Separation and Purification Technology 124 (2014) 141–147

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Separation and Purification Technology

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The kinetics of extraction of the medicinal ginger bioactive compoundsusing hot compressed water

1383-5866/$ - see front matter � 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.seppur.2014.01.008

⇑ Corresponding author at: Centre of Lipid Engineering Applied Research (CLEAR),Universiti Teknologi Malaysia, Jalan Semarak, 54100 Kuala Lumpur, Malaysia. Tel.:+60 326154317.

E-mail address: azian.morad@gmail.com (N.A. Morad).

Mohd Sharizan Md Sarip a,b, Noor Azian Morad a,b,⇑, Nor Azah Mohamad Ali c, Yasmin Anum Mohd Yusof d,Mohd Azizi Che Yunus e

a Centre of Lipid Engineering Applied Research (CLEAR), Universiti Teknologi Malaysia, Jalan Semarak, 54100 Kuala Lumpur, Malaysiab Malaysia – Japan International Institute of Technology (MJIIT), Universiti Teknologi Malaysia, Jalan Semarak, 54100 Kuala Lumpur, Malaysiac Forest Research Institute of Malaysia (FRIM), Kepong, 52109 Selangor, Malaysiad Department of Biochemistry, Faculty of Medicine, Universiti Kebangsaan Malaysia, 54100 Kuala Lumpur, Malaysiae Centre of Lipid Engineering Applied Research (CLEAR), Universiti Teknologi Malaysia, 81310 Johor Bahru, Malaysia

a r t i c l e i n f o

Article history:Received 2 August 2013Received in revised form 9 December 2013Accepted 1 January 2014Available online 19 January 2014

Keywords:Hot compressed water6-Gingerol6-ShogaolKineticsMass transfer model

a b s t r a c t

Zingiber officinale or ginger known for its high medicinal compounds is extracted using hot compressedwater (HCW). The two most important bioactive compounds namely 6-gingerol and 6-shogaol in the gin-ger extracts are analyzed using HPLC. The effects of temperature and time of extraction on 6-gingerol and6-shogaol are studied using HCW extraction. It is found that HCW extraction can extract these two bio-active compounds; 6-gingerol at 130 �C and 30 min whilst 6-shogaol at 170 �C and 20 min. This findingshows that HCW extraction is potentially used in selective extraction of bioactive compounds at differentconditions of HCW. The kinetics of extraction for both bioactive compounds is studied from the optimumtemperatures obtained. The overall mass transfer coefficient which represents the extraction efficiency iscalculated using mass transfer model. The optimum values of the overall mass transfer coefficient (k) for6-gingerol is 8 � 10�7 m/s at 130 �C whilst for 6-shogaol, is 18 � 10�7 m/s at 170 �C using HCW extrac-tion. The relationship between the overall mass transfer coefficient and the dielectric constant of varioussolvents for 6-gingerol is identified. Similar relationship is identified for 6-shogaol using HCW as solvent.The dielectric constant does not contribute to the extraction efficiency of 6-gingerol and 6-shogaol.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

Ginger or zingiber officinale is known to be anticancer [1,2], anti-oxidants [2] and anticarcinogenic [3]. The main bioactive com-pounds that exhibit these medicinal properties are the series ofgingerols, shogaol and paradol. The most important medicinalcompounds identified are 6-gingerol, 10-gingerol and 6-shogaol.6-Gingerol, the most abundant compound in ginger rhizome hasbeen proven to give positive response in mediating cardiac con-tractile, act as antioxidant [4], antiproliferative and also apoptosis[2]. 10-Gingerol has significant medicinal effect such as antibacte-rial [5] and antimicrobial activities [6]. 6-Shogaol is another pre-dominant pungent constituent in ginger is proven to reduce celldeath and restores motor function in rat spinal cord injury [7]and lessen the human oral cancer [8]. Furthermore, pharmacoki-netics study of 6-gingerol, 10-gingerol, and 6-shogaol shows that

all these compounds are absorbed quickly in the serum withmajority detected as glucuronide metabolites [9].

