Food Chemistry 212 (2016) 585589

Contents lists available at ScienceDirect

Food Chemistry
journal homepage: www.elsevier.com/locate/foodchem

Analytical Methods

Determination of the acid values of edible oils via FTIR spectroscopy

based on the OAH stretching band
Xiuming Jiang , Shen Li, Guoqiang Xiang, Qiuhong Li, Lu Fan, Lijun He, Keren Gu
School of Chemistry & Chemical Engineering, Henan University of Technology, Zhengzhou 450001, PR China

a r t i c l e i n f o a b s t r a c t

Article history: A new method for determining the acid values (AVs) of edible oils based on the OAH stretching band was
Received 1 November 2015 developed. The oil sample was diluted with carbon tetrachloride and was placed in a quartz cuvette with
Received in revised form 16 May 2016 a thickness of 1 cm to record the FTIR spectrum. The peak at 3535 cm
1, which corresponds to the OAH
Accepted 13 June 2016
stretch of the carboxyl group in free fatty acids, together with the peak valley at 3508 cm
1 and the spec-
Available online 14 June 2016
tral data in the range of 33403390 cm
1 were used to determine the AV of the edible oil. The excellent
linear relationship between the AVs measured in this work and those measured using a titration method,
Chemical compounds studied in this article:
with a correlation coefficient (R) of 0.9929, indicates that the present procedure can be applied as an
Carbon tetrachloride (PubChem CID: 5943)
Stearic acid (PubChem CID: 5281)
alternative to the classic method for determining the AVs of edible oils.
Potassium hydroxide (PubChem CID: 2016 Elsevier Ltd. All rights reserved.
23665647)
Tertiary butylhydroquinone (PubChem CID:
16043)
Monostearin (PubChem CID: 24699)

Keywords:
Acid value
FTIR spectroscopy
Quartz cuvette
Edible oil

1. Introduction improved methods have been developed, including automatic pho-

tometric titration (Crispino & Reis, 2013), flow injection analysis
Edible oils are important raw materials for home cooking and (Ayyildiz & Kara, 2014; Saad, Ling, Jab, & Lim, 2007), and potentio-
the food industry. The quality of such oils will become poor and metric titration (Osawa, Goncalves, & Ragazzi, 2006). Various
deteriorate during storage, transportation and use, particularly instrumental methods have also been used to determine the AVs
when used for frying, even to the extent that such edible oils can- of edible oils, including gas chromatography (Tan, Ghazali,
not be used for food. Therefore, it is necessary to monitor oil qual- Kuntom, Tan, & Ariffin, 2009), high-performance liquid chromatog-
ity parameters, such as acid value, peroxide value, iodine value, raphy (Li et al., 2011), capillary electrophoresis (Balesteros et al.,
moisture content, anisidine value and carbonyl value. Acid value 2007), electrochemical impedance spectroscopy (Grossi, Lecce,
(AV), which represents the content of free fatty acids produced Toschi, & Ricco, 2014), ultravioletvisible spectroscopy (Zhang
from the hydrolysis of triglycerides in edible oils, is typically con- et al., 2015), Raman spectroscopy (Muik & Lendl, 2003), 1H NMR
sidered to be one of the main parameters that reflect the quality of (Satyarthi, Srinivas, & Ratnasamy, 2009; Skiera, Steliopoulos,
edible oils. The official method (AOCS, 1989) for determining the Kuballa, Diehl, & Holzgrabe, 2014), infrared spectroscopy, and
AVs of edible oils is titration analysis, based on the chemical reac- near-infrared spectroscopy (Adewale, Mba, Dumont, Ngadi, &
tion between free fatty acids and potassium hydroxide. To over- Cocciardi, 2014; Cantarelli, Funes, Marchevsky, & Camina, 2009;
come the major disadvantages of this chemical method, such as Ng, Wehling, & Cuppett, 2007).
being time-consuming and labour-intensive, requiring large In recent decades, Fourier transform infrared spectroscopy has
amounts of organic solvents, and difficulty in distinguishing the been widely used for classifying and quantifying oils and fats.
end-point with samples containing coloured substances, some When measuring the FTIR spectra of edible oils, the samples are
generally placed in a simple transmission cell composed of two
Corresponding author. halide crystals, such as KBr, NaCl, CaF2 or BaF2, plus a spacer to
E-mail address: jxm1965@sohu.com (X. Jiang). provide a defined path length. Although the standard IR cell is

