Jingjing Xio,Jingzhi Wu,Yn Cho,Rukun Liu,Chngzhu Li,*,Zhihong Xio,*
a State Key Laboratory of Utilization of Woody Oil Resource,Hunan Academy of Forestry,Changsha 410004,China
b School of Chemistry and Chemical Engineering,University of South China,Hengyang 421001,China
Keywords:Cornus wilsoniana fruit oil Subcritical n-butane extraction Pressing extraction Soxhlet extraction Thermal stability Rheological property
A B S T R A C T Cornus wilsoniana fruit oil is a very important woody oil and is the main raw material of biodiesel.In this study,the oil yield,physicochemical properties,fatty acid composition,rheological properties,thermal stability,and Fourier transform infrared(FTIR)spectra of C.wilsoniana fruit oil obtained by subcritical n-butane extraction(SBE)and conventional methods such as pressing extraction(PE)and Soxhlet extraction(SE)were determined to study the influence of different extraction methods on the quality and yield of C.wilsoniana fruit oil.The oil yield of SBE(19.47%)was higher than that of PE(9.93%)but slightly lower than that of SE(21.08%).All of the extracted oils exhibited similar physicochemical properties,and the SBE oil was richer in polyunsaturated fatty acids(PUFA)than that of the PE oil,with an approximate 1:2 ratio of total saturated fatty acids against unsaturated fatty acids.The results of rheological behavior and thermal stability showed that all extracted oils had Newtonian flow characteristics,wherein the SBE oil exhibited lower viscosity and higher thermal stability.Furthermore,scanning electron microscopy(SEM)images of the surface topography indicated that different oil extraction methods will affect the residual oil content of the C.wilsoniana fruit powder.Compared with PE,the pores on the surface of the C.wilsoniana fruit powder after oil extraction were clearly visible,indicating that the driving force of SBE for oil extraction is stronger than that of PE.Based on the above results,it is implied that SBE is the best of the three methods for extracting C.wilsoniana fruit oil and can be potentially applied to extract other edible oils.
Cornus wilsoniana,commonly known as Wilson"s dogwood,is a member of the Cornaceae family[1].This plant is an important ecological and economic tree species in China and is also a rare,high-quality and multi-purpose woody oil species with high yield and oiliness.The fruit ofC.wilsonianais spherical in shape with a diameter of 7 mm and contains 10%-17% oil;its sarcocarp contains 55%-59%oil.[2].C.wilsonianafruit oil has been used as edible oil for more than 100 years and contains mainly three fatty acid compositions:palmitic acid(～30%),oleic acid(～30%),and linoleic acid(～30%)[3,4].Recently,C.wilsonianafruit oil has been widely developed as a highquality biodiesel product and has been extensively studied as a renewable biofuel feedstock[5].
The extraction method is a key factor affecting the yield and quality ofC.wilsonianafruit oil.Commonly used methods for oil extraction are pressing and chemical solvent methods[6,7].However,the pressing extraction(PE)method has the disadvantages of high residual oil content(＞6%)and local high temperature caused by blockage of the pressing chamber by the plant sample[8].Among solvent extraction techniques,the Soxhlet extraction(SE)method usingn-hexane as the solvent has been employed widely owing mostly to its benefit of high oil yield.However,functional components in the plant fruit are destroyed by the high temperatures present in the process of oil and meal desolventization during SE,and harmful components such as benzopyrene,trans-fatty acids,polycyclic aromatic hydrocarbons,and glycidyl esters are produced[9].Modern extraction techniques such as supercritical CO2extraction have also been explored for oil extraction.Compared with chemical solvent extraction,supercritical CO2extraction has a wide range of applications owing to its advantages of wide availability,environmental safeness,and non-toxicity[10,11].However,supercritical CO2extraction is not suitable for industrial application owing to its low processing capacity,long extraction time,high production cost,high pressure requirement,and low triglyceride solubility[12].
