食用油检测掺杂加热

Analytical MethodsAuthentication of edible vegetable oils adulterated with used frying oil by Fourier Transform Infrared SpectroscopyQing Zhang,Cheng Liu,Zhijian Sun,Xiaosong Hu,Qun Shen ⇑,Jihong Wu ⇑National Engineering &Technology Research Center for Fruits and Vegetables Processing,Key Laboratory of Fruits and Vegetables Processing,Ministry of Agriculture,College of Food Science and Nutritional Engineering,Box 112,China Agricultural University,No.17,Qinghua East Road,100083Beijing,Chinaa r t i c l e i n f o Article history:Received 16November 2010Received in revised form 5May 2011Accepted 30November 2011Available online 8December 2011Keywords:Edible vegetable oils Used frying oil AdulterationFourier Transform Infrared Spectroscopy Cluster analysisDiscriminant analysisLinear regression analysisa b s t r a c tThe application of Fourier Transform Infrared (FTIR)Spectroscopy to authenticate edible vegetable oils (corn,peanut,rapeseed and soybean oil)adulterated with used frying oil was introduced in this paper.The FTIR spectrum of oil was divided into 22regions which corresponded to the constituents and molec-ular structures of vegetable oils.Samples of calibration set were classified into four categories for corn and peanut oils and five categories for rapeseed and soybean oils by cluster analysis.Qualitative analysis of validation set was obtained by discriminant analysis.Area ratio between absorption band 19and 20and wavenumber shift of band 19were treated by linear regression for quantitative analysis.For four adulteration types,LODs of area ratio were 6.6%,7.2%,5.5%,3.6%and wavenumber shift were 8.1%,9.0%,6.9%,5.6%,respectively.The proposed methodology is a useful tool to authenticate the edible veg-etable oils adulterated with used frying oil.Ó2011Elsevier Ltd.All rights reserved.1.IntroductionDeep-fat frying is a welcome food processing method and has been prevailed for centuries.Palatable fried products and time-saving process course are the two main reasons for its prevalent-ness in people’s life.Nevertheless,many harmful reaction prod-ucts,such as trans configuration (Tsuzuki,Matsuoka,&Ushida,2010),polymers (Choe &Min,2007),etc.are produced during the deep-fat frying course and then exist in the products.The more deep-fat frying times conduct,the more harmful to consumers’health.How to deal with these used frying oil (UFO)is a big prob-lem for the industry.The UFO is supposed to be a very good renew-able resource like the raw material of biodiesel,but now it becomes one of the threats of people’s health.This is because that some unscrupulous traders add it to qualified edible vegetable oils just for high profits while the government does not have the corre-sponding legal regulations.This problem is particularly serious in some areas.Adulteration of edible oil has been a chronic illness in food adul-teration for many years.It not only causes a potential harm or threat to the health of consumers,but also undermines the integ-rity and orderly economy.There are many adulteration ways,forexample,high priced oil adulterated with lower priced oil,edible oil adulterated with non-edible oil and qualified vegetable oils adulterated with waste cooking oil.Fortunately,though the adulteration means become compli-cated and diverse,the corresponding detection techniques go bet-ter and are improved faster which have benefited from the development and application of advanced instrumentation.Fast,non-destructive,non-polluting methods of instrumental analysis have replaced the traditional chemical titration in the concept of modern detection means.Therefore,there are many references which focused on the edible oil adulteration.So far,there have been lots of methods such as nuclear magnet resonance (Agiomyrgianaki,Petrakis,&Dais,2010;Rezzi et al.,2005),dielectric spectroscopy (Lizhi,Toyoda,&Ihara,2010),gas chromatography (Al-Ismail,Alsaed,Ahmad,&Al-Dabbas,2010;Brodnjak-Voncina,Kodba,&Novic,2005),high performance liquid chromatography (Cunha &Oliveira,2006;Park,Chang,&Lee,2010),mass spectrometry (Lerma-Garcia,Herrero-Martinez,Ra-mis-Ramos,&Simo-Alfonso,2008;Vaclavik,Cajka,Hrbek,&Haj-slova,2009),fluorescence spectroscopy (Poulli,Mousdis,&Georgiou,2007;Sikorska,Gorecki,Khmelinskii,Sikorski,&Koziol,2005),near infrared spectroscopy (Downey,McIntyre,&Davies,2002),mid-infrared spectroscopy (Gurdeniz &Ozen,2009;Vlachos et al.,2006),Raman