423.5 kJ/kg based on the dry raw material. Accordingly the energy input is 124.7 kJ within 150 min. When the temperature increases up to 47 °C, the heat dissipation would require continuous heat input for stabilizing temperature. It’s difficult to estimate this heat dissipation. Even though the dissipation is not considered, the energy consumption of solvent extraction is almost 2.8 times higher than that of circulating method. In conclusion, in this study a HVED circulating extraction system with a novel electrode couple (needle to ring electrode) has been successfully developed and has shown a good performance. It has been applied to investigate the influences of five factors (extraction time, liquid to solid ratio, flow rate, ethanol concentration and discharge voltage) on polyphenols extraction from SCG using single-factor experiments and the maximum reached up to 59.83 ± 1.53 mg GAE/g DM (at 20 min extraction time, 50 mL/g liquid to solid ratio, 200 mL/min flow rate, 24 % ethanol concentration and 11 kV discharge voltage) which is 11.78 % higher than the yield of “converged electric field type” method with 91.6 min extraction time and 20.03 % higher than the yield of solvent extraction with 150 min extraction time. The energy consumption of circulating extraction is 1962.1 kJ/kg, which is far lower than those of the 18


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“converged electric field type” method (6015.9 kJ/kg) and the solvent extraction (5423.5 kJ/kg). With the optimized protocol, the circulating extraction of polyphenols from SCG enormously ameliorates the operation conditions and improves the polyphenols yield with much lower energy consumption. Thus, the circulating extraction system could be efficiently and economically carried out, indicating that HVED circulating extraction was a feasible and promising method for extraction at a high throughput. Sincerely, we anticipate that numerous potential applications of the new separation system will be explored and promote the development of extraction industry in the future.

AUTHOR INFORMATION Corresponding Author Phone: +86 28 85405209. Fax: +86 28 85405209. E-mail address: [email protected]. Funding Sources This work is financially supported by the National Natural Science Foundation of China (No. 21376150), Postdoctoral Science Foundation of China (No. 2013M530400, 2014T70871) and the Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20100181120076). Notes The authors declare no competing financial interest.

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Figures:

Figure 1. Coffee beans and spent coffee grounds

Treatment chamber

High-voltage pulse generator

Oscilloscope Ground

M

Flow meter

Untreated product

Cooling system

Peristaltic pump

Treated product

Outlet Treatment region Electrode Insulating plate with a hole Electrode

Inlet

Figure 2. Schematic sketch of the HVED continuous extraction system based on “converged electric field type” treatment chamber.

27



High voltage pulse generator

A

B

Voltage measurement Ground Agitator Material inlet

Treatment chamber M

Liquid level

Pipeline Tank

Valve 1

M

Valve 2 Screw pump

Outlet

Figure 3. Schematic sketch of HVED circulating extraction system (A) and physical apparatus (B). A

Columnar electrode

Venthole

B

Needle electrode Inlet

Insulating wall

Discharge region

Ring electrode Outlet

C

D Needle electrode

Ring electrode

Figure 4. Schematic sketch of the treatment chamber (A), discharging of “needle to ring electrode“ of the treatment chamber (B), “needle to ring” electrode (C), discharging of “needle to ring“ electrode (D). 28


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Polyphenools yield, mg GAE/g DM

A 70 60 50 40 30 20 10 0 10

15

20

B 70 60 50 40 30 20 10 0

25

30



C 70 60 50 40 30 20 10 0 67

133

200

40

50

60

Lquid to solid ratio, mL/g

Extraction time, min

D 70 60 50 40 30 20 10 0

267

0

Circulating flow rate, mL/min Polyphenools yield, mg GAE/g DM

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 29 of 31

12

24

36

Ethanol concentration, %

E 70 60 50 40 30 20 10 0 8

11

14

17

Discharge voltage, kV

Figure 5. The effect of different extraction time (A), liquid to solid ratio (B), circulation flow rate (C), ethanol concentration (D) and discharge voltage (E) on the extraction yield of polyphenols obtained using the HVED circulating extraction system.

29



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Tables: Table 1. Independent variables and levels for polyphenols extraction. Levels Independent variable

Unit 1

2

3

4

Extraction time

min

10

15

20

25

Liquid to solid ratio

mL/g

30

40

50

60

Flow rate

mL/min

67

133

200

267

Ethanol concentration

%

0

12

24

36

Discharge voltage

kV

8

11

14

17

30


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TOC Graphic The performances (polyphenols yield, energy consumption, etc.) of circulating extraction, “converged electric field type” method and solvent extraction were compared.

“Converged electrical field tape” method

Solvent extraction (maceration)

Polyphenols Yield

Contrast of polyphenols yields of three methods

Evaluation

1


Circulating Polyphenols Extraction System with High-Voltage

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