Hydrothermal Hydrolysis of Hesperidin Into More ... - ACS Publications

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Hydrothermal Hydrolysis of Hesperidin Into More Valuable Compounds Under Supercritical Carbon Dioxide Condition Duangkamol Ruen-ngam,† Armando T. Quitain,*,‡ Mitsuru Sasaki,‡ and Motonobu Goto†,§ †

Bioelectrics Research Center, Kumamoto University, Kumamoto, Japan Graduate School of Science and Technology, Kumamoto University, Kumamoto, Japan

‡

ABSTRACT: Synergistic effects of utilizing subcritical water and supercritical carbon dioxide were investigated for the hydrolysis of hesperidin into more valuable compounds at a pressure range of 10−25 MPa and temperature of 110−140 °C. The effect of operating conditions such as pressure, temperature, reaction time, and addition of cosolvent such as ethanol were found to affect the hydrolysis rate. Higher yields were obtained at higher carbon dioxide pressure owing to an increase in formation of carbonic acid. Moreover, the rate of reaction was also found to increase with increasing temperature, obtaining higher yields of the main products, hesperetin-β-glucoside and hesperetin. Addition of ethanol as a cosolvent could increase solubility of hesperidin, however, it inhibited the reaction and formation of carbonic acid. At the maximum temperature of 140 °C and pressure of 25 MPa investigated in this work, the highest concentration of hesperetin-β-glucoside of 4.2 mol/L was obtained in a reaction time of 2 h, whereas for hesperetin the highest was 6.9 mol/L obtained in 3 h. Among the hydrolysis methods investigated, the proposed method was the most selective toward formation of the target compounds.

1. INTRODUCTION Citrus processing byproducts, especially those obtained from immature fruits, contain substantial amount of beneficial compounds such as bioflavonoids.1−5 These bioflavonoids possess some antioxidant properties, and their presence in diet can have preventive or therapeutic effects especially on the treatment of numerous diseases such as cardiovascular-related illness and cancer.6,7 Other than the above-mentioned properties of flavonoids, clinical studies had also shown antiatherosclerotic, anti-inflammatory, antitumor, antithrombogenic, antiosteoporotic, and antiviral effects. The mechanisms of action and potential applications of these flavonoids had been reviewed by Nijveldt et al.4 Hesperidin (hesperetin-7-O-rutinoside, hereby referred to as HPD), having a chemical structure shown in Figure 1(a), is one of many bioflavonoids that can be found mostly in immature citrus fruits, its composition can reach more than 40% on a dryweight basis. This tasteless compound can be a good source of more useful chemical compounds such as hesperetin, its aglycone and hesperetin-7-β-D-glucoside. Hesperetin-7-β-Dglucoside (hereby referred to as HBG) as shown in Figure 1(b), an intermediate hydrolysis product. This is known to be more valuable than HPT because it can be easily converted into an intensely sweet compound by simple alkaline hydrogenation. Clinical tests showed that the compound could increase circulation limitation time during digestion, and could also decrease toxicity.4 Another useful compound that can be obtained from the hydrolysis of hesperidin is hesperetin (hereby referred to as HPT) as its structure shown in Figure 1(c). Generally, HPT can be used as a starting material for the preparation of dyes and sweeteners that has value-added medicinal functions such as analgesic, anti-inflammatory, and antioxidant properties.8−11 Hydrolysis of HPD was first carried out in 1881 by Tiemann et al. as reported in the U.S. patent application of Wingard.12 © 2012 American Chemical Society

The hydrolysis was carried out in aqueous sulfuric acid at elevated temperatures in the presence of an alcohol as cosolvent. To date, no significant modification to the method has been applied, and most of the reported modern techniques utilize sulfuric acid as catalyst. Grohmann et al.3 utilized dilute sulfuric acid at a concentration of about 0.05% to hydrolyze hesperidin under elevated temperature, obtaining highest yield of glucose and rhamnose as products from hydrolysis reaction at 140 °C. Acid-catalyzed hydrolysis under subcritical water conditions at the temperature range of 150 to 320 °C has also been applied to the decomposition of some biochemical compounds including monosaccharides, disaccharides, and cellulosic biomass.13−18 However, with the risks posed by the use of such strong acid, especially if the products are intended for human consumption, alternative safe methods are currently being explored. Coupling supercritical carbon dioxide (ScCO2) and hydrothermal conditions for the hydrolysis of hesperidin without employing any harmful catalyst could be a better alternative. This method employs the idea that the formation and dissociation of carbonic acid from the reaction of H2O and CO2 especially at elevated temperatures and pressures serves as a catalyst for the reaction, thereby, enhancing hydrolysis rate.19−22 The catalytic effects of carbonic acid on hydrolysis have been investigated by Van Walsum et al.23 on hot water treatment of corn stover up to a pressure of 5.5 MPa and temperature range of 180−220 °C. Addition of carbonic acid enhanced the occurrence of xylose and furan in hydrolysate. Moreover, Rogalinski et al.22 used supercritical carbon dioxide for the hydrolysis of biopolymers in subcritical water Received: Revised: Accepted: Published: 13545

