Chapter 35 Microwave Volatilization of Aroma Compounds Ted R. Lindstrom and Thomas H. Parliment Kraft General Foods, 555 South Broadway, Tarrytown, NY 10591
Dielectric constants and dielectric loss factors were measured for a series of seven and eight carbon alcohols, aldehydes, ketones and esters, having the similar boiling points. The microwave heating rates of these compounds were also measured and found to be proportional to the microwave power absorption calculated from the measured dielectric properties. Both the microwave absorption and microwave heating rates were found to decrease in the series benzaldehyde, octanone, heptanol, octanal, octanol, methyl hexanoate. Volatilization of these compounds by microwave heating was determined by headspace analysis of mixtures dispersed in fat and heated in a microwave oven. For all mixtures, the headspace concentrations of microwave heated samples were the same as control samples heated on a steam bath. There was no selective microwave volatilization of the chemicals even though the microwave heating rates of the individual compounds varied by a factor of ten.
The concept that microwave heating occurs through the absorption of microwave energy by specific molecules has long enticed flavor chemists into compounding microwave flavors on the basis of dielectric and microwave heating properties. A prominent example of this approach is the Delta Τ theory proposed by Shaath (i). Shaath has cataloged over 500 flavor compounds on the basis of relative microwave heating rates of the pure flavor compounds. Other studies suggest that the dielectric properties of the pure flavor compounds are not important. Steinke et al (2,3), in a study of the microwave volatilization of organic acids, concluded that the chemical and physical properties of the complete food system controlled the volatilization of individual flavor compounds. The purpose of this study is to understand the role of dielectric properties (4) in microwave volatilization. The dielectric constants and dielectric loss factors for
0097-6156/94/0543-0405$06.00/0 © 1994 American Chemical Society
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six flavor compounds possessing similar boiling points were measured. The microwave heating rates of the individual flavor compounds were measured and related to the dielectric properties and microwave absorption. The volatilization of these flavor compounds was determined by measuring the relative headspace concentrations of the flavor compounds over mixtures heated in a microwave oven and on a steam bath.
Materials and Methods Materials The flavor compounds were purchased from Aldrich Chemical Co. (Milwaukee, WI) and used without further purification. The boiling points reported by Aldrich are: 1-heptanol 176°, methyl heptanoate 172-173.5°, benzaldehyde 178179°, 2-octanol 174-181°, 2-octanone 173°, and octanal 171°. The fat used was hydrogenated coconut/palm kernal oil manufactured by Karlshamns Food Ingredients (Columbus, OH). Dielectric Measurements The dielectric constants and dielectric loss factors were measured at 51 frequencies between 0.3GHz and 20GHz using a Hewlett-Packard HP8720A network analyzer equipped with a HP85070A dielectric probe kit. A l l measurements were made at 25°C. Microwave Heating of Pure Flavors Ten grams of each flavor compound in a 25mL glass vial were heated in a Litton Generation II microwave oven (630w) at full power for two minutes. 200g of room temperature water in a 400mL glass beaker were heated simultaneously with each flavor compound to serve as a simulated food load. Both the sample vial and the water load were positioned in exactly the same location in the microwave oven for each heating test. The temperatures of the flavor compounds were measured continuously during microwave heating using a Luxtron model 750 Fluoroptic Thermometry System (Mountain View, CA) equipped with Luxtron MIW fiber optic probes. Two probes were used in each flavor compound vial; one in the center and one on the side, each 1cm above the bottom of the vial. The surface temperatures of the vial were also monitored continuously during microwave heating using a Hughes Aircraft Co. (Carlsbad, CA) model 4100 Probeye infra red video camera to detect uneven heating patterns across the sample vial. No uneven heating was detected. Heating of Flavors in Fat Matrix Equal weight binary, ternary, and quaternary mixtures of the flavor compounds were mixed with the hydrogenated fat so that each flavor compound was present at 2%(w/w) of the total flavor-fat mixture. 10g of each flavor-fat mixture were sealed in a serum screw cap 25mL vial and heated either on a steam bath for 3.5 minutes (to 95°C) or in the Litton microwave oven containing a 60g water load for 3.5 minutes (also to 95°C). The 60g water load was used to make the microwave heating rate of the flavor-fat mixture comparable to the steam bath heating rate.