http://dx.doi.org/10.1016/j.foodchem.2016.06.035
0308-8146/ 2016 Elsevier Ltd. All rights reserved.
586 X. Jiang et al. / Food Chemistry 212 (2016) 585589

relatively simple, it has some disadvantages with respect to oil 2.2. Acid value determination
analysis. Because halide crystals are hygroscopic and fragile, thus
making the flow cell somewhat awkward to work with, an alterna- The AV of the edible oil was determined by the AOCS titration
tive to flow cells would be welcomed in many situations. From a method. The result was expressed as the number of milligrammes
sample handling perspective, the attenuated total reflectance of KOH required to neutralize the free fatty acids in 1 g of sample
(ATR) attachment, which allows one to simply place the oil onto (AOCS, 1989).
an exposed crystal surface and record the spectrum, is one of the
most viable alternatives (Filho, 2014; Sherazi, Mahesar, Bhanger,
2.3. Fourier transform infrared spectroscopy analysis
Voort, & Sedman, 2007). Polyethylene and polytetrafluoroethylene
films, which contain microcrystalline pores capable of absorbing
The infrared spectra of edible oils were recorded using an IR
liquids into the polymer matrix via capillary action, have also been
prestige-21 Fourier transform infrared spectrometer (SHIMADZU
employed as single-use, disposable infrared-transparent substrates
Corporation, Ltd.) equipped with a cuvette rack. Prior to FTIR anal-
for measuring the infrared spectra of oils (Dong, Li, Sun, Chen, & Yu,
ysis, all edible oils were diluted with carbon tetrachloride. The
2015; Xu, Zhu, Chen, Sun, & Yu, 2015).
dilution procedure simply involved weighing approximately 0.1 g
According to Lambert-Beers law, exactly controlling the optical
of oil sample into a 10 ml graduated test-tube with a glass stopper,
path length of the sample is very important for quantitatively
followed by adding carbon tetrachloride to scale. The tube was
determining the free fatty acid content in edible oils. Although
then capped with the stopper and shaken. The oil solution was
the transmission flow cell could control the optical length and the
placed in the infrared quartz cuvette with an optical path length
AV of the oil sample was directly calculated from the peak height
of 1 cm. The cuvette was mounted in the instrument, and all
or area (Aryee, Voort, & Simpson, 2009) of the characteristic band,
absorption spectra were recorded from 4000 to 2500 cm
1 at a res-
several quantitative calibration techniques have been developed
olution 4 cm
1. Pure carbon tetrachloride was used to collect the
to improve the measurement accuracy. By adding a spectral maker,
background spectrum.
such as methylcyclopentadienyl manganese tricarbonyl (MMT), to
oils, the relative intensity of the peak at 1942 cm
1, corresponding
to the C@O stretching of MMT, could be used for spectral reconsti- 3. Results and discussion
tution to remove the spectral contributions from diluents, to elim-
inate the influence of oil film thickness on the spectra and to correct 3.1. Spectral analysis of the oil sample
the characteristic band of free fatty acids (Yu, Voort, Sedman, & Gao,
2011). To avoid the necessity of using a marker, Dong and co- The characteristic peak at 1711 cm
1, attributed to C@O
workers normalized the path length of each oil film to a fixed path stretching in the carboxyl acid group of free fatty acids, is typically
length of 0.15 mm with the absorbance at 4334 cm
1, which is used to determine the AVs of edible oils via FTIR spectroscopy (Yu,
fairly constant in intensity, irrespective of the type of oil being anal- Du, Voort, Yue, & Li, 2009). Because of the band at 1750 cm
1 due
ysed and corresponds to a maximum of the broad absorption band to C@O stretching in triglyceride ester, the C@O group in carboxylic