Recently,subcritical butane/propane extraction methods have been developed with several advantages in addition to those included in all conventional technologies[8,13-15].Many investigations have demonstrated that subcritical butane/propane extraction is attractive,efficient,and feasible for extracting plant oils[16-18].Furthermore,the operation conditions of subcritical butane/propane extraction are milder with lower temperature and pressure requirements,which guarantees oil quality with minimal damage[19].Subcritical butane/propane extraction has gradually become a dominant technology in the field of oil production,including that from plant sources[20,21].
In this work,C.wilsonianafruit oil was produced using PE,subcriticaln-hexane extraction(SBE),and SE,and the yields and physicochemical properties of the oils obtained by the three methods were compared.In addition,thermogravimetric(TG)-differential scanning calorimetry(DSC)and Fourier transform infrared(FTIR)spectroscopy were employed to reveal slight differences in the compositions of oil extracted by PE and SBE.Moreover,scanning electron microscopy(SEM)was employed to reveal the morphologies ofC.wilsonianafruit powders before and after oil extraction.The feasibility and related indicators of the different oil extraction methods for the preparation ofC.wilsonianafruit oil were then evaluated.This research will provide a theoretical basis and technical practice for extraction of other edible oils.
2.1.Materials
TheC.wilsonianafruit,harvested in November 2021,was obtained from the Experimental Forest Farm of Hunan Academy of Forestry(Changsha,Hunan Province,China).Fresh mature fruits were selected and then dried in a hot air oven at 80°C for 12 h to obtain dry fruit(moisture content～8%).Finally,the dried fruits were packed and stored for further treatment.
Food-graden-butane was purchased from Puyang Red Sun Materials Co.,Ltd.,(Puyang,Henan province,China).All other reagents were of analytical reagent grade obtained commercially with no further purification.
2.2.C.wilsoniana fruit oil extraction
TheC.wilsonianafruit oil was obtained by PE,SBE,and SE.The oil extraction yield was calculated according to following formula:
wheremoandmfare the mass of the extracted oils and theC.
wilsonianafruit,g.
2.2.1.PE method
The dry fruits were fed to a screw press through a hopper on demand by gravity at a screw speed of 50 r/min using a KOMET PE producer(CA59G,IBG Monforts,Mönchengladbach,Germany).Then,the oil was obtained at an outlet temperature of 45°C.
2.2.2.SBE method
SBE was performed using laboratory equipment(Fig.1)purchased by Henan Subcritical Biological Technology Co.,Ltd.(Anyang,China).Before extraction,n-butane was pumped into the vaporizing tank of the subcritical extraction equipment under vacuum for reserving.The fruits were crushed by a swing grinder(DFY-500,Shanghai Xinnuo Instrument Group Co.,Ltd.,China).Then,1400 g of the fruit powder was transferred to the extractor,and 3000 mL ofn-butane was added at a sample-to-n-butane ratio of about 1:2(g/mL).The extraction process was conducted at 45 °C and 0.5 MPa.After 40 min,then-butane was removed to a separator by reduction vaporization,and the extractedC.wilsonianafruit oil was weighed and stored in a stoppered glass bottle for further analysis.The same process was repeated three times to ensure maximum extraction,and the final products were combined after three extractions.
2.2.3.SE method
SE was conducted in a solvent extractor(SZF-06A,Shanghai Xinjia Electron Co.,Ltd.,Shanghai).Briefly,10 g of theC.wilsonianafruit powder was placed in 60 mL ofnhexane at 55°C and was held for 6 h.After exhaustive extraction,n-hexane was recovered,and the extraction oil was collected and weighed.The extractions were performed in triplicate,and the results were obtained as mean value±standard deviation.Because SE is not suitable for large-scale oil production,only PE and SBE were used in subsequent experiments except for physicochemical properties and extraction yield analysis.
Fig.1.Subcritical n-butane extraction equipment.
2.3.Qualitative and quantitative analyses of C.wilsoniana fruit oil
2.3.1.Gas chromatograph(GC)analysis
The fatty acid compositions ofC.wilsonianafruit oil were measured by a GC instrument(GC-2010 Plus,Shimadzu,Kyoto,Japan)equipped with a flame ionization detector and an HP-88 capillary column(100 m×0.25 mm×0.20 μm;Agilent Co.,Santa Clara,California,USA).Prior to the GC analysis,the oil fraction of the fruit was converted into its fatty acid methyl ester as described by the GB5009.168-2016 standard.The fruit oil samples were then diluted withn-heptane,and GC analysis was conducted as described in GB/T GB5009.168-2016.The area normalization method was used to determine the fatty acid content,and the relative percentage of each fatty acid composition was calculated by dividing its peak area by the total area.
