Determination of unsaturation grade andtrans isomers generatedduring thermal oxidation of edible oils and fats by FTIR

M.C.M. Moya Morenoa,* , D. Mendoza Olivaresb, F.J. Amezquita Lopezb,J.V. Gimeno Adelantadoa, F. Bosch Reiga

aDepartment of Analytical Chemistry, Faculty of Chemistry, University of Valencia, C/Dr. Moliner 50, 46100 Burjassot, Valencia, SpainbDepartment of Chemistry, Instrumental Laboratory, Faculty of Chemistry, University of Guanajuato, Noria Alta, 36050 Guanajuato, Mexico

Received 24 August 1998; received in revised form 30 November 1998; accepted 30 November 1998

Abstract

The oxidative deterioration of culinary oils and fats during episodes of heating associated with normal usage (808C–3008C,20–40 min) was monitored by FTIR spectroscopy. The thermal oxidation of polyunsaturated fatty acids during heating wasstudied by the determination of unsaturation percentage andtrans isomers at various temperatures and heating times. Oilsfrequently used in food frying such as olive oil, sunflower oil, corn oil and seeds oil (sunflower, safflower andcanola seed), andlard were studied. TheAbsorbance Correction Methodis proposed to correct the spectral interference and allows the analyticuse of signal which would not be initially valid forquantitativeanalysis. The results show that there is a decrease in unsaturationand an increase intransisomers starting at 1508C and becomes more pronounced at temperatures around 2508C. This variationin unsaturation grade and conformation provides evidence of the transformation of essential polyunsaturated fatty acids andsubsequent decrease in the oils’ nutritional value.q 1999 Elsevier Science B.V. All rights reserved.

Keywords:Edible oils; Polyunsaturated fatty acids;Transisomers; FTIR

1. Introduction

Oils and fats are an important part of the human dietand more than 90% of global production is used asfood or as ingredients in food products. Oils and fatsin the diet are a rich source of energy, they containcertain fatty acids, which are nutritionally indispen-sable and their functional and textural characteristicsadd to the flavour and acceptability of many naturaland processed foods. The polyunsaturated fatty acids(PUFA) which cannot be synthesised by the human

body must be present in the diet to avoid symptomscaused by shortage. They can be considered asvitamin factors and linoleic, and linolenic acidsmust be considered specifically as they are polyunsa-turated fatty acids which are necessary for the normaldevelopment and functioning of human tissues andare known as essential fatty acids [1]. Oils and fatsbegin to decompose from the moment they areisolated from their natural environment when changesoccur causing a disagreeable taste and smell. Atmo-spheric oxidation is the most important cause of dete-rioration in fats. This oxidative rancidity isaccelerated by exposure to heat, light, humidity andthe presence of trace transition metals [2]. The intensefrying of oil causes an oxidizing thermal degradation

Journal of Molecular Structure 482–483 (1999) 551–556

0022-2860/99/$ - see front matterq 1999 Elsevier Science B.V. All rights reserved.PII: S0022-2860(98)00937-5

* Corresponding author. Tel.:1 34 6 386 4533; fax:1 34 6 3864436.

E-mail address:montse.moya@uv.es (M.C.M. Moya Moreno)

with the formation of decomposition products [3–6],and change in physical properties [7]. Spectroscopicmethods can be employed to evaluate deterioration inoils subjected to intense heat [8].

Fourier Transform Infrared (FTIR) Spectroscopyprovides a quick and accurate way of evaluatingthermal degradation in these lipids. This articlereports a methodology to evaluate the variation incomposition in edible oils (sunflower oil, corn oil,olive oil and seeds oil) and lard when it is subjectedto intense heat (808C–3008C, 20–40 min). The unsa-turation percentage andtrans isomers percentage wasdetermined by FTIR at various temperatures andheating times, usingn-valeronitrile as an internal stan-dard, which quickened sample preparation and did notnecessitate the determination of optical path length.

Quantitative analysis by IR can be performed withtechniques based on the addition of a standard to thesample in order to measure absorbance quotients sothat a variable and unknown spacer may be used asmeasurements are independent of the optical pathlength. An analyte band and a standard band whichare free from interference are required. When theunknown band overlaps with one of the standardbands this spectral interference may be corrected bythe method developed by the authors known as theAbsorbance Correction Method(ACM).

1.1. Absorbance correction method: theoreticalbackground

The ACM permits the analytic use of signals whichinterfere with the signals from the standard. The basisfor this method is as follows:

Let AA;n�n1�, be the absorbance of the analyte atfrequencyn1 in a prepared samplen (sample1 stan-dard) andAP;n�n1� be the absorbance of the standard atthe same frequency, thus giving rise to a spectral inter-ference in the measurement of the analytical signal.The total measured absorbance atn1 is denoted here asAn�n1�, so that

An�n1� � AA;n�n1�1 AP;n�n1�: �1�Note that in the infrared spectrum the absorbance

bands owing to the same compound show a constantrelation even when the concentration of thiscompound or sample thickness is varied, it is possibleto predict the absorbance atn1 resulting from the

standard by relating it with the absorbance at anotherauxiliary frequencyn2 at which only the standardabsorbs and not the analyte, i.e., knowing the absor-bance arising exclusively from the standard at anotherfrequencyn2, it is possible to predict the absorbance atthe frequency of interestn1, as the relation betweenthe both is constant.

