JOURNAL OF FOOD SCIENCE
CHEMISTRY/BIOCHEMISTRY
Fat Reduces Volatiles Production in Oil
Emulsion System Analyzed by Purge-and-Trap
Dynamic Headspace/Gas Chromatography
C. Jo and D.U. Ahn
ABSTRACT
gently, and used as a standard mix.
INTRODUCTION
FLAVOR IS GENERALLY UNDERSTOOD AS
the perception of volatile compounds released from food while eating (Lubbers et
al., 1998). Volatility and affinity of volatile
compounds in a food matrix control the effectiveness of aroma compounds as flavoring agents. The volatility of aroma compounds depends on the vapor-liquid partitioning of volatile compounds, which determines
the affinity of volatile molecules for each
phase (Buttery et al., 1973). Most volatile
aromas are hydrophobic, and would be preferentially partitioned into the lipid phase
(Harrison et al., 1997) with the reduced gasliquids partition coefficient decreasing the
volatility or amount of flavor compounds in
lipids (Buttery et al., 1973). McNulty and
Karel (1973) reported that increased viscosity of oil decreased the release rate of octanol
from silicone oils. From a study using diacetyl and heptan-2-one, Harrison et al.
(1997) reported that the initial release rates
of these compounds were faster for emulsions
with low oil content. The equilibrium concentrations depended upon the nature of flavor compounds and the volume fraction of
oil in the emulsion.
Many researchers have shown that 2thiobarbituric acid reactive substances
(TBARS) values correlated well with the
amount of volatile compounds and sensory
characteristics of meat products (Salih et al.,
1987, Shahidi and Pegg, 1994). Studies on
the production of volatile compounds in irAuthors Jo and Ahn are affiliated with the Dept. of
Animal Science, Iowa State Univ., Ames, IA 500113150. Address inquiries to Dr. D.U. Ahn.
© 1999 Institute of Food Technologists
radiated meat or meat products (Ahn et al.,
1998) indicated that samples with higher fat
content produced smaller amounts of volatile compounds than low fat meat with similar TBARS values. The interactions with other ingredients such as carbohydrates and proteins would affect the release of volatile compounds in foods. Hydrocolloids such as gums
and thickeners affect the rate of flavor release
due to their viscosity, which hinders transport to the vapor phase (Godshall, 1997).
Physicochemical conditions of foods, which
influence conformation of proteins, also are
closely related to flavor release (Lubbers et
al., 1998).
The objective of this study was to determine the effects of fat content on the production and release of volatiles in oil emulsions.
MATERIALS & METHODS
Chemicals
Pure standard volatile compounds were
purchased from Chromatography Research
Supplies Inc. (Addison, IL). Compounds
were selected after comparison of retention
times from individual standards to produce
separate peaks for exact quantification. Eight
hydrocarbons (heptane, 1-heptene, octane, 1octene, nonane, 1-nonene, decane, 1-decene),
five aldehydes (propanal, 2-methylpropanal,
propenal, hexanal, octanal), four alcohols (2methylpropanol, 1-pentanol, 1-hexanol, 1heptanol) and seven ketones (2-pentanone,
4-methyl-2-pentanone, 3-heptanone, 2-heptanone, cyclopentanone, cyclohexanone)
were selected, and 20 µL of each compound
were taken into a 2-mL sample vial, mixed
Sample preparation
Oil emulsions were prepared by mixing
200 mL of deionized distilled water (DDW)
with 0, 0.5, 1, 2, 4, or 8% (w/w) of soybean
oil (Hain Food Group Inc., Uniondale, NY)
using a Waring Blendor for 7 min at high
speed. Soybean oil containing vitamin E was
used to minimize lipid oxidation during preparation and analysis. Oil emulsion (2 mL)
was taken into a 40 mL sample vial and 1 µL
of volatile standard mix was added by a micropipette (10 µL). To avoid losing the compounds to the air, the micropipette needle was
inserted into the oil emulsion while adding
standard volatile compounds. For the interaction of volatile standards with water, standard mixture (1 µL) was added either in dry
sample vials or vials with 2 mL of water.
Volatile compounds analysis
Precept II and Purge-and-Trap concentrator 3000 (Tekmar-Dohrmann, Cincinnati,
OH) were used to purge and trap added volatile compounds as described by Ahn et al.
(1998) with modifications. A GC (Model
6890, Hewlett Packard Co., Wilmington, DE)
with a mass selective detector (MSD, Model
5973, Hewlett Packard Co., Wilmington, DE)
was used to qualify and quantify volatile
compounds.
