Microwave-assisted extraction of phenolics from Hibiscus sabdariffa calyces: Kinetic modelling and process intensification

Industrial Crops & Products 137 (2019) 528–535

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Microwave-assisted extraction of phenolics from Hibiscus sabdariffa calyces: Kinetic modelling and process intensification

T

⁎

Oluwaseun Ruth Alara , Nour Hamid Abdurahman Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, Lebuhraya Tun Razak, 26300, Gambang, Pahang, Malaysia

A R T I C LE I N FO

A B S T R A C T

Keywords: Microwave-assisted extraction Hibiscus sabdariffa Second-order rate law Effective diffusivity Power law LC-ESI-MS-MS

Hibiscus sabdariffa is a multi-cropping system endowed with wide ranges of benefits to humans. Being a plant rich in phenolic compounds, the process intensification of extraction method used for recovering the phenolic compounds from this essential plant is inevitable to easily proffer keen information on the extraction behaviours. Thus, the process intensification and kinetic behaviour of microwave-assisted extraction (MAE) for recuperating total phenolics (TP) content from H. sabdariffa calyces were investigated. The impacts of MAE factors such as solid/sample ratio, microwave power and temperature at varied irradiation time were studied. The effective diffusivity was also estimated. In addition, the extract was characterized to tentatively assign the phenolics using Liquid chromatography tandem mass spectrometry of quadrupole time-of-flight (LC-ESI-MS-MS). The achieved results clearly indicated that the MAE process intensification was acheived with TP content of 70.53 mg GAE/g extract in 3 min at solvent/sample ratio of 14:1 mL/g, microwave power of 500 W and 60 °C of temperature where the highest effective diffusivity coefficient was obtained. Out of the two considered kinetic models (second-order rate and power law), the second-order rate model best describes the MAE process intensification with higher coefficients of determination (R2 > 0.99). Moreover, an aggregate of 77 phenolic compounds was assigned in the extract of H. sabdariffa calyx; signifying the wider potentials of the extract industrially. Thus, this study clearly outlined that H. sabdariffa calyx is an embodiment of diverse phenolic compounds; indicating its potential in pharmaceutical and food industries.

1. Introduction Hibiscus sabdariffa is a well-known flowering plant that belongs to the family Malvaceae. It is mostly called roselle, red sorrel or hibiscus. This plant is widely found in South-east Asia (including Thailand and Malaysia), West and Central Africa countries. The branched annual shrub can grow up to 3.5 m above sea level (Alara and Abdurahman, 2019a,b; Inikpi et al., 2014). It possesses darken to reddish leaves with reddish stems. Due to the red colour of its flowers, it is commonly employed as a food colouring agent. Most importantly, the calyces of H. sabdariffa are ingested in several parts of the world in form of cold or hot tea, jams, jellies, beverages, syrup, sherbets/ice-cream, and other desserts. In fact, its extracts are being used in several cosmetic products including skin lotions and shampoos (Villani et al., 2013). The reddish colouring obtained from H. sabdariffa is being utilized as a natural colourant in poultry and meats. Additionally, H. sabdariffa is known to be multi-cropping systems because apart from its viability as food, it is can be used as a fibre for manufacturing ropes, cords and burlap (DaCosta-Rocha et al., 2014; Villani et al., 2013). Other than the

⁎

importance in food and cosmetic industries, the extracts from H. sabdariffa calyces are endowed with several health-beneficial properties such as cancer-preventive, antioxidant, anti-hypertensive, anti-diabetic, anti-obesity, delayed puberty, antibacterial, anti-inflammatory, and many more (Da-Costa-Rocha et al., 2014). The presence of these healthbeneficial activities which are mainly due to the occurrence of phenolic compounds in the extract might have been responsible for the potential usages of H. sabdariffa calyces in the cosmetic, pharmaceutical and food industries; therefore, improving the economic importance and minimize the adverse influences of synthetic-based products (Owoade et al., 2016). The extraction of phenolics from plant materials is performed by using several techniques and solvents; this wholly depends on nature and their distribution in the plant samples. Microwave-assisted extraction (MAE) is part of the advanced methods employed in recovering phenolics from plant samples. It is widely used because of its lesser costeffectiveness, yielding excessive phenolic compounds in lesser extraction time with the use of a minimal solvent in relative to conventional techniques (Alara et al., 2019, 2018; Shams et al., 2015). Thus,

Corresponding author. E-mail address: [email protected] (O.R. Alara).

https://doi.org/10.1016/j.indcrop.2019.05.053 Received 23 February 2019; Received in revised form 8 May 2019; Accepted 19 May 2019 Available online 28 May 2019 0926-6690/ © 2019 Elsevier B.V. All rights reserved.

Industrial Crops & Products 137 (2019) 528–535

O.R. Alara and N.H. Abdurahman

Irradiation time (1–10 min), Irradiation power (300, 400, 500, 600, and 700 W), solvent/sample ratio (10:1, 12:1, 14:1, 16:1, and 18:1 mL/g), and temperature (40, 50, 60, 70, and 80 °C) were considered. During the MAE, 10 g of H. sabdariffa sample was mixed with distilled water depending on the varied solvent/sample ratio, the mixture in the 250mL conical flask was placed in the microwave system; the irradiation time, power and temperature were set to begin the experiment. This experimental process was continued by varying a factor at a time while fixing others. After then, the extract was screened by utilizing filter paper and concentrated to dryness through a rotary evaporator. The phenolic contents in the extract were then estimated.

