Eur Food Res Technol (2013) 236:1031–1040 DOI 10.1007/s00217-013-1960-7
ORIGINAL PAPER
Application of a new sampling device for determination of volatile compounds released during heating olive and sunflower oil: sensory evaluation of those identified compounds Ignacio Ontano´n • Laura Cullere´ • Julian Zapata Beatriz Villanueva • Vicente Ferreira • Ana Escudero
•
Received: 14 November 2012 / Revised: 24 January 2013 / Accepted: 28 February 2013 / Published online: 30 March 2013 Springer-Verlag Berlin Heidelberg 2013
Abstract This work has applied a system for characterizing volatile compounds generated during the heating of oils, based on solid-phase extraction–gas chromatography– mass spectrometry. The system has been applied to sunflower and extra virgin olive oil at kitchen conditions, heating from room temperature to 80 above their smoking point. The work identified twenty-three compounds, mainly saturated and unsaturated aldehydes, and alkanes; carboxylic acids were present in lower concentrations. During heating, the stability of each oil was found to be different. The alkanals were produced in greater quantities in the sunflower oil. Alkanes were found in higher concentration in the olive oil and showed no significant variation in either of the oils as the temperature changed. And a relationship between the quantity of aldehydes released at the highest temperature and the percentage of fatty acids from which those aldehydes originated in each oil was found. The importance of the unsaturated aldehydes in the deep-fried odor was also exposed in this study. Keywords Deep-fried odor Olive oil Sunflower oil Volatile compounds
I. Ontano´n L. Cullere´ J. Zapata V. Ferreira A. Escudero (&) Laboratory for Flavor Analysis and Enology, Department of Analytical Chemistry, Institute of Research on Engineering of Aragon (I3A), Universidad de Zaragoza, 50009 Zaragoza, Spain e-mail:
[email protected] B. Villanueva BSH Electrodomesticos Espan˜a S.A., Avda. de la Industria 49, 50016 Zaragoza, Spain
Introduction Frying is a process whereby food is cooked by its immersion in hot oil. This cooking method is one of the most commonly used methods worldwide because of its simplicity and because it generates pleasurable aromas, appearance, and textures in the cooked foods. The types of oils used and the manner of employing them vary with different cultures, but it is principally their use at high temperatures in the presence of atmospheric oxygen that causes the alteration in their physical characteristics (color [1], viscosity [2], and chemical characteristics [3, 4]). Other factors such as heating time, the surface to volume relationship of the oil, the materials used for cooking, and the presence of antioxidants or metallic ions [5–8] can also affect the characteristics of the oils after cooking. Different reactions happen in the oxidation of oils: formation of alkyl radicals, formation of alkylperoxyl radicals, formation and decomposition of hydroperoxide [9]. As a consequence of these reactions, an infinite number of chemical compounds of different chemical families (alkanes, fatty acids, aldehydes, ketones, polycyclic aromatic hydrocarbons…) are generated. Various studies [10–22] link the compounds generated during heating to the teratogenic or mutagenic character of the fumes released during the cooking process. For this reason, countries have developed different regulations prohibiting the use of deteriorated oils and fats [7] and a variety of methods of control and evaluation of the oils used [23–26]. Polycyclic aromatic hydrocarbons and aldehydes are among the compounds released by the heating of oils that are the most noxious to humans. The typical odor released during the frying process is related to the compounds generated by the degradation of the triglycerides, principally by that of linoleic acid [27].
