Nur Atirah Ibrahim,Muhammad Abbas Ahmad Zaini*

Centre of Lipids Engineering&Applied Research(CLEAR),Ibnu-Sina Institute for Scientific&Industrial Research,Universiti Teknologi Malaysia,81310 UTM Johor Bahru,Johor,Malaysia School of Chemical&Energy Engineering,Faculty of Engineering,Universiti Teknologi Malaysia,81310 UTM Johor Bahru,Johor,Malaysia

Keywords:Castor seed Castor oil Dielectric properties Microwave-assisted solvent extraction Oxidation stability

A B S T R A C T This study was aimed at evaluating the physicochemical properties and oxidation stability of castor oil using microwave-assisted solvent extraction(MAE).MAE was performed using 5%ethanol in hexane as solvent at different extraction times,power intensities and solvent-to-feed(S/F,ml of solvent to gram of feed)ratios.The process parameters were optimized by statistical approach using historical data design of response surface method(RSM).The oils were characterized for yield,physicochemical properties,dielectric properties and oxidation stability,and comparison was also made with oil extracted using Soxhlet method.Results show that the maximum oil yield of 37%was obtained at 20 min with microwave power intensity of 330 W and S/F ratio of 20.The main fatty acid composition of castor oil is ricinoleic acid.The density,refractive index,dielectric properties and oxidation stability of oils are not affected by the extraction methods and extraction parameters of MAE.However,the MAE-extracted oil is more viscous compared to that by Soxhlet method.With extra caution on oil oxidation,MAE could be a promising solvent extraction method with an 86%less in processing time and a higher yield.

1.Introduction

Castor seeds are poisonous due to the presence of ricin,ricinine and other allergens that are toxic to human and animals[1].However,it has more than 700 uses in the field of cosmetics,plastics,manufacturing of biodiesel,lubricants and medicine[2,3].The oil content of castor seeds ranged from 30%to 50%[4,5].Generally,about 30%to 35%of oil from various fruits and seeds of plants can be extracted by different types of extraction techniques[6].Nevertheless,quest for a better extraction method has become a subject of considerable interest to improve the performance of conventional solvent extraction.The conventional solvent extraction often requires more than an hour to complete the extraction depending on the solvent rates of diffusion,and furthermore it is an energy-intensive process[7].

Microwave-assisted solvent extraction(MAE)is among the modern extraction techniques for essential oils,fats and oils,and has advantages over conventional solvent extraction[8].Fast extraction ability with small amount of solvent consumption and minor implication on thermolabile constituents are some of the attractive attributes of MAE[9].Besides,the operation of MAE is simple with low equipment cost compared to ultrasound-assisted extraction and supercritical fluid extraction[10].

In MAE,the electromagnetic waves and the inherent nature of microwave heating may have some influence on the quality and physicochemical properties of oils[11].The effect may vary depending on the types of materials,solvents and process parameters.Also,despite many studies on MAE,the information on dielectric properties of solvent and oil,and the oxidation stability of extracted oil are still lacking in much of published literature.Therefore,the present study was embarked and aimed at evaluating the effect of process parameters of MAE on the yield and physicochemical properties of castoroil.The comparison was made using the Soxhlet extraction.Also,the dielectric properties of oils and solvent,and the oxidation stability of extracted oil using both techniques were discussed to shed a better insight on the performance of MAE.

2.Materials and Methods

2.1.Materials

Castor bean was purchased from Ancient Green field Pvt.Ltd.,India.Hexane,petroleum ether,distilled water and ethanol were used as extraction solvents.Diethyl ether,ethanol,potassium hydroxide and phenolphthalein were used for oxidation stability tests.The chemicals were purchased from Fisher Scientific Chemical(Loughborough,UK)and Merck(Darmstadt).All chemicals are analytical-grade reagents,and were used without purification.