Therefore, the Standard Correction FactorFc�n1; n2� is defined at the frequencyn1 usingn2 asauxiliary frequency from the infrared spectrum of thepure standard compound,

Fc�n1; n2� � AP;0�n1�=AP;0�n2�; �2�where,AP;0�n1� andAP;0�n2� are the measured absor-bance values atn1 andn2, respectively, in the spectrumof the pure standard compound.

The Standard Correction Factormultiplied by theabsorbance at the auxiliary frequencyn2 in theprepared samplen;AP;n�n2�, gives the absorbance atthe frequency of interestn1 in the same sample whichis only because of the standard,AP;n�n1�AP;n�n1� � AP;n�n2�·Fc�n1; n2�: �3�

Arranging the terms in Eq. (1) and substituting Eq.(3) will provide the corrected absorbance atn1 whichis only because of the analyte

AA;n�n1� � An�n1�2 AP;n�n1�AA;n�n1� � An�n1�2 AP;n�n2�·Fc�n1; n2�:

�4�

Once the corrected absorbance value is obtained,the chosen quantitative method can then be applied.

2. Experimental

2.1. Instrumentation

Perkin-Elmer 1600,Model 1605 FTIR Spectro-meter with deuterated triglycine sulphate (DTGS)detector. Number of the co-added scans:16; Resolu-tion: 4 cm21. Data acquisition and processing soft-wareSpectrum for Windows, Perkin-Elmer.

2.2. Reagents

n-Valeronitrile Aldrich analysis grade; nujolAldrich IR grade; methyl linoleateAldrich analysis

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grade; methyl elaidateAldrich analysis grade; andchloroformBakeranalysis grade.

2.3. Samples

Oils from the Mexican and Spanish market wereanalysed: Virgin Olive oil (Domeq), Corn oil(Mazola), Sunflower (Cristal), Seeds oil (Sunflower,Safflower and Canola,As), and lard.

2.4. Procedure

Samples were heated in a porcelain capsule andwere subjected to increasing heat on an electricdevice, up to a temperature of 3008C, during thisprocess heating was maintained for 20 and 40 min atthe following temperatures: 808C, 1508C, 2008C,2508C and 3008C ^ 58C. The samples and internalstandardn-valeronitrile were mixed in a test tube,weighed on an analytical scale and the sample wasshaken manually to ensure total homogenisation.The proportion in weight ofn-valeronitrile in theprepared samples was between 35% and 65%. Toobtain the infrared spectra one or two drops of theprepared samples were placed, using a Pasteur pipette,between two circular pieces of well polished potas-sium bromide crystals without using a spacer sosample thickness was not controlled but varied in

each prepared sample. In the IR spectra of theprepared samples absorbance was measured as thepeak height at 3008 cm21 with respect to the baseline tangent at 3116–2718 cm21, at 970 cm21 withrespect to the baseline 990–944 cm21 and the peak2246 cm21, corresponding ton-valeronitrile, withrespect to the base line tangent at 2408–2144 cm21.

Eight mixtures of known composition ofn-valero-nitrile, methyl linoleate and nujol were prepared toobtain the calibration straight line for the determina-tion of unsaturation grade. In these mixtures, theabsorbance quotient 3008 cm21/2246 cm21 wasmeasured with respect to the earlier mentioned base-lines. In the same way eight mixtures of knowncomposition ofn-valeronitrile, methyl elaidate andnujol were prepared to obtain the calibration straightline for the determination oftrans isomers and theabsorbance quotient (970 cm21/2246 cm21) wasmeasured.

3. Results and discussion

The infrared spectra show how the intense fryingproduces a variation in composition of the oils’nature, causing a decrease in unsaturated componentsand an increase intrans isomers (Fig. 1). The

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Fig. 1. Infrared spectra of sunflower oil unheated (…) and heated at 3008C during 40 min (—).

percentage of unsaturation expressed as a percentagein weight of methyl linoleate (l) and trans isomersexpressed as a percentage in weight of methyl elaidate(e) was obtained for each type of oil at differentheating. The internal standard method was used withn-valeronitrile (v) as internal standard. The infraredspectrum of n-valeronitrile shows an absorbanceband at 2246 cm21, owing to the stretching vibrationof the triple CxN link which can be used as referenceband as it does not interfere with any of the samplebands. The analytical signal used to determine theunsaturation was at the peak 3008 cm21, caused bythe stretching vibration of the C–H link adjoiningthe double CyC link. From eight mixtures of

n-valeronitrile/methyl linoleate/nujol, the calibrationstraight line was obtained representing the quotient[A(3008)/A(2246)] against theCl =Cv concentrationquotient, whereCl is methyl linoleate concentrationand Cv the n-valeronitrile concentration bothexpressed as %(w/w). Linear regression providedthe following behaviour with a correlation index0.998:A�3008�=A�2246� � 0:38881 2:0326Cl =Cv.