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These compounds belong to the family of unsaturated aldehydes, and a study [27] suggests that (E,E)-2,4-decadienal is primarily responsible for this odor. Nevertheless, as previously stated, the heating of the different types of oil generates another series of compounds belonging to other chemical families that can modify the characteristic odor of frying [28, 29], making it more rancid or green. There are numerous studies that examine the quality of the oil, as well as its composition and characteristics after the heating or frying process [1, 14, 15, 24, 30–32]; nevertheless, fewer studies examine the composition of the fumes generated during this process [4, 33, 34]. The latter studies have developed various sampling methods based, in many cases, on purge and trap systems. Umano and Shibamoto [35, 36] developed two sampling systems in which the volatiles were moved by a current of air or nitrogen through a solution of morpholine, or through deionized water. In the latter case, they were subsequently extracted with dichloromethane. A sampling pump has also been used to collect volatiles on filter paper so that they can then be extracted with acetone [18, 19]. In this way, it is possible to study the polycyclic aromatic hydrocarbons and the amines present in the fumes of the oils. The condensation of the generated vapors and subsequent dissolution with petroleum ether have also been used [37]. In 2004, Fullana developed two distinct sampling methods [4, 33]. In one of them, purging was done with air, and the volatile compounds were adsorbed in Tenax and subsequently desorbed thermally. In the second system, purging was done with nitrogen, and the fumes were collected in 500 mL Tedlar bags (Supelco, Bellefonte, PA, USA). The samples of collected gas were analyzed via GC–MS. In 2010, another sampling procedure was developed [5] in which oil was heated in a balloon flask, and the fumes were moved by a current of purified air. The volatile compounds were adsorbed in C18 cartridges and impregnated with an acidic dissolution of 2, 4-dinitrophenyl hydrazine and subsequently eluded with acetonitrile and analyzed by liquid chromatography. Most of the systems were based on purge and trap systems such as the system applied in this work; however, the best advantage of our method is the use of resin Lichrolut EN which has better properties to retain volatile compounds than other sorbents [38]. The Lichrolut EN trapping capacity is up to 1,000-fold higher than that of Tenax, for example. The principal objective of this study is the application of a sampling system to identify and follow the evolution of the released compounds of olive and sunflower oil at kitchen conditions heating from room temperature to 80 above their smoking point. In the same way, another objective of this study is to examine the contribution that the identified compounds in real cooking conditions make
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to the characteristic odor released during the frying process.
Materials and methods Materials, chemicals, and standards High-oleic sunflower oil and extra virgin olive oil were purchased from a local supermarket. Samples were protected from light during storage and a new bottle was opened each day. Pentanal C97 %, heptanal C92 %, hexanoic acid C98 %, octanal C99 %, nonanal C95 %, (E)-2-octenal C94 %, heptanoic acid C99 %, (E)-2-nonenal 97 %, octanoic acid C98 %, (E)-2-decenal C95 %, (E)-2-undecenal C90 %, (E, E)-2,4-decadienal 85 %, (E, E)-2,4-heptadienal 88 %, and pentafluorophenyl hydrazine were supplied by Sigma–Aldrich, Spain (Madrid, Spain). Hexanal C97 %, (E)-2-hexenal C97 %, pentanoic acid C99 %, (E)-2-heptenal C96 %, and nonanoic acid C97 % were from Fluka, Spain (Madrid, Spain). An alkane solution (C8–C20), 40 mg L-1 in hexane supplied by Fluka, Spain (Madrid, Spain), was employed to calculate the linear retention index (LRI) of each analyte. Dichloromethane and methanol were of chromatographic