intensifying the process involves in MAE is essential because of the environmental and energy evaluations to improve the recovery of phenolics from plant sample with minimal inputs. It should be noted that the process intensification of MAE accords the transfer of microwave radiation into the plant materials, leading to the generation of heat emanating from the interactions with polar components (Patil and Akamanchi, 2017). The synergistic actions of concentration and temperature gradients working in the same directions accelerate the extraction process in MAE (Alara et al., 2018). During the MAE, the combination of microwave power and heating can be adjusted at varying or constant temperature to achieve an effective extraction process. In the process of MAE, there is a pressure difference between the outer and inner parts of the plant cells to facilitate the ejection of bioactive compounds to the surrounding solvent leading to an efficient mass transfer coefficient. Hence, to improve the MAE efficiency, several kinetic models have been postulated for the effective component diffusion. In engineering, mathematical modelling can ease the process control, optimization, design of a process, and providing rightful information on the equipment scale-up procedure (Tao et al., 2014). Moreover, the kinetic laws proffer keen information on the extraction behaviours. A lot of extraction kinetic models have been previously reported which include second-order rate law, power law, two-site kinetic law, chemical kinetic-based equations, and diffusion model based on the Fick’s law (Alara and Abdurahman, 2019a,b; Patil and Akamanchi, 2017; Yedhu Krishnan and Rajan, 2016). Nevertheless, most studies have reported that the second-order rate model can best describe the kinetics of modern and traditional techniques of extractions (Alara and Abdurahman, 2019a,b; Patil and Akamanchi, 2017). Even though the MAE of phenols from H. sabdariffa calyces had previously been investigated (Nizar et al., 2014); however, the process intensification and kinetic studies had not been reported for the recovery of phenolics from H. sabdariffa calyx. Thus, this study was carried out to investigate the MAE process intensification to examine the influences of the extraction factors which include solvent/sample ratio, microwave power and extraction temperature on the recovery of total phenolic (TP) content from H. sabdariffa calyces. The second-order rate and power-law models were employed to study the kinetics. Furthermore, the effective diffusivity coefficient was estimated, and the extract was characterized using LC-ESI-MS-MS analysis for tentative assignment of the phenolic compounds.

2.4. Assessment of total phenolic (TP) in the calyx extract of H. sabdariffa The TP content in the calyces extract of H. sabdariffa was evaluated by utilizing FC assay as previously presented (Alara et al., 2018). In concise, to a 0.1 mL of the extract, 0.2 mL of FC was added followed by the addition of 0.6 mL of 0.2 mM Na2CO3 preceding when the mixture had been left for 5 min. This mixture was incubated for 120 min and the absorbance was evaluated at 765 nm. The TP content was estimated from the calibration curve of gallic acid and the result was presented as mg GAE/g extract. 2.5. Characterization of the extract using LC-ESI-MS/MS analysis A Waters Acquity ultra-performance liquid chromatography (UPLC HSS TS, USA) machine comprising a binary pump, 10 μL and autosampler was employed in the analysis. The phytochemical compounds were separated using an analytical column C18 (100 mm × 2.1 mm × 1.8 μm; Waters, USA) at 30 °C. The gradient elution and operating conditions for negative and electrospray ionization source interface (ESI) had previously been reported in the authors’ published article (Alara et al., 2018). The tentative assignments of phytochemicals in the extract of H. sabdariffa calyx were performed through the MS/MS fragmentation pattern by comparing the obtained spectra with Waters library reference standards. 2.6. Kinetic modelling 2.6.1. Second-order rate kinetic model Prominently, the second-order kinetic model is often employed in the solid-liquid extraction modelling because of its suitability in representing the process (Alara and Abdurahman, 2019a,b; Chan et al., 2015; Kusuma and Mahfud, 2017; Patil and Akamanchi, 2017; Yedhu Krishnan and Rajan, 2016). The analytical solutions generated from the integral analysis can be utilized to determine the parameters of this kinetic model. Hence, this kinetic model was employed to examine the MAE of TP contents in H. sabdariffa calyces. Eq. (1) expresses the second-order rate kinetic model.

2. Materials and methods 2.1. Preparation of H. sabdariffa samples The dried calyces of H. sabdariffa grown in Malaysia were bought from a local distributor in Kuala Lumpur. These samples were further dried under the room temperature for two weeks to achieve about 10% moisture content. Thereafter, the H. sabdariffa samples were crushed by utilizing a domestic electrical grinder and sieved to achieve an average particle size of 0.105 mm. The crushed samples were fastened in a dark container and kept at 4 °C until used.

dCt = k (Cs − Ct )2 dt

(1)

where k denotes the second-order rate kinetic constant in millilitre per milligram of gallic acid equivalent per seconds (mL/mg GAE s); Cs represents the saturation concentration of phenolics in milligram gallic acid equivalent per millilitre (mg GAE/mL); and Ct represents concentration of phenolics in milligram of gallic acid equivalent per millilitre (mg GAE/mL) per time t. Eq. (1) can be integrated by utilizing the boundary conditions Ct = 0 at t = 0 and Ct = Ct at t = t to achieve a final Eq. (4) as illustrated in Eqs. (2,3).

2.2. Chemicals and reagents The gallic acid, Folin-Ciocalteu reagent and sodium carbonate were supplied by Sigma Aldrich Sdn. Bhd., Selangor. 2.3. Extraction procedure An ethos E microwave extractor system (Milestone, Italy) was utilized for the extraction process as provided in the Supplementary materials (Fig. 1S). This microwave system was equipped with an easy control compartment comprises of temperature, time and power control. In order to investigate the effects of different MAE process factors:

1 1 − = kt (Cs − Ct ) Cs

(2)

The reorganization of Eq. (2) gives Eqs. (3) and (4), respectively. 529

Industrial Crops & Products 137 (2019) 528–535

O.R. Alara and N.H. Abdurahman

Ct = Cs −

Ct =

Cs 1 + Cs kt

Initialconditions: C = C0, t = 0at 0 ≤ r ≤ R (3)

Boundaryconditions : C = 0, t > 0atr

(4)

dC = 0 t ≥ 0 at r = 0 dr

= R (Assume no solubility lim itation)

Cs2 kt 1 + Cs kt

Thereafter, Eq. (4) was reorganized in its linear form to achieve Eq. (5).