2.2.Extraction of oil

Castor bean was cleaned,winnowed and dried prior to extraction.The dried material was crushed and separated by a vibrator sieve into a nominal particle size of 2 mm.The particles were stored in a refrigerator at-4°C before extraction.

Microwave-assisted solvent extraction(MAE)of castor oil was performed using a multimode microwave(Sharp R6460).A mixture of 200 ml solvent(5%ethanol in hexane)and crushed seeds was added into a capped Teflon mold(1 L)inside the microwave.The mold is connected to a condenser outside the microwave by a PTFE tube.For each extraction run,10 g of sample was used(S/F ratio of 20 ml·g-1).The MAE was carried out at different power levels of 160 W,230 W and 330 W for 5,10,20 and 30 min.The solvent was recovered using a rotary evaporator(Hei VAP Value,Heidolph),and the yield of castor oil was calculated.This method was repeated for S/F ratios of 10 and 40 ml·g-1,while the power level and extraction time were kept constant at 330 W and 20 min,respectively.

The conventional solvent extraction using Soxhlet method was employed for comparison.Fifteen grams of crushed seeds was extracted using 300 ml of different solvents,i.e.,petroleum ether,water,hexane,ethanol,and a 5%ethanol in hexane for 2.5 h(25 re fluxes).The solvent was recovered using a rotary evaporator to obtain the oil yield.

2.3.Optimization using response surface method

Historical data was chosen as experimental design of response surface method(RSM,Design Expert 6.0.4 software).A quadratic regression polynomial model was assumed to predict the response(oil yield).The proposed model is Y=P0+P1X1+P2X2+P11X11+P22X22+P12X12,where P0is a constant;P1and P2are the linear coefficients;P12is the cross product coefficient;and P11and P22are the quadratic coefficients.The model was evaluated by the coefficient of determination,R2and the ANOVA for the goodness of fit.

2.4.Characterization of oil

The viscosity of oils was determined using viscometer(Anton Paar Stabinger,SVM 3000 and Brook field DV-II+Pro).The specific gravity of oils was estimated by weighing the desired volume of oil with reference to water.The refractive index of oils was measured using refractometer(model RX-5000α,ATAGO).The pH of oil was measured using a pHmeter(model Hanna,HI8424)according to the method proposed by Danlami(2015).GC–MS analysis based on PORIM Test Method(1984)was conducted using an Agilent 6890 N machine(Agilent Technologies,Wilmington,USA).The measurement was repeated 3 times,and the average values were reported.The dielectric properties of oils,and solid–solvent mixtures were measured using a HP 85070D openended coaxial probe attached to a computer controlled HP 8720B vector network analyzer with frequency ranging from 0.2 to 10 GHz.Analysis of the fatty acids content was conducted using an Agilent 6890 N gas chromatography(Agilent Technologies,Wilmington,USA).Helium was used as a carrier gas(1.0 ml·min-1).A 1.5 ml of methylated samples of ethanol,n-hexane or petroleum ether was injected using the split mode(50:1).The injector and detector temperatures were maintained at 235 °C and 250 °C,respectively.The column temperature was initially 160°C,and the heating was programmed to increase by 2 °C·min-1to 195 °C for 1 min and a subsequent increase of 1°C·min-1to 210 °C.

2.5.Oxidation stability of oil

The oxidation stability of oils was conducted using an accelerated oxidation process or Schaal oven test.The oils were placed in an oven at70°C for 2 weeks.The oil samples were collected at different time intervals,t=0,t=2 days,t=1 week and t=2 weeks[12-14].A desired volume of oil was withdrawn for the determination of pH,viscosity,acid value and free fatty acid value.

Acid value can be defined as the number of milligrams of caustic potassium required to neutralize the acid in 1 g of oil.1 mol·L-1KOH solution and a mixture of equal volume of diethylether and ethanol were prepared.Two grams of oil and 50 ml of the mixture of diethylether and ethanol were added into a 250 ml conical flask,and stirred.Two drops of phenolphthalein indicator were added into the mixture.The acid titration of oil was performed using potassium hydroxide solution via burette.The titration was stopped,and the required volume of KOH solution was recorded as the dark pink color appeared indicating the neutralization end-point[15].The oxidation stability is based on the acid value,fatty acid composition and viscosity of the oxidized oil.