Sample (2 mL) was purged by using an
auto sampling unit (Precept II) equipped with
a robotic arm. Samples were purged at 40°C
with helium (99.999% purity, 40 mL/min) for
11 min. Volatiles were trapped by using a
Tenax/silica/charcoal column (Tekmar-Dohrmann, Cincinnati, OH), desorbed for 1 min
at 220°C. The temperature of transfer lines
was maintained at 135°C. A split inlet (split
ratio, 99:1) was used to inject volatiles into
an HP-wax bonded polyethylene glycol column (60 m, 250 m i.d., 0.25 m nominal),
and ramped oven temperature was used
(32°C for 1 min, increased to 40°C at 2°C/
min, to 50°C at 5°C/min, to 70°C at 10°C/
min, to 140°C at 20°C/min, to 200°C at 30°C/
min and held for 5 min). Helium was the carrier gas at constant flow of 1.2 mL/min. The
ionization potential of MS was 70 eV, scan
range was 50 to 550 m/z, and data acquisition rate was 2.94 scan/sec. The identification of volatiles was achieved by comparing
Volume 64, No. 4, 1999—JOURNAL OF FOOD SCIENCE
641
CHEMISTRY/BIOCHEMISTRY
Release of volatiles in oil emulsion systems containing various fat contents was
determined using purge-and-trap dynamic headspace/gas chromatography. Oil
emulsions were prepared using 0, 0.5, 1, 2, 4, or 8% oil, and a standard mixture
was prepared using hydrocarbons, aldehydes, ketones, and alcohols. Fat content in oil emulsions reduced the volatility of added volatile standards, and the
amount of volatiles released from oil emulsion correlated negatively with fat content (r2 = 0.92 in total volatiles). Among these tested, the volatilization of ketones
was least influenced by fat content. The release of nonpolar hydrocarbons was
not influenced but polar compounds (aldehydes, ketones, and alcohols) were
greatly influenced by water.
Key Words: volatiles, oil emulsion, fat content, purge-and-trap, headspace
Fat Reduces Volatiles Production in Oil Emulsion . . .
Table 1—Collected amounts of hydrocarbons from oil emulsions with different fat contents
Lipid content (%)
Volatiles
Heptane
1-heptene
Octane
1-octene
Nonane
1-nonene
Decane
1-decene
0
0.5
1276a
1287a
1689a
1793 a
2429a
2263a
3087a
3171a
1296a
1308 a
1763a
1840a
2338a
2208a
2293b
2563b
1
2
Total ion counts (× 105)
1233a
1089a
1235a
1168 a
1492b
1297b
1652a
1416b
1477b
1686b
1536b
1476b
1611bc
1412 cd
1551c
1934c
4
8
SEM
1080a
1125 a
1273b
1316b
1286b
1177 b
1273d
1357c
721b
739b
800c
847c
761c
712c
583e
626d
52.5
47.8
55.5
68.8
111.2
99.5
166.5
145.6
a-eMeans within a row with no common superscript differ (P<0.05). n = 24.
Table 2—Collected amounts of aldehydes from oil emulsions with different fat contents
Lipid content (%)
Volatiles
CHEMISTRY/BIOCHEMISTRY
Propanal
2-methylpropanal
Propenal
Hexanal
Octanal
0
98a
165a
277a
156a
1625a
0.5
93a
162a
267a
155a
1542a
1
2
Total ion counts (× 105)
90ab
82b
160a
150ab
273a
230ab
162a
160a
1595a
1430b
4
8
SEM
69c
139ab
217b
147a
842c
67c
116b
188b
122b
542d
2.7
8.6
12.2
3.9
26.9
a-dMeans within a row with no common superscript differ (P<0.05). n = 24.
Table 3—Collected amounts of alcohols from oil emulsions with different fat contents
Lipid content (%)
Volatiles
0
0.5
1
2
4
8
SEM
25
257b
462c
474c
25
234b
388d
355d
2.9
8.6
12.0
15.2
5
2-methylpropanol
1-pentanol
1-hexanol
1-heptanol
33
299a
635a
916a
36
313a
626a
914a
Total ion counts (× 10 )
34
34
311a
298a
a
615
561b
872a
708b
a-dMeans within a row with no common superscript differ (P<0.05). n = 24.
Table 4—Collected amounts of ketones from oil emulsions with different fat contents
Lipid content (%)
Volatiles
0
0.5
1
2
4
8
SEM
298a
33b
1252a
1205a
330
383b
241b
26b
923b
672b
315
356b
9.3
4.3
44.7
68.5
21.5
9.6
5
2-pentanone
4-methyl-2-pentanone
3-heptanone
2-heptanone
Cyclopentanone
Cyclohexanone
305a
66a
1449a
1518a
361
463a
300a
65a
1459a
1495a
380
479a
Total ion counts (x 10 )
304a
290a
66a
68a
1426a
1413a
1486a
1482a
380
376
478a
457a
a,bMeans within a row with no common superscript differ (P<0.05). n = 24.
mass spectral data with those of the Wiley
library (Hewlett Packard Co., Wilmington,
DE). The peak area (total ion counts x 105)
was reported as the amount of volatiles released.