grade and were purchased from Merck (Darmstadt, Germany). Paraffin oil was purchased from Sigma– Aldrich, Spain (Madrid, Spain). Lichrolut EN resins (styrene/divinylbenzene copolymer) were supplied by Merck. Glass cartridges were handmade. Study via SPE–GC–MS of the volatiles released during heating Experimental system to retain the volatile compounds during heating The experimental system used in this work [39] was developed from that devised by Bueno et al. [40]. In our case, we have adapted this system to evaluate the volatile compounds released from edible oils subjected to high temperature, and it is shown in Fig. 1. Some studies of optimization were made: quantity of resin, quantity of oil, and time of extraction with the pump (data not shown). In this system, the oil is heated in a frying pan with a ceramic hot plate which is controlled by a temperature controller. The frying pan is covered with a glass lid which has two holes where two pieces of Teflon, with a hole in each one, are introduced. One of the pieces must be in the center of the lid and the other piece near the first piece. In the central piece, a micro air sampler pump PAS-500 (Supelco,
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injector with a pulse of pressure of 30 psi during the 3 min splitless time. The carrier gas was He (1 mL min-1) and the column was a DB-WAX ETR (J and W Scientific), 60 9 0.25 mm I.D., with 0.25 lm film thickness, preceded by a precolumn, 3 m 9 0.25 mm I.D. The chromatographic oven was held at 40 C for 10 min then to 220 C at 3 C min-1 and finally the temperature was held at 220 C for 15 min. The obtention of the extracts for each oil sample was made by triplicate. The quantification of the identified compounds was carried out using response factor, knowing, of this way, the mass of the collected analytes. The response factor was calculated by the GC–MS analysis of dichloromethano-methanol solutions containing known amounts of the standards. Knowing the volume of extracted fumes, the concentration of the compounds present in the fumes generated during heating of oil is known. Fig. 1 Sampling system for determining the volatile compounds released
Bellefonte, PA, USA) and a holder are situated. Into the holder, a glass cartridge is introduced, the cartridge contains 100 mg of resin Lichrolut EN (previously conditioned with 4 mL of dichloromethane and dried under vacuum), enclosed by frits and glass wool. The other Teflon piece has also a hole which is used to introduce a thermocouple to control the temperature of oil in each moment. The oils (200 g) were heated from room temperature to 80 above their respective smoking point (190 C for the olive oil and 220 C for the sunflower oil) in 12 min. The sampling was made during heating, at five different temperatures distributed in 12 min. The pump was previously calibrated at 270 mL min-1 (maximum flow) and the sampling time for each extract was 1 min during heating (over a temperature range), so 270 mL of headspace of frying pan were sampling. Cartridges were eluted with 1 mL of dichloromethane containing 5 % of methanol [41]. These extracts were frozen until their injection. Gas chromatographic conditions Two microliters of each extract was injected in a gas chromatograph Varian GC CP-3800 Saturn 2,200 with an Ion-Trap mass spectrometric detection system. The mass analyzer was operated in SCAN mode from 40 to 400 m/z, and the characteristic ions used for quantification purposes are shown in Table 1. The ion source, electronic impact was used at 70 eV, the temperature of the trap was 170 C, and the interface was kept at 220 C. Splitless mode injection was used at a temperature of 250 C in a PTV
