Release of Volatile Oxidation Products from Sunflower Oil and Its Oil

Chapter 25

Release of Volatile Oxidation Products from Sunflower Oil and Its Oil-in-Water Emulsion in a Model Mouth System 1

2

Saskia M. van Ruth and Jacques P. Roozen 1

Department of Food Science and Technology, Division of Nutritional Sciences, University College Cork, Cork, Ireland Department of Food Technology and Nutritional Sciences, Wageningen Agricultural University, P.O. Box 8129, 6700 EV Wageningen, the Netherlands Downloaded by GEORGETOWN UNIV on April 6, 2017 | http://pubs.acs.org Publication Date: September 7, 2000 | doi: 10.1021/bk-2000-0763.ch025

2

The amounts secondary lipid oxidation products formed in vegetable oils, which are available for perception, are determined by both lipid oxidation rates and release from the matrix. In the present work, the influence of fatty acid composition and emulsification on lipid oxidation rates and release were determined during oxidation of vegetable oils at 60 °C. Volatile compounds were released from sunflower oil, a blend of sunflower and linseed oil, and from their 40 % oil-in-water emulsions in a model mouth system and were analyzed by gas chromatography-sniffing port analysis. Higher polyunsaturated fatty acid content and emulsification of the oil resulted in more odor active compounds and greater amounts available for perception. Higher concentrations in the oil phase of these samples revealed increased lipid oxidation rates. Reference compounds added to the samples demonstrated an increase in aroma release from the oil under mouth conditions for most of these compounds. In contrary, the higher static headspace concentrations of reference compounds were higher for the emulsion than for the oil.

A major part of flavor research has dealt with analysis of volatile compounds, which is not surprising since the nose is capable of detecting hundreds of different odors. To elicit a response, an aroma compound must achieve a sufficient concentration in the vapor phase to stimulate the receptors in the nasal cavity during eating. The rate of volatilization depends upon the partition coefficient of the compound, molecular interactions between aroma compounds, the ambient temperature, the composition and viscosity of the food material, and binding to components of the food (7).

© 2000 American Chemical Society

Roberts and Taylor; Flavor Release ACS Symposium Series; American Chemical Society: Washington, DC, 2000.

309

Downloaded by GEORGETOWN UNIV on April 6, 2017 | http://pubs.acs.org Publication Date: September 7, 2000 | doi: 10.1021/bk-2000-0763.ch025

310

Analysis of volatiles in the gaseous headspace above food samples has been widely used to determine factors affecting their partitioning between the product and vapor phase (2-6). A variety of methods have been used to sample and/or isolate the trace volatiles present in such headspace. However, the term "headspace" as applied to these sampling techniques has been used to convey a number of different meanings. Headspace could be defined as the gaseous mixture surrounding a sample within a closed system at equilibrium. However, many "headspace" determinations involve the passage of a gas over the sample to sweep the volatiles into a trapping device. Under these non-equilibrium conditions, the composition of the sample as subsequently determined, i.e. the ratios of the individual volatiles, may bear little relation to the real headspace composition. As measurements in the equilibrium headspace provide information regarding factors influencing the partitioning of aroma compounds between a product and vapor phase, the dynamic headspace measurements might be useful to simulate the aroma release from products when food is sniffed prior to eating. In actual eating situations, aroma concentrations are determined kinetically rather than thermodynamically because equilibrium is not very likely (7). Only a few instrumental methods of flavor release have incorporated the crushing, mixing, dilution, and temperature conditions required to simulate aroma release in the mouth. Lee (8) reported an instrumental technique for measuring dynamic flavor release. A mass spectrometer was coupled with a dynamic headspace system, i.e. a vial with several small metal balls. The vial was shaken and the balls simulated chewing, while the headspace was flushed with helium gas in order to displace volatile compounds, one of which was analyzed directly by mass spectrometry. Roberts and Acree (7) reported a "retronasal aroma simulator", a purge-and-trap device made from a blender. It simulated mouth conditions by regulating temperature to 37 °C, adding artificial saliva, and using mechanical forces. Napl et al. (9) described a "mouth imitation chamber", which consisted of a thermostated 800 mL vessel with a stirrer, while artificial saliva was added to the system. The authors of the present work presented their model mouth system for the first time in 1994 (10). The model mouth system has a similar volume as the human mouth. It is temperature controlled, and the sample is salivated and masticated. Later, it was shown that the release of volatiles from rehydrated vegetables in this model mouth did not differ significantly from release in the mouth of volunteers (11). Both oil-in-water (e.g. milk, cream, salad dressings, and mayonnaise) and waterin-oil types (e.g. butter, margarine) emulsions are common in foods (12). Fats modify the perception of flavor compounds by influencing partitioning between the food product, saliva, and vapor phase within the oral cavity (13). The general effects of food composition and fat phase volume on flavor release have been reviewed (14), and several theoretical physicochemical models have been developed (15-16). Initially, studies were carried out to determine the partition coefficients of aroma compounds between air and water or oil (2). It soon became apparent that simple partition coefficients did not explain the release of volatiles from foods. Volatile lipid oxidation products are important for the aroma of oils and emulsions. The perception of aroma of the latter depends on the formation of the aroma compounds as a result of lipid oxidation as well as on the release of these