Statistical analysis
This experiment was designed to determine the effects of oil content on production
of volatile compounds. Analyses of Variance
by SAS program (SAS Institute Inc., 1989)
were used to find significance and StudentNewman-Keul’s multiple range test was used
642
to compare differences among mean values.
Mean values of peak area (×105) from chromatogram and standard errors of the mean
(SEM) were reported. Significance was defined at P <0.05.
RESULTS & DISCUSSION
THE PEAK AREAS FROM OIL EMULSION
samples containing volatile standards indicated that increase of oil content decreased
production of all volatile compounds in the
oil emulsion (Tables 1–5). This result agreed
well with those of Buttery et al. (1971, 1973)
JOURNAL OF FOOD SCIENCE—Volume 64, No. 4, 1999
who found that the increase of oil percentage reduced the rate of air-to-solution partition coefficient of volatile compounds in water, vegetable oil, and water-oil mixtures. As
carbon number (molecular weight) of volatile compounds increased, the reduction rate
of volatile compounds increased because increase of carbon number sharply decreased
air-to-solution partition coefficient (Buttery
et al., 1973).
The amount of individual volatile compound released was expected to be similar in
all samples because amounts of individual
standards added were the same. However,
results indicated that certain compounds such
as propanal, 2-methylpropanol, and 4-methyl-2-pentanone had extremely low response
and other compounds such as 2-methylpropanal, propenal, hexanal, 1-pentanol, and 2pentanone had somewhat higher response.
One reason could be the high volatility of
small compounds (carbon number 6 or less)
which might have been volatilized into headspace of the vial (2 mL) during and after preparation of the standard mixture. This would
result in lower concentrations of those in the
liquid phase of the standard mixture.
The measurement of 1-heptene decreased
43% and that of decane decreased 81% as
the content of fat in oil emulsion increased
from 0 to 8%. Hydrocarbons collected relatively large amounts of volatiles compared
with alcohols or ketones. The molecular
weights of hydrocarbons (C7 – C10) we used
were higher than those of alcohols (C4 – C7)
and ketones (C5 – C7). Within aldehydes, the
reduction of octanal was the greatest (67%)
and that of propenal was the lowest (22%).
Among the four groups, the volatility of ketones was influenced least by fat content in
the oil emulsion (Table 5).
Ahn et al. (1998) observed that cooked
meat patties prepared from L. dorsi (6.64%
fat) produced less hexanal or total volatiles
than L. psoas (2.38% fat) and R. fermoris
(1.83% fat). Jo et al. (1999) reported that
cooked pork sausage with 15.8% fat had
higher TBARS values but had the same or
lower volatile content than that with 4.7%
fat. This result could have been caused by
low gas-liquid partition coefficient of volatile compounds in the high fat meat sample.
De Roos and Graf (1995) reported that lipids influenced both the physical and chemical stability of flavors. A reduction in fat content resulted in higher flavor loss during processing and storage due to increase in flavor
volatility. The volatility of standard compounds decreased linearly (r2 of aldehydes,
alcohols, ketones, and hydrocarbons were
0.93, 0.92, 0.91, and 0.85, respectively) with
the increase of fat content (Table 5) in the oil
emulsion system. Among the volatile groups,
ketones had the smallest change (proportional) caused by fat content.
Among the volatile standards, 9 volatile
compounds with low areas (volatility) were
CONCLUSION
FAT CONTENT IN OIL EMULSIONS INFLUENCED THE VOLATILITY of compounds.
High fat content in the oil emulsion reduced
the release of volatile compounds when determined using the purge-and-trap dynamic
headspace/GC method. Results indicated that
the production of volatiles would be influenced not only by the polarity and partition
coefficients of volatile compounds but also
by the composition and characteristics of the
Table 5—Collected amounts of hydrocarbons, aldehydes, alcohols, and ketones from oil
emulsions with different fat contents
Lipid content (%)
Volatiles
Hydrocarbons
Aldehydes
Alcohols
Ketones
Total
0
16461a
2321a
1883a
3857a
24827a
0.5
15607a
2217a
1889a
3877a
23889a
1
2
Total ion counts (x 105)
11786b
12205b
2279a
2051b
1832a
1601b
3834a
3795 a
20034b
19941b
4
8
SEM
9887b
1413 c
1218 c
3202b
16016c
5838 c
1034d
1000d
2290 c
10403d
764.1
41.5
25.3
105.0
758.6
medium in model foods.