The proven medicinal roles of each ginger compound have pro-moted the search for selective extraction to give an added value tothe ginger oleoresin. Ginger in its fresh form is made up of isolatedcells containing oleoresins comprising of gingerols and shogaols[10]. In dried ginger, the oleoresin cell wall is ruptured and ex-posed. This can facilitate the extraction of ginger bioactivecompounds.

The use of water in subcritical region as a ’green’ solvent has at-tracted the interest of numerous researchers from all over theworld. Thermodynamically, water in its liquid state below the crit-ical point of 374 �C and 22 MPa is referred to as subcritical water.Meanwhile, hot compressed water (HCW) specifically refers tosubcritical water above the normal boiling point of 100 �C [11].HCW extraction has been successfully utilized for herbal extractionas demonstrated in the extraction of cumin [12], zataria multiflora[13], centella asiatica [14], thymbra spicata [15], bitter melon [16]and oregano [17].

HCW does not only act as a ‘green’ solvent but it also has the po-tential use for selective extraction [18]. Wiboonsirikul and Adachihave reported that the main parameters that affect the extraction

142 M.S. Md Sarip et al. / Separation and Purification Technology 124 (2014) 141–147

efficiency during HCW extraction process are temperature andtime [19]. Meanwhile, pressure is proven to have no significant ef-fect on the extraction efficiency [17]. The effect of temperature andtime for the extraction of 6-gingerol and 6-shogaol from zingiberofficinale using HCW are explored in this study for the potentialuse in selective extraction.

There is no known mass transfer data available for 6-gingeroland 6-shogaol which is required for design purposes in particularfor the optimization of HCW extraction process. The overall masstransfer model is applied to 6-gingerol and 6-shagaol extractionusing HCW to determine the overall mass transfer coefficient, k.The k value represents the extraction efficiency of a process. Thek value is affected by the properties of the solute as well as the sol-vent. The dielectric constant is a fundamental property of a solventand it has been shown that it is affected significantly by tempera-ture and insignificantly by pressure [20]. Dielectric constant varieswith different types of solvent and generally, organic solvents suchas hexane, acetone and chloroform have lower dielectric constantswhich increase the solubility of organic solutes. The relationshipbetween the dielectric constant of different types of solvent onextraction efficiency of 6-gingerol is confirmed in a study by Shad-mani et al. [21]. The effect of the dielectric constant of HCW on thek value of 6-gingerol and 6-shogaol is investigated in this study.

2. Material and method

2.1. Material

Dried and ground ginger was supplied by a local supplier fromRanau, Sabah, East Malaysia. The ginger standards of 6-gingerol(85.8% w/w), 10-gingerol (95.1% w/w) and 6-shogaol (96.4% w/w)were purchased from ChromoDex Inc., CA, USA. Acetonitrile andmethanol were of High Performance Liquid Chromatography(HPLC) grade, supplied by MERCK, Germany. Distilled water wasused in the HCW extraction.

2.2. Hot compressed water extraction

HCW extraction was done batch wisely using the fabricatedequipment in this laboratory as shown in Fig. 1. Prior to this, pre-liminary experiments on temperature effect were done using 32 mlextraction cell of Accelerated solvent extractor (ASE 200, Dionex,USA) to verify the result of the fabricated equipment. The solventof sample ratio used in this equipment was scaled up from theASE as recommended by the manufacturer. The fabricated equip-ment consisted of two vessels with one liter volume each; theextraction and the cooling cells connected by a 1=4 in. stainless steelpipeline. 75 g of ground ginger was weighed and loaded into a cov-ered stainless steel mesh cylinder before being placed into theextraction cell. 700 ml of distilled water was added into the cell.The cell was then securely covered with a stainless steel lid. N2

gas was then passed through the cell for 2 min to purge out air

Fig. 1. Schematic diagram of

and dissolved oxygen. Excess pressure was relieved through the re-lease valve. The temperature was set according to the requiredexperiment. The electrically jacketed extraction cell took 3–5 minto achieve the desired temperature. The extraction time startedonce the set temperature was achieved as indicated by the temper-ature indicator in the extraction cell.