that oils exhibit in the CH combination band region (Dong et al., acids presents a shoulder when the concentration of free fatty
2015). Another quantitative calibration technique is the chemo- acids is small, as in many edible oils. By ratioing against the spec-
metric method, which is capable of providing precision, accuracy, trum of fatty acid-free oil, the spectral contribution of the oil could
and analytical information and has been widely used in recent be eliminated, leaving only the spectrum of the fatty acids (Ismail,
years. One or more parameters (Talpur et al., 2014), such as perox- Voort, Emo, & Sedman, 1993). To distinguish between bands of the
ide value (Yu, Voort, & Sedman, 2007), total polar compounds (Ng, carbonyl groups of free acid and ester, Lanser, List, Holloway, and
Wehling, & Cuppett, 2011), iodine value (Hendl, Howell, Lowery, & Mounts (1991) estimated the free fatty acid content in oil by
Jones, 2001), free fatty acid, conjugated diene and triene (Innawong, deconvoluting the absorption bands between 2000 and
Mallikarjunan, Irudayaraj, & Marcy, 2004), and carbonyl value 1600 cm
1 (Lanser et al., 1991). However, this characteristic peak
(Wang, Yu, Chen, Yang, & Zhang, 2014), could be determined by is not applicable for the determination of the AVs of fats and oils
establishing the relationship between the chemical parameters that have been extensively oxidized or thermally stressed (Ismail
and spectral data of oils, using chemometric methods involving et al., 1993). Oxidative degradation products, such as hydroperox-
partial least squares (PLS), multiple linear regression (MLR), and ides, aldehydes and ketones, are commonly present in such sam-
principal component regression (PCR) analyses. ples, with aldehydes and ketones having absorption bands in the
In the present work, an infrared quartz cuvette, a common 17301670 cm
1 region. For oxidized/thermally stressed oils, an
accessory for UVvis and NIR spectrophotometers, was applied indirect FTIR method that includes converting the fatty acids to
during the collection of FTIR spectra of edible oils to hold the sam- the corresponding carboxylates by reaction with an alkaline sub-
ple solution and control the optical path length. The objective of stance (Al-Alawi, Voort, & Sedman, 2004; Caada, Medina, &
this work is to determine the AV of edible oils, using the character- Lendl, 2001; Li, Garca-Gonzlez, Yu, & Voort, 2008), such as
istic band caused by the OAH stretching of carboxyl groups in free KOH, sodium hydrogen cyanamide, or potassium phthalimide
fatty acids. and subsequently measuring the carboxylate anion band at
1570 cm
1, was established.
2. Materials and methods When the infrared quartz cuvette was applied during the collec-
tion of the FTIR spectrum of the oil, the band corresponding to the
2.1. Reagents and materials C@O stretching in the carboxyl acid group of free fatty acids could
not be observed, due to the complete absorption of infrared radia-
All oils, including 45 peanut, 25 rapeseed, 20 soybean and 15 tion with wavenumbers of less than 2500 cm
1 by quartz. Another
palm oils, were obtained from supermarkets or oil companies characteristic band caused by OAH stretching of fatty acids is in
and had been stored in brown bottles at room temperature for the wavenumber range of 25004000 cm
1. However, this band
between 1 week and 2 years. All chemicals and reagents used were is a large, broad peak (Safar, Bertrand, Robert, Devaux, & Genot,
of analytical grade. Infrared quartz cuvettes with an optical path 1994) in the region of 35002500 cm
1 and was unavailable unless
length of 1 cm were purchased from Yixing Jingke Optical Instru- the FTIR spectrum was measured in a dilute solution to avoid the
ment Company, Limited. formation of fatty acid dimers
 X. Jiang et al. / Food Chemistry 212 (2016) 585589 587