April 23, 2012 August 29, 2012 October 1, 2012 October 1, 2012 dx.doi.org/10.1021/ie301056b | Ind. Eng. Chem. Res. 2012, 51, 13545−13551

Industrial & Engineering Chemistry Research

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investigated by elucidating the effects of various parameters such as reaction time, temperature, pressure, and addition of cosolvent such as ethanol.

2. EXPERIMENTAL SETUP AND METHODOLOGY 2.1. Chemicals and Reagents. Most chemicals and reagents used in the experiments, including standard samples of hesperidin (92%) and hesperetin (97%) for quantitative analysis of the reaction products, and solvents such as acetic acid (99.9%) and acetonitrile were purchased from Wako Pure Chemical Industries Ltd. (Kyoto, Japan). Carbon dioxide (99.9%) and nitrogen (99.99%) gases were purchased from Uchimura-Sanso Co. Ltd. (Kumamoto, Japan). 2.2. Experimental Apparatus and Methodology. The schematic diagram of the experimental apparatus is shown in Figure 2. It consists of a high-pressure pump that delivers liquefied carbon dioxide into a 30-mL high pressure vessel that serves as a reactor. Six heating probes were attached to the reactor. A thermocouple was also attached at the center of the reactor and connected to a temperature controller to monitor and control the reaction temperature. Carbon dioxide was preheated prior to its introduction into the reactor. The pressure of the system was controlled by a back-pressure regulator (hereby referred to as BPR). In a typical experiment, about 1 mg of hesperidin was dissolved in 20 mL distilled water (50 mg/L or 0.082 mmol/L), then loaded into a high-pressure 30-mL batch reactor with a Teflon-coated stirrer chip placed inside. The reactor was closed and placed on top of a magnetic stirrer as shown in the schematic diagram in Figure 2. While heating to the set reaction temperature in the range of 100−140 °C, liquefied CO2 was pumped into the reactor until the desired pressure of 10−25 MPa was reached. The temperature was controlled automatically by a temperature controller, while the pressure was adjusted using BPR when necessary. The content of the reactor was kept at the desired reaction temperature and pressure for up to 4 h of reaction time at constant stirring. After the reaction time has elapsed, the reactor was gradually cooled down to room temperature (25 °C). The samples were taken from the reactor for analysis thereafter. If subsequent analysis could not be carried out right after each experimental run, then samples were stored in a refrigerator at a temperature of about 5 °C. When necessary, three trials were performed to check reproducibility of the results. 2.3. Analysis of Products. Quantitative amount of hesperidin (HPD), hesperetin-β-glucoside (HBG), and hesperetin (HPT) in the samples were analyzed using a high performance liquid chromatography (HPLC) apparatus

Figure 1. Chemical structure of (a) hesperidin (HPD), (b) hesperetinβ-glucoside (HBG), and (c) hesperetin (HPT).

at temperatures of 240−280 °C, taking the liquefaction of cellulose as an example. In this case, the carbonic acid formed by mixing H2O and CO2 aids in the hydrolysis of cellulose. Results obtained at temperatures up to 260 °C showed an enhancement in liquefaction rates of cellulose in hydrothermalScCO2 mixed solvents at elevated pressure compared with only water. This result implied that the cleaving of glucosidic bond of disaccharides was more effective under acidic conditions at elevated temperatures. A number of studies on the application of the technique to hydrolysis of natural compounds have been reported in literatures, but no reports could be found regarding its application to hydrolysis of citrus bioflavonoids. In this study, application of the technique to hydrolysis of HPD into more valuable compounds (HBG and HPT) was