311

compounds. The authors have shown previously the development of aroma compounds in sunflower oil and its emulsion during storage, which was related to aroma generation and aroma release (17). Fatty acid composition of the lipid phase and emulsification can both influence formation and release of compounds. In the present work, the influence of fatty acid composition and emulsification on lipid oxidation rates and release were determined during oxidation of vegetable oils.

Experimental Procedures


Experimental Samples Sunflower oil (SFO), and a blend (VEGO) of SFO (85/100 WAV) and linseed oil (15/100 W/W) and their 40 % oil-in-water emulsion (SFO-E and V E G O - E ; 40/100 W / W sunflower oil, 59/100 W / W deionized water, 1/100 W / W Tween 60) were supplied by Unilever Research Vlaardingen (Vlaardingen, the Netherlands). The C I 618 fatty acid composition determined by gas chromatography of methyl esters was for SFO: 6.0 % 16:0, 4.3 % 18:0, 23.6 % C 1 8 : l , 64.3 % C18:2, 0.12 % C18:3, and for V E G O 5.8 % C16:0, 0.01 % C 1 6 : l , 4.2 % C18:0, 23.0 % C 1 8 : l , 56.9 % C18:2, 8.5 % C18:3. SFO contained 716 mg α-, 26 mg β-, 7 mg γ-, and < 5 mg δ-tocopherol per kg oil, V E G O contained 609 mg α-, 22 mg β-, 73 mg γ-, and < 5 mg δ-tocopherol per kg oil (AOCS Official Method Ce 8-89, 1992). The emulsion was prepared using a homogenizer (APV Gaulin model L A B 40-10 RBFI, A P V Gaulin GmbH, Liibeck, B R D ) at 150 bar for 10 min.*The average particle size in the emulsion was 1.0 pm (Coulter Laser measurements) and stable during storage at 60 °C for 8 days. For lipid oxidation experiments, samples (65 mL) were stored in glass jars (350 mL) in the dark at 60 °C for 4 days. For volatility and aroma release experiments, propanal, butanal, pentanal, hexanal, octanal (aldehydes: PolyScience, Niles, IL), 1-pentanol (Sigma-Aldrich, Steinheim, Germany), 3-pentanol (Aldrich, Milwaukee, WI), l-penten-3-one (SigmaAldrich, Steinheim, Germany), l-penten-3-ol and l-octen-3-ol (both: Janssen Chimica, Geel, Belgium) were added to the fresh oils and emulsions in duplicate (0.1 % V / V ) . The solutions were incubated for 24 h at 4 °C in the dark before being subjected to analysis. Volatile compounds formed in the control samples, i.e. oils and emulsions without added compounds stored under the same conditions, were determined and their amounts subtracted.

Static Headspace Analysis For static headspace gas chromatography (SHGC), 2 mL oil or emulsion, or 1 m L oil or emulsion and 1 mL artificial saliva (18) were transferred into a 10 mL vial and incubated at 60 °C for 10 min in the headspace unit of a Carlo Erba M E G A 5300 G C (Interscience bv, Breda, the Netherlands). The G C was equipped with a DB-Wax column (J&W Scientific, Folsom, CA), 30 m length, 0.53 mm i.d., film thickness l p m


312

and a flame ionization detector (FID) at 275 °C. A n initial oven temperature of 60 °C for 5 min was used, followed by a rate of 3 °C min" to 110 °C and then by 4 °C min" to 170 °C. Each (stored) sample was analyzed in duplicate vials. Peak areas were standardized with known concentrations in oils and emulsions. Results were calculated as mmoles per kilogram oil. 1