spectroscopy (Zou et al.,2009),differential scanning calorimetry (Angiuli et al.,2009;Ferrari et al.,2007),etc.Meanwhile,it had obtained satisfactory adulteration detection0308-8146/$-see front matter Ó2011Elsevier Ltd.All rights reserved.doi:10.1016/j.foodchem.2011.11.129⇑Corresponding authors.Tel.:+861062737524;fax:+861062737392(Q.Shen),tel.:+86106273743416;fax:+86106273743412(J.Wu).E-mail addresses:shenqun@ (Q.Shen),wjhcau@ (J.Wu).effectiveness by combining with chemometrics analysis to process the data of these instrumental techniques.These chemometrics in-clude principal component analysis(Lizhi et al.,2010),linear dis-criminate analysis(Rezzi et al.,2005),partial least square (Gurdeniz,Ozen,&Tokatli,2010),multiple linear regression(Ler-ma-Garcia et al.,2008),counterpropagation artificial neural net-works(Brodnjak-Voncina et al.,2005),etc.All these methodologies were of extremely important significance in the field of edible oil study.FTIR is one of the best instruments of detection and has been interested by many researchers from the beginning of discriminat-ing work.Fast spectrum acquisition,easy to operate and needing no complex sample preparation are its advantages(Maggio et al., 2009).FTIR was used to authenticate hazelnut oil mixed with dif-ferent types of oils,the spectral data was analysed by discriminant analysis and partial least-squares analysis.The detection level and correlation coefficient for the PLS model were2%,0.99,respectively (Ozen&Mauer,2002).In addition to its application in the detection of edible oil adul-teration,FTIR was also used in the studies of thermal stability of edible oils.(Moros,Roth,Garrigues,&de la Guardia,2009;Pinto, Locquet,Eveleigh,&Rutledge,2010).These studies proved the method was a promising research approach for the rapid analysis of the thermal degradation of oils.The oxidised fatty acid concen-tration in virgin olive was obtained by analysing the result of FTIR spectra using multiple linear regression(Lerma-García,Simó-Al-fonso,Bendini,&Cerretani,2011),the method was effective and the result showed satisfactorily.When the edible oils were heated at high temperature for a long time,the amount of trans fatty acids increased(Tsuzuki et al.,2010),which could be the basis for the authentication of edible vegetable oils adulterated with UFO.Due to the lack of reports that focused on the authentication of edible vegetable oils adulterated with UFO,the aim of this work was to develop and validate an analytical method based on FTIR spectroscopy,in conjunction with multivariate calibration meth-odologies,for the qualitative and quantitative analysis of the edible vegetable oils adulterated with UFO.2.Experimental2.1.MaterialsThe test materials used during the study were corn oil(CO), peanut oil(PO),rapeseed oil(RO),soybean oil(SO)and UFO.The former four oils were purchased from local supermarket;they were all pure and qualified products.The UFO was collected from the sales stand of twisted cruller of the local street,its origin was mainly soybean oil.In this study,in order to get the deep oxidation of the collected UFO we simulated the frying process of twisted cruller forfive times and each time it cost2h.Potassium bromide(KBr)(Chemical Reagent Beijing Co.,Ltd.) that was used to compress a KBr pellet was analytically pure.2.2.ApparatusThe instrument used for the spectrum acquisition was a Perkin Elmer(precisely)100FTIR spectrometer(Perkin Elmer Corpora-tion,Norwalk,CT,USA)equipped with a room temperature deuter-ated triglycine sulphate(DTGS)detector and interfaced to a personal computer operating with Windows-based IR Spectros-copy Version6.1.0(PerkinElmer,Inc.).The apparatus adopted to acquire the KBr pellet was an YP-2tablet press(ShanYue Scientific Instrument Ltd.,Shanghai,China)attached with a disc notch mod-el.An oven(Binder ED115,Germany)was used to dry the KBr.2.3.Sample arrangementAccording to different adulteration proportion of the qualified edible vegetable oils mixed with UFO,two sets had been arranged: calibration set and validation set.For the calibration set,the adul-teration proportions were0%,1%,2%,3%,4%,5%,6%,7%,8%,9%,10%, 20%,30%,40%,50%,60%,70%,80%,90%and100%.For the validation set,twelve samples of different adulteration proportions for each qualified vegetable oil mixed with UFO were arranged.The codes of these twelve samples were letters of A,B,C,D,E,F,G,H,I,J, K,L.2.4.Spectra