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Headspace Analysis Immediately after heating, l m L of headspace was removed from the vial with a gas-tight syringe and injected into a Perkin Elmer model 3920 gas chromatograph. A 30m χ 0.53mm i.d. fused column coated with a Ιμπι film of DB225 (cyanopropyl phenyl methyl silicon) was used. The following gas chromatograph oven parameters were employed: 2 minutes at 80°C then 4°C/minute to 130°C with a final hold of 1 minute. Peak area analysis was performed by a Perkin Elmer Nelson model 2600 Chromatography Data System. Experiments were performed in duplicate and the results averaged. Results and Discussion The six compounds were chosen for this study since they are used or found in various flavors in food products. They represent six chemical classes yet possess the one characteristic of similar boiling point. The frequency dependence of the dielectric constants of the aldehydes and ketone is shown in Figure 1. Benzaldehyde has the highest dielectric constant in this frequency range. The dielectric constants for the alcohols and ester are shown in Figure 2. These dielectric constants of the alcohols and ester are lower then those of the aldehydes and ketone shown in Figure 1. The frequency dependence of the dielectric constants of the alcohols is much different from the other compounds. Instead of having a characteristic dispersion at about 8 to 10 GHz, the two alcohols have a much lower frequency dispersion out of the measuring range of the instrument. The corresponding frequency dependence for the dielectric loss factors of the aldehydes and ketone is shown in Figure 3. Again, benzaldehyde has the highest dielectric loss factors rising to a maximum value of 8.1 at its critical frequency (4) of 5.7GHz. Octanone has a maximum dielectric loss factor of 4.2 at a critical frequency of 8.0GHz. Octanal has a maximum dielectric loss factor of 2.4 at a critical frequency of 8.6GHz. Figure 4 shows the dielectric loss factors for the alcohol and ester. The dielectric loss factors of the alcohols rise towards a maximum value at a critical frequency that lies outside the measuring range of the instrument. This suggests that the dipolar rotational motion of the alcohols is much slower than the other compounds in this study which is consistent with the ability of the alcohols to form intermolecular hydrogen bonds. Table I lists both the dielectric constant ε' and the dielectric loss factor ε" at 2.45GHz for the six flavor compounds. Benzaldehyde has the highest dielectric constant and loss factor. 2-Octanol has the lowest dielectric constant and methyl heptanoate has the lowest dielectric loss factor. A useful way to relate the dielectric properties to the absorption of microwave energy is through the concept of penetration depth (5). The half-power penetration depth is the distance into the absorbing material at which half the microwave power has been absorbed. The half power penetration depth is a convenient measure to compare the relative microwave absorbing characteristics of materials and to understand readily the effects of dielectric properties and geometry on microwave heating.
408
THERMALLY GENERATED FLAVORS 20 r
.
Ο • A
BENZALDEHYDE 2-OCTANONE OCTANAL
ιππτίΓΠ QTjjp I
S
4 -
10
10*
10
Frequency (GHz) Figure 1. Dielectric constants of benzaldehyde, 2-octanone and octanal as a function of frequency at 25°C.
υ
15
S
1
10
e
10
,Q
Frequency (GHz) Figure 2. Dielectric constants of 1-heptanol, 2-octanol and methyl heptanoate as a function of frequency at 25°C.
12 Γ Ο
•Δ
BENZALDEHYDE 2-OCTANONE OCTANAL
Frequency (GHz) Figure 3. Dielectric loss factors of benzaldehyde, 2-octanone and octanal as a funtion of frequency at 25°C.
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Table I. Dielectric Properties at 2.45GHz and 25°C, Calculated Microwave Penetration Depths, and Microwave Heating Rates of Flavor Compounds
Flavor Compound
Dielectric Constant ε]
Benzaldehyde 2-Octanone 1-Heptanol Octanal 2-Octanol Methyl Heptanoate
Dielectric Half Power Loss Factor Penetration ε^ Depth (cm)
16.42 10.54 3.14 7.61 2.76 4.59
1.04 2.48 3.06 3.73 4.05 9.43
5.34 1.78 0.79 1.00 0.58 0.31
Heating Rate (°C/sec)
2.98 1.34 1.13 0.77 0.76 0.28
The half power penetration depth in centimeters d ^ is calculated from the measured dielectric properties by the following equation:
'/2
0.9563e'
pli 2
-1/2
(1)
in which ε' is the dielectric constant and ε" is the dielectric loss factor. The half power penetration depths were calculated for the six flavor compounds from the measured dielectric properties at 2.45GHz and 25°C and are listed in Table I. Short penetration depths indicate that the microwave power is absorbed more readily. Benzaldehyde has the shortest penetration depth; methyl heptanoate has the longest penetration depth. Table I lists the flavor compounds in order of increasing penetration depth or decreasing microwave power absorbance. Note that the penetration depth or power absorbance is primarily dependent on the dielectric loss factor. The microwave heating rates of the pure flavor compounds are shown in Figures 5 and 6. Benzaldehyde heats the fastest reaching 130°C in 1 minute. Methyl heptanoate heats the slowest with the temperature rising only 15° in 1 minute. Heating rates were calculated as the initial slopes of the curves in Figures 5 and 6. These heating rates are listed in the fourth column of Table I and are expressed in degrees per second. The heating rates have the same rank order as the microwave power absorption expressed as the penetration depth. These heating rates are graphed as a function of the reciprocal half power penetration depth in Figure 7. The reciprocal half power penetration depth is directly proportional to the microwave power absorbed per distance. Figure 7 clearly shows that the microwave heating rates of these six flavor compounds are
410
THERMALLY GENERATED FLAVORS
Ο • Δ
ο
1-HEPTANOL 2-OCTANOL METHYL HEPTANOATE
ο
υ "β
10
10'
10
Frequency (GHz)
Figure 4. Dielectric loss factors of 1-heptanol, 2-octanol and methyl heptanoate as a function of frequency at 25°C.