The effect of temperature was studied independently from 100to 200 �C with 10 �C increment and referred to as the first set ofexperiments. The extraction time was kept constantly at 30 min.For example, at 100 �C, the first experiment in the first set wasrun for 30 min, the second experiment was run at 110 �C for30 min and so on up to 200 �C for 30 min, which the temperatureincreased but the time was constant.

The effect of extraction time was studied at 10, 20, 30, 40, 50and 60 min at the determined optimum temperature from thetemperature effect experiment and referred to as the second setof experiments. For example, if the optimum temperature for 6-gingerol was T1, the first experiment in the second set was T1for 10 min, the second experiment was run at T1 for 20 min andso on up to 60 min at T1. All experiments were carried out at con-stant pressure of 3.5 MPa. Once the extraction process was com-pleted, the extractant was transferred into the cooling cell at25 �C and 1 MPa within 1 min to ensure rapid cooling.

Ginger extracts were collected and subjected to further analysisusing HPLC. Each experimental condition was run triplicate. Theaverage absolute deviation, AAD was applied for each set of exper-iment using Eq. (1).

AAD ¼ 1N

Xn

i¼1

jxi � �xjxi

ð1Þ

where i, n is the no. of runs for one experimental condition; N thetotal no. of runs; xi the data for one experimental condition; �x isthe average for one experimental condition.

The percent recovery of each bioactive compound was calcu-lated using Eq. (2).

Percent recovery ¼ci

lg bioactiveg dried ginger

coi; lg bioactive

g dried ginger

� 100 ð2Þ

where ci is the species concentration in the bulk solution; coiis the

species initial concentration obtained using eight hours soxhletextraction with ethanol.

2.3. HPLC analysis for ginger bioactive compounds

The analysis were carried out using HPLC (Waters Corp., MA,USA) equipped with Photodiode Array (PDA) detector and theLichrocart 250-4, 6 Purospher Star RP-8E (5 Mym) column (MERCK,Germany). In this analysis, 10 min run time was used with 8 min asan injection delay. Two mobile phase solutions were used whichwere A; 100% acetonitrile and B; 65% (v/v) methanol in water.The mobile phase ratio A to B increased gradually during the sep-aration process from 20:80 to 50:50 (volume of A/volume of B) ata constant flow rate of 1.20 ml/min. 10 ll of sample extracts were

HCW extraction facility.

M.S. Md Sarip et al. / Separation and Purification Technology 124 (2014) 141–147 143

filtered using syringe filter (PTFE, 0.45 lm, Whatman, USA) beforeinjected into the HPLC. The standard curve of 6-gingerol, 6-shogaoland 10-gingerol were established using 10 ll of standards injectedinto the HPLC. The ginger bioactive compound concentration wasestablished through the calibration of HPLC chromatograms usingthe standard curves. The method was adapted from Schwertnerand Rios [22].

2.4. Mass transfer coefficient model

The experimental data on ginger bioactive compounds concen-tration was used to describe the extraction mechanism through themass transfer coefficient model. The overall mass transfer coeffi-cient, k was calculated based on the mass balance for the processwith the assumption that the amount transferred was proportionalto the concentration difference and the interfacial area [23]. Basedon the mass transfer model used as demonstrated in Sarip andMorad [24], two different first order expressions of mass transfercoefficients were used in this work. The assumption on the gingerparticles size and total surface area using this model was explainedin Sarip and Morad [24]. The overall mass transfer coefficient, k andabsolute overall mass transfer coefficient, kabs are shown in Eqs. (3)and (4) [23,25].

k value is expressed as : ki ¼ �VL

Atln

coi

coi� ci

ð3Þ

kabs value is expressed as : kabsi¼ ln

coi

coi� ci

� �ð4Þ

where At is the surface area for particle; coithe species initial con-

centration measured using eight hours soxhlet extraction with eth-anol at 4:1 (ml:g) solvent to sample ratio, lg bioactive/g driedsample; ci the species concentration in the bulk solution, lg bioac-tive/g dried sample; i the species, 6-gingerol and 6-shogaol; VL isthe volume of the solution.