acquisitionKBr(powder)was dried at130°C for4h,and then used to make the KBr pellet by the tablet press.KBr of0.3g was accurately weighed to the notch of the model;pressure was set at27MPa for4–5min.Afilm of small amount of oil sample(approximately 2l L)was daubed evenly on a side of the homemade smooth disc of KBr.It used the smooth KBr pellet as the background before ob-tained the spectra of samples.The spectra were recorded from 4000to450cmÀ1,the number of scans being1at a resolution of 4cmÀ1.Spectrum acquisition of each sample repeated three times in the same condition.The operation of infrared spectra acquisition was performed under the ambient temperature.2.5.Data treatment and statistical analysisAll graphs and data treatments were obtained by Origin7.5 (OriginLab corporation,Northampton,England)and SPSS17.0 (SPSS corporation,Chicago,USA)respectively.2.5.1.Data treatmentFor knowing well about the relation between FTIR spectrum of vegetable oil and its chemical component structures and achieving the convenient analysis of FTIR spectrum,the work divided the en-tire infrared absorption area into22regions and all the specific information,including absorption range and corresponding func-tional group and vibration mode,was showed in Table1,whose train of thought was derived from former study(Lerma-García et al.,2011)and target was to know the ralationship between chemical constitution of vegetable oils and their FTIR spectrum.The area of each absorption peak or shoulder was calculated by IR Spectroscopy Version6.1.0.According to the change of each area,the smallest area change(standard deviation(SD)was 0.0048,the minimum among the22SDs)of band14was selected as the divisor,dividing another21areas.So there were21area ra-tios,which were used as the parameters for the cluster analysis.2.5.2.Qualitative analysisFor achieving qualitative analysis of the adulteration,cluster analysis was used.This work was accomplished by the calibration set.The feasibility of this methodology was validated according to discriminatory analysis by validation set.2.5.3.Quantitative analysisQuantitative analysis was carried out by remarkable area ratio between two absorption peaks and wavenumber shift of a typical absorption band.This work was completed according to construct linear regression(LR)equations by the calibration set.1608Q.Zhang et al./Food Chemistry132(2012)1607–16133.Results and discussion3.1.Qualitative analysis of adulterationThe FTIR spectra offive pure oils were presented in Fig.1-a. There were less absorption bands in functional groups region (4000–1650cmÀ1),but more complicated absorption profiles in fingerprint region(<1650cmÀ1).As Fig.1-a showed,there were not any significant differences among thefive spectra,in other words,absorption band positions and absorption intensities of the same wavenumber were very similar.Nevertheless,subtle dis-crepancies among these spectra did exist.As long as the spectra were observed carefully,small differences about the absorption band position and absorbency intensity of the same band ap-peared.When the absorption band was amplified,the differences emerged as shown in Fig.1-b.Many complex chemical reactions(hydrolysis,oxidation and thermal reaction etc.)happen during the process of food frying, especially in the repeated and long time deep frying.Accord-ingly,many kinds of products are produced.The amount of free fatty acids,mono-and di-acylglycerols,and glycerols is increased by hydrolysis in UFO.Oxidation produces hydroperoxides and then low molecular volatile compounds such as aldehydes,ketones,carboxylic acids,and short chain alkanes and alkenes. Triacylglyceride monomers,Dimers and polymers,cyclic and epoxy compounds are result from thermal and cyclisation reac-tions(Sahin&Sumnu,2009).When UFO was added into the qualified vegetable oils,the chemical constitution of qualified oil was changed.These changes in constituent might generate the FTIR spectrum diversities between the UFO and qualified vegetable oils.At the basis of the above-mentioned comparisons among the spectra offive oil samples,a series of adulteration proportions be-tween UFO and four qualified oils were carried out.Soybean oil adulterated with UFO was used as the instance to elucidate the adulteration discrimination.The FTIR spectra of different adultera-tion proportion samples were showed by Fig.1-c.It was hard to find any