Fatty acids analysis The fatty acid composition of oil samples was determined by triplicate as methyl esters by GC-FID, according to the IUPAC standard method [42]. A Varian CP-3800 gas chromatograph was used. The column (30 m 9 0.32 mm and 0.5 lm film thickness) was a DB-WAX from J and W Scientific (Folsom, CA, USA). Samples (1 lL) were injected in split mode (1:30). The injection temperature was 250 C and the oven temperature program was as follows: 160 C (held for 5 min) and 3 C min-1 to 220 C (held for 20 min). Helium was used as carrier gas at a flow rate of 2.2 mL min-1 and detection was by FID (250 C). Sensory studies Estimation of odor thresholds The sensory panel was formed by ten judges aged 23–40, with previous experience in sensory analysis. The tastings were carried out in a conditioned tasting room. In all cases, samples (20 mL) were served presented in amber wide mouth glass amber jars with cap. The determination of odor thresholds was carried out according to the Spanish Norm (AENOR 87-006-92) by means of triangle tests, presenting to the panelists solutions containing the tested group of odorants progressively diluted (dilution factor was 1:3). Panelist had to decide which jar smelt different compared to the two others. The number of correct answers was compared with tabulated values to decide whether significant differences (95 %) exist due to the added amount of the mixture of odorants. A solution of acids (pentanoic, hexanoic, heptanoic, octanoic, and nonanoic acid), other solution of saturated aldehydes (pentanal,
123
123
1,841 1,949
2,059
2,166
Hexanoic acid Heptanoic acid
Octanoic acid
Nonanoic acid
1,100
1,200
1,300
1,600
Undecane
Dodecane
Tridecane
Hexadecane
a
1,179 (345)b
1,093 (320)b
b
c a
1,456
1,560
1,660 1,772
(E)-2-octenal
(E)-2-nonenal
(E)-2-decenal (E)-2undecenal
(E,E)-2,4heptadienal
1,529
1,337
(E)-2-heptenal
Alkedienals
1,229
(E)-2-hexenal
Alkenals
1,402
81
70 69
70
83
TIC
69
57
4,77E-06
2,21E-05 2,37E-05
3,78E-05
2,88E-05
1,62E-06
1,97E-05
1,29E-05
1,45E-06
b
\61
b
\61b
980 (225) 788 (188)b
\89
b
\62
b
101 (21)b
63 (8)
c
1,831 (442)
b
4,775 (1,313)
823 (144)
b
b
b
488 (81)
b
1,514 (249)b
2,103 (483) 943 (221)b
\89
b
424 (71)
b
531 (112)b
192 (24)
b
4,193 (911)
b
9,587 (2,635)
949 (166)
561 (56)b b
Nonanal
TIC
291 (29)b b
1,297
1,21E-05
2,342 (184)
Octanal
70
1,439 (113)
1,192
2,27E-05
Heptanal
56
1,088
Hexanal
b
569 (38)b b
\25c
986
10,299 (1,985)a
2,210 (114)
529 (12)
97 (22)b
1,252 (137)
a
a
Pentanal
6,612 (1,274)b
3,305 (170)
571 (13)
b
113 (26)b
497 (54)
b
14,387 (2,270)
643 (28)b
768 (33)c 11,098 (1,537)
628 (28)b 224 (12)b
245 (12)
701 (31)b 260 (14)b
268 (13)
b
2,919 (425)b
3,090 (411)b b
Heating T : 214–232 C
a
Heating T : 101–133 C
a
Extra virgin olive oil
14,008 (3,079)b
1,85E-05
6,69E-07
5,90E-07
3,89E-06
7,57E-06
8,17E-07
1,38E-05
1,22E-05
8,84E-06 9,51E-06
8,46E-06
Response factor#
7,328 (1,599)b
44
TIC
TIC
57
43
TIC
60
60
60 60
60
m/z
Alkanals
Aldehydes
1,000
Decane
Alkanes
1,733
LRI
Pentanoic acid
Acids
Chemical compound
a
a b
a
a a
a a
3,735 (618)
a
11,967 (1,967)a
51,147 (11,743) 31,251 (7,338)a
4,811 (1,156)
5,205 (868)
a
5,029 (1,057)a
507 (63)
a
97,950 (22,225)
30,263 (8,319)
5,325 (933)
a
4,789 (476)a
11,403 (895)
3,712 (247)a
55,492 (10,870)a
7,087 (1,366)b
3,301 (170)
1,010 (23)
367 (84)a
1,273 (139)
a
13,038 (1,782)
4,530 (1,327)a
1,395 (60)a
1,295 (58)a 571 (32)a
416 (20)
a
8,207 (1,497)a
Heating T : 255–264 C
a
b
b
c
b
\61
a
172 (28)c
4,314 (991) \87b
2,067 (497)
2,542 (424)
\78b
\46
b
a
a
b
8,923 (1,912)
b
17,450 (4,797)
2,175 (381)
699 (69)c
754 (59)
\25c
a
21,078 (5,306)c
\71a
1,875 (96)
\8
a
\85c
279 (31)
b
2,154 (127)
778 (228)c
\16c
443 (20)c 256 (14)c
179 (9)
c
1,656 (269)c
Heating T : 101–133 C
a
a
a
a
\61
a
4,002 (656)b
a
a
b
3,930 (902) 1,801 (423)a
2,295 (552)
2,606 (435)
\78b
\46
b
10,632 (2,312)
b
27,055 (7,437)
3,114 (546)
b
2,296 (228)b
18,813 (1,476)
b
17,370 (1,155)b
68,548 (10,842)b
\71a
2,175 (112)
\8
a
304 (70)a
439 (48)
a
2,918 (230)
a
2,608 (764)b
742 (32)b
724 (32)b 440 (24)b
232 (11)
b
4,746 (863)b
Heating T : 214–232 C
High-oleic sunflower oil
\61a
7,614 (1,248)a
3,872 (775)a \87b
3,223 (775)a
6,191 (1,033)a
38,151 (8,017)a
3,607 (447)a
55,044 (11,047)a
27,246 (7,490)a
7,629 (1,337)a
8,039 (798)a
49,106 (3,853)a
46,408 (3,085)a
138,428 (16,563)a
\71a
2,191 (113)a
\ 8a
118 (27)b
488 (53)a