The kabs values of 6-gingerol extracted using various organicsolvents which were benzene, hexane, chloroform, ethanol, meth-anol, acetone, dichloroethane, acetonitrile, diethyl ether, ethyl ace-tate, isopropanol and dichloromethane were taken from Shadmaniet al. [21]. The respective dielectric constant values of the organicsolvents were obtained from Timmermans [26]. These kabs valueswere compared with the kabs value of 6-gingerol extracted usingHCW. The dielectric constant of water at 3.5 MPa was interpolatedfrom the experimental data between 2.5 and 5 MPa provided byUematsu and Frank [20].

3. Results and discussion

3.1. The effect of temperature using HCW on ginger bioactivecompounds

The effect of the extraction temperature on each ginger bioactivecompound concentration (lg bioactive/g dried ginger) is shown inFig. 2. The effect can only be seen for the two main ginger bioactivecompounds which were the 6-gingerol and 6-shogaol. Meanwhile,10-gingerol was not detected in the HCW extracts under these con-ditions through HPLC analysis. Therefore, it can be considered thatduring the HCW extraction 10-gingerol was not extracted withinthe experimental temperature range of 100–200 �C at 3.5 MPa.

Fig. 2 shows that from 100 to 120 �C, the modest increase of the6-gingerol concentration was observed from 610.31 to721.98 ± 2.19 lg 6-gingerol/g dried ginger; however, 6-shogaolwas not extracted. HCW extraction at temperatures from 120 to130 �C indicated a rapid increase of 6-gingerol concentration from721.98 ± 2.19 lg 6-gingerol/g dried ginger to its maximum

concentration peak of 1741.54 ± 2.19 lg 6-gingerol/g dried gingerat 130 �C. 6-Gingerol with the existence of hydroxide group inthe structure was more polar compared to 6-shogaol. The principletheory of ‘like dissolve like’ was applied during the extraction pro-cess. The advantage of using HCW as a solvent was that it had awide range of polarity which varied with temperature as indicatedin Fig. 2. From 120 to 130 �C, the dielectric constant (measure ofpolarity) of water was about 50 [20] and it was indicated that itwas the most suitable condition for extracting the relatively polar6-gingerol rather than 6-shogaol.

Fig. 3a shows the HPLC chromatogram, which indicates that at130 �C, only 6-gingerol has been extracted. Fig. 3b indicates thatboth 6-gingerol and 6-shogaol peaks were detected in the gingerextracts using HCW at 170 �C. It was found that extraction usingHCW at 130–170 �C was not favorable for 6-gingerol as the concen-tration rapidly decreased from 1741.54 to 340.84 ± 2.19 lg 6-ging-erol/g dried ginger. However, for 6-shogaol, it can be observed thatthe bioactive compound concentration steadily increased from theextraction temperature of 120–170� using HCW. The optimumextraction temperature was 170 �C with a maximum concentrationof 541.78 ± 2.95 lg 6-shogaol/g dried ginger. The increasing of 6-shogaol and the decreasing of 6-gingerol concentrations were re-lated to the decreasing of the dielectric constants of water from50 at the temperature of 120 �C to 39 at 170 �C [23]. At 170 �C,the extraction condition favoured the extraction of 6-shogaolwhich was also less polar than 6-gingerol.

The other contributing factor for the decline of 6-gingerol wasthe increase in 6-shogaol due to the degradation of 6-gingerol. b-hydroxy keto group in the 6-gingerol molecule dehydrates to form6-shogaol and water molecules as illustrated in Fig. 4. This was thekey feature that made 6-gingerol thermally labile undergo faciledehydration at high temperature and acidic condition [27,28]. Eventhough it was a reversible reaction, the degradation affects 6-ging-erol since this bioactive was extracted out in abundance at a lowertemperature where as 6-shogaol was significantly absent at below150 �C.