3.Results and Discussion

3.1.Effect of solvent-to-feed(S/F)ratio on castor oil yield

The solvent-to-feed(S/F)ratio is one of the important parameters in MAE.Apositive effect of S/F ratio for MAE of castor oil was found to be at 20 ml·g-1(37%)due to a higher oil yield compared to S/F ratios of 10 ml·g-1(18%)and 40 ml·g-1(31%).The increase in S/F ratio has resulted in an increase of castor oil yield because the use of large solvent volume increases the extraction recovery.The solvent volume must be sufficient to ensure that the particles are immersed and in intimate contact with the solvent throughout the process,especially for a matrix that swells during the extraction[16–18].On the other hand,insufficient solvent volume can cause low recovery due to non-uniform distribution and exposure to microwaves.If the S/F ratio is too high,the oil yield may also decrease because more energy and time would be required to re flux the extraction solution,hence the optimum cycles for extraction could not be reached[19].

3.2.Fitting of the response surface model

The yield of castor oil by MAE at different operating conditions is summarized in Table 1.Response surface method(RSM)was employed to determine the interactions between the independent variables(power intensity and extraction time)and response(castor oil yield)in order to optimize the extraction conditions.The design of experiments(DOE)was based on historical data design.The experimental data were used to calculate the coefficients of the second order polynomial equation.The second order polynomial model showed the relationship between the independent variables of power(X1)and extraction time(X2),and the dependent variable which is castor oil yield(Y)as,

Table 1Yield of MAE-extracted castor oil at different operating conditions

The ANOVA table as shown in Table 2 is significant to determine the adequacy of the second order polynomial model by applying the F-value test at a 95%confidence level.The calculated F-value for castor oil yield is 7.219,that is greater than the tabulated F-value,F(5,6,0.05)of4.39.It indicates the significant relationship between the variables.Hence,the results of R2and ANOVA show that the second order polynomial model developed for castor oil yield is adequate and defined well the true behavior of the system.

Table 2Analysis of variance for castor oil yield

The magnitude and sign of the regression coefficients show the different influences on the response.The regression coefficient with positive sign indicates that the response is directly proportional to the independent variable while the negative sign implies that the relationship between the response and the independent variable is inversely proportional.On the other hand,the higher the magnitude for regression coefficient shows the greater influence on the response,and vice versa.As can be seen in Fig.1,the experimental data(observed value)scattered closely to the predicted value by the model.The coefficient of determination(R2)is 0.8561,suggesting that the experiment data are well fitted with the model.

Fig.1.Experimental data(observed value)versus predicted value for castor oil yield.

3.3.Effects of operating conditions on MAE-extracted castor oil yield

Fig.2 shows the response surface of the castor oil yield expressed as the function of power and time.For a constant extraction time of 5 min,the increase of power level results in the increase of castor oil yield within the range of experimental values.The increase of microwave power facilitates the rate of extraction and so enhances the extraction yield.At a constant power intensity,the oil yield also increases with extraction time,and then slightly decreases after24 min.The RSMoptimization is in line with the experimental conditions that yield maximum oil extraction at 330 W microwave power and 20 min extraction time as discussed earlier.

Fig.2.Response surface of castor oil yield expressed as the function of power and time.

The oil yield is ranging between 25.4%and 37%,and it clearly shows that the yield increased with increasing microwave power level from 160 W to 330 W.This is also true for different extraction times.Attempt has been made to use microwave power level higher than 330 W in MAE.At this operating condition,the solvent splashed beyond the condenser as a result of aggressive and uncontrolled heating.Hence,the effect of power level was limited to 330 W.

