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Dielectric properties of six different species of starch at 2450 MHz
Food Research International, Vol. 31, No. 1, pp. 43±52, 1998 # 1998 Canadian Institute of Food Science and Technology Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain PII: S0963-9969(98)00058-1 0963-9969/98/$19.00+0.00
Dielectric properties of six dierent species of starch at 2450 MHz M. K. Ndife,a G. S,umnu,b* & L. Bayindirlib a
Department of Food Science and Technology, The Ohio State University, Columbus, OH 43210, USA b Food Engineering Department, Middle East Technical University, 06531, Ankara, Turkey Dielectric properties at 2450 MHz were determined for tapioca, corn, wheat, rice, waxymaize and amylomaize starches in granular (moisture content 1 and 13%) and in solution forms [starch±water ratio:1:1, 1:1.5, 1:2 (w/w)] at temperatures from 30 to 95 C. Dielectric properties were shown to be dependent on temperature, moisture content and starch type. The increase in moisture content increased dielectric constant and loss factor of all types of starches. Increasing temperatures decreased the dielectric values for starch solutions (high moisture content) but increased the dielectric values for granular starches (low moisture content). Quadratic equations were developed to show the relationship between the changes in dielectric properties with temperature for starch species. There were signi®cant dierences in dielectric properties of dierent types of starches. # 1998 Canadian Institute of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved Keywords: dielectric properties, dielectric constant, loss factor, starch.
INTRODUCTION
of insucient gelatinization of starch during microwave baking of starch based products will be minimized. Dielectric properties depend on moisture content and temperature of the food (Calay et al., 1995). Dielectric constant and loss factor increase with moisture content are common (Roebuck et al., 1972; Nelson et al., 1991; Calay et al., 1995). The rate of change of dielectric constant and loss factor with temperature depends on the free and bound water content of the material (Calay et al., 1995). Nelson et al. (1991) showed that dielectric properties of potato starch increased with moisture content and temperature. Dielectric properties of potato starch in both gelatinized and granular form were compared at 25 C for solutions having 40±100% water and gelatinized starch was shown to have higher E00 values than the granular form (Roebuck et al., 1972). RyynaÈnen et al. (1995) compared dielectric properties of potato, wheat, corn and waxy corn starch solutions at 2750 MHz and at low concentrations of starch (5±30%) and no signi®cant dierence in dielectric properties was detected between dierent kinds of starch. The important factor which in¯uenced the dielectric properties was found to be the water content.
Dielectric properties are the key factors in the understanding of the interactions of microwaves with food materials. Dielectric constant (E0 ) is a measure of the ability of a material to store electromagnetic radiation and dielectric loss factor (E00 ) is a measure of the ability of a material to dissipate electrical energy into heat. Penetration depth shows how far a wave will penetrate before it reduces to 1/e of its intensity (Mudgett, 1986). Although many foods are produced by microwave heating; less satisfactory results are obtained in baked products in the microwave oven. This is due to the short heating time that is not enough for complete starch gelatinization. In order to improve quality of a microwaveable baked product, in the formulation it is necessary to use a starch type that can be quickly heated and gelatinized in the microwave oven. Dielectric properties of starch will give an idea which type of starch will be heated in a microwave oven quickly. Thus, the problem
*To whom correspondance should be addressed. Fax: +90312-2101270; e-mail: [email protected] 43
44
M. K. Ndife et al.
The objective of this study was to compare the dielectric properties of starches from dierent plant origins in solution form and granular form. This will be helpful to ®nd out the type of starch that will be heated in microwave oven faster which can be further used in the development of high quality microwave baked products. In order to predict thermal eects of the microwave heating process, equations that correlate the eects of variations in moisture content and temperature on dielectric constant and loss factor are determined in this study.