2.3.2.Determination of physicochemical properties
American Oil Chemists"Society(AOCS)standard methods Cd 3-25,Cd 8b-90,and Cd 3d-63,and Cd 1d-92 were used to determine the saponification value,peroxide value(POV),acid value(AV),and iodine value,respectively.The refractive index was tested at room temperature(25 °C)using a refractometer(Abbe 60,Bellingham and Stanley,Ltd.,Tunbridge Wells,Kent,UK).
2.3.3.FTIR analysis
The functional groups of theC.wilsonianafruit oil were tested using an FTIR spectrometer(Nicolet,Thermo Scientific,Madison,Wisconsin,USA)with an attenuated total reflectance(ATR)device.In the experiment,a droplet of the fruit oil was placed on the detector.The spectra recording range was 500-4000 cm-1,and the resolution was 4 cm-1.
2.3.4.Thermal analysis
Simultaneous thermal analysis of theC.wilsonianafruit oil was conducted using a TG analyzer(STA 449 F3 Jupiter,NETZSCH,Selb,Germany)with aluminum crucibles under a nitrogen atmosphere to obtain the TG,first derivative of the TG(DTG),and DSC data.The weight of each sample was about 15 mg.The experiment was conducted at 50-650°C at a constant heating rate of 10°C/min.
2.3.5.Rheological measurements
The rheological characteristics of theC.wilsonianafruit oil were determined using a rheometer(HAAKE MARS 60,Anton Paar Germany,Ostfildern,Germany)equipped with a 27-mm(inner diameter)cylinder.The rheological data were collected using Rheoplus software(Version 3.21,Anton Paar,Graz,Austria).The relationship between viscosity and shear stress was plotted at 25°C in a shear rate range of 0.1-1000 s-1.To investigate the effect of temperature,the viscosity was plotted against the temperature at a heating rate of 2°C/min and a shear rate of 50 s-1over a temperature range of 5-85 °C.The Arrhenius equation was used to fit the obtained data as where μ is viscosity(Pa·s),Ais the Arrhenius constant,Eais activation energy(kJ/mol),Ris the universal gas constant(8.314 J·mol-1·K-1),andTis temperature(K).
2.4.Characterization of C.wilsoniana fruit powder
An FTIR spectrometer was employed to determine the chemical nature of theC.wilsonianafruit powder before and after extraction using the ATR device over a spectral range of 500-4000 cm-1.
The morphology of the fruit powder was observed by SEM(QUANTA 200,FEI Co.,Hillsboro,Oregon,USA).In the experiment,the fruit powder samples before and after extraction were coated with a gold-palladium alloy through gold sputtering.
3.1.Physicochemical properties and extraction yield of C.wilsoniana fruit oil
As shown in Table 1,the extraction yields of theC.wilsonianafruit oil obtained by PE,SBE,and SE were 9.93%,19.47%,and 21.08%,respectively.These results demonstrated that SE had the highest extraction efficiency.Moreover,the residual oil content(1.54%)in the fruit powder after SE was lower than those after SBE(1.87%)and PE(11.15%).Similar results were observed by Shuai et al.[22],which showed that the extraction rate of macadamia oil byn-hexane was higher than that by other methods.In addition,the PE method has a certain effect on the quality of oil,as reported previously[23].Therefore,the effects of the different extraction methods on theC.wilsonianafruit oil quality were further investigated.