In general,the extraction yield increases proportionally with increasing microwave power level to a limit before the increase becomes insignificant or decline.A high microwave power intensity could also cause poor quality of extraction yield due to the degradation of thermal-sensible compounds.At a high power level,the purity of extracted material may reduce substantially because of rapid rupture of cell wall at a higher heating rate.As a result,the impurities also leached out into the solvent.At a low power level,the cell wall rupture might take place gradually to enable selective extraction[16].The microwaves can also accelerate the extraction through the desorption of the oil from the material matrix,and may as well induce decomposition of some target molecules if the power intensity is too high[20].

From Fig.2,for all power levels studied,the yield of castor oil increased from 5 to 20 min,and slightly decreased with the increment of irradiation time exceeding 20 min.The maximumyield of 37%of castor oil was obtained at 330 MW for 20 min.It signifies that microwave can accelerate the extraction of castor oil from castor seeds in a short period of time(20 min).Generally,as time of extraction increases,the extraction yield also increases.However,this statement is nullified for a longer extraction time.The excessive and prolong exposure to microwave may result in the degradation and oxidation of oil,leading to the decrease of oil yield even at low temperature or power level[21].

3.4.Physicochemical characteristics of MAE-extracted castor oil

3.4.1.Viscosity of castor oil

Fig.3 illustrates the effects of different power levels and extraction times on the viscosity of castor oil.The viscosity of MAE-extracted oil displayed an increasing trend when power intensity and extraction time increased.The increasing pattern of viscosity is more prevalent and occurs at a faster rate at 330 W.

Fig.3.Effects of different power levels and extraction times on the viscosity of castor oil.

The viscosity of castor oil increased gradually with increasing power intensity and extraction time.The values were recorded between 357 mPa·s and 517 mPa·s.Castor oil is among the oils of high viscosity and density in comparison with fossil fuel and other vegetable oils[22,23].It is among the viscous vegetable oils;by comparison,the viscosities of palm oil,sun flower oil and coconut oil at room temperature are 46 mPa·s,41 mPa·s and 29 mPa·s,respectively[23].

3.4.2.pH of castor oil

Fig.4 represents the effects of different power levels and extraction times on the pH of castor oil.The pH values decreased if power intensity and extraction time increased.The decrease in pH was found to be rapid at high microwave power level used.The result may be owing to the increment of free fatty acids(FFA)in oil due to auto-oxidation as the increase of microwave power intensity and extraction time is associated with the prolong exposure to high temperature[24].

Fig.4.Effects of different power levels and extraction times on the pH of castor oil.

Oxidation is a process of complex reaction whereby the oil molecules are breaking down into smaller chains,and the newly formed compounds increase the insoluble matter and oil acidity.The oxidation mechanism encompasses initiation,propagation,branching and termination[25].Nevertheless,high percentage of unsaturated fatty acid(ricinoleic acid)keeps the castor oil in liquid form even at low temperature,from which the sign of oxidation was not visibly seen.

On the other hand,the density and refractive index of castor oil are almost identical,from which no clear pattern with respect to the MAE process parameters can be observed.

3.5.Comparison of castor oil by MAE and Soxhlet extraction

3.5.1.Yield and characteristics of castor oil

Table 3 summarizes the yield and characteristics of castor oil by MAE and Soxhletextraction using a 5%ethanolin hexane as solvent.The yieldof castor oil by MAE is 37%for 20 min re fluxes compared to a 17.4%for 150 min re fluxes by Soxhlet method.The values of refractive index of castor oil by both methods are within the range of 1.476 to 1.479 according to the American Standard for Testing Materials[26].Other physiochemical properties are in the range of acceptable standards[27–29].

Table 3Properties ofcastor oil by MAE and Soxhlet extraction using a 5%ethanol in hexane as solvent(MAE conditions;power=330 W,t=20 min,S/F=20 ml·g-1)

The MAE-extracted oil is more viscous,has a lower pH value and higher acid value and viscosity compared to Soxhlet-extracted oil.However,the oxidation stability of oil extracted by MAE and Soxhlet has no significant difference.It indicates that MAE is a promising extraction method without affecting the quality of oil.