Notably, the use of the quartz cuvette can not only provide an the other two peaks, at 3618 and 3690 cm
1, are almost
accurate and long optical path length for measuring the FTIR spec- unchanged, indicating that the band at 3535 cm
1 is related to
tra of oils but also enables determination of the AV of the oil by the free fatty acids in the oil. The FTIR spectra of oil samples with
using the OAH stretching band because the sample can be pre- different AVs (see Fig. S1 in the Supplementary Material) also show
pared as a diluted solution. When the oil was diluted to a 1% solu- that the absorbance at 3535 cm
1 increases with the AV. Moreover,
tion with carbon tetrachloride and the AV of the oil was not more FTIR spectra of oil solutions, in which tertiary butylhydroquinone
than 5.6 mg KOH g
1, the concentration of free fatty acids in the (TBHQ) or monostearin was added (Fig. S2 in the Supplementary
solution was less than 1  10
3 M, a very dilute solution, and the Material), show that the band in the wavenumber range of
fatty acids in the solution would exist as monomers rather than 36003700 cm
1 is caused by the OAH groups in TBHQ, a com-
dimers. The OAH stretching peak of the monomer fatty acid would pound containing phenolic hydroxyl groups usually added to the
appear at a wavenumber around 3550 cm
1. (Brown, Floyd, & edible oil as an antioxidant, and monostearin, a compound con-
Sainsbury, 1988; Silverstein, Bassler, & Morrill, 1991). taining aliphatic hydroxyl groups from the hydrolysis of the edible
Fig. 1 presents a typical FTIR spectrum of a 1% solution of edible oil, and does not interface with the peak at 3535 cm
1. Therefore,
oil in the 4000 to 2500 cm
1 region. There are four peaks in the the peak at 3535 cm
1 was used as the characteristic band for
range of 33003800 cm
1 (see the inset in Fig. 1), and the peak determining the AV of the edible oil.
at 3470 cm
1 is attributed to the overtone of the C@O stretching.
The other three peaks at 3535, 3618, and 3690 cm
1 are likely 3.2. Spectral parameters for calibration
related to the OAH stretching vibration, and one of these peaks
should be caused by the OAH stretching in monomer free fatty Because the peak at 3535 cm
1 is a characteristic band of the
acid. To verify which band was attributed to the OAH stretching OAH bond in free fatty acids, the intensity of the band should repre-
in the carboxyl group, 0.25, 0.50 and 0.75 ml aliquots of a solution sent the AV of the oil sample. However, the spectral data of oils show
of stearic acid in carbon tetrachloride (0.2%, w/v) were added to the a poor relationship between AVs and peak heights at 3535 cm
1.
oil solution, and the spectra were recorded. These spectra are After carefully observing FTIR spectra of oil samples, it is easy to find
shown in Fig. 2. Fig. 2 shows that the intensity of the peak at that the AV of the oil is related, not only to the absorbance at
3535 cm
1 increases with the addition of stearic acid and that 3535 cm
1, but also to the absorption curve, which is a straight line,

0.16
3470

0.16
1.6
0.12
Absorbance

Absorbance

0.12
Absorbance

1.2 3618
3535

0.08 3690
3508
0.08
0.8 3655
3580
0.04
3800 3600 3400 3200

0.4 Wavenumber/cm
-1
0.04

3800 3700 3600 3500 3400 3300 3200

-1
0.0 Wavenumber/cm
4000 3800 3600 3400 3200 3000 2800 2600
-1 Fig. 3. FTIR spectra of edible oils (1% in CCl4) with similar acid values. From bottom
Wavenumber/cm
to top, the acid values of the edible oils are 0.33, 0.34, 0.36 and 0.35 mg g
1,
respectively.
Fig. 1. Typical FTIR spectrum of the edible oil (1% in CCl4) in the wavenumber range
of 40002500 cm
1. The inset shows peaks in the range of 38003200 cm
1.