It was proven that 6-gingerol decreased when HCW extractionwas conducted from 140 to 200 �C. On the other hand, for the sameHCW extraction conditions, 6-shogaol concentration increased inthe ginger extract as shown in Table 1. The changes in lmol for6-gingerol and 6-shogaol respectively were calculated starting at130 �C. Subsequent changes in the lmoles of the bioactives werecalculated at an increment of 10 �C. Since the dehydration of 6-gingerol to 6-shogaol should be in accordance with the stoichiom-etry, the obvious reason for further decline in 6-gingerol content atextraction temperatures above 130 �C was attributed to otherforms of degradation [27]. Apart from the degradation of 6-ginger-ol to 6-shogaol, the further increase in 6-shogaol can be attributedto the preference of bioactive compound being extracted from gin-ger samples at higher temperature as indicated in Table 2.

It can be observed that the concentration of both bioactive com-pounds declined at 180–200 �C. The decrease was due to both bioac-tive compounds degradation at these temperatures. Further studyon ginger bioactive stability is required to understand the effectsof HCW extraction at higher temperatures. Thus, two different opti-mum extraction temperatures were identified for the extraction of6-gingerol and 6-shogaol which were at 130 �C and 170 �C respec-tively at 30 min extraction time and 3.5 MPa. Further investigationon the suitable extraction time was required to verify the appropri-ate optimum extraction time for both bioactive compounds.

3.2. Effect of the extraction time on the bioactive compoundsconcentration in the ginger extract

Another important parameter involved in this HCW extractionstudy was the extraction time or treatment time. The effects of

Fig. 2. The effect of temperature, �C in the ginger bioactive compounds concentration using HCW at constant time of 30 min and constant pressure of 3.5 MPa.

Fig. 3. HPLC profile for HCW extract. (a) 130 �C and (b) 170 �C: (—) Sample, (- --) Standard.

Fig. 4. The dehydration of 6-gingerol to 6-shogaol [28].

144 M.S. Md Sarip et al. / Separation and Purification Technology 124 (2014) 141–147

the extraction time in this study focused on the optimum temper-atures that were 130 �C and 170 �C for 6-gingerol and 6-shogaolrespectively. At 130 �C, the effect of extraction time can be

observed for 6-gingerol since only traces of 6-shogaol wasextracted from the ginger as shown in Fig. 5. The high rate of theextraction giving 800.33 ± 2.61 lg 6-gingerol/g dried ginger canbe observed after 10 min. When HCW extraction was conductedfor 20 min, a moderate extraction rate with a concentration of949.38 ± 2.61 lg 6-gingerol/g dried ginger was noted; an incre-ment of 149.05 lg 6-gingerol/g dried ginger in 10 min. The highestrate of extraction giving a maximum concentration of1741.54 ± 2.61 lg 6-gingerol/g dried ginger was achieved for30 min of extraction time.

During the first 10 min of extraction time, the mass transferwithin the solid matrix is the dominating factor. The low

Table 1Degradation of 6-gingerol to 6-shogaol.

Temperature (�C) Decrease of 6-gingerol (lmol)a Increase of 6-shogaol (lmol)a Difference in changes (lmol)a

130–140 0.82 1.11 �0.29140–150 2.20 0.03 2.17150–160 0.93 0.20 0.73160–170 0.81 0.62 0.19

a Basis 1 g of dried ginger.

Table 2Compound initial concentration, co, optimum HCW extract yield, ci and maximum recovery of bioactive at 3.5 MPa.

Compound Optimum HCW condition co, (lg/g) HCW extract conc., ci (lg/g) Max recovery of bioactive, %

T (�C) Time (mins)

6-Gingerol 130 30 8406.99 1741.54 20.716-Shogaol 170 20 716.76 609.51 85.0410-Gingerol None None 903.41 None None

Fig. 5. The effect of the extraction time on the ginger bioactive compoundsconcentration at a constant temperature of 130 �C and a constant pressure of3.5 MPa.