140
° 1-Heptanol -Ô
ε
Δ
2-Octanol
D
Methyl Heptanoate
80 +
40
20
10
-h
-h
-+-
20
30
40
50
60
Time (seconds)
Figure 5. Microwave heating rates of heptanol, octanol and methyl heptanoate.
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Microwave Volatilization
140
120
u
° Octanal
Β
° Benzaldehyde
W)
α
•α «ο Δ
2-Octanone
2
eu ε
40
Time (seconds) Figure 6. Microwave heating rates of octanal, benzaldehyde and octanone.
3.5
T
Reciprocal Penetration Depth (1/cm) Figure 7. Microwave heating rates as a function of the reciprocal of the half power penetration depth.
412
THERMALLY GENERATED FLAVORS
directly related to the microwave power absorbed, calculated from the dielectric properties. Table II lists the normalized gas chromatograph peak areas of the headspace over the flavor-fat mixtures heated to 95°C (3.5 minutes) on either a steam bath or microwave oven. Data for four different mixtures are shown. In the binary mixture (Mixture 1) of benzaldehyde and methyl heptanoate, the ratio of benzaldehyde to methyl heptanoate in the headspace is essentially the same whether the flavor compounds were volatilized on a steam bath or in the microwave oven. If anything, the benzaldehyde concentration decreases in the microwave where it would be expected to increase. Similar results are found for Mixtures 2, 3, and 4. In each case where the flavor compounds are dispersed in fat, the headspace concentrations of flavor compounds are the same regardless of the form of heating. Compounds having higher microwave absorption and higher microwave heating rates are not preferentially volatilized by microwave heating compared to heating over a steam bath.
Table II. Normalized G C Peak Areas of Headspace over Four Flavor Compound Mixtures Heated to 9 5 ° C (3.5 minutes) on Steam Bath or in Microwave Oven
Mixture Number
Composition
Normalized Peak Areas Microwave Oven Steam Bath
1
Benzaldehyde Methyl Heptanoate
67 100
63 100
2
Benzaldehyde 2-Octanol Methyl Heptanoate
62 24 100
64 25 100
3
Benzaldehyde 2-Octanone 1-Heptanol Methyl Heptanoate
67 85 34 100
68 87 31 100
4
Benzaldehyde Octanal/2-Octanol Methyl Heptanoate
64 95 100
64 93 100
Conclusions These results demonstrate that the microwave heating rates of pure flavor compounds are dependent on the dielectric properties of the compounds. The
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measured heating rates were found to be related to the dielectric constant and loss factor in a way described by the equations for microwave power absorption by dielectric materials. Across this series of flavor compounds having similar specific heats, those having high dielectric loss factors heat more rapidly than compounds having low dielectric loss factors. The volatilization by microwave heating of the flavor compound mixtures dissolved in fat does not depend on the relative microwave heating rates of the pure compounds. The headspace concentrations of the flavor compounds is dependent on the temperature of the total system rather than the absorption of microwave energy by individual molecules. The aroma of foods heated in a microwave oven will be dependent on the temperature profile of the food itself and not on the dielectric properties of the individual aroma compounds.
Literature Cited 1.
Shaath, N.A.; Azzo, N.R. In Thermal Generation ofAromas, Parliment, T.H.; McGorrin, R.J.; Ho, C.-T., Eds.; ACS Symposium Series 409; American Chemical Society: Washington, DC, 1989; pp 512-518.
2.
Steinke, J.A.; Frick, C.M.; Gallagher, J.A.; Strassburger, K.J. In Thermal Generation of Aromas, Parliment, T.H.; McGorrin, R.J.; Ho, C.-T., Eds.; ACS Symposium Series 409; American Chemical Society: Washington, DC, 1989; pp 519-525.
3.
Steinke, J.A.; Frick, C.; Strassburger, K.; Gallagher, J. Cereal Foods World, 1989, 34, 330-332.
4.
Mudgett, R.E. In Microwaves in the Food Processing Industry, Decareau R.V., Ed.; Academic Press: New York, 1985; pp 15-37.
5.
Buffler, C.R.; Stanford, M.A. Microwave World, 1991,12,15-23.
RECEIVED February
9, 1993