The physicochemical properties of theC.wilsonianafruit oil obtained by the three methods were shown in Table 1.The POV and AV of the oils showed significant differences,at 0.29-0.49 mmol·kg-1and 3.35-3.80 mg KOH·g-1,respectively.The AV values were very close to the value of 3.75 mg KOH·g-1measured in our previous study[4],which indicated that different oil extraction methods had little effect on AV values.The small POV values indicated that only small amounts of hydroperoxides and peroxides were generated in the fruit oil during extraction.The POV value of oil obtained by SBE is smaller than that of PE,implying that SBE oil is more stable than PE oil and has a longer shelf life.Furthermore,the oils extracted by the three methods had similar refraction index,iodine value,and saponification value.
3.2.Fatty acid composition
As shown in Table 2,the main fatty acids in the PE and SBE oils were composed of linoleic acid(C18:2,35.90%-39.40%),palmitic acid(C16:0,31.00%-35.80%),oleic acid(C18:1,22.26%-27.80%),palmitoleic acid(C16:1,1.60%-2.10%),linolenic acid(C18:3,0.615%-2.10%)and stearic acid(C18:0,0.216%-0.993%).The ratio of the total amount of saturated fatty acids(SFA)to unsaturated fatty acids(UFA)was about 1:2.The results displayed that theC.wilsonianafruit oils was mainly composed of unsaturated fatty acids.The fatty acid contents in PE and SBE oils were not significantly different,indicating that the SBE method had little effect on the fatty acid composition of theC.wilsonianafruit oils,which was consistent with the previous results of physicochemical properties.At the same time,compared with the previous results[4],the fatty acid composition was similar but the content was different,which might be related to the variety,maturity,growth conditions,and geographical location,and did not affect the limitation of different extraction methods on theC.wilsonianafruit oils.
Table 1Extraction yield and physicochemical properties of C.wilsoniana fruit oil extracted by PE,SBE and SE.
Table 2Fatty acid compositions(mg/g)of C.wilsoniana fruit oils obtained by PE and SBE.
3.3.Rheological behaviors
The rheological behaviors of theC.wilsonianafruit oil extracted by PE and SBE at 25°C were listed in Fig.2.All extracted oils showed similar flow patterns,and their shear rate and shear stress values had a linear relationship(Fig.2(A)),which demonstrated that theC.wilsonianafruit oil containing long chain molecules had Newtonian flow characteristics.These results were consistent withthose reported for other plant oils such as yellow horn seed oil and macadamia oil[22,24].Furthermore,the viscosities ofC.wilsonianafruit oil calculated by the slopes of the flow curves were 68.48 mPa·s and 53.61 mPa·s for the PE and SBE oils respectively,demonstrating that the latter had a lower viscosity than the former in this study.The lower viscosity reflected the higher content of polyunsaturated fatty acids(PUFA)[25];the total content of linoleic and linolenic acids in the SBE oil was higher than that in the PE oil.
The effects of temperature on the viscosity of theC.wilsonianafruit oil extracted by the three methods were shown in Fig.2(B).As the temperature increased,the viscosity of the oil decreased,with all types showing similar patterns.This phenomenon occurred because the increase in temperature intensified the movement of molecules in the oil,reduced the interaction between the molecules,and destroyed the aggregates in the oils[24].The SBE oil had lower viscosity from 5°C to 85°C,which agreed with the results obtained by steady shear[22].The activation energy ofEafor theC.wilsonianafruit oil extracted by the different methods was calculated by using the Arrhenius equation.The values of ln(viscosity)versus 1/Tplotted in Fig.2(C)revealed a good linear relationship(R2＞0.99).Furthermore,theEaof the oils of PE and SBE methods was calculated to be 28.19 kJ/mol-1and 25.50 kJ/mol-1(Table 3),respectively.The PE oil had a higherEavalue,indicating that it was more sensitive to temperature than the SBE oil.Similar findings were reported by other researchers[24].Therefore,we concluded that the processing method affected the rheological properties of theC.wilsonianafruit oil.
Table 3Arrhenius model parameters of the Cornus wilsoniana fruit oils obtained by PE and SBE.
3.4.Thermal stability
Fig.2.Rheological behaviors of C.wilsoniana fruit oils obtained by PE and SBE.