Figure 2. Schematic diagram of the experimental apparatus. 13546

dx.doi.org/10.1021/ie301056b | Ind. Eng. Chem. Res. 2012, 51, 13545−13551


Article

coupled with a diode array detector (DAD) (Shimadzu Corporation, Japan). Samples were pretreated using a 0.45 μm filter, then placed in vial and arranged in a sample rack for automatic injection of about 10 μL. Separation was carried out using an InertsilODS-3 column (250 × 4.6 mm i.d., 4 μm particle size) (GL Sciences Inc., Japan) at a temperature of 35 °C. Gradient elution was performed using a mobile phase solvent A consisting of 0.1% acetic acid in water and solvent B consisting of 0.1% acetic acid in 75% acetonitrile at a total flow rate of 1.0 mL/min. The ratios of solvent A and B were adjusted as follows: 0 min (78:22), 10 min (72:28), 17 min (62:38), 30 min (52:48), 36 min (32:68), 40 min (0:100), 45 min (0:100), and 60 min (78:22). Compounds were detected at a wavelength of 285 nm. The amounts of HPD and HPT in the samples were quantified using a standard curve prepared by dissolving standard compounds in methanol. The retention time for hesperetin-βglucoside (HBG) was determined using the compound synthesized by the methods of Grohmann et al.3 This was further verified by comparing the UV spectra of the peak with those reported in literatures.3,24 The calibration curve obtained for HPD was also used for quantitative analysis of HBG. 2.4. Calculation of Products. To study the extent of reaction at various conditions, expressed in terms of conversion and selectivity of the products, it is necessary to know the amount of each product in mol/L. The following molecular weights were used in the calculation: HPD = 610.6 g/mol, HBG = 480.4 g/mol, and HPT = 302.3 g/mol. The equations used for the calculation of conversion and selectivity are as follows: conversion of HPD =

[HPD]0 − [HPD]t × 100 [HPD]0

Figure 3. Comparison of HPLC-DAD chromatographs of products obtained at 140 °C and reaction time of 2 h.

Table 1. Solubility of CO2 in Water at 140°C and Pressure of 10 to 25 MPa20 P (MPa)

solubility (mole CO2/mol water)

10 15 20 25

1.4 1.8 2.2 2.4

(1)

selectivity of HBG =

[HBG]t × 100 [HBG + HPT]t

(2)

selectivity of HPT =

[HPT]t × 100 [HBG + HPT]t

(3)

where: [HPD] is concentration of hesperidin, mol/L [HBG] is concentration of hesperetin-β-glucoside, mol/ L [HBG + HPT] is the total concentration of hesperetin-βglucoside and hesperetin, mol/L subscript “0” means initial reaction time subscript “t” means any reaction time t

Figure 4. Pressure dependency of the products obtained at 140 °C and 2 h.

to use in the succeeding hydrolysis experiments. This concentration was almost similar to 53 mg/L used by Grohmann et al.3 in the study of hesperidin conversion into hesperetin-β-glucoside. Moreover, stability tests on 1 mg of hesperidin in 20 mL of water showed no decomposition of hesperidin took place when the mixture was kept at the abovementioned temperatures for 4 h as a result of HPLC analysis described in section 2.3. This implies that hesperidin is stable at this temperature range, and reaction would not proceed without adding a suitable catalyst. 3.2. Preliminary Tests on Catalytic Role of Carbon Dioxide at Elevated Pressure. As previously mentioned in the introduction, the proposed method employs the concept that the formation and dissociation of carbonic acid from the reaction of H2O and CO2 according to eq 4 especially under

3. RESULTS AND DISCUSSION 3.1. Solubility and Stability of Hesperidin in H2O at Elevated Temperatures. Preliminary tests on solubility of HPD in water at elevated temperatures in the range of 110− 140 °C were carried out to determine suitable concentration for the experiments. About 1 to 200 mg of hesperidin standard was dissolved in 20 mL of water, and then heated to reach the above-mentioned temperatures. After heating, the solution was allowed to cool down gradually to room temperature, and then kept overnight in the refrigerator at a temperature of 5 °C prior to analysis. No precipitates were seen forming from the solution even after cooling, thus based on these preliminary solubility tests a concentration of 50 mg/L was found suitable 13547



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Figure 5. Conversion of hesperidin (left axis) and selectivity of hesperetin-β-glucoside (right axis) at 140 °C, 10−25 MPa, and 2 h reaction time.