significant differences among the curves in Fig.1-c.For thoroughly utilising the information performed by the infrared spectrum of oil and fat,the FTIR spectrum of each sample(calibra-tion and validation set)was divided into22regions for the quali-tative analysis.The two-step cluster was chosen to classify the adulteration proportions.After the clustering method,discriminant analysis was used by the validation set on the basis of the cluster results. The classifications of other three vegetable oils adulterated with1610Q.Zhang et al./Food Chemistry132(2012)1607–1613UFO were resolved in the same procedure.The results of clustering and classifying were listed in Table 2.The entire classified catego-ries were the optimised classification.As displayed in Table 2,the cluster results were acceptable basi-cally,except for that of the PO +UFO and RO +UFO.For PO +UFO,samples with the proportions of 0%,1%,2%and 9%were assigned to the first category while with the proportions of 3,4,5,6,7,8(<9%),10,and 20%were assigned to the second category.For RO +UFO,samples with the proportions of 1%,2%,and 7%were assigned to the first category,but those of 3,4,56(<7%),8,9,10,and 20%were assigned to the second category.For CO +UFO,samples with the proportions of less than 6%were assigned to the first category.So it was hard to discriminate the adulteration when the propor-tion was low.This may attribute to the similar fatty acids compo-sition and the nearly the same amount of some fatty acids in vegetable oils (Sherazi,Talpur,Mahesar,Kandhro,&Arain,2009).For SO +UFO,adulteration could be distinguished when the pro-portion is as low as 2%,which was a satisfactory cluster result.When it came to the results of validation set by discriminant analysis,it showed there were some erroneous judgements in the four mixed ways.However,the classification results showed all the samples were differentiated besides sample A and D of CO +UFO and sample A and B of PO +UFO.In summary,the detect-abilities of the four adulteration ways were 83.33%,83.33%,100%and 100%,respectively.In other words,by means of this method,qualitative analysis of vegetable oils authentication may get an effective result.3.2.Quantitative analysis of adulterationThrough the observation of Fig.1-d,the absorption intensity of the spectrum increased accompanied with the increase of adulte-ration proportion except 2%.After comparing the areas of band 19and 20and wavenumber of band 19of different proportions,it could be found that:(1)the area of band 19increased and the area of band 20decreased while the adulteration proportion in-creased.(2)When the adulteration proportion increased,the peak’s wavenumber of band 19shifted toward the higher wave-number position.With the increase in the number of frying times,a series of chemical reactions took place in vegetable oils,including the transformation from cis-configuration to trans-configuration (Tsuzuki et al.,2010).That’s to say,the amount of trans-configura-tion increased and the amount of cis-configuration decreased.Meanwhile,along with the increase of the amount of trans-config-uration,the absorption of trans-configuration in the mid-infrared spectrum needed a higher wavenumber.For further quantitative analysis of adulteration,these two above-mentioned points were used to construct linear regression equations by the calibration set.Fig.2showed that there were sig-nificant linear correlations between the selected area ratio (A19/A20),the wavenumber shift of band 19and adulteration propor-tion of four mixed ly,with the increase in the propor-tion of adulteration,both A19/A20and wavenumber of band 19increased.The limit of detection (LOD)was obtained according to the following equation:LOD =(Y LOD Àintercept)/slope,where:Y LOD =intercept +3Sb,Sb means standard error of the regression statistics (Vlachos et al.,2006).All results of linear regression anal-ysis were listed in Table 3.In Table 3,it was interesting to notice that there were highest coefficient of determination (R 2=0.9988,0.9971)and lowest LOD (3.6,5.6%)in both A19/A20and wavenumber shift for SO +UFO.By contrast,PO +UFO had the lowest coefficient of determination (R 2=0.9951,0.9924)and highest LOD (7.2,9.0%)in both A19/A20and wavenumber shift.The differences in the R 2and detection lim-its could be explained by the origin of UFO used in this study and the differences of component of fatty acids among