The thermo-oxidative processes occurring in theC.wilsonianafruit oil as a function of temperature and mass were explained by the TG,DTG,and DSC.In nitrogen atmosphere,as shown in Fig.3,the TG and DTG curves of all samples extracted by PE and SBE during dry heating were similar.For the two oil samples,three main stages inthe degradation process were observed.The onset of weight loss(about 2%)occurred at 218°C and 212°C for the fruit oil extracted by PE and SBE in the first stage,respectively.Then,a second stage with 50% of mass loss occurred at 405°C and 408°C,and a third and more severe mass loss stage(＞98%)occurred at 451°C and 464°C,respectively.The mass loss in the three stages were mainly caused by the degradation of PUFA,monounsaturated fatty acids(MUFA),and SFA followed by volatilization and pyrolysis of the polymer products[26].
Fig.3.Simultaneous thermal analysis TG-DSC of the C.wilsoniana fruit oils obtained by PE(A)and SBE(B).
The DTG curve can provide accurate information about each thermal degradation stage,such as the reaction termination temperature,maximum reaction rate temperature,and maximum temperature.The maximum temperature of the oil was directly proportional to the content of MUFA,SFA,and representative fatty acids and was inversely proportional to the content of PUFA[27].Because the mass loss was caused by the release of highly volatile components with high moisture content,the temperature with a weight loss of 2%was defined as the highest temperature[8,28].The maximum temperature(218°C)and spike peak value(15.4%)of theC.wilsonianafruit oil obtained by SBE were lower than those obtained by PE,indicating that the PE oil contained fewer small molecules than the SBE oil.A sharp reduction in the mass of all oils occurred from 310 °C to 490°C,which is presumably attributed to the breakdown of heavier molecular hydrocarbons to simpler and lighter CO,CO2,and hydrocarbon molecules[29].
The DSC curves showed the thermal degradation stages of theC.wilsonianafruit oils.Very obvious exothermic absorption occurred in the temperature range of 50-650°C and peaked at about 425°C,which might be attributed to the oxidation of lipid molecules forming peroxides that quickly decomposed into further products[27].
3.5.FTIR analysis
Fig.4.FTIR spectra of C.wilsoniana fruit oils obtained by PE and SBE(A)and C.wilsoniana fruit powder before and after extraction(B).
The spectroscopic behaviors of theC.wilsonianafruit oils obtained by PE and SBE were investigated using FTIR spectroscopy.As shown in Fig.4(A),the FTIR spectra of the fruit oils extracted by the two methods were strongly similar.The peaks at about 3010 cm-1are attributed tocis-olefinic-HC=CH-bond.Its position depended intensely on the number of conjugated C=C bonds in the fatty acids[24],which are generally used to detect UFAs.The strong peaks at 2922-2852 cm-1corresponded to the C-H stretching of-CH3and-CH2in the fruit oil[22].Moreover,a peak at about 3470 cm-1occurred in all FTIR spectra in relation to the O-H stretching vibration present in proteins,carbohydrates,and fatty acids[30].For the lowwavenumber regions of all FTIR spectra,a very strong peak occurred at 1743 cm-1.This was attributed to the C=C carbonyl stretching of lipid and fatty acid ester groups,which reflected the total numbers of lipids in theC.wilsonianafruit oil[24].Moreover,a weak peak appeared at 1653 cm-1that corresponded to UFA C=C stretching[31,32].Peaks occurring at 1466 cm-1and 1373 cm-1represented the combination of the methyl and methylene group deformation modes[24].In addition,two peaks occurring at 1169 cm-1and 1105 cm-1are attributed to the combined asymmetric stretching of C-C(=O)-O and O-CC bonds and C-O-C symmetric stretching of triglycerides and cholesterol esters,respectively.Finally,a strong peak occurred in all FTIR spectra in relation tocis-C=C out-ofplane bending at 720 cm-1[33].