M.S. Md Sarip et al. / Separation and Purification Technology 124 (2014) 141–147 145

concentration of 6-gingerol in the bulk liquid causes the high rateof mass transfer since the driving force is high with large concen-tration gradient in accordance with the overall mass transfer mod-el. For the next 10 min, more 6-gingerol is transferred out into thebulk liquid but at a much reduced concentration about149.04 ± 2.61 lg 6-gingerol/g dried since most of it is trappedwithin the bulk liquid just outside the solid matrix. This will re-duce the concentration gradient and consequently reduce masstransfer process. As the extraction time is prolonged to 30 min,6-gingerol starts to diffuse further into the bulk liquid and this im-proves the mass transfer and increase by 792.16 ± 2.61 lg 6-ging-erol/g dried of 6-gingerol. The dominating effect of diffusion of 6-gingerol from outer surface of solid matrix into the bulk liquid isillustrated in this region. From 40 to 60 min of extraction, the con-centration of 6-gingerol is decreased from 1741.54 to

Fig. 6. The effect of the extraction time on the ginger bioactive compoundsconcentration at the temperature of 170 �C at constant pressure of 3.5 MPa.

933.44 ± 2.61 lg 6-gingerol/g dried ginger due to prolonged heatexposure. 6-Gingerol being thermally labile underwent the dehy-dration and degradation as discussed earlier. At 130 �C, 6-shogaolis not significantly extracted as indicated in Fig. 5.

The effect of extraction time on the ginger bioactive compoundsat a constant temperature of 170 �C was observed in Fig. 6. At170 �C and extraction time of 10 min, both ginger bioactive com-pounds 6-gingerol and 6-shogaol are in high concentration of969.91 ± 10.68 lg 6-gingerol/g dried ginger and 427.79 ± 6.92 lg6-shogaol/g dried ginger respectively. The 6-gingerol concentra-tion however, decreases steadily for extraction times of 20 min tothe concentration of 227.07 ± 10.68 lg 6-gingerol/g dried gingerat 60 min.

Meanwhile, 6-shogaol concentration increases for the extrac-tion time of 20 min at 609.51 ± 6.92 lg 6-shogaol/g dried gingerand remains almost constant for the extraction times of 30 and40 min. At the extraction times of 50 and 60 min, the 6-shogaolindicated the reduction to the concentration of 157.13 ± 6.92 lg6-shogaol/g dried ginger due to the possible degradation to otherforms.

Therefore, it was found that HCW extraction at a constanttemperature of 130 �C gave the maximum 6-gingerol 1741.54 ±2.61 lg 6-gingerol/g dried ginger at the extraction time of30 min. At this temperature, it was obvious that selective extrac-tion took place in which only 6-gingerol was extracted whilst6-shogaol was significantly absent in the ginger extract.Meanwhile, 6-shogaol was at its highest concentration of609.51 ± 6.92 lg 6-shogaol/g dried ginger at the extraction timeof 20 min compared to 541.78 ± 2.95 lg 6-shogaol/g dried gingerat 30 min of extraction. This indicates that the optimum condi-tion for 6-shogaol extraction is at 170 �C with 20 min extractiontime.

The optimum HCW extracts and the initial concentration, co

of 8406.99, 716.76 and 903.41 lg bioactive/g dried ginger for6-gingerol, 6-shogaol and 10-gingerol is tabulated in Table 2.The percent recovery of 6-gingerol, 6-shogaol and 10-gingerolusing HCW extraction is 20.71, 85.04 and 0%; respectively basedon the co. The higher recovery for 6-shogaol is due to the addi-tional degradation of 6-gingerol to 6-shogaol which contributeto its higher concentration. This consequently reduces the over-all recovery for 6-gingerol. Further improvement in extractionmethod need to be studied to increase the extraction efficiency.This includes the addition of co-solvent in the HCW, introduc-tion of turbulent mixing between sample and solvent and inclu-sion of external factors to improve extraction such as sonicenergy.