the differentT a b l e 2C l u s t e r a n d d i s c r i m i n a n t r e s u l t s f o r q u a l i t a t i v e a n a l y s i s o f v e g e t a b l e o i l s a d u l t e r a t i o n .C O +U F OP O +U F O R O +U F OS O +U F ON o .o f c l u s t e rI I I I I I I V I I I I I I I VII II I II VVI I II I II VVC a l i b r a t i o n s e t (%)0,1,2,3,4,5,67,8,9,10,20,30,4050,60,70,8090,1000,1,2,93,4,5,6,7,8,10,2030,40,50,6070,80,90,10001,2,73,4,5,6,8,9,10,2030,40,50,60,7080,90,1000,12,3,4,5,6,7,8,910,20,30,4050,60,70,8090,100V a l i d a t i o n s e t (%)aA (3.94),D (17.50)B (7.78),C (12.37),E (23.43)F (36.00),G (48.34),H (55.98),I (65.14),J (67.39),K (84.83)L (92.96)A (2.99),B (8.13)C (13.15),H (56.00)D (17.90),E (23.51),F (36.29),G (49.10),I (64.53)J (67.57),K (83.75),L (93.08)A (3.96)B (7.68),C (14.58),D (18.64),E (23.11)F (36.02),G (48.51),H (55.27),I (64.62),J (67.23),K (84.49)L (92.68)A (4.03),B (7.68),C (14.58)D (18.64),E (23.11),F (36.08),G (49.86)H (54.80),I (64.76),J (74.08),K (84.10)L (85.24)aL e t t e r s o f A ,B ,C ,D ,E ,F ,G ,H ,I ,J ,K ,L w e r e t h e c o d e n a m e s o f t h e s a m p l e s s e t i n t h e v a l i d a t i o n s e t .Q.Zhang et al./Food Chemistry 132(2012)1607–16131611vegetable oils.In other hand,the R 2and LOD for A19/A20were bet-ter than that for wavenumber shift of band 19,which could be attributed to the subtle change in the wavenumber shift of band 19and comparatively large change in the area ratio between band 19and 20.This methodology obtained a satisfactory authentica-tion result.For validating the reproducibility of the regression analysis,pre-dicted value of adulteration proportion of validation set was calcu-lated by the equation of the regression analysis.Then the simple distribution of scatter plots between the actual value and predicted value were obtained and showed by Fig.3,which corresponded to A19/A20and the wavenumber shift of band 19respectively.There was good reproducibility between the predicted values and actual values for A19/A20of the four mixed forms in Fig.3(a–d);but in Fig.3(e–h),there were certain divergences between the predicted values and actual values of the four mixed ways.This could be explained by assuming that the small change among the wave-number shifts of band 19of the different adulteration proportions caused the biggish error between the predicted values and actual values.4.ConclusionsThe FTIR spectrum of vegetable oils was divided into 22regions according to the different absorption peaks.Cluster analysis for calibration set and discriminant analysis for validation set were accomplished on the basis of areas of these regions and got a rea-sonable vegetable oils authentication result.The area ratio be-tween band 19and band 20and wavenumber shift of band 19were used to construct linear regression equations,the LODs of CO,PO,RO and SO adulterated with UFO were 6.6%,7.2%,5.5%and 3.6%on the condition of area ratio and 8.1%,9.0%,6.9%and 5.6%on the condition of wavenumber shift,respectively.So edibleTable 3The results of linear regression analysis between A19/A20,the wavenumber shift of band 19and adulteration proportion of four mixed forms.ItemA19/A20Wavenumber shift of band 19Regression equationR 2LOD (%)Regression equation R 2LOD (%)CO +UFO y =0.0693x +1.9610.9959 6.6y =0.0156x +966.70.99388.1PO +UFO y =0.0525x +2.0660.99517.2y =0.0182x +966.50.99249.0RO +UFO y =0.0405x +3.7040.9971 5.5y =0.0128x +967.00.9955 6.9SO +UFOy =0.0727x +1.4170.99883.6y =0.0164x +966.70.99715.61612Q.Zhang et al./Food Chemistry 132(2012)1607–1613vegetable oils adulterated with UFO could be detected by the com-bination of the information of FTIR spectrum and chemometrics according to the result of qualitative and quantitative analysis ob-tained in this study.What specific components are produced after repeatedly frying of vegetable oils and what the relation is be-tween the infrared spectrum of vegetable oils and the specific chemical substances need a further study.AcknowledgementsThe research is supported by project of oil quality and safety control technology research and industrialisation demonstration (2009BADB9B08)of the‘‘11-5’’Technology Support Program.The authors would like to thank teacher Zhang of Academy of Science of China Agricultural University and upperclassman who helped us during the experiment and the course of paper writing. 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