For determination of thetrans isomers,n-valeroni-trile gives rise to a spectral interference in themeasurement of the peak at 970 cm21, which can becorrected by the proposedACM. The StandardCorrection Factor Fc(970, 2246), was obtained as0.0175, providing the corrected absorbance value

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Fig. 2. Calibration straight line to obtain thetrans isomers percentage as methyl elaidate, usingACM.

Fig. 3. Infrared spectrum of a prepared sample of the unheated corn oil.

AC(970 cm21). In the same way, from eight mixturesof n-valeronitrile/methyl elaidate/nujol, the calibra-tion equation obtained was:AC(970)/A(2246) �0.01611 1.3423Ce/Cv, r � 0.998, whereCe is themethyl elaidate concentration (Fig. 2).

The unsaturation percentage andtrans isomerswere calculated from the spectra of the samplesprepared withn-valeronitrile (Fig. 3) using the corre-sponding calibration equations, the values obtainedare shown in Table 1. The results obtained showhow the intense heating of oils causes a decrease inunsaturation and increase intrans isomers. It is

possible to establish that in all the oils studied thisdegradation starts becoming considerable after1508C and at higher temperatures, a more significantvariation is apparent. The peroxide index obtained foreach sample by the iodimetric method [9] reveals thatthese changes coincide with the decomposition ofhydroperoxides (Fig. 4).

4. Conclusions

FTIR Spectroscopy provides a very useful

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Table 1Unsaturation percentage (%Un), expressed as methyl linoleate (%w/w) andtrans isomers percentage (%t-I), expressed as methyl elaidate(%w/w) in oils and lard, obtained by internal standard method in FTIR spectroscopy

Sample Heating (8C/min) Olive oil Sunflower oil Corn oil Seeds oil Lard

%Un %t-I %Un %t-I %Un %t-I %Un %t-I %Un %t-I1 Tamb 18.91 0.56 29.59 0.23 27.43 0.35 23.23 0.47 11.21 0.792 80-20 18.83 0.29 29.34 0.21 27.84 0.70 23.32 0.43 10.99 0.623 80–40 18.24 0.41 29.71 0.25 27.01 0.85 22.51 0.45 10.86 0.924 150–20 18.30 0.42 29.21 0.23 27.10 0.92 22.04 0.46 11.53 1.145 150–40 18.53 0.53 29.22 0.29 26.39 0.62 21.17 0.84 11.17 1.296 200–20 16.71 0.62 28.34 1.05 24.60 0.87 20.71 0.86 11.07 1.597 200–40 12.41 0.64 26.86 1.10 24.12 0.96 19.65 0.83 10.90 1.558 250–20 11.85 1.08 26.34 2.01 24.00 0.87 18.83 2.48 8.73 1.269 250–40 11.54 1.73 24.89 3.20 23.20 1.24 18.09 2.13 8.85 2.2310 300–20 9.81 4.23 19.03 7.19 18.67 3.19 16.41 5.57 8.78 2.7911 300–40 5.65 4.37 11.89 11.45 18.29 5.32 10.94 5.41 8.59 3.62

Fig. 4. Variation in composition (unsaturation grade,trans isomers and peroxide value) of sunflower oil at different thermal treatment.

information on the composition and the extent ofthermal self-oxidation in oils and lard subjected tointense heat, equivalent to that used in the ordinarypreparation of food. The methodology developed inthis study permitted determination of the percentageof unsaturated components andtrans isomers in thesamples at different heating temperatures, with noprior treatment. The use of absorbance quotientmeasurements permitted quantitative analysis withoutrequiring a constant and known cell thickness thusfacilitating sample handling and reducing analysistime. Further it permits quantification in cases whereit is impossible to know the optic path length accu-rately. ACM, permits the analytical use of signalswhich would initially not be valid because of the spec-tral interference and in this particular case permits themeasurements of signals which are significant for theinformation provided on the state of decomposition oftriglycerides.

It can be said that when an edible oil is subjected toa heating process or frying, the initial nature of thetrigliceride changes significantly and the decrease inunsaturation and increase intransisomers is evidenceof the transformation occurring in the essential poly-unsaturated fatty acids which the oil initially contains,thus indicating a decrease in the oils’ nutritionalvalue.

Acknowledgements

Authors are very grateful to the Department ofForeign Affairs in Mexico for a research grant forone of the authors (Moya Moreno, M.C.M.).

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