REFERENCES
Ahn, D.U., Olson, D.G., Lee, J.I., Jo, C., Wu, C., and
Chen, X. 1998. Packaging and irradiation effects on
lipid oxidation and volatiles in pork patties. J. Food
Sci. 63: 15-19.
Buttery, R.G., Bomben, J.L., Guadagni, D.G., and Ling,
L.C. 1971. Some consideration of the volatiles of organic flavor compounds in food. J. Agric. Food Chem.
19: 1045-1048.
Buttery, R.G., Guadagni, D.G., and Ling, L.C. 1973. Flavor compounds: Volatilities in vegetable oil and oilwater mixtures. Estimation of odor thresholds. J. Agric. Food Chem. 21: 198-201.
De Roos, K.B. 1997. How lipids influence food flavor.
Food Technol. 51: 60-62.
De Roos, K.B. and Graf, E. 1995. Nonequilibrium partition model for predicting flavor retention in microwave and convection heated foods. J. Agric. Food
Chem. 43: 2204-2211.
Godshall, M.A. 1997. How carbohydrate influence flavor. Food Technol. 51: 63-67.
Harrison M., Hills, B.P., Bakker, J., and Clothier, T. 1997.
Mathematical models of flavor release from liquid
emulsion. J. Food Sci. 62: 653-658, 664.
Jo, C., Lee, J.I., and Ahn, D.U. 1999. Lipid oxidation,
color, and volatiles changes in irradiated cooked pork
sausage with different fat content and packaging during storage. Meat Sci. 51: 353-361.
Lubbers, S., Landy, P., and Voilley, A. 1998. Retention
and release of aroma compounds in foods containing
proteins. Food Technol. 52: 68-74, 208-214.
McNulty, P.B., and Karel, M. 1973. Factors affecting flavor release and uptake in O/W emulsions. II. Stirred
cell studies. J. Food Technol. 8: 319-331.
Salih, A.M., Smith, D.M., Price, J.R., and Dawson, L.E.
1987. Modified extraction 2-thiobarbituric acid method for measuring lipid oxidation in poultry. Poultry
Sci. 66: 1483-1488.
SAS Institute, Inc. 1989. SAS User’s Guide. SAS Institute, Inc., Cary, NC.
Shahidi, F. and Pegg, R.B. 1994. Hexanal as an indicator
of meat flavor deterioration. J. Food Lipids 1: 177186.
Ms received 10/3/98; revised 3/15/99; accepted 3/29/99.
Journal paper No. J - 17959 of the Iowa Agriculture and Home
Economics Experiment Station, Ames, IA 50011. Project No. 3322,
and supported by Hatch Act and the Food Safety Consortium.
a-dMeans within a row with no common superscript differ (P < 0.05). n = 24.
Table 6—Percent (%) release of volatile standards added in water a
1-heptene
Control
Water
100
97.3
propanal
100
25.8
propenal
100
65.7
1-nonene
1-pentanone
2-methyl2-pentanone
Total ion counts (×105)
100
100
106.4
60.7
100
60.7
hexanal
propanol
1-pentanol
100
63.8
100
48.6
100
36.3
aRelative to those added in dry sample vials (control). n = 4.
Reprinted from J. Food Sci. 64(4): 641–643
©1999 Institute of Food Technologists
Volume 64, No. 4, 1999—JOURNAL OF FOOD SCIENCE
643
CHEMISTRY/BIOCHEMISTRY
compounds, therefore, contributes to the
slower perception of fat-soluble volatile compounds than the water-soluble compounds.
In oil-water emulsion systems, proteins added as emulsifiers can influence the volatility
of the aroma compounds because proteins are
important in molecule transport also through
the oil-water interface (Lubbers et al., 1998).
Proteins can bind aroma compounds, and the
binding intensity depends on the composition and structure of the protein.
selected to determine their interactions with
water. The release of nonpolar hydrocarbons
(1-heptene and 1-nonene) was not influenced
by water (Table 6), but aldehydes, ketones,
and alcohols were greatly influenced. When
mixed in water, the volatilized amount of
these polar compounds ranged from 34.3%
to 74.2% of those added in dry purge vials.
Lubbers et al. (1998) indicated that the volatility of flavor compounds in water could be
increased or decreased according to their
chemical nature. Except for propanal, the
measurement of volatiles decreased as the
polarity increased. De Roos (1997) reported
that flavor release from the lipid phase of a
food proceeds at a lower rate than that from
the aqueous phase. This was attributed to
higher resistance to mass transfer in fat and
oil than in water. Flavor compounds must be
released from a lipid phase to an aqueous
phase in an emulsion system before they are
released from aqueous phase to headspace.
This delayed release of fat-soluble flavor