1


Isolation of Volatile Compounds Volatile compounds were isolated from the oil or emulsion in a model mouth system as described previously (10). Artificial saliva (4 mL) was transferred to the sample flask (70 mL) of the model mouth system, which was kept at 37 °C, and a sample of oil and emulsion (4 mL) was added. The headspace was flushed with purified nitrogen gas (100 mL min" ) for 2 min to trap the volatile compounds in 0.1 g Tenax T A (diameter 0.25-0.42 mm, Alltech Nederland bv, Zwijndrecht, the Netherlands). During isolation of the volatiles, a plunger made up and down screwing movements in order to simulate mouth movements. 1

Gas Chromatography/Sniffing Port Analysis (GC/SP) In GC/SP, desorption of volatile compounds from Tenax was performed by a thermal desorption (245 °C, 5 min)/cold trap (-120 °C/260 °C) device (Carlo Erba T D A S 5000, Interscience bv, Breda, the Netherlands). Gas chromatography was carried out on a Carlo Erba M E G A 5300 (Interscience bv, Breda, the Netherlands) equipped with a Supelcowax 10 capillary column, 60 m length, 0.25 mm i.d., film thickness 0.25 pm and a FID at 275 °C. A n initial oven temperature of 40 °C was used, followed by a rate of 2 °C min" to 92 °C and then by 6 °C min" to 272 °C. At the end of the column the effluent was split 1:2:2 for FID, sniffing port 1, and sniffing port 2, respectively. In volatility/release experiments, the sniffing ports were not occupied and the FID response was used only (20 % of the effluent). In regular GC/SP sessions two assessors were sniffing and FID response was recorded simultaneously. Ten assessors were selected based on their sensitivity, memory, ability to recognize odors, and availability. These assessors were trained on the technique of sniffing prior to sniffing the effluent of the oil and emulsion samples. Assessors used portable computers with a program in Pascal for data collection. The data were converted from the field discs into Microsoft Excel software in order to process the raw data. Aroma descriptors were generated during preliminary GC/sniffing experiments and clustered after group sessions of the panel, resulting in a list of 19 descriptors (green, mushroom, spicy, fruity, sweet, flowers, fatty, oil, rancid, rotten, musty, chemical/glue, nuts, almond, burned, caramel, chocolate, vanilla, sharp/irritating). These descriptors and "other/I do not know" had to be used for each compound detected by the assessors at the sniffing port. Tenax tubes without adsorbed volatile compounds were used as blank samples for determining the signal-to-noise level of the group of assessors. For identification 1

1


313

odor descriptors and the number of assessors perceiving a compound at the sniffing port were compared with those of authentic compounds at similar concentrations.


Gas Chromatography/Mass Spectrometry Volatile compounds were isolated as described above and were identified by combined G C (Varian 3400, Varian, Walnut Creek, C A , USA) and mass spectrometry (MS; Finnigan M A T 95, Finnigan M A T , Bremen, Germany) equipped with a thermal desorption/cold trap device (TCT injector 16200, Chrompack bv, Middelburg, the Netherlands). Capillary column and oven temperature program were the same as those used in "gas chromatography of isolated volatile compounds". Mass spectra were obtained with 70 eV electron impact ionization, while the mass spectrometer was continuously scanning from ml ζ 24 to 400 at a scan speed of 0.7 s/decade (cycle time 1.05 s).

Statistical Analysis Analysis of variance ( A N O V A ) was used to determine significant differences between the means of quadruple analysis of aroma compounds released in the model mouth. If significant differences were found, Fisher's Least Significant Difference tests (LSD) were performed (19). The GC/SP data were subjected to Friedman two-factor ranked analysis of variance (19). Significance level is ρ < 0.05 throughout the study.

Results and Discussion The volatile compounds of SFO, V E G O , and their emulsions were isolated in the model mouth system and analyzed by GC/SP after four days of storage at 60 °C. Figure 1 represents the chromatogram of aroma compounds of V E G O - E obtained by sniffing port detection. GC/SP revealed 14 compounds possessing detectable odors. The aroma compounds were identified by G C / M S , and by comparison of their retention times, odor descriptions, and number of assessors perceiving the compounds with those of authentic compounds. The aroma compounds were characterized by their FID peak areas and the odors described by the assessors of the sniffing panel (Table I). G C sniffing of dummy samples showed that detection of an odor at the sniffing port by one or two of ten assessors can be considered as "noise". Odor descriptors and numbers of assessors perceiving the odor active compounds are listed for the various samples in Table I. The authors showed in a previous experiment that numbers of assessors perceiving compounds are related to the perceived intensity at the sniffing port (20). A number of odor active compounds were in common: hexanal, octanal, and l-octen-3-one. Across all samples, pentanal, hexanal, and l-octen-3-one were most frequently detected by the assessors. Many of the odor active compounds identified can be formed in autoxidation of linoleic acid,