0.16

0.16
0.12
Absorbance
Absorbance

0.12
0.08

0.08 0.04

3800 3700 3600 3500 3400 3300 3200

3800 3700 3600 3500 3400 3300 3200 Wavenumber/cm
-1
-1
Wavenumber/cm
Fig. 4. FTIR spectra of edible oils (1% in CCl4) with similar peak intensities at
Fig. 2. FTIR spectra of edible oils (1% in CCl4) with the addition of 0.00, 0.25, 0.5 and 3535 cm
1. From bottom to top, the acid values of the edible oils are 1.16, 0.52, 0.91
0.75 ml (from bottom to top) of a 0.2% solution of stearic acid in CCl4. and 0.76 mg g
1, respectively.
588 X. Jiang et al. / Food Chemistry 212 (2016) 585589

in the range of 33403390 cm
1. Because of the different slopes of

the straight lines in the range of 33403390 cm
1, the FTIR spectra
of some oils with similar AVs (Fig. 3) display different peak intensi- 3.5
ties and the larger the slope of the straight line, the higher is the

-1
Acid value with FTIR (AVFTIR) / mg g
peak at 3535 cm
1. Additionally, some oils with different AVs 3.0
(Fig. 4) show similar peak heights and the higher the AV, the smaller
is the slope. Moreover, the band at 3535 cm
1 overlaps with the 2.5
peak at 3470 cm
1, and the absorbance in the valley at 3508 cm
1
2.0
represents the effect of the band at 3470 cm
1 on the peak at
3535 cm
1. Therefore, four spectral parameters, namely, the absor- 1.5
bance of the peak at 3535 cm
1, the absorbance of the valley at
3508 cm
1, the slope and the intercept of the linear regression equa- 1.0
tion of the absorption curve in the range of 33403390 cm
1, were
used to predict the AV of the edible oil. 0.5

0.0
3.3. Multivariate linear regression
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
To investigate the practicability of utilizing FTIR spectral data to Acid value with titration (AVAOCS) / mg g
-1

determine the AVs of edible oils, 105 oil samples were classified
into the calibration standard, which included 30 oil samples, and Fig. 6. Plots of acid values measured by FTIR spectroscopy against those measured
the validation set, which included 75 oil samples. The FTIR spectra using the titration method.
of 30 calibration standards, including 20 peanut and 10 rapeseed
oils, are shown in Fig. 5. Regarding the effect of the concentration
and AOCS methods. The plot of AVFTIR, which represents the AV
of the oil solution in carbon tetrachloride on the infrared spectrum,
determined with the FTIR method, against AVAOCS, which repre-
after fitting the FTIR spectral data from 3340 to 3390 cm
1 with a
sents the AV determined with the AOCS method, is shown in
linear equation to obtain the slope and intercept, the multiple lin-
Fig. 6. The regression equation for the composite data presented
ear regression equation expressing the relationship between AVs
in Fig. 6 is AVFTIR = 0.979AVAOCS + 0.025, with a correlation coeffi-
and FTIR spectral parameters was established as Eq. (1) with a cor-
cient (R) of 0.9929 and a standard error (SD) of 0.095 AV units,
relation coefficient, R2, of 0.9921.
indicating an excellent linear relationship between the AV deter-
AV  m 0:0521 9:5145Ap
8:2296Av
4203:53S
1:0722I mined using the present FTIR method and that obtained from the
1 standard procedure with the slope and correlation coefficient
being close to 1.0.
where m, Ap, Av, S and I represent the mass of the oil weighed for A plot of the absolute deviation of the AVFTIR and IVAOCS, calcu-
preparing the carbon tetrachloride solution, the peak absorbance lated with (IVFTIR-IVAOCS), as a function of the IVAOCS for all 75 sam-
at 3535 cm
1, the valley absorbance at 3508 cm
1, and the slope ples (see Fig. S3 in the Supplementary Material), shows that the
and the intercept of linear regression equation of the absorption absolute deviation is not greater than 0.1 AV units for most of
curve in the range of 33403390 cm
1, respectively. The AV of the the oil samples (61 of 75 samples), and it is less than 0.2 AV units
oil can be calculated from its FTIR spectral parameters using Eq. (2). for more than 95% of the oil samples (73 of 75 samples) regardless
AV 0:0521 9:5145Ap
8:2296Av
4203:53S
1:0722I=m of the types of oils that were determined, indicating that the AV
determined using the proposed method is consistent with that
2
determined using the official method.