Ct = t

Thus,

Ct 6 D π 2t = 1 − 2 exp ⎛− e 2 ⎞ Cs π R ⎠ ⎝

1

(

1 kCs2

+

⎜

t Cs

)

(5)

(6)

Thus, Eq. (7) can be achieved as follows:

t 1 t = + Ct m Cs

ln

⎜

⎟

(12)

2.7. Data analysis and model verification All the experimental procedures were repeated thrice, and the obtained results were provided in the form of mean ± standard deviation with a significant level at p < 0.05. IBM® SPSS® Statistics V22.0 (IBM SPSS, United States) was employed to analyse the kinetic models. The predicted model values and experimental values were related using the residual sum of squares (RSS) and a correlation coefficient of determination (R2). The lower RSS values and higher R2 indicate better fitness of the model to experimental data.

2.6.2. Power law model This kinetic model has previously been employed in the extraction process (Patil and Akamanchi, 2017). Eq. (8) represents the power law model equation. (8)

where Ct stands for the concentration of TP content in the extract of H. sabdariffa calyx (mg GAE/mL) at any time t (s), B represents extraction coefficient (mL/mg GAE s) and n indicates the power law exponent (< 1). Eq. (8) can further be simplified to obtain Eq. (9).

ln Ct = ln B + n ln t

Cs π2 D π2 = ln ⎛ ⎞ + e 2 t Cs − Ct R ⎝6 ⎠

(7)

Hence, the initial MAE rate coefficient (m), saturation concentration of phenolics (Cs), second-order rate constant (k) can be determined from the intercept and slope by a plot of t/Ct against t.

Ct = Bt n

(11)

where Ct represents the TP content (mg GAE/g extract) extracted from H. sabdariffa calyx at any time t (min), Cs represents the TP content (mg GAE/g extract) extracted from H. sabdariffa calyx at saturation and R is the distance from the centre of spherical H. sabdariffa calyx particles (m). Thus, De can be determined by further simplifying Eq. (11) to obtain Eq. (12). Hence, ln (Cs/(Cs-Ct)) can be plotted against time.

Hence, MAE rate (Ct/t) can be determined using Eq. (5). By representing the initial MAE rate m (mg GAE/mL s) with Ct = t as time t tends toward zero. Then,

m = kCs2

⎟

3. Results and discussion 3.1. Effect of extraction time

(9)

Extraction time is one of the key variables impacting the retrieval of phenolic-rich extracts from plant substances. Hence, it is essential to select an adequate and proper time of extraction to ensure a comprehensive ejection of phenolic compounds from the tested plant matrix so as to steer clear of the bioactive compounds’ degradation and reduced cost that can be incurred due to extended process time. The effect of MAE time on the yield recovery of TP content from H. sabdariffa calyces is presented in Fig. 1. This MAE process variable was studied at a fixed microwave power of 300 W, solvent/sample ratio of 10:1 mL/g and 40 °C of extraction temperature as the time was varied between 1 and 10 min. It can be observed in Fig. 1 that there are two stages in the kinetics of extracting TP content from H. sabdariffa calyces. The first

By plotting ln Ct against ln (t), the values for the B and n can be obtained from the intercept and slope of the linear plot. 2.6.3. Estimating the effective diffusion coefficient The diffusion model using Fick’s second law was employed to estimate the effective diffusion coefficient of phenolics from H. sabdariffa calyx. The following assumptions were made before using Fick’s second law in studying the MAE process, they are: i The dried H. sabdariffa calyx particles were spherical in shape with a 0.105 mm average diameter indicating the average particle size of the plant sample. ii The resistance from the liquid phase was insignificant. iii There were neither degradation nor chemical reaction in the phenolics extracted through MAE. Thus, the Fick’s second law for MAE of phenolics from spherical particles as provided in Eq. (10).

dC 1 d dC = De ⎡ 2 ⎛r 2 ⎞ ⎤ ⎢ dt r dr ⎝ dr ⎠ ⎥ ⎦ ⎣

(10)

where C denotes the concentration of phenolics in H. sabdariffa calyx particles (mg GAE/mL), De represents the effective diffusion coefficient of phenolics, r represents the distance from the centre of spherical H. sabdariffa calyx particles (m), and t denotes the time (s) (Yedhu Krishnan and Rajan, 2016). Eq. (10) can further be simplified by utilizing the boundary and initial conditions as previously reported to obtain Eq. (11) (Krishnan et al., 2015; Tao et al., 2014; Yedhu Krishnan and Rajan, 2016).

Fig. 1. Influence of extraction time on kinetics of MAE of TPC yield from H. sabdariffa calyces during MAE at fixed microwave power of 300 W, solvent/ sample ratio of 10:1 g/mL and temperature of 40 °C. 530

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stage reflects a swift improvement in the concentration of TP content referred to as washing mechanism which occurred between 0 until 1 min; this might be due to the quick dissolution of solutes emanating from the surface of plant sample (Krishnan and Rajan, 2017). Furthermore, the second stage is called the diffusion phase; this occurred between 1 and 3 min which showed a steady increase in the TP content extracted. The highest TP content was achieved in 3 min of extraction time as seen in Fig. 1. Beyond this time, the yields started to decline gradually. This occurrence of declination might be related to the effect of degradation emanating from over-exposure to microwave irradiation (Shams et al., 2015). Additionally, the highest yield of TP content was achieved in 3 min; the reduced time might be associated with the importance of MAE technique; it consistently disrupt the plant cell wall within a shorter time of exposure resulting in process intensification (Alara et al., 2018; Alara and Abdurahman, 2019a,b; Patil and Akamanchi, 2017). Therefore, 3 min was selected as optimum. Fig. 3. Influence of solvent/sample ratio on kinetics of MAE of TPC yield from H. sabdariffa calyces during MAE at fixed microwave power of 500 W and temperature of 40 °C.