3.5.2.Fatty acid composition

Table 4 shows the composition of fatty acids in castor oil by MAE and Soxhlet extraction.There are seven leading fatty acids in castor oil,namely palmitic acid,linoleic acid,oleic acid,valeric acid,propanoic acid,ricinoleic acid and tridecanoic acid.The main fatty acid constituent in castor oil is ricinoleic acid(C18H34O3),and this is true for different types of solvents and extraction techniques used in this work.Castor oil is the only commercial source of a hydroxylated fatty acid(ricinoleic acid)and it renders the oil suitable for paints,coatings,inks,lubricants,plasticizers and cosmetics.The high level of purity by single fatty acid content making the oil unique among other naturally occurring fats and oils[30].

Table 4Fatty acid composition in castor oil by MAE and Soxhlet extraction

3.6.Oxidation stability of castor oil

The oxidation mechanism consists of initiation,propagation,branching and termination.Oxidation is initiated by the departing of proton from α-methanilic carbon to the unsaturated bond of glyceride molecule,forming the free radicals.The free radicals attack oxygen to form unstable peroxy free radicals(ROO*).Lipid molecules are then attacked by the peroxy free radicals to form hydroperoxides(ROOH)and another free radicals,thus propagating the oxidation process.Branching stage is described by the breaking down of hydroperoxide into more free radicals that trigger auto oxidation of unsaturated bonds[31].

The formation of hydroperoxides and secondary oxidation products is caused by the exposure of oil to convective and microwave heating.This often results in significantal terations to oil quality and composition[32].Normally,the oxidation of oil increased by microwave treatment because of excessive exposure to high heating rate that is also associated with high temperature.Castor oil contains polyunsaturated fatty acids,and the speed at which the oil deteriorates depends strongly on its production and storage conditions[33].

Fig.5 shows the changes in oil viscosity at fixed speed rotations when the oil was placed in oven at70°C for14 d.As the exposure period in the oven increased,the viscosity of castor oil shows an increment trend for both types of extraction methods,MAE and Soxhlet.The MAE-extracted oil experienced a dramatic increase in viscosity especially after 14 d in oven.This behavior could be due to the acceleration of castor oil oxidation,as the presence of reactive radicals are increased by the prolong exposure to heat and microwave during extraction[34,35].

Fig.5.Viscosity profiles of castor oilbefore and after storage at70°C(speed rotation=0.6 r·min-1).

Increment in oil viscosity at elevated temperature is an indication of oxidation,and such increase would result in an excess energy consumption to overcome fluid friction and heat generation in lubricating applications[36].Viscosity of Soxhlet-extracted oil also inclined as time increases.This could be interpreted as the state of propagation in the oxidation mechanism.However,after 7 d exposed in the oven,the viscosity was slightly dropped.It is believed that the oil degradation has occurred in 7 d,and the branching stage at molecular level has been completed[34].It is obvious that the MAE-extracted oil is more viscous compared to that by Soxhlet.

The acid value is an indication of oil spoilage,and represents the FFA contentdue to enzymatic activity.It can also be used to evaluate the oxidation occurrence in the oil[34,37].Fig.6 shows the acid value of castor oil before and after storage at 70°C.The acid value of castor oil showed an increment of acid concentration as the time elapsed.A high acid value indicates the presence of FFA,while a low acid value is due to high oxidative stability[38].Castor oil by MAE shows a higher acid value than that by Soxhlet extraction.This could be associated with the hydrolysis of triacylglycerols by microwaves to produce more FFA[39].Nonetheless,the acid value of castor oil still falls within the allowable limits for edible oils.An acceptable maximum level,as recommended by the international standards is 4 mg KOH·(g oil)-1[40].