Statistical analysis
MATERIALS AND METHODS
Dielectric properties of starch solutions were shown to be dependent on water content (Tables 1 and 2). The increase in starch±water ratio from 1:1 to 1:2 increased dielectric constant and loss factor of starch solutions. This was expected since when there was much free water in the system, electric ®eld would see more polarizable dipole moments per unit volume of the sample. Dielectric constant and loss factor of starch in granular form also increased with increasing moisture content. A representative ®gure of this is given by wheat (Fig. 1) and corn starches (Fig. 2) for dielectric constant and loss factor respectively. Variation of tapioca, rice, waxymaize and amylomaize starches with temperature are given in Table 3. In granular starches, water is adsorbed on the starch and refered as irrotationally bound. The heating of granular starch can be explained by the bound water relaxation mechanism. The bound water relaxation phenomena result from the interaction between surface charges and centers of partial positive and negative charge on water dipoles (Mudgett, 1995). Although the mechanism of heating of starch solutions is dierent from that of starch granules the eects of moisture on dielectric properties are the same (Tables 1±3 and Figs 1 and 2). The change of dielectric constant and loss factor of dry granular samples (moisture content: 1%) with temperature was linear (Figs 1 and 2). The temperature dependence of both dielectric constant and loss factor increased, when the water activity of the granular starch was increased to 0.6 (moisture content:13%) (Figs 1 and 2). Nelson et al. (1991) also showed that temperature dependence increased markedly with increasing moisture content in various hydrocolloids, potato starch, locust bean gum, gum arabic, carrageenan and carboxy-metyl cellulose in granular form. Regression analysis was performed to relate the dielectric responses of wheat, rice, corn, tapioca, waxymaize and amylomaize solutions to temperatures between 30 and 95 C using the least square ®t. Dielectric constant and loss factor of all starches were found to vary quadratically with temperature. Equations are in the form of:
Materials Wheat starch (Midsol 50, Midwest Grain Products, Inc., Atchison, KS), tapioca starch (Actobind 2000, Advance Food Systems Inc., NJ), corn (amaizo, American Maize Products Company, Hammond, Indiana), waxy maize (amioca, American Maize Products Company, Hammond, Indiana), amylomaize (amylomaize VII, American Maize Products Company, Hammond, Indiana) and rice starch (Remy Neutral Dr, A and B Ingredients, Fair®eld, NJ) were used. Methods For preparation of starch solutions, starch was mixed with dierent levels of water to obtain a starch±water ratio of 1:1, 1:1.5 and 1:2 (w/w) which are within the typical concentrations used in baked products. To prepare granular starches at dierent water activities starch was placed in small containers and kept in a desiccator at 25 C. The bottom of the desiccator was ®lled with saturated sodium bromide solution to obtain relative humidity of 60%. Water activities of the samples were adjusted in the desiccator for 2 weeks before dielectric properties were measured. Low moisture level was obtained by oven drying of starch for 1 h at 130 C (Nelson et al., 1991). Water activity of the samples was measured by using a water activity meter (Aqualab-CX2, Decagon Devices Inc., Pullman, Washington). Dielectric properties were determined by using a dielectric probe and a network analyzer at 2450 MHz (HP 85070, Santa Rosa, CA). Dielectric measurements were taken in the temperature range of 30±95 C at 5 C intervals. Bulk density of the powdered samples were determined at 30 C before dielectric properties were determined. Weight of the samples were recorded. The reading of sample length from sample height gage and cross sectional area of sample holder provided sample volume and bulk density was calculated as g cmÿ3 (Nelson et al., 1991).