3.2. Effect of microwave power Microwave power is a key variable that differentiates MAE from other techniques; this process variable was studied to examine its effect on the TP content from H. sabdariffa calyces. Microwave power is associated with the creation of localized heating, adsorption and dissemination of energy from extracting solvent to the plant sample; resulting to the disruption of the cell wall to eject the bioactive compounds (Chen et al., 2008; Patil and Akamanchi, 2017). It has been reported that microwave and temperature are interrelated because the increase in microwave power enhances and increases the temperature; resulting in the improvement of the yields. However, it was further reported that this mechanism can only occur to a particular value of microwave power and temperature before degradation of phenolic compounds set in; causing insignificant increase or reduction in the yields (Alara et al., 2018; Patil and Akamanchi, 2017; Veggi et al., 2013). The impacts of microwave powers on the MAE process intensification at constant solvent/sample ratio of 10:1 mL/g, 40 °C of temperature and varied time between 30 to 180 s were investigated. As presented in Fig. 2, the TP content improved speedily as the microwave power increased in the washing phase and further increase at the diffusion phase. However, the improvement in the yields when considered microwave power from 300 to 500 W proceeded as the driving forces increased. In addition, further increase in power causes a decline in the TP content yield; indicating that 500 W was the appropriate microwave power to achieve highest phenolics from H. sabdariffa calyces at fixed

extraction time of 3 min. 3.3. Effect of solvent/sample ratio Studies have shown that a higher volume of solvent might favour and enhance the mass transfer rate. Nevertheless, an excessive volume of solvent might require more energy; thus, it is important to estimate the actual volume needed in extracting bioactive compounds from a plant sample. Fig. 3 shows the results obtained for the effects of different solvent/sample ratio on the yield of TP content from H. sabdariffa calyces at fixed 500 W of microwave power, 40 °C of temperature and varied extraction time between 30 and 180 s. It was observed that TP content yields improved in both the washing and diffusion phases. Moreover, the yields increased as the solvent/sample ratio increased from 10:1 mL/g through 14:1 mL/g where the highest yield was obtained. However, the yields declined as the solvent/sample ratio was beyond 14:1 mL/g. The reason for this might be related to the fact that a larger volume of solvent requires more energy and time to attain the equilibrium yield of TP content. Additionally, the concentration of solute was reduced at higher solvent/sample ratio; resulting in an increasing driving force for diffusion and dissolution. Moreover, the driving force for adsorption will be reduced in lower solute concentration (Krishnan and Rajan, 2017). Therefore, 14:1 mL/g of solvent/sample ratio was selected. 3.4. Effect of temperature Extraction temperature is part of the process variables influencing the leaching of phenolic compounds from plant samples when using MAE. The solvent movement into the inner part of the plant sample can be enhanced by increased temperature, causing a higher yield of extract. Based on the Einstein equation, the solvent viscosity tends to decline with increasing temperature, resulting in a higher diffusion rate (Ciğeroğlu et al., 2017). Nevertheless, phenolic compounds can degrade when overexposed to a higher temperature (Alara et al., 2018). The yield of TP contents tends to improve with increasing temperature from 40 to 60 °C where the highest yield was achieved (Fig. 4). However, the TP content yields were rapidly increased at the washing stage but gradually improved in the diffusion phase. The yields of TP contents were gradually improved by increasing temperature from 40 through 60 °C; slight changes in the yields were observed as the temperature was above 60 °C. In general, a positive impact of temperature is always observed on the rate and efficiency if it is not too high, as the higher temperature can cause a reduction in the extracting solvent, causing the degradation of bioactive compounds (Krishnan et al., 2015). Additionally, there might be vaporization of volatile compounds during

Fig. 2. Influence of microwave power on kinetics of MAE of TPC yield from H. sabdariffa calyx during MAE at fixed solvent/sample ratio of 10:1 g/mL and temperature of 40 °C. 531

Industrial Crops & Products 137 (2019) 528–535

O.R. Alara and N.H. Abdurahman

Table 2 Effective diffusion coefficient of MAE of phenolic from H. sabdariffa calyces. MAE fixed conditions

Temperature (ºC)

De × 10−12 (m2/s)

R2

S/S ratio = 14:1 mL/g Microwave power = 500 W

40 50 60

2.042 2.066 2.130

0.967 0.967 0.962

MAE = microwave-assisted extraction; De = effective diffusion coefficient; R2 = coefficient of determination.

correlation coefficient of determination (R2) and the residual sum of squares (RSS) to determine the relationship between predicted and experimental values. The lower RSS values and higher R2 indicate better fitness of the model to experimental data. It can be observed that the second-order kinetic model portrait the best fits compared to experimental data, reflecting the highest R2 values (0.993-0.998). This showed clear suitability of second-order rate equation for the good fitness of the MAE kinetics of TP content from H. sabdariffa calyces. However, the values of the second-order kinetic coefficient (k) and concentration of TP content at saturation (Cs) used in calculating the initial extraction rate were evaluated from the plots of t/Ct against t (Eqs. 6 and 7) as illustrated in Table 1. These values were determined for different microwave power, solvent/sample ratio and temperature. The obtained results reflected that the highest initial extraction rate was achieved at microwave power of 500 W; however, the initial extraction rate declined beyond 500 W. The reason might be due to the degradation of the phenolic compounds in the extract of H. sabdariffa calyces which can be associated with the overexposure to microwave radiation (Alara et al., 2018; Bouras et al., 2015). Similarly, the influences of solvent/sample ratio on the initial extraction rate were as well presented in Table 1. The lowest initial extraction rate was obtained at 14:1 mL/g; the reason might be because the initial extraction is controlled by the washing stage whereby the solute was washed out rapidly. During the MAE, the interkinesis mas transfer transpires in both directions; the solute concentration gradient enhances the mass transfer from the plant sample to the extracting solvent through diffusion and dissolution. Moreover, the solute can non-preferentially adsorb on the surface of the plant sample (Krishnan