Table 3k, kabs and e values of 6-gingerol and 6-shogaol at different temperatures.

Temperature, �C kabs, min�1(�103) k, m/s (�107) Dielectric constant of water, e[23]

6-Gingerol 6-Shogaol 6-Gingerol 6-Shogaol

110 6.10 N.A 2.46 N.A 53.00130 20.10 9.10 8.12 3.68 49.03150 6.80 10.90 2.75 4.40 39.17170 2.20 45.50 0.89 18.38 35.00

Fig. 7. The relationship between the absolute overall mass transfer coefficient, kabs

and dielectric constant of solvents for 6-gingerol.

Fig. 8. The relationship between the absolute mass transfer coefficient, kabs anddielectric constant of HCW for 6-shogaol.

146 M.S. Md Sarip et al. / Separation and Purification Technology 124 (2014) 141–147

3.3. Kinetics of the HCW extraction process using mass transfercoefficient model

The k and kabs values for 6-gingerol and 6-shogaol at the extrac-tion temperatures of 110 �C, 130 �C, 150 �C and 170 �C at the con-stant pressure of 3.5 MPa were calculated and tabulated in Table 3.The values of dielectric constant, e of water at the respective tem-peratures are also included in the Table 3.

Generally, the higher k value indicates a higher rate of solutetransfer from the solid matrix into a solvent [29]. Thus, k value rep-resents the extraction efficiency which is related to the solvent andsolute properties. One of the solvent properties which is widely re-lated to the extraction efficiency is the dielectric constant. In the-ory, the extracting power increases as the dielectric constantdecreases. Therefore, in this study, the relationship between k va-lue and dielectric constant of solvents including HCW wasinvestigated.

Comparisons were made between the k values derived for SFEand HCW extraction processes. For SFE process, the k value derivedfor 6-gingerol is 6 � 10�5 m/s [30] which is higher compared to thek value of 8 � 10�7 m/s for HCW extraction process in this work. Apossible explanation for this variation in the k values is due to thedifferent initial concentrations used for both calculations. In SFEprocess, the initial concentration, co was 2.4 wt% (on a dry solidsbasis) [30] meanwhile for HCW extraction was 0.8 wt% (on a drysolid basis). The difference on these initial concentrations wasdue to the different sample origins, sample preparations andextraction methods. The higher co values contribute to the higherk value in SFE extraction. Furthermore, SFE process operates at alow temperature of 40 �C thus excluding the dehydration and deg-radation of 6-gingerol to other products. Supercritical CO2 which isthe common supercritical fluid as a solvent most likely promotesbetter mass transfer process for thermal labile compounds suchas 6-gingerol compared to HCW. The comparisons between k val-ues of SFE and HCW extraction is not completely indicative of abetter process in 6-gingerol extraction using SFE. For best compar-ison, the co values for both SFE and HCW extraction should be thesame using same samples, similar preparation and similar initialconcentration extraction method.

For 6-shogaol, it can be observed that the k value increases withtemperature to its maximum value of 18.3764 � 10�7 m/s at170 �C since extraction above this temperature indicates degrada-tion of the bioactive compound. The k value for 6-shogaol is highercompared to k value of 6-gingerol through HCW extractionprocess. Apart from the naturally existing 6-shogaol in the gingersample, the addition of 6-shogaol through dehydration of6-gingerol contributes to the increased concentration at highertemperatures. However, the k value for 6-shogaol cannot be com-pared to other means of extraction since there is no such knownexperimental data.

For the purpose of identifying the relationship between extrac-tion efficiency and dielectric constant, kabs is utilized instead of kvalues. This is due to the availability of kabs data for various solventextractions to be used for comparison purposes with HCW extrac-tion in this work. The kabs value of 6-gingerol increases from 110 �Cto the highest value of 0.0201 min�1 at 130 �C and decline from

150 �C to 170 �C. The decrease of kabs values for 6-gingerol from150 �C to 170 �C is inconsistent with the decrease of water dielec-tric constant at the same temperatures as demonstrated in Table 3.