314

10 2 ο

4 +

1


10

20

30

40

50

Retention time [min] Figure 1. Sniffing chromatogram of a the 40 % oil-in-water emulsion of a blend of sunflower and linseed oil after 4 days of storage at 60 °C which is the major fatty acid of SFO and V E G O , e.g. pentanal, hexanal, octanal, pentanol, l-octen-3-one, and 1- octen-3-ol (27-25). Despite the common compounds, the numbers of assessors perceiving individual compounds differed significantly between the various samples (Friedman two-factor ranked analysis of variance, ρ < 0.05). Higher values were observed for the emulsions than for the oils, as well as for the oil with the higher concentration unsaturated fatty acids (VEGO). Figure 2 represents the relative FID peak areas of the formed compounds released from the different samples. The same trends are shown as for the sniffing data in Table 1, except for hexanal where SFO was not found to be less intense than SFO-E by sniffing. However, relative differences are generally greater for the FID data, which is probably due to the log linear relationship between the physical concentration of a compound in the G C effluent and the number of assessors perceiving an odor active compound (24). The differences observed between the samples, which originated from emulsification and fatty acid composition, could result from differences in formation of the compounds, as well as from differences in release from the matrix. In order to study the formation aspect, the release of compounds formed was related to known concentrations in the oil and emulsions (calibration curve) and the amounts formed in the oil phase were calculated for those compounds with sufficient FID response (Table II). A N O V A of the amounts formed in the oil phase showed significant differences (p < 0.05) between the samples. As before, V E G O - E showed highest values, followed by SFO-E, V E G O and SFO, respectively (LSD, ρ < 0.05). The differences due to emulsification are in agreement with results of Frankel et al. (25), which showed increased formation of volatile secondary lipid oxidation products in emulsions in comparison with bulk oils. In contrary to the one phase oil matrix, in the emulsion the various molecules partition themselves between the three different regions of the emulsion system according to their polarity and surface activity. The precise molecular environment of a molecule may have a significant effect on its chemical reactivity or other properties in such a system (26). Another



6

5

4

3

2

'SFO = sunflower oil. SFO-E = 40 % sunflower oil-in-water emulsion. VEGO = blend of vegetable oils. VEGO-E = 40 % blend of vegetable oils-in-water emulsion. At or below detection level (Ω two assessors perceiving an odor). Unk = unknown, not identified.

6

1

2

Table I. Odor active compounds released from sunflower oil, a blend of sunflower and linseed oil, and their emulsions after 4 days of storage at 60 °C. odor descriptions and numbers of assessors perceiving the compounds in GC/sniffing port analysis VEGO-E? VEGCT SFO-E SFO Odor description Compound _5 4 4 Fruity, sweet, chemical 1. Pentane 8 Chemical, sweet, oil 2. Heptane 3 10 Chemical, fruity, fatty, sweet, oil rancid, 3. Propanal sharp/irritating Oil, spicy, chocolate, fatty 1 9 4. Butanal 9 6 9 Chemical, rancid, green, fatty, caramel 5. Pentanal 3 Chemical, oil 6. Unk 6 9 Chemical, oil, musty 7. l-Penten-3-one 8 9 7 7 Green, flowers, fatty, oil, chemical 8. Hexanal 9 4 5 Chemical, rancid 9.3-Pentanol 5 5 Chemical, sweet, green 10. Unk 3 3 Chemical, rancid 11. 1-Pentanol 3 5 3 5 Fruity, oil 12. Octanal 4 9 6 4 Mushroom, musty, rancid 13. l-Octen-3-one 4 4 Musty, oil, rancid, almond 14. l-Octen-3-ol


316

c* 1000000

o ts S *


ο ο

fi~"

100000 •

10000

SFO

• VEGO

1000 T3

100

s

10

II M i l i i l

• SFO-E • VEGO-E

/////yy/y

1

fi fi o α S o

9f

V

V

Figure 2. Relative release of odor active compounds in a model mouth system, which were formed in sunflower oil (SFO), a blend of sunflower and linseed oil (VEGO) and their 40 % oil-in-water emulsions (SFO-E and VEGO-E) during storage at 60 "Cfor 4 days, based on GC/FID analyses. Release of odor active compoundfrom SFO = 100.

1

Table II. Amounts of odor active compounds formed in oils and emulsions 2

Compound

SFO

Propanal Butanal Pentanal l-Penten-3-one Hexanal 3-Pentanol 1-Pentanol Octanal l-Octen-3-ol

Release of Volatile Oxidation Products from Sunflower Oil and Its Oil

Recommend Documents