Fig. 4. Influence of temperature on kinetics of MAE of TPC yield from H. sabdariffa calyces during MAE at fixed microwave power of 500 W and solvent/ sample ratio of 14:1 g/mL.

the MAE of phenolics from H. sabdariffa calyces at the elevated temperature which can be due to an increase in solvent losses. Thus, 60 °C was selected to be the optimum temperature where the maximum yields of TP content (70.53 mg GAE/g extract) was obtained in 3 min at the solvent/sample ratio of 14:1 mL/g and microwave power of 500 W. This result showed an improvement over a recently published article on the MAE for H. sabdariffa bioactive compounds where the optimal extraction conditions to achieve maximum extraction yields were 22 min of extraction time, 164 °C and 60% of ethanol concentration (Pimentelmoral et al., 2018).

3.5. Process kinetic modelling In order to interpret the mass transfer, diffusion and thermodynamic process variables influencing the extraction rate, it is essential to study the extraction kinetic models. The kinetic parameters for both considered models in this study (second-order rate and power law) are presented in Table 1. The model fittings were validated using a

Table 1 The kinetic parameters comparing second-order rate and power law models for MAE of TP content from H. sabdariffa calyces. MAE conditions

Second-order model R

2

Power law

Residual Sum of Squares

Initial extraction rate (m) (mg GAE/mL s)

Conc. of TPC at saturation Cs (mg GAE/mL)

k (L/mg GAE s)

R2

Residual Sum of Squares

B (mL/mg GAE s)

n

Microwave power (W) 300 400 500 600 700

0.996 0.993 0.998 0.998 0.996

0.021 0.031 0.007 0.009 0.026

3.486 3.511 3.644 3.385 3.425

51.161 58.302 66.296 53.773 46.588

1.332 1.033 0.829 1.171 1.578

0.985 0.985 0.996 0.996 0.968

0.001 0.001 0.000 0.000 0.001

23.151 20.304 24.111 21.769 22.665

0.139 0.184 0.179 0.160 0.125

Solvent/sample ratio (mL/g) 10:1 12:1 14:1 16:1 18:1

0.998 0.997 0.994 0.997 0.998

0.007 0.010 0.001 0.009 0.007

3.726 3.482 3.415 3.668 3.733

66.176 69.242 73.333 67.250 64.049

0.851 0.726 0.635 0.811 0.910

0.987 0.988 0.984 0.985 0.986

0.000 0.000 0.001 0.000 0.000

24.702 23.850 24.548 24.576 24.113

0.174 0.187 0.191 0.177 0.173

Temperature (ºC) 40 50 60 70 80

0.994 0.994 0.993 0.994 0.994

0.019 0.018 0.016 0.017 0.019

3.408 3.491 3.820 3.473 3.446

73.366 73.886 76.903 73.927 72.928

0.633 0.639 0.646 0.635 0.648

0.984 0.984 0.985 0.984 0.981

0.001 0.001 0.001 0.001 0.002

24.558 25.125 24.595 24.643 24.576

0.190 0.188 0.199 0.192 0.190

MAE = microwave-assisted extraction; R2 = coefficient of determination; k = second-order rate kinetic constant; B = extraction coefficient; n = power law exponent (< 1). 532

Demethoxycurcumin

2,3,5,4'-Tetrahydroxystilbene-2,3-O-β-D-glucopyranoside

Mulberrofuran F Dendrocandin G

Blestrianol D Tubuloside A Dendrocandin I

Cistanoside B Feralolide 3,4-Dihydroxyphenothyl-3-O-β-D-glucopyranoside Cinchonain Ia Moracin H Blestrin A (3R,4R)-3,4-trans-7,3′-Dihydroxy-2′,4′-dimethoxy-4-[(3R)2′,7-dihydroxy-4′-methoxyisoflavan-5′-yl]-isoflavan Isohypericin Erigoster A Kuwanon I Mulberrofuran G 1-(4,7-Dihydroxy-2,6-dimethoxy-9,10dihydrophenanthrenyl)-4,7-dihydroxy-2,6-dimethoxy-9,10dihydrophenanthrene Moracin M-3′-O-β-D-glucopyranoside Verbascoside 2,3,5,4'-Tetrahydroxystilbene-2-O-β-D-glucopyranoside Mulberrofuran Q Echinacoside (3R,4R)-3,4-trans-7,2′,3′-Trihydroxy-4′-methoxy-4-[(3R)-2′,7dihydroxy-4′-methoxy-isoflavan-5′-yl]-isoflavan Moracin C Nilocitin 2,4,6-Tri-O-galloyl-β-D-glucose 1-(4-Hydroxybenzyl)-4-methoxy-2,7-dihydroxyphenanthrene Lavandulifolioside 2-((3R,4R)-7-Hydroxy-4-(4-hydroxy-5-((R)-7hydroxychroman-3-yl)-2-methoxyphenyl) chroman-3-yl)-5methoxycyclohexa-2,5-diene-1,4-dione (1R,2S,3R,6'R,7'R)-3, 7'-Bis(3,4-dihydroxy-phenyl)1,1',2,2',3,3', 4,4'-octahydro-1,1'-binaphthyl-2,2',4',6,6',8hexaol Anthranol Tribulusamide B Dendrocandin B 2′-Hydroxy-3′,4′-dimethoxy-isoflavan-7-O-β-D-glucoside 2,4,6-Trihydroxyacetophenone-2,4-di-O-β-D-glucopyranoside Asebotin Filixic acid ABA 3,7-Dihydroxy-2,4-dimethoxyphenanthrene-3-O-glucoside

1.

2.

3. 4.

5. 6. 7.

8. 9. 10. 11. 12. 13. 14.