2,797 (193)a
4,535 (1,329)a
946 (41)a
1,250 (56)a 720 (40)a
344 (17)a
7,795 (1,483)a
Heating T : 255–264 C
a
NS
*
NS
*
Oil type
NS
*
*
*
* NS
Ta
*
*
*
NS
*
Ta * Oil type
Two-factor ANOVA
Table 1 Concentration of volatile compounds (ng L-1) in fumes of extra virgin olive oil and high-oleic sunflower oil heated at different temperatures ranges (sampling time 1 min)
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Ta * Oil type Oil type 7,614 (1,248)a
Ta
The response factor is mgL1 ðCH2 Cl2 Þ /area of specific m/z #
Different superscripts indicate significant differences (95 %) in ANOVA of one factor (temperature) for each of the two oils
* Significant (95 %)
Standard deviation is given in parenthesis
1,37E-06 TIC 1,835 (E,E)-2,4decadienal
a, b, c
4,002 (656)b 172 (28)c 1,026 (168)b \61b
8,232 (1,349)a
Heating T : 255–264 C Heating T : 214–232 C Heating T : 101–133 C
a a
LRI linear retention index, m/z m/z of the ion used for quantification, TIC total ion current, NS not significant (95 %)
Heating T : 101–133 C
Contribution to deep-fried odor
a
Extra virgin olive oil
Response factor# m/z LRI Chemical compound
Table 1 continued
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hexanal, heptanal, octanal, and nonanal), and other solution of unsaturated aldehydes ((E)-2-hexenal, (E)-2-heptenal, (E)-2-octenal, (E)-2-nonenal, (E)-2-decenal, (E)-2undecenal, and (E, E)-2, 4-decadienal) were made in paraffin oil. In each solution, the compounds were in the same ratio which was released at 300 C in sunflower oil. Odor thresholds of each one of these mixtures were estimated.
a
High-oleic sunflower oil
a
Heating T : 214–232 C
a
Heating T : 255–264 C
Two-factor ANOVA
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To evaluate the contribution of each family to the characteristic deep-fried odor, a sort task and a ranking test were carried out. Sort task Each panelist completed one session in individual. The samples were presented randomly in jars with cap at room temperature. First, the panelists were asked to smell the nine samples once in the proposed order, in order to minimize any bias introduced by the order of presentation. Afterward, they could smell the samples in any order. The panelists were asked to sort the samples into groups on the basis of odor. The number of similar odor components in the group was free. Groups could be formed by a sample or for all samples. A simple description of the group smell was necessary. The nine samples were a reference (paraffin oil without any addition) and eight solutions (mixtures of the three families in paraffin oil). These 8 solutions were as follows: iSA, isA, ISA, IsA, ISa, Isa, iSa, and isa. The lowercase letters indicate the concentration of that family in the mixture is the same as the concentration of the odor threshold calculated on the previous section, and the capital letters indicate the concentration of that family in the mixture is ten times the concentration of the odor threshold. I and i are the family of unsaturated aldehydes, S and s are the family of saturated aldehydes, and A and a are the family of acids. Ranking test The group of samples (four) were characterized by an aromatic descriptor like fried, and the reference was evaluated in a ranking test (UNE 87023:1995). The aim of this test was to order by the characteristic deepfried odor. The ranking was made by the summation of the ranks established by each panelist: almost no odor (0), weakest odor (1), second least intense odor (2), second most intense odor (3), and most intense odor (4). The judges were the same as the judges in the previous test and the way of presenting the samples too. Data treatment ANOVA tests were made with SPSS Statistics 19. Hierarchical cluster analysis with the Ward criterion was applied to the data obtained from the sort task. Data from the ranking test were processed with Friedman test to know
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the significance of the detected differences by the judges between samples.