The relationship between the dielectric constant and kabs valuefor 6-gingerol in the HCW extraction is shown in Fig. 7 togetherwith the experimental values from Shadmani et al. [21] using var-ious solvents at lower temperatures. Two different linear fits fordata from Shadmani et al. [21] and the HCW extraction data of thiswork are presented. The relationship between the dielectric con-stant and kabs for HCW extraction process of 6-gingerol gives anr2 value of 0.0256 and has a low slope of 0.0001. This indicate poorrelationship between dielectric constant and kabs since a good lin-ear fit will have r2 value approaching 1. A slope of 0.0001 indicatesalmost a nonexistence relationship between dielectric constantand kabs value. Similar trend is observed for the extraction of 6-gingerol using various solvents with r2 value of 0.0186 and slopeof �0.00004 [21]. This suggests that the dielectric constant is notthe parameter affecting the extraction efficiency of 6-gingerol.

A more important condition affecting extraction efficiency ismost likely the solvent–solute interaction which includes the sol-vent shift and the cavity as reported by Horák and Plíva [31]. More-over, the effect of dielectric constant for different types of solvent

M.S. Md Sarip et al. / Separation and Purification Technology 124 (2014) 141–147 147

on 6-gingerol may have different solute solvent interaction. Thus,6-gingerol would be effectively extracted out at a certain favorabledielectric constant suited to the 6-gingerol solvent interaction andnot necessarily affected by lower dielectric constant values. Fur-thermore, another factor affecting the kabs values of 6-gingerolwas confirmed to be the degradation of 6-gingerol to the othercompounds including the 6-shogaol as discussed earlier.

The relationship between the dielectric constant with the kabs

value for 6-shogaol is shown in Fig. 8. It is observed in 6-shogaolthat the changes of kabs towards the dielectric constant is more sig-nificant when compared to those of 6-gingerol with the r2 value of0.5754 and slope of �0.0022. The r2 value suggests that there issome linear correlation between dielectric constant and kabs, how-ever dependency of kabs on dielectric constant value is still verylow as indicated by low slope. The higher slope value of �0.0022in 6-shogaol compare to 6-gingerol at 0.0001 is contributed bythe accumulation of 6-shogaol concentration in the sample extractdue to dehydration process. This shows that similar to the 6-ging-erol, dielectric constant does not contribute to the extraction effi-ciency of 6-shogaol.

4. Conclusion

HCW offers an alternative solvent for the extraction of 6-ginger-ol and 6-shogaol. It was observe that the extraction was selectivewith the optimum conditions for 6-gingerol of 130 �C and 30 minwhilst 6-shogaol of 170 �C and 20 min with percent recovery of20.71% and 85.04% respectively. At higher temperatures from180 �C, the degradation of both compounds occurred. The dielectricconstant of solvent was not directly affecting the extraction effi-ciency. The overall mass transfer coefficient, k for 6-gingerol is8 � 10�7 m/s whilst for 6-shogaol is18.3764 � 10�7 m/s at theoptimum conditions using HCW extraction. HCW extraction hasthe potential to be use as an alternative extraction method pro-vided further improvements in operating conditions avoiding thedegradation of the bioactive compound is addressed. Further studyof using co-solvent or additives to HCW extraction process mayavoid operating in degradation region and also improve the per-cent recovery of the bioactive compounds.

Acknowledgements

The research is funded by Malaysian Ministry of Agriculture(MOA) under Science Fund Grant No. 05-01-06-SF1004. The re-search infrastructure provide by Universiti Teknologi Malaysia isgreatly appreciated. The assistant of En Radzi Ahmad from FRIMin using HPLC analysis is greatly acknowledged. Other individualswho have contributed to this research are also duly acknowledged.

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