Experiments at 140 °C and reaction time of 2 h were performed at various pressures up to 25 MPa. Results in Figure 4 show decreasing trend of HPD concentration (as represented by diamond symbol) with increasing pressure. This indicates positive dependency of hydrolysis rate with increasing carbon dioxide pressure, evident from the increase in concentration of products as a result. The conversion of HPD was close to 52% as shown in Figure 5, while the reaction was more selective toward HBG formation. At a higher pressure of 25 MPa, the conversion increased, but the selectivity of HBG decreased due most likely to its further decomposition to HPT as a result of the cleavage of the glucosidic bond. 3.4. Relation of Hydrolysis Reaction on Temperature and Time. Reactions under hydrothermal conditions are extremely dependent on temperature and time. It is expected that at relatively lower temperatures, the hydrolysis would proceed slowly, thus it would take longer time for the reaction to reach equilibrium or completion. In contrast, at higher temperatures the reaction rate would be fast and could reach equilibrium in shorter time. This behavior is evident from the results of the experiments at a pressure of 25 MPa conducted on the effect of time up to 4 h and temperature of 110−140 °C on the amount of each compound in hydrolysate as shown in Figure 6(a)−(c). The initial rate of reaction indicates by the slope of the graph for the change in concentration of hesperidin with time at various temperatures increases with increasing temperature. As a result, the formation of two main products, especially HPT, increases with increasing temperatures from 110 to 140 °C. The highest concentration of HBG around 4.2 mol/L was obtained at 2 h reaction time and an operating temperature of 140 °C as shown in Figure 6(b). At longer reaction time, the amount of HBG decreased gradually due likely to its decomposition to other compounds such as HPT as indicated by the results shown in Figure 6(c). The highest concentration of HPT, about 6.9 mol/L, was obtained at reaction time of 3 h, attaining reaction equilibrium thereafter. The conversion of HPD reached the maximum at 73.3% in 4 h, while the selectivity to HBG was high initially at 71.1%, and then the conversion of HPD continuously decreased to about 39.8% at reaction time of 4 h. The mass balances before and after

elevated temperature and pressure serves as a catalyst for the reaction.22,25 CO2 + H 2O ↔ H 2CO3 ↔ H+ + HCO−3 ↔ 2H+ + CO32 −

(4)

Carbonic acid, even though considered as a weak acid, may promote hydrolysis of HPD consisting of glycosidic bonds that seems to be weaker than those of cellulose.22 Brito-Arias26 had confirmed that phenolic glycosides can be decomposed even in dilute acid solution, close to the acidic conditions being employed in this work. It was also reported that the glucosidic bond could be cleaved under carbonic acid catalyzed subcritical water.15,22 Similarly, cleavage of rhamnosidic and glucosidic bonds of HPD might also take place obtaining main products such as HBG and HPT, respectively. In this regard, the catalytic effect of adding carbon dioxide at elevated pressures on reaction under hydrothermal condition was first verified by carrying out experiments at a pressure and temperature of 10 MPa and 140 °C. The results were compared with those obtained using N2, an inert gas, instead of carbon dioxide and in water without carbon dioxide. The catalytic effect of adding carbon dioxide is evident from the obtained chromatograms in Figure 3(a). The peaks of the products (i.e., HBG and HPT at residence times of 23 and 38 min, respectively) are noticeably large in the presence of carbon dioxide compared to that of water alone in Figure 3(c), or when pressurized with an inert gas such as N2 at 10 MPa in Figure 3(b). This indicates that hydrolysis of hesperidin took place in the presence of carbon dioxide at high pressure, confirming the catalytic effect of its addition. 3.3. Dependency of Hydrolysis Reaction on Carbon Dioxide Pressure. The solubility data of carbon dioxide in water at a pressure range of 10−25 MPa and temperature range of 110−150 °C, as reported by Takenouchi and Kenney,20 demonstrated that the solubility of carbon dioxide increases with increasing pressure as shown in Table 1. Even at a low pressure of 10 to 25 MPa, the solubility of CO2 in water may increase up to 60%. This increase in solubility of carbon dioxide at elevated pressures enhances the formation of carbonic acid, which serves as a catalyst for the reaction.20−22,24 13548



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Figure 7. Speculated general reaction pathway for the hydrolysis of HPD under hydrothermal conditions.

Figure 8. Effect of ethanol−water mole ratio on the composition of the main products at 140 °C, 25 MPa, and 2 h reaction time.

Figure 9. Comparison of the products obtained from various hydrolysis methods. Figure 6. Effect of temperature and reaction time on the concentration of HPD, HBG and HPT at 25 MPa.

reactions were also in good agreement, as shown in the results in Table 2.

Table 2. Summary of the Results of Reaction at 140°C and 25 MPa at Various Reaction Times run

reaction time (h)

1 2 3 4 5 6 7

0.5 1.0 1.5 2.0 2.5 3.0 4.0

HPD (mg) 0.69 0.60 0.49 0.39 0.32 0.28 0.21

± ± ± ± ± ± ±

0.09 0.07 0.04 0.01 0.01 0.02 0.02

HBG (mg) 0.12 0.18 0.26 0.39 0.34 0.37 0.37

± ± ± ± ± ± ±

0.00 0.05 0.09 0.05 0.04 0.01 0.11

HPT (mg)

total mass (HPD+HBG+HPT) (mg)

conversion (%)

selectivity of HBG (%)

selectivity of HPT (%)