533

33. 34. 35. 36. 37. 38. 39. 40.

32.

26. 27. 28. 29. 30. 31.

20. 21. 22. 23. 24. 25.

15. 16. 17. 18. 19.

Component name

No

193.0664 637.2196 481.1867 463.1626 537.1472 449.1469 611.2115 431.1347

619.1816

1.05

1.11 1.15 1.16 2.17 2.18 2.19 2.34 2.35

355.1176 483.0789 635.0893 345.1123 801.2474 601.1701

403.1015 669.2022 451.1245 591.1299 831.2554 557.1828

503.0777 603.1358 677.2388 561.1574 541.1874

813.2818 389.0869 333.0816 497.1095 383.1152 527.171 571.1989

497.1592 873.2686 589.2069

675.2245 529.1852

567.17

383.1153

Observed m/z

0.8 0.85 0.85 0.88 0.93 0.98

0.64 0.64 0.65 0.65 0.72 0.73

0.62 0.62 0.62 0.63 0.64

0.48 0.48 0.5 0.52 0.61 0.62 0.62

0.47 0.47 0.48

0.46 0.46

0.45

0.45

Observed retention time (min)

Table 3 The phenolics in extract of H. sabdariffa calyces with potential biological activities.

8237

−0.9

78 352 516 428 195 557 972 375

158 153 426 152 1499 481

−3.2 1.9 0.5 −2.8 1.9 −2.4

2.9 0.7 −0.2 3.6 2 3.5 −3.2 −0.2

106 326 520 199 23586 345

−4.8 −2.2 −0.2 0.5 −1.2 1.9

261 2325 615 219 321 267 214

−0.5 −2.4 −3.5 1 4.1 −0.2 2.7 126 142 279 206 554

333 260 107

−2.8 1.9 −1.8

1 0.4 −0.6 3.3 1.1

793 197

224

−3.4 1.4 −3

204

Response

4.3

Mass error (ppm)

–

– Hepatocytic – Anti-diabetic, antimicrobial – – –

−H −H −H, +HCOO −H +HCOO −H −H −H

– – – Cytotoxic Locomotor –

+HCOO

+HCOO −H −H −H +HCOO +HCOO

−H +HCOO +HCOO −H +HCOO −H

– Prooxidant Antioxidant, anti-platelet, anti-inflammatory – Antioxidant –

– – – – –

−H +HCOO −H −H −H

−H +HCOO +HCOO +HCOO +HCOO +HCOO −H

+HCOO +HCOO +HCOO

– Antioxidant, antimutagenic, anti-inflammatory, antiplatelet aggregation immunomodulator, antimicrobial, and antitumor – – Antioxidant, antimutagenic, anti-inflammatory, antiplatelet aggregation immunomodulator, antimicrobial, and antitumor – – – – – Antibacterial –

Antioxidant, anti-aging, anti-platelet

Antioxidant, anti-inflammation, anti-proliferative

Potential biological activities

+HCOO −H

−H

+HCOO

Adducts [MH]-

(continued on next page)

– Li et al. (1998) – Semwal et al. (2009) – Nkengfack et al. (2006) –

–

Takasugi et al. (1979) – – Liu et al. (2016) Mil et al. (2002) –

Takasugi et al. (1979) Aquila et al. (2014) Xiang et al. (2014) – Cunqin et al. (2009) –

– – – – –

Kobayashi et al. (1984) Speranza et al. (1993) – – Takasugi et al. (1979) Bai et al. (1990) –

– – Lam et al. (2015)

Jayaprakasha et al. (2006); Sandur et al. (2007) Ling et al. (2016); Xiang et al. (2014) Fukai et al. (1985) Lam et al. (2015)

References

O.R. Alara and N.H. Abdurahman

Industrial Crops & Products 137 (2019) 528–535

Polydatin

Torachrysone-8-O-β-D-glucopyranoside Ciwujiatone 2′-Acetylacteoside Tubuloside C 10-O-Methylprotosappanin B 3′,4′-Dimethoxy-isoflavan-7,2′-di-O-β-D-glucoside β-Hydroxyacteoside Digupigan A (3R,4R)-3,4-trans-7,2′-Dihydroxy-4′-methoxy-4-[(3R)-2′,7dihydroxy-4′-methoxy-isoflavan-5′-yl]-isoflavan Moracin E Dihydrocurcumin 3-Hydroxy-1-(4-hydroxy-3,5-dimethoxyphenyl)-2-propanone Cistanoside C 2,7-Dihydroxy-4-methoxyphenanthrene-2-O-glucoside 6′-O-Galloyl-homoarbutin Apocynin B

Forsythoside A Sinapaldehyde Agrimol C Mulberrofuran N Vanillin Octahydrocurcumin Tellimagrandin Ⅱ Apocynin

Coniferol Feroxidin Phlorofucofuroeckol A Corilagin

Mulberrofuran D Laevigatin A Mallotinic acid Tamarixinol Protohypericin 3,3′-Dihydroxy-5-methoxy-2,5′,6-tri(4-hydroxyphenyl) bibenzyl Blestritin B Blestrin C

41.

42. 43. 44. 45. 46. 47. 48. 49. 50.

58. 59. 60. 61. 62. 63. 64. 65.

66. 67. 68. 69.

70. 71. 72. 73. 74. 75.

534

76. 77.

51. 52. 53. 54. 55. 56. 57.