Results and discussion Study by SPE–GC–MS of the volatiles released during heating During heating time, different extracts were collected (at the same temperature conditions and during the same time for both oils) in which twenty-three different compounds were identified and quantified, among them alkanes and carboxylic acids, as well as saturated and unsaturated (both monounsaturated and diunsaturated) aldehydes. Table 1 shows the detected levels (ng L-1) of all the identified compounds for each of the studied oils at three different temperature ranges: temperatures well below the smoking point of both oils (101–133 C), temperatures slightly above it (214–232 C), and temperatures well above it (255–264 C). Two additional samples were taken for each oil, but data were not shown. The three final columns of the table show the results obtained when carrying out an ANOVA of two factors (temperature and type of oil). Influence of temperature and type of oil First, regarding the effect that the heating temperature has on the release of volatiles, one can conclude that it causes a clear variation practically in all the compounds in both oils. In the olive oil, none of the compound families underwent a significant increase in concentration when the temperature was raised from the first to the second range. At the third range, there was a significant increase in the emission of all the volatiles except the alkanes. In the sunflower oil, there was a significant increase in the quantity of all the identified families, except the alkenals and alkanes, across all three temperature ranges. This difference in behavior indicates a different level of stability during heating within these temperature ranges. Virgin olive oil has a high resistance to deterioration due to both a triacylglycerol composition low in polyunsaturated fatty acids and a group of phenolic antioxidants (minor compounds) composed mainly of polyphenols and tocopherols [9]. After applying the ANOVA of two factors to the families of compounds, it is possible to conclude that acids, alkanals, and alkadienals vary significantly with temperature. It is also possible to say that the quantity of acids and alkadienals does not vary with the type of oil, although an important difference in the composition of both oils is (E,E)-2,4-heptadienal, which was only detected in the olive
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oil. Nevertheless, the fumes of the olive oil and of the higholeic sunflower oil differed significantly in their levels of alkanes (higher values in olive oil), of alkanals (much higher in the high-oleic sunflower oil), and of alkenals. In this last case, the measured levels are much higher in the sunflower oil at the two lower temperature ranges; yet, at the third temperature range, this behavior is inverted, and much higher levels are obtained in the olive oil, owing primarily to the elevated content of E-2-decenal and E-2undecenal. This result is very different to that found by Fullana [33] in which E-2-undecenal was not detected. Most works studying fumes of oil [4, 5] heat oil over long periods of time; however, in this study, fumes produced during the first cycle of cooked are analyzed; therefore, the concentration of all identified compounds is much less than in other papers. In our knowledge, no studies have been conducted at kitchen conditions, in real cooking conditions. In this discussion, it is also important to keep in the mind the possible interactions between the oil type factor and the temperature factor, which are evaluated by a twofactor ANOVA test. It was deduced that the release of each family of compounds at different temperatures was significantly different depending on the type of oil, with the exception of the alkanes. Effect of the heating temperature on different families of compounds Figures 2 and 3 show the percentages of each of the families identified in the fumes released when the two oils were heated. Distinct letters were used on the bars of the families to indicate where there was significant percentage variation at differing temperature ranges. The greatest differences were produced by the olive oil, where the percentage of alkadienals increased significantly with the increase in temperature, while the percentage of acids decreased. Regarding the compositional profile of the fumes, it was
Fig. 2 Percentage of each chemical family identified in the smoke sampled at three different temperatures in extra virgin olive oil
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Fig. 3 Percentage of each chemical family identified in the smoke sampled at three different temperatures in high-oleic sunflower oil