Analysis of variance was used to determine signi®cant dierences between starch treatments. Treatment means were compared by using Duncan's Multiple range comparison test. Three samples per treatment were used in each analysis. Correlations were obtained to relate dielectric constant and loss factor to temperature for each starch and concentration (SAS, 1988). RESULTS AND DISCUSSION
Temperature ( C) 30 35 40 45 50 55 60 65 70 75 80 85 90 95
Tapioca
Corn
Wheat
Rice
Waxymaize
Amylomaize
Starch±water ratio (w/w) 1:1c* 1:1.5b 1:2a
Starch±water ratio (w/w) 1:1b 1:1.5b 1:2a
Starch water ratio (w/w) 1:1b 1:1.5b 1:2a
Starch water ratio (w/w) 1:1c 1:1.5b 1:2a
Starch water ratio (w/w) 1:1.5c 1:2b
Starch water ratio (w/w) 1:1.5c 1:2b
42.7 41.0 39.0 36.5 32.4 30.1 27.5 25.0 23.4 20.9 21.0 23.9 25.2 27.4
52.0 51.1 49.8 45.6 40.0 36.0 35.0 34.5 34.0 32.8 23.3 21.6 21.6 24.8
54.3 54.0 52.2 51.9 41.7 40.9 36.6 33.8 33.6 33.6 32.4 31.5 34.0 36.5
45.6 45.8 45.9 46.5 47.1 48.2 48.2 47.4 40.0 37.2 36.3 36.1 37.0 37.0
52.8 54.9 54.0 53.5 51.7 51.2 49.2 43.3 43.0 41.0 35.2 33.9 35.2 35.5
53.4 53.1 49.8 41.3 38.7 36.3 34.5 31.4 25.1 22.1 20.8 21.8 31.5 31.6
52.2 51.9 51.2 48.0 40.8 36.3 34.7 34.2 33.0 30.2 31.2 32.2 34.5 39.4
64.1 63.4 60.4 59.1 47.2 43.7 35.7 32.9 29.3 24.6 25.5 26.6 28.8 36.0
65.2 62.3 61.5 61.0 60.0 54.0 47.4 43.5 41.2 36.1 36.1 32.6 34.2 36.1
41.0 41.7 42.4 42.9 43.4 43.3 43.3 43.2 43.0 42.8 42.4 42.4 43.0 43.2
48.8 49.2 49.3 49.2 48.8 48.3 47.2 41.5 41.8 42.8 43.5 43.4 42.5 40.5
58.4 58.4 57.3 57.0 56.7 55.7 54.8 54.4 51.5 51.8 52.0 52.1 51.7 51.2
59.1 53.2 46.9 43.8 38.4 34.1 33.0 31.0 30.4 29.3 24.8 21.5 22.3 25.5
54.2 54.8 56.0 51.0 50.8 44.0 35.9 34.5 31.9 27.9 24.9 24.5 25.4 30.0
54.2 54.7 54.1 51.7 51.0 50.2 49.1 47.9 46.5 41.3 41.4 41.3 41.1 41.0
*Means water levels with dierent letters (a, b, c) have dielectric values signi®cantly dierent from the others. (Each measurement is mean of three replications).
63.1 63.4 64.0 64.4 64.7 65.3 64.6 64.7 63.0 61.6 59.0 59.7 61.1 60.8
Dielectric properties of six dierent species of starch at 2450 MHz
Table 1. The eects of temperature on dielectric constant of dierent starches at dierent water levels
45
46
Table 2. The eects of temperature on loss factor of dierent starches at dierent water levels Tapioca
Corn
Wheat
Rice
Waxymaize
Amylomaize
Temperature Starch±water ratio (w/w) Starch±water ratio (w/w) Starch±water ratio (w/w) Starch±water ratio (w/w) Starch water ratio (w/w) Starch water ratio (w/w) ( C) 1:1c 1:1.5b 1:2a 1:1c* 1:1.5b 1:2a 1:1c 1:1.5b 1:2a 1:1c 1:1.5b 1:2a 1:1.5b 1:2a 1:1.5b 1:2a 8.7 8.6 8.3 7.2 5.7 5.6 5.6 4.1 3.7 3.5 3.2 3.0 4.0 4.4
9.1 8.8 8.3 7.7 6.8 6.3 5.8 5.6 5.5 5.4 5.4 4.3 3.0 3.9
10.4 10.1 9.7 8.8 7.8 7.0 6.6 6.4 6.2 5.8 5.3 5.5 5.7 5.9
9.1 8.7 7.7 6.3 6.1 5.6 5.3 4.7 3.8 3.3 4.0 4.3 4.7 5.0
8.1 8.0 7.8 7.7 7.2 7.1 7.0 6.5 6.5 6.1 6.0 6.0 6.0 5.6
9.4 9.4 8.9 8.6 8.3 8.0 7.8 7.3 7.2 7.0 6.8 6.5 6.2 6.2
10.7 9.6 9.0 8.4 8.0 6.4 6.0 5.8 5.9 6.3 6.4 6.4 6.2 6.2
10.4 9.5 9.4 8.9 8.2 7.6 6.8 6.0 5.6 5.5 6.4 6.9 7.2 7.5
10.4 10.4 10.3 9.4 8.9 8.2 7.1 6.3 6.1 5.6 5.6 5.8 5.9 6.2
8.7 8.2 7.6 7.2 7.0 6.7 6.6 6.4 6.3 6.1 5.9 5.8 5.6 5.4
10.0 9.6 9.4 9.1 8.8 8.5 8.2 7.5 7.4 7.3 7.2 7.0 6.6 6.2
10.0 9.5 9.2 9.2 9.0 8.8 8.6 8.5 8.3 8.2 8.0 7.9 7.7 7.3
8.4 7.4 6.6 6.1 5.1 4.5 4.3 3.9 3.7 3.4 2.9 2.7 2.7 3.2
8.4 8.1 7.9 7.0 6.3 5.4 5.1 4.8 4.3 4.2 3.6 3.2 3.3 3.4
7.2 6.5 6.2 5.5 4.9 4.5 4.2 3.8 4.7 4.7 4.5 4.5 4.0 3.8
*Means water levels with dierent letters (a, b, c) have loss factor values signi®cantly dierent from the others, (each measurement is the mean of three replications).
7.8 7.4 7.0 6.8 6.4 5.9 5.3 4.8 4.2 4.1 4.0 3.4 2.7 3.5
M. K. Ndife et al.
30 35 40 45 50 55 60 65 70 75 80 85 90 95
Dielectric properties of six dierent species of starch at 2450 MHz
47
Fig. 1. Variation of dielectric constant of granular wheat starch at dierent moistures (~): dry, (^): aw=0.6, (Ð):model. Three replications were used to generate each data point.
Fig. 2. Variation of loss factor of corn starch solutions. (~): dry, (^): aw=0.6, (Ð):model. Three replications were used to generate each data point.
0 a bT cT2
1
00 a bT cT2
2
where E0 is dielectric constant, E00 is loss factor, a, b and c are constants, T is temperature. These quadratic equations are used to determine the dielectric constant and loss factor of starch species at
the desired temperature. Moreover, when experimental data are inadequate, or in order to avoid developing temperature dependent equations for starch based products, the quadratic equations (eqns 1 and 2) may be used by incorporating them within a microwave-heating model. Equations together with experimental data were shown in Figs 1±5. Constants and coecients of the equations of starch species are given in Table 4.
48
M. K. Ndife et al. Table 3. Variation of dielectric properties of granular starches with moisture content
Temperature ( C) Dielectric property Dielectric constant
Starch type Tapioca Corn Wheat Rice Waxymaize Amylomaize
Loss factor
Tapioca Corn Wheat Rice Waxymaize Amylomaize
30
35
40
45
50
55
60
65
70
75
Dry aw=0.6b Drya aw=0.6b Drya aw=0. 6b Drya aw=0.6b Drya aw=0.6b Drya aw=0.6b
2.25 3.11 2.74 2.25 2.42 4.43 1.25 2.50 2.25 3.06 2.42 3.23
2.18 3.15 2.56 2.26 2.49 4.40 1.20 2.52 2.26 3.02 2.33 3.24
2.14 3.21 2.58 2.68 2.52 4.39 1.18 2.54 2.68 3.00 2.30 3.29
2.06 3.28 2.61 2.69 2.55 4.40 1.22 2.60 2.69 3.20 2.30 3.42
2.03 3.33 2.69 2.77 2.60 4.43 1.28 2.65 2.77 3.49 2.44 3.76
2.06 3.41 2.73 2.87 2.74 4.75 1.35 2.57 2.87 3.68 2.52 3.87
2.24 3.49 2.75 2.85 2.8 4.86 1.37 2.60 2.85 4.00 2.58 3.96
2.30 3.59 2.81 2.84 2.83 4.97 1.39 2.70 2.84 3.94 2.67 4.08
2.38 3.79 2.80 2.86 2.91 5.09 1.46 3.04 2.86 4.20 2.77 4.2
2.49 4.12 2.86 2.86 2.99 5.39 1.53 4.40 2.84 6.72 2.86 4.93
2.55 2.66 2.79 5.07 6.46 9.00 2.90 2.92 2.96 3.50 4.99 5.80 3.11 3.21 3.33 6.33 6.98 7.89 1.61 1.81 2.66 5.07 6.20 8.00 2.91 2.99 3.04 8.25 10.70 12.00 2.93 2.98 3.09 6.91 10.23 12.66
Drya aw=0.6b Drya aw=0.6b Drya aw=0.6b Drya aw=0.6b Drya aw=0.6b Drya aw=0.6b
0.08 0.25 0.14 0.26 0.05 1.13 0.00 0.00 0.43 0.43 0.37 0.48
0.08 0.27 0.17 0.27 0.06 1.14 0.00 0.00 0.41 0.41 0.38 0.50
0.07 0.26 0.19 0.31 0.07 1.13 0.00 0.00 0.41 0.41 0.38 0.49
0.07 0.26 0.19 0.34 0.10 1.10 0.00 0.00 0.41 0.41 0.37 0.51
0.06 0.26 0.18 0.44 0.11 1.10 0.00 0.00 0.42 0.42 0.36 0.61
0.06 0.27 0.19 0.51 0.13 1.10 0.00 0.00 0.44 0.44 0.37 0.62
0.05 0.29 0.20 0.57 9.13 1.10 0.00 0.00 0.44 0.44 0.38 0.62
0.06 0.30 0.20 0.64 0.14 1.13 0.00 0.00 0.45 0.45 0.38 0.66
0.07 0.34 0.21 0.78 0.19 1.17 0.00 0.00 0.44 0.44 0.39 0.70
0.10 0.39 0.21 1.53 0.21 1.23 0.00 0.05 0.42 0.42 0.4 0.80
0.10 0.58 0.22 2.00 0.24 1.40 0.00 0.10 0.43 0.43 0.41 1.07
a*
80
85
0.13 0.88 0.24 2.30 0.26 1.50 0.05 0.25 0.45 0.45 0.42 1.60
90
0.16 1.55 0.23 2.60 0.25 1.63 0.08 0.50 0.45 0.45 0.44 2.05
95 2.80 11.94 3.00 9.00 3.32 10.19 3.11 10.00 3.10 14.00 3.21 14.33 0.16 3.18 0.24 3.20 0.26 1.98 0.10 1.00 0.46 0.45 0.46 2.20
*Means starches with dierent moisture contents have signi®cantly dierent dielectric values or loss factors. (Each measurement is the mean of three replications.)
Fig. 3. Variation of dielectric constant of starches with temperature for starch-water ratio of 1:2. (&): waxy maizee, (*):amylomaizea, (&):corne, (): wheatc, (*): tapiocad, (x):riceb, (Ð):model. Starches with dierent letters have signi®cantly dierent dielectric constant values. Three replications were used to generate each data point.
Dielectric properties of six dierent species of starch at 2450 MHz
49
Fig. 4. Variation of loss factor of starches with temperature for starch±water ratio of 1:2. (&): waxy maized, (*):amylomaized, (&): cornb, (): wheatb, (*): tapiocac, (x): ricea, (Ð):model. Starches with dierent letters have signi®cantly dierent loss factor values. Three replications were used to generate each data point.
Fig. 5. Variation of loss factor of dry starches with temperature. (&): waxy maizea, (*):amylomaizeb, (&): cornc, (): wheatd, (*): tapiocae, (x): ricef, (Ð):model. Starches with dierent letters have signi®cantly dierent loss factor values. Three replications were used to generate each data point.
As temperature increased the dielectric constant and loss factor of all starch solutions decreased (Figs 3 and 4). This was expected since the free water content has a negative temperature coecient at microwave
frequencies (Calay et al., 1995). Dielectric constants of starches were signi®cantly dierent from each other (p 0.05) (Fig. 3). ANOVA results showed that loss factor of corn, wheat and rice starch were signi®cantly
50
M. K. Ndife et al. Table 4. Equation constants and coecients of quadratic equations for dierent starch species between temperatures 30±95 C Dielectric constant
Starch type Tapioca Tapioca Tapioca Corn Corn Corn Wheat Wheat Wheat Rice Rice Rice Waxy maize Waxy maize Amylomaize Amylomaize
Starch±water ratio
a
b c a bx cx2
1:1 1.0:1.5 1.0:2.0 1:1 1.0:1.5 1.0:2.0 1:1 1.0:1.5 1.0:2.0 1:1 1.0:1.5 1.0:2.0 1.0:1.5 1.0:2.0 1.0:1.5 1.0:2.0
73.30 76.72 95.68 38.15 59.98 107.4 89.19 90.54 94.00 36.81 54.98 65.24 90.54 99.40 63.02 59.24
ÿ1.193 ÿ0.847 ÿ1.520 0.410 ÿ0.072 ÿ2.030 ÿ1.408 ÿ1.160 ÿ0.950 0.193 ÿ0.160 ÿ0.228 ÿ1.160 ÿ1.164 ÿ0.254 0.211
0.0071 0.0028 0.0092 ÿ0.005 ÿ0.0024 0.0127 0.0087 0.0049 0.0031 ÿ0.0014 0.0001 0.0008 0.0049 0.009 0.000 ÿ0.0022
Table 5. Bulk density and dielectric properties of dry granular starches at 30 C Starch type
Bulk density (g cm3)
Dielectric constant
Loss factor
Corn Rice Tapioca Wheat Waxymaize Amylomaize
0.810b* 0.678a 0.808b 0.790b 0.902c 0.886c
2.74a 1.25b 2.25a 2.42b 2.81b 2.42b
0.14c 0.00a 0.08b 0.05b 0.43e 0.37d
*Means columns with dierent letters are signi®cantly dierent. Each measurement is the mean of three replications.
dierent than waxy, amylomaize and tapioca starch (p 0.05) (Fig. 4). The parameter that in¯uences microwave heatability the most is the dielectric loss factor (Chen et al., 1993). The higher the loss factor the faster will be the heating in microwave oven. Therefore, wheat, rice and corn starches are expected to be heated and gelatinize faster than the other starches in the microwave oven. It is logical to select these starches for microwave baked products. The variation of dielectric properties of starches with respect to temperature in granular form is totally different from the variation in solution forms. Although dielectric constant and loss factor of starches in solution forms decreased during heating, both of these properties increased for starches in granular form. Fig. 5 shows the change of loss factor of the dry powdered starches with respect to temperature. Calay et al. (1995) reported that the increase in loss factor for low moisture materials was due to the bound water relaxation mechanism where it was stated that bound water had a positive
Coefficient of determination r2 0.95 0.94 0.93 0.79 0.92 0.91 0.93 0.91 0.94 0.71 0.78 0.93 0.91 0.98 0.94 0.66
Loss factor a 17,950 12,946 17,237 18,901 9,679 11,723 19,294 19,372 18,137 11,445 12,277 11.64 15,272 14,143 11,479 11,447
b a bx cx2 ÿ0.339 ÿ0.139 ÿ0.26 ÿ0.384 ÿ0.0548 ÿ0.079 ÿ0.351 ÿ0.356 ÿ0.27 ÿ0.115 ÿ0.082 ÿ0.066 ÿ0.278 ÿ0.207 ÿ0.182 ÿ0.126
c
0.002 0.0004 0.00144 0.0025 0.00014 0.0002 0.00231 0.0024 0.00147 0.00056 0.000206 0.00025 0.00156 0.00096 0.0011 0.00038
Coefficient of determination r2 0.94 0.94 0.97 0.94 0.98 0.99 0.96 0.84 0.93 0.98 0.99 0.98 0.99 0.98 0.87 0.98
temperature coecient at the microwave frequencies. Dry starches exhibited small loss factor (Fig. 5). Rice starch had the smallest loss factor compared to other starches, and this is most likely associated with the bulk density (Table 5). It is known that loss factor of granular materials are dependent on bulk density (Nelson, 1983). At a given moisture content loss factor was shown to vary linearly with bulk density (Sokhansanj and Nelson, 1988). The reason why rice starch has the smallest dielectric loss factor may be due to the fact that it had the lowest bulk density. A correlation coecient of 0.90 was found between bulk density and loss factor while the correlation coecient showing the relation between bulk density and dielectric constant was determined as 0.85. Penetration depth of a material is a very important parameter in characterizing microwave heating. Penetration depth is important for microwave baking since it shows the depth at which heat is dissipated uniformly. The increase in penetration depth will increase the uniformity in microwave heating. Penetration depth for a material was calculated by using the following formula (Metaxas and Meredith, 1983). 8" 9 00 2 #0:5 =ÿ0:5 < l0 1 0 ÿ1 Dp ; 2 20 0:5 :
3
where Dp is the penetration depth (cm), l0 is the wave length in free space 12.237 cm at 2450 MHz, E0 is the dielectric constant and E00 is the loss factor. The change of penetration depth of dierent kinds of starch with respect to temperature are given in Fig. 6. The increase in temperature increased penetration depth of especially amylomaize and waxy maize starches.
Dielectric properties of six dierent species of starch at 2450 MHz
51
Fig. 6. Variation of penetration depth starches for starch-water ratio of 1.0:1.5. ( ): ricea, (&): wheata, (&): corna, ( ): waxy maizeb, ( ), tapiocaa ( ): amylomaizec. Starches with dierent letters have signi®cantly dierent loss factor values. Three replications were used to generate each data point..
Amylomaize and waxy maize starches were found to have signi®cantly higher penetration depth than the other starches where as no signi®cant dierence was detected between the other starches (p 0.05). This was expected since with materials showing high loss factors microwave energy does not penetrate deeply (Buer, 1993). This shows that a microwave will penetrate deeper when amylomaize or waxymaize starches are used in formulations. CONCLUSIONS Dielectric constant and loss factor of starches were shown to be dependent on moisture content, temperature and starch type. Dielectric values were directly related with moisture content. Dielectric constant and loss factor of granular starches (low moisture content) increased while those of starch solutions (high moisture content) decreased with temperature increase. Wheat, corn and rice starches may be selected to be used in microwave cake formulations since loss factor of these starches were found to be high compared to others. ACKNOWLEDGEMENTS Financial support was provided by The Ohio Agricultural Research and Development Center and by
NATO A-2 Fellowship of The Scienti®c and Technical Research Council of Turkey (TUBITAK). REFERENCES Buer, C. (1993) Microwave Cooking and Processing: Engineering Fundamentals for the Food Scientist, pp. 157±158. Avi Book, New York. Calay, R. K., Newborough, M., Probert, D. and Calay, P. S. (1995) Predictive Equations for dielectric properties of foods. International Journal of Food Science and Technology 29, 699±713. Chen, M., Siochi, E. J., Ward, T. C. and Mcgrath, J. E. (1993) Basic Ideas of Microwave processing of polymers. Polymer Engineering and Science 33, 1092±1109. Metaxas, A. C. and Meredith, R. J. (1983) Industrial Microwave Heating, Peter Peregrinus, London. Mudgett, R. E. (1986) Microwave properties and heating characteristics of foods. Food Technology 40, 84±93. Mudgett, R. E. (1995) In Engineering Properties of Foods, Rao, M. A. and Rizvi, S. S. H. pp. 389±455. Marcel Dekker Inc., New York. Nelson, S. (1983) Density dependence of dielectric properties of particulate materials. Transactions of the ASAE 26, 1823± 1825, 1829. Nelson, S., Prakash, A. and Lawrence, K. (1991) Moisture and temperature dependence of the permitivities of some Hydrocolloids at 2.45 GHz. The Journal of Microwave Power and Electromagnetic Energy 26, 178±185. Roebuck, B. D., Goldblith, S. A. and Westphal, W. B. (1972) Dielectric properties of carbohydrate-water mixtures at microwave frequencies. Journal of Food Science 37, 199±204.
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M. K. Ndife et al.
RyynaÈnen, S., Risman, P. O. and Ohlsson, T. (1995) The dielectric properties of Native starch solutions-A research note. The Journal of Microwave Power and Electromagnetic Energy 31, 50±53. SAS (1988) SAS User's Guide Statistics, SAS Institute, Inc., Raleigh, NC.
Sokhansanj, S. and Nelson, S. O. (1988) Dependence of dielectric properties of whole-grain wheat on bulk density. Journal of Agricultural Engineering Research 39, 173±179.
(Received 30 September 1997; accepted 29 July 1998)
Dielectric properties of six dierent species of starch at 2450 MHz M. K. Ndife,a G. S,umnu,b* & L. Bayindirlib a
Department of Food Science and Technology, The Ohio State University, Columbus, OH 43210, USA b Food Engineering Department, Middle East Technical University, 06531, Ankara, Turkey Dielectric properties at 2450 MHz were determined for tapioca, corn, wheat, rice, waxymaize and amylomaize starches in granular (moisture content 1 and 13%) and in solution forms [starch±water ratio:1:1, 1:1.5, 1:2 (w/w)] at temperatures from 30 to 95 C. Dielectric properties were shown to be dependent on temperature, moisture content and starch type. The increase in moisture content increased dielectric constant and loss factor of all types of starches