3.4. FTIR determination of edible oil samples 4. Conclusion

To determine the efficacy of the developed methodology, the In this work, the AVs of edible oils were successfully deter-
AVs of 75 validation samples, including 25 peanut, 15 rapeseed, mined via FTIR spectroscopy by using the peak at 3535 cm
1 corre-
20 soybean and 15 palm oils, were measured, using both FTIR sponding to the OAH bond in the carboxyl group of free fatty acids.
To measure the FTIR spectrum, the edible oil was diluted to a 1%
solution in carbon tetrachloride and was placed in an infrared
quartz cell with an optical path length of 1 cm. As an alternative
0.16
method to the official titrimetric method, the developed approach
has some advantages over other infrared methods.
First, determining the AVs of edible oils using the OAH stretch-
Absorbance

0.12
ing band could avoid the severe interference from other peaks. In
previously reported FTIR works, the AV was typically measured
using the peak at approximately 1711 cm
1, caused by the C@O
0.08
stretching in free fatty acids, which generally appears as a shoulder
on the very strong peak at approximately 1746 cm
1 attributed to
the C@O stretching in triacylglycerol. Peak overlap interferes with
0.04
accurately determining the AV and had to be eliminated by devel-
oping oil-specific calibrations or by ratioing the spectrum against
3800 3700 3600 3500 3400 3300 3200 that of the same oil free of fatty acids.
Wavenumber / cm
-1 Second, the infrared quartz cell can provide a long optical path.
Because the sample can be diluted to a low concentration and the
Fig. 5. FTIR spectra of 30 edible oil calibration standards (1% in CCl4). weak interactions, such as hydrogen bonds, between sample mole-
 X. Jiang et al. / Food Chemistry 212 (2016) 585589 589

cules will weaken or even completely disappear, more spectral Dong, X., Li, Q., Sun, D., Chen, X., & Yu, X. (2015). Direct FTIR analysis of free fatty
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to the sample thickness using complex technologies. Moreover, the determination of frying oil quality using Fourier transform infrared
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Saad, B., Ling, C. W., Jab, M. S., & Lim, B. P. (2007). Determination of free fatty acids in
The authors gratefully acknowledge financial support from the palm oil samples using non-aqueous flow injection titrimetric method. Food
National Natural Science Foundation of China (21205028) and Chemistry, 102, 14071414.
Safar, M., Bertrand, D., Robert, P., Devaux, M. F., & Genot, C. (1994). Characterization
from the Henan University of Technology (Special Funds for Intro-
of edible oils, butters and margarines by Fourier transform infrared
duction of Talent 2015RCJH08). spectroscopy with attenuated total reflectance. Journal of the American Oil
Chemists Society, 71, 371377.
Satyarthi, J. K., Srinivas, D., & Ratnasamy, P. (2009). Estimation of free fatty acid
Appendix A. Supplementary data content in oils, fats, and biodiesel by 1H NMR spectroscopy. Energy Fuel, 23,
22732277.
Supplementary data associated with this article can be found, in Sherazi, S. T. H., Mahesar, S. A., Bhanger, M. I., Voort, F. R. V. D., & Sedman, J. (2007).
Rapid determination of free fatty acids in poultry feed lipid extracts by SB-ATR
the online version, at http://dx.doi.org/10.1016/j.foodchem.2016. FTIR spectroscopy. Journal of Agriculture and Food Chemistry, 55, 49284932.
06.035. Silverstein, R. M., Bassler, G. C., & Morrill, T. C. (1991). Spectrometric identification of
organic compounds (5th ed. p. 117). New York: John Wiley and Sons.
Skiera, C., Steliopoulos, P., Kuballa, T., Diehl, B., & Holzgrabe, U. (2014).
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