Fig.6.Acid value of castor oil before and after storage at 70°C.

Fig.7 shows the pH values of castor oil before and after storage at70°C.The pH of castor oil exhibits a decreasing trend as exposure time increases,suggesting the formation of acidic oxidation products as the storage progressed from 0 to 14 d.The MAE-extracted castor oil displays a lower degree of oxidation stability at time 0 compared to Soxhlet-extracted oil due to the degradation in oil composition as a result of greater lipid oxidation under acidic medium[41].

Fig.7.pH values of castor oil before and after storage at 70°C.

3.7.Dielectric properties of oil

Fig.8 shows the dielectric properties of castor oil extracted using MAE and Soxhlet at microwave frequencies.Generally,the castor oil demonstrates insignificant change in dielectric constant(ε′)and loss tangent(tan δ)irrespective of extraction methods and MAE operating parameters.Types of material,composition,moisture content and temperature can influence dielectric properties at a given frequency[42].Rudan-Tasic and Klofutar[43]investigated the dielectric properties of eleven edible oils and reported that the dielectric constant lies in the range of 3.0–3.2(at 298.15 K),and for most oils,the dielectric constant increased with increasing unsaturation(iodine value)and decreased with increasing temperature.

Fig.4 illustrates the profiles of ε′and tan δ of castor oil at different operating conditions,and solid–solvent mixtures with frequencies.In Fig.4(a),the ε′of castor oil is between 2.8 and 4.2,which is lower than 8.0 indicating that the material is highly non-polar[44].The patterns of ε′of castor oil by different extraction techniques are nearly uniform with varying frequencies.Mathew et al.[45]claimed that the combined effect of the molecular rotations of all fatty acids present in the triglyceride molecules of oil,from which the presence or absence of any single fatty acid would not make much difference.Hence,it can be concluded that the dielectric properties of non-polar castor oil are unaffected by the small changes in oil composition,partly due to the extraction methods(Table 2).

The dielectric properties of solid–solvent mixtures before and after the extraction are closely identical to one another,having similar values to that of 5%ethanol in hexane(solvent),but lower than that of castor oil.It implies that the dielectric properties of miscible oil in solvent could not be detected,unless the solvent has been completely removed.This also implies the effectiveness of the solvent used in MAE,as the heat still can be dissipated by microwaves during the course of extraction.Fig.4 also reveals that the castor oil possesses a better microwave heating efficiency(higher tan δ)than the solid–solvent mixtures.For comparison,oils from peppermint and palm shell displayed tan δ of 0.32 and 0.12 respectively[46,47].It also indicates that there is no unpredictable dielectric behavior and aggressive heating during extraction of castor oil using MAE[48].

Fig.8.(a)Dielectric constant of castor oil at different operating conditions;(b)loss tangent of castor oil at different operating conditions.

4.Conclusions

The extraction yield increases proportionally with increasing microwave power up to a limit before the increase becomes insignificant or decline.The same pattern was also observed for the effect of extraction time on oil yield.This result is in line with the statistical findings using response surface method(RSM).The maximum yield of 37%of castor oil by MAE was obtained at 330 MW and 20 min.It signifies that microwave can accelerate the extraction of castor oil from castor seeds in a short period of time(20 min)compared to Soxhlet(150 min).However,extra care should be exercised as excessive exposure to microwave may result in the degradation and oxidation of oil even at low power intensity.The findings also suggest that there is no unpredictable dielectric behavior and aggressive heating during the extraction of castor oil using MAE.Besides that,the MAE-extracted oil is more viscous and has a higher acid value compared to that of Soxhlet.The increase of oil acid value may be attributed to the hydrolysis of triacylglycerols by microwaves to produce more free fatty acids.However,the acid values are still below the acceptable maximum level of 4 mg KOH·(g oil)-1.

Acknowledgement

N.A.Ibrahimis thankfulto the Ministry of Higher Education Malaysia for the MyMaster scholarship.References

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