observed that the olive oil below and at the smoking point contains primarily saturated compounds (aldehydes and alkanes). When the temperature is increased, the alkanes are no longer found in high proportion, and the fumes are formed primarily by saturated and monounsaturated aldehydes. This compositional profile coincides with the profile of the sunflower oil fumes before and after the smoking point. At the temperature near the smoking point, the sunflower oil fumes release primarily saturated aldehydes (80 %). Relation between fatty acids and released volatile compounds It is well known that when oil is heated, different aldehydes are generated [4, 33, 34, 43–45], among other compounds, as a consequence of the degradation of the oil. These generated compounds are thought to be related to the fatty acids contained in the oils [43, 46]. The percentage of fatty acids of the studied oils appears in Table 2. The relationship of the released aldehydes (sampling at higher temperature, 255–264 C) to the acids from which they originate can be seen in Table 3. The percentage of oleic acid is significantly higher in olive oil; although in the case of the sunflower oil studied here, the quantity of oleic acid present is also very high: above 70 %. Comparing the quantity of aldehydes originating from the decomposition of the same acid, it will be observed, as expected, that there
is a greater quantity in the olive oil. Nevertheless, the difference in the level of aldehydes between the two oils is greater than the difference in the percentage of oleic acid. The greatest differences can be seen in (E)-2-decenal and (E)-2-undecenal, since both oils contain very similar quantities of the rest of the aldehydes. This comparison is also valid for the degradation of linoleic and linolenic acids. In both cases, the greater percentage of acid corresponds to the greater quantity of aldehydes generated as a consequence of their decomposition. In the case of linoleic acid, it is important to highlight that the great differences in the presence of pentanal, hexanal, and (E)-2-heptenal and, in the case of linolenic acid, the absence of (E,E)-2,4-heptadienal in the sunflower oil produce the differences in the sums of the aldehydes originating from those acids. Sensory analysis Four chemical families were detected when heating the sunflower oil. Only three showed aromatic characteristics: carboxylic acids, saturated aldehydes (alkanals), and unsaturated aldehydes (alkenals and alkadienals). In order to know the sensory role of each in the frying odor, the olfactory thresholds were first calculated. The estimated values of the global olfactory threshold for the three studied families were 17.5 lg L-1 for the saturated aldehydes, 18.5 lg L-1 for the unsaturated aldehydes, and 750 lg L-1 in the case of the acid family (pentanoic acid: 14.8 lg L-1; hexanoic acid: 52.7 lg L-1; heptanoic acid: 71.4 lg L-1; octanoic acid: 163.7 lg L-1; nonanoic acid: 447.4 lg L-1). The components and the proportions of each compound in the mix of aldehydes are shown in Table 4. Focusing on the threshold and the quantities found in the fumes, both the saturated aldehydes family and the unsaturated aldehydes appear to be principally responsible for the odor generated during the heating of the oil (acid concentrations do not exceed the odor threshold). Using the previous data (thresholds of families) and those published by Meijboom [47] (individual thresholds), aroma values (OAV) were then calculated. In the case of the unsaturated aldehydes, the sum of the OAV of each of the components
Table 2 Fatty acid composition (%) of high-oleic sunflower oil and extra virgin olive oil Myristic (14:0)
Palmitic (16:0)
Stearic (18:0)
Oleic (18:1)
Linoleic (18:2)
Linolenic (18:3)
Arachidic (20:0)
Behenic (22:0)
Sunflower
0.04 (3.7)
4.25 (1.5)
3.60 (0.43)
73.09 (0.045)
16.92 (0.28)
0.64 (5.8)
0.32 (1.7)
1.15 (5.1)
Olive
0.01 (2.0)
10.58 (1.3)
3.17 (0.15)
80.38 (0.14)
4.20 (0.22)
1.20 (4.2)
0.37 (2.0)
0.10 (0.92)
All compounds are different significative (95 %) between oils Relative standard deviation (%) is given in parenthesis. n = 3
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1038
Eur Food Res Technol (2013) 236:1031–1040
of the mix is 0.097, while the OAV of the mix taking into consideration the global threshold was estimated as 27.027. The sum of the OAV of each saturated aldehyde was 0.041 and the OAV of the mix was 1.143. In both cases, there was a synergistic effect; although in the case of the unsaturated aldehydes, this effect was much greater. The results of the sort task of the nine samplings appear in Fig. 4. Two groups can be seen. One is formed by those Table 3 Relation between fatty acids composition of both oils studied and the level of aldehydes (lg L-1) released and present in the fumes analyzed at 255–264 C (sampling time 1 min) Olive oil Oleic acid composition (%) Heptanal
High-oleic sunflower oil
80.4
73.1
4.8
8.0
Octanal
5.3
7.6
Nonanal
30.3
27.2
(E)-2-Decenal
51.1
3.9
(E)-2-Undecenal
31.3
0.09 (\LOD)
Total Linoleic acid composition (%)
122.8 4.20
46.8 16.92
Pentanal
3.7
46.4
Hexanal
11.4
49.1