± ± ± ± ± ± ±

0.87 0.91 1.10 0.80 0.93 1.00 0.94

12.4 28.1 37.6 52.5 59.7 64.6 73.3

71.1 63.1 53.4 53.3 44.5 39.8 39.8

28.9 36.9 46.6 46.8 55.5 60.2 60.2

0.06 0.13 0.14 0.23 0.27 0.36 0.36

0.01 0.01 0.01 0.03 0.13 0.01 0.00 13549



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CO2 pressure owing to the increased formation of carbonic acid. The highest yields of hesperetin-β-glucoside (HBG) and hesperetin (HPT) were obtained at the maximum investigated pressure of 25 MPa and temperature of 140 °C. The highest concentration of HBG of about 4.2 mol/L was obtained in 2 h of reaction time, whereas the highest concentration of 6.9 mol/ L of HPT was obtained in 3 h. The highest conversion of HPD was close to 52%. HBG and HPT were the main products obtained by the cleavage of rhamnosidic and glucosidic bonds of HPD, respectively. Addition of ethanol (EtOH) as cosolvent inhibited hydrolysis reaction and the formation of carbonic acid. Thus, addition of EtOH as cosolvent is not recommended in this method. Using the proposed method, the reaction was also found more selective to the formation of the target compounds better than the other hydrolysis methods reported in literatures.

On the basis of these results, the hydrolysis of HPD is speculated to consist of complex reactions involving consecutive reactions via HBG (1) route to its final decomposition productHPT (2) as shown in Figure 7. Parallel reaction 3 involving direct cleavage of the glucosidic bond in HPD to form HPT is also likely to occur. The degradation behavior can be further explained by the fact that an increase in temperature decreases activation energy of the reaction, making the hydrolysis faster. However, at lower temperature of 110 °C, it is reported that HPD is quite resistant to acid-catalyzed hydrolysis,2,24 thus obtaining lower yield of the products. 3.5. Inhibitory Effect of Adding Ethanol As Cosolvent. It was reported in the patent by Wingard12 that HPD is hardly soluble in aqueous media, and addition of lower primary alkanol such as ethanol (EtOH) was recommended to further increase solubility of HPD and even carbon dioxide in water.27,28 It is likely that the yield of the products would also increase with an increase in solubility of the reactants. In this regard, the effect of addition of EtOH up to EtOH− H2O mole ratio of 39 was investigated. The fraction of the products decreased with increasing amount of added EtOH, while the content of unreacted HPD increased based on the results shown in Figure 8. Addition of EtOH may increase the solubility of HPD, however, its presence in the solution only inhibits hydrolysis reaction of HPD to HBG and HPT under ScCO2−H2O conditions, contrary to what was speculated to occur. As reported in literature,12 decomposition of HPD may take place under acidic condition using lower primary alkanol such as EtOH as a solvent. However, in aqueous media pressurized by carbon dioxide, addition of EtOH has an inhibitory effect on reaction. It may also hinder formation of carbonic acid that serves as a catalyst for the reaction. Thus, its addition for the purpose of increasing solubility of the reactant is not recommended. 3.6. Comparison of the Proposed Technique with Various Hydrolysis Methods. The proposed technique was compared with various hydrolysis methods and the results are presented in Figure 9. On the basis of the results, using strong acid such as H2SO4 according to the method of Grohmann et al.,3 the conversion of HPD was relatively high as indicated by the significant decrease in its concentration. However, the reaction was not selective to the formation of the target compounds. It is most likely that further decomposition of HBG and HPT took place in the presence of strong acid. Using a relatively weaker acid such as formic acid, slightly increased the selectivity toward HBG and HPT. Other than the benefits of using the combined hydrothermalScCO2 methods for being green and safe solvents, the proposed technique is favorable to selective formation of the target compounds. The conversion may not be as high as those using formic or sulfuric acid, but the concentration of HBG and HPT were the highest among the methods investigated. Moreover, the reaction can also be easily controlled by CO2 pressure adjustments.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address §

Department of Chemical Engineering, Nagoya University Furo-cho, Chikusa, Nagoya 464-8603 Japan. Tel: +81-52-7893392. E-mail: [email protected] Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was partly supported by the Kyushu Bureau of Economy, Trade, and Industry. REFERENCES

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4. CONCLUSIONS The catalytic effect of adding carbon dioxide under supercritical conditions was verified on hydrothermal hydrolysis of hesperidin (HPD), a bioflavonoid predominantly present in immature citrus fruits. Detailed experiments carried out in a temperature range of 110−140 °C and pressures up to 25 MPa showed a positive dependency of hydrolysis rate with increasing 13550



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