Component name

No

Table 3 (continued)

7.09 7.25

6.02 6.35 6.65 6.92 7.02 7.08

5.82 5.86 5.87 5.96

4.42 4.46 4.52 4.66 5.06 5.25 5.42 5.74

3.48 3.48 3.59 3.59 3.83 3.97 4.36

2.41 2.42 2.56 2.82 3.06 3.16 3.18 3.22 3.25

2.38

Observed retention time (min)

485.196 479.1519

445.2375 801.0797 831.0902 445.4062 505.094 561.2275

225.0778 239.0932 601.0616 679.0789

623.198 253.0717 667.2772 391.1908 197.0447 421.1879 983.1019 211.0613

307.0985 415.1391 271.0837 683.2198 401.1225 437.1076 513.1033

453.142 479.1576 711.2159 953.2926 363.1074 625.2126 639.1915 479.1389 587.1928

435.1297

Observed m/z

383 95 179 362 216 313 249 284 138 1410 207 254

−2.1 0.6 0.5 2.5 2.3 −1.3 −1.9 3.9

171 97 200 116 164 311 280 86

−0.3 −0.3 1.7 −1.7 −4.2 2.5 1.2 0.7 4.1 3.1 −1.4 0.1

737 161 363 325 1087 413 246

238 268 704 218 135 218 696 189 144

138

Response

3.1 −1.9 4.9 0.8 −4.2 −3.1 −1.1

3.9 3.5 2.4 −0.7 −3.2 −2 −2.4 −3.7 0.9

0

Mass error (ppm)

-H -H

-H -H +HCOO -H -H -H

+HCOO +HCOO -H +HCOO, -H

-H +HCOO -H -H +HCOO +HCOO +HCOO +HCOO

-H +HCOO +HCOO +HCOO -H -H +HCOO

+HCOO +HCOO, −H +HCOO −H +HCOO -H -H +HCOO +HCOO

+HCOO

Adducts [MH]-

Antioxidant, antibacterial Antibacterial

– – – Anti-tumor, anti-inflammatory, hepatoprotective, antihyperalgesic – – – – – –

Anti-endotoxin – – – Flavour Hepatic-protective – Antioxidant, neuroprotective

– – – – – – Antioxidant, neuroprotective

Hepatoprotective, lung protective, neuroprotective, antiarteriosclerosis, anti-inflammatory, anti-shock, anti-tumor, antioxidant, anti-tumor, Immunoregulatory – – – – – – – – –

Potential biological activities

Shah et al. (2018) Bai et al. (1990)

– – – – – –

Takasugi et al. (1979) – – Kobayashi et al. (1984) Bai et al. (1990) Zheng et al. (2017) Agnes et al. (2008; Van den Worm et al. (2001) Zeng et al. (2017) – – – Walton et al. (2003) (Luo et al., 2019) – Agnes et al. (2008; Van den Worm et al. (2001) – – – Li et al. (2018)

– – Lei et al. (2001) – Bahtiar and Han (2017) – Tayfun et al. (2002) – –

Du et al. (2013)

References

O.R. Alara and N.H. Abdurahman

Industrial Crops & Products 137 (2019) 528–535

Industrial Crops & Products 137 (2019) 528–535

O.R. Alara and N.H. Abdurahman

Appendix A. Supplementary data

and Rajan, 2017). At higher solvent/sample ratio, the concentration of TP content was reduced. This can cause an increasing driving force for diffusion and dissolution. The concentration of TP contents at saturation, initial extraction rate and second-order rate constant improved with increasing temperature from 40 through 60 °C. Beyond the extraction temperature of 60 °C, the concentration of phenolics in the extract tends to reduce. This outcome is similar to the previously reported result for the extraction of camptothecin from Nothapodytes nimmoniana stems (Patil and Akamanchi, 2017). Increasing the temperature of the system in MAE can improve the vapour pressure and decline the surface tensing, causing a reduction in the release of energies emanating from the electromagnetic wave and microwave radiation. Moreover, the effective diffusivities determined for the temperatures between 40 and 60 °C are presented in Table 2. The effective diffusivity increased with temperature from 2.042 × 10−12 to 2.130 × 10−12. This is in accordance with the Einstein equation that postulated that increase in diffusivity can be facilitated by increasing the temperature (Tao et al., 2014). Beyond 60 °C of temperature, the effective diffusivity declines; this is supported with the previous studies that reported that the effective diffusivity of phenolic compounds was higher at a reduced temperature of extraction (Tao et al., 2014).

Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.indcrop.2019.05.053. References Alara, O.R., Abdurahman, N.H., 2019a. GC-MS and FTIR analyses of oils from Hibiscus sabdariffa, Stigma maydis and Chromolaena odorata leaf obtained from Malaysia: potential sources of fatty acids. Chem. Data Collect. 100200, 1–7. Alara, O.R., Abdurahman, N.H., 2019b. Kinetics studies on effects of extraction techniques on bioactive compounds from Vernonia cinerea leaf. J. Food Sci. Technol. 56, 580–588. Alara, O.R., Abdurahman, N.H., Ukaegbu, C.I., Azhari, N.H., 2018. Vernonia cinerea leaves as the source of phenolic compounds, antioxidants, and anti-diabetic activity using microwave-assisted extraction technique. Ind. Crop. Prod. 122, 533–544. Alara, O.R., Abdurahman, N.H., Abdul Mudalip, S.K., 2019. Optimizing microwave‐assisted extraction conditions to obtain phenolic compounds‐rich extract from Chromolaena odorata leaves. Chem. Eng. Technol. https://doi.org/10.1002/ceat. 201800462. Bouras, M., Chadni, M., Barba, F.J., Grimi, N., Bals, O., Vorobiev, E., 2015. Optimization of microwave-assisted extraction of polyphenols from Quercus bark. Ind. Crops Prod. 77, 590–601. Chan, C.-H., Lim, J.-J., Yusoff, R., Ngoh, G.-C., 2015. A generalized energy-based kinetic model for microwave-assisted extraction of bioactive compounds from plants. Sep. Purif. Technol. 143, 152–160. Chen, L., Song, D., Tian, Y., Ding, L., Yu, A., Zhang, H., 2008. Application of on-line microwave sample-preparation techniques. TrAC - Trends Anal. Chem. 27, 151–159. Ciğeroğlu, Z., Kırbaşlar, İ., Şahin, S., Köprücü, G., 2017. Optimization and kinetic studies of ultrasound-assisted extraction on polyphenols from Satsuma Mandarin (Citrus unshiu Marc.) leaves. Iran. J. Chem. Chem. Eng. 36, 163–171. Da-Costa-Rocha, I., Bonnlaender, B., Sievers, H., Pischel, I., Heinrich, M., 2014. Hibiscus sabdariffa L. - A phytochemical and pharmacological review. Food Chem. 165, 424–443. Inikpi, E., Lawal, O.A., Ogunmoye, A.O., Ogunwande, I.A., 2014. Volatile composition of the floral essential oil of Hibiscus sabdariffa L. From Nigeria. Am. J. Essent. Oils Nat. Prod. 2, 4–7. Krishnan, R.Y., Rajan, K.S., 2017. Influence of microwave irradiation on kinetics and thermodynamics of extraction of flavonoids from Phyllanthus emblica. Brazilian J. Chem. Eng. 34, 885–899. Krishnan, K.R., Babu, P.A.S., Babuskin, S., Sivarajan, M., 2015. Modeling the kinetics of antioxidant extraction from Origanum vulgare and Brassica nigra. Chem. Eng. Commun. 202, 1577–1585. Kusuma, H.S., Mahfud, M., 2017. The extraction of essential oils from patchouli leaves (Pogostemon cablin Benth) using a microwave air-hydrodistillation method as a new green technique. RSC Adv. 7, 1336–1347. Nizar, S., Elhadi, M., Algaili, M.A., Hozeifa, Mohamed Hassan, Mohamed, O., 2014. Determination of total phenolic content and antioxidant activity of Roselle (Hibiscus sabdariffa L.) calyx ethanolic extract. Stand. Res. J. Pharm. Pharmacol. 1, 034–039. Owoade, A.O., Adetutu, A., Olorunnisola, O.S., 2016. Identification of phenolic compounds in Hibiscus sabdariffa Polyphenolic Rich extract (HPE) by chromatography techniques. Br. J. Pharm. Res. 12, 1–12. Patil, D.M., Akamanchi, K.G., 2017. Microwave assisted process intensification and kinetic modelling: Extraction of camptothecin fromNothapodytes nimmoniana plant. Ind. Crops Prod. 98, 60–67. Pimentel-moral, S., Borrás-linares, I., Lozano-sánchez, J., 2018. Microwave-assisted extraction for Hibiscus sabdariffa bioactive compounds. J. Pharm. Biomed. Anal. 156, 313–322. Shams, K.A., Abdel-azim, N.S., Saleh, I.A., Hegazy, M.F., El-missiry, M.M., Hammouda, F.M., Bohouth, E., Tahrir, E., 2015. Green technology: economically and environmentally innovative methods for extraction of medicinal and aromatic plants (MAP) in Egypt. J. Chem. Pharm. Res. 7, 1050–1074. Tao, Y., Zhang, Z., Sun, D.W., 2014. Kinetic modeling of ultrasound-assisted extraction of phenolic compounds from grape marc: influence of acoustic energy density and temperature. Ultrason. Sonochem. 21, 1461–1469. Veggi, P.C., Martinez, J., Meireles, M.A., 2013. Microwave-assisted extraction for bioactive compounds, microwave-assisted extraction for bioactive compounds: theory and practice. Food Eng. Series. 4, 15–52. Villani, T., Juliani, H.R., Simon, J.E., Wu, Q., 2013. Hibiscus sabdariffa: phytochemistry, quality control, and health properties. Discoveries and Challenges in Chemistry, Health, and Nutrition. American Chemical Society, Washington, DC, pp. 209–230. Yedhu Krishnan, R., Rajan, K.S., 2016. Microwave assisted extraction of flavonoids from Terminalia bellerica: study of kinetics and thermodynamics. Sep. Purif. Technol. 157, 169–178.

3.6. LC-ESI-MS/MS analysis The tentative assigned phenolic compounds in the extract of H. sabdariffa calyces at extraction of 3 min, solvent/sample ratio of 14:1 mL/g, microwave power of 500 W, and temperature of 60 °C are presented in Table 3. The BPI plot for the tentatively assigned phenolics in the extract is presented in Fig. 2S (Supplementary material). An aggregate number of 77 compounds were identified in the extract. The potential biological activities of individual identified phenolic compounds as previously reported have been provided with references in Table 3. The larger number of phenolic compounds tentatively assigned were reported for H. sabdariffa calyces in this study. All the identified phenolics showed mass errors below 5 ppm, hence, confirming their presence. The aggregate influences of these phenolic compounds might have been responsible for the diverse biological activities of the extract from H. sabdariffa calyces. 4. Conclusion Even though the MAE of phenols from H. sabdariffa calyces had previously been investigated; however, this study investigated the process intensification and kinetic studies for the recovery of phenolics from H. sabdariffa calyces. The considered MAE factors which include irradiation time, solvent/sample, microwave power, and temperature played essential roles in recovering phenolics from H. sabdariffa calyces and the kinetic studies. The obtained results reflected that second-order rate law best explain the kinetic process involved in the MAE of TP content from H. sabdariffa calyces with R2 greater than 0.99. The MAE process intensification showed that the highest yield of TP content (70.53 mg GAE/g extract) was obtained at extraction time of 3 min, solvent/sample ratio of 14:1 mL/g, microwave power of 500 W, and temperature of 60 °C. Moreover, the effective diffusivity falls within the range of 2.042 × 10−12 to 2.130 × 10−12. Additionally, an aggregate of 77 phenolic compounds most of which are reported for the first time was assigned in the extract of H. sabdariffa calyces; indicating extensive industrial potentials of the extract. Thus, H. sabdariffa calyx can potentially be explored in food and pharmaceutical industries. Conflict of interest We declare none.

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