Heptanal (E)-2-Heptenal
4.8 5.0
8.0 38.2
(E)-2-Octenal
5.2
6.2
(E)-2-Nonenal
4.8
3.2
(E,E)-2,4-Decadienal
8.2
7.6
43.1
158.7
Total Linolenic acid composition (%) (E)-2-Hexenal
1.20
0.64
0.5
3.6
(E,E)-2,4-Heptadienal
3.7
0.06 (\LOD)
Total
4.2
3.7
LOD limit of detection
Table 4 Odor activity values of different mixtures of aldehydes in paraffin oil Mixture saturated aldehydes
a
Odor threshold calculated in the laboratory (explication of the estimation in Sect. 2.4.1). b Individual odor threshold (in paraffin oil) taken from [47]. c Initial concentration of each compound/individual odor threshold
123
Mixture unsaturated aldehydes
samplings that possess a concentration of acids 10 times the olfactory threshold (A) (group 1), with a cheese aromatic descriptor. The second group (group 2) is formed by the other five samplings whose aromatic descriptors are rancid and fried. Therefore, it can be concluded that the presence of high concentrations of acids released with the heating masks the deep-fried odor. As stated previously, the samples from group 2 underwent a ranking test to order them according to the deepfried odor intensity. The samples valued at an intensity significantly higher than this odor (Friedman test, 95 %) were those that contained a concentration of unsaturated aldehydes 10 times above the threshold value (I), that is, the Isa and ISa samples, followed by the iSa and isa samples and the reference (paraffin oil). To avoid any deep-fried odor (independently of the food introduced in the heated oils), it is advisable to fry with extra virgin olive oil at a not excessively high temperature. This conclusion is possible comparing these results with the volatile compounds quantified in the fumes. At temperatures below 230, the antioxidant power of polyphenols is favored and the generation of unsaturated aldehydes is reduced. These data are in accord with those presented by Warner, Neff and van Loon [27, 48, 49] in which the frying odor was associated with different unsaturated aldehydes, which included many of those identified in this present study. However, in the study of Warner, it was not mentioned that the saturated aldehydes could influence the deep-fried odor. The presence of saturated aldehydes could be the cause of the rancid description of this group, as other studies indicate [50, 51], because it has been reported that the quantification of hexanal and nonanal is sufficient to test the oxidation of the oils.
Compounds
Odor threshold in the mixture (lg L-1)a
Pentanal
3.1
Hexanal
3.7
Heptanal
1.2
2,760
0.001
Octanal
1.8
275
0.007
Nonanal
7.7
11,610
0.001
(E)-2-Hexenal
0.49
8,600
0.002
(E)-2-Heptenal
5.83
12,040
0.013
(E)-2-Octenal
1.53
6,020
0.007
(E)-2-Nonenal
1.43
2,752
0.014
(E)-2-Decenal
2.79
29,068
0.003
(E)-2-Undecenal
2.41
129,000
0.001
(E,E)-2,4-Decadienal
4.02
1,849
0.059
Individual odor threshold (lg L-1)b
Individual OAVc
OAVmixture/ P OAVi
206
0.017
1.1/0.041
275
0.016
27.1/0.097
Eur Food Res Technol (2013) 236:1031–1040
1039 Compliance with Ethics Requirements This article does not contain any studies with human or animal subjects.
References
Fig. 4 Hierarchical cluster analysis of samples spiked at different levels of target compounds (i,I unsaturated aldehydes, s,S saturated aldehydes, a,A acids)
Conclusions This study has applied a new sampling system based on SPE–GC–MS for determining the volatile compounds generated during the heating of two oils with different smoking points (sunflower and extra virgin olive oil). Twenty-three different compounds, belonging to distinct chemical families, were identified at kitchen conditions, heating from room temperature to 80 above their smoking point. Both oils were found to have a different level of stability in relation to temperature. Only the alkane family had a similar behavior with both types of oil (no significant variation with temperature), although they were more concentrated in the olive oil fumes. The quantity of acids and aldehydes with double unsaturation was not significantly different between the types of oil. This did not occur with unsaturated or saturated aldehydes; the latter were produced in greater quantity in the sunflower oil. It was possible to relate the content of fatty acids of each of the oils to the proportion of aldehydes generated in each case. Also, the sensory study has shown that there is a synergistic effect between the different aldehydes, some of which are responsible for the characteristic odor of the frying process, principally the unsaturated aldehydes. These are present in lower quantities in the extra virgin olive oil than in the high-oleic sunflower oil, when these oils are heated to temperatures used in frying processes. Acknowledgments I.O. has received a grant from BSH Electrodome´sticos Espan˜a S.A. Authors thank E. Campo for helping with sensory analysis. Conflict of interest
None.
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