To investigate the oil extraction efficiency of PE and SBE methods,the FTIR spectra of theC.wilsonianafruit powder before and after oil extraction were evaluated,as shown in Fig.4(B).Before extraction,peaks occurred at 3010,2926,2857,1743,and 1156 cm-1,which corresponded to the stretching vibrations of the functional groups in the fruit oil appearing in the FTIR spectra of the fruit powder.This is attributed to the richness of oil in theC.wilsonianafruit powder.After PE,the FTIR characteristic absorption peaks of the fruit oil remained in the fruit powder,which indicated that oils were still present in the fruit powder.When the fruit powder was treated directly by SBE,the FTIR characteristic peaks of the fruit oil were markedly weaker in the powder,which indicated that the content of the residual oil in the extracted powder was much lower,and the extraction efficient of SBE was higher than PE.
3.6.Surface characteristics of C.wilsoniana fruit powder before and after extraction
SEM images showed the surface topography of theC.wilsonianafruit powder(Fig.5).As shown in Fig.5a-c,the color of the fruit powder before extraction was grayishblack,and the oil content was as high as 22.68%.However,after PE or SBE,the color changed to brown or light gray,and the residual oil content was reduced to 11.15% or 1.87%,respectively.The calculation results of the residual oil content were consistent with these FTIR analysis results.This phenomenon demonstrated that the different extraction methods affected the oil content,color,and morphology of the fruit powder.
Fig.5.Morphology of C.wilsoniana fruit powder before extraction(a,d)and after extraction by PE(b,e)and SBE(c,f).
SEM was employed to detect microscopic changes in the fruit powder before and after extraction(Fig.5d-f).Before extraction,the SEM image of the fruit powder showed that a large amount ofC.wilsonianafruit oil bound the fruit powder together(Fig.5d).The crushing process decreased the diameter of the fruit and increased the contact surfaces,which facilitated the mass transfer of the fruit oil.After PE treatment,the fruit oil and powder were still bound together(Fig.5e),and the oil content retained high.This may be attributed to the fact that oil from the fruit of theC.wilsonianatree was pressed to the outside under external force.Meanwhile,Fig.5b showed that the pressed cake contains larger-sized particles,indicating that the overall structure of theC.wilsonianafruit was destroyed by pressing,and the oil was squeezed out from the inside of the fruit under external mechanical force.However,due to the difference in the direction and uniformity of the force exerted on the fruit during the pressing process,the cell walls of theC.wilsonianafruit were not completely destroyed,resulting in oil remaining.The SEM images of Fig.5f clearly showed significant structural changes in the fruit powder obtained by the SBE method,with oil disappearance and the development of holes on the surface.These holes might be related to the release of lipid material as the extraction proceeded.These SEM results were consistent with the morphology analysis of theC.wilsonianafruit powder.
In this study,the yield and physicochemical properties of oil extracted fromC.wilsonianafruit by SBE were compared with those obtained by conventional PE and SE methods.The order of oil yield was SE＞SBE＞＞PE,and the physicochemical properties of oils extracted by three different methods were similar.Owing to their applicability to large-scale production,SBE and PE were selected for subsequent analysis.
The fatty acid composition,rheological behavior,thermal stability,and infrared spectroscopy of theC.wilsonianafruit oil extracted by SBE and PE were compared.The PUFA content of the SBE oil was higher than that of the PE oil,with an approximate 1:2 ratio of total saturated fatty acids against unsaturated fatty acids.The results of rheological behavior and thermal stability experiments indicated that the SBE oil had lower viscosity and an activation energy value of 25.50 kJ·mol-1with better thermal stability.Moreover,infrared spectra analysis revealed similar characteristic functional groups in the two oil samples.
SEM images revealed light-gray surface topography in theC.wilsonianafruit powder extracted by SBE,and the oil content was lower than that obtained by crushing or pressing.Therefore,SBE is an effective method for extracting oil fromC.wilsonianafruit and can be applied for extraction of other edible oils.
Author Contributions
Jingjing Xiao:Investigation,Methodology,Supervision,Data curation,Writing-original draft;Jingzhi Wu:Investigation,Supervision,Data curation;Yan Chao:Investigation,Software,Validation;Rukuan Liu:Investigation,Supervision;Changzhu Li:Investigation,Methodology,Supervision;Zhihong Xiao:Investigation,Supervision,Validation,Data curation.
Conflicts of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to thank the National Key Research and Development Projects of China(2019Y FB1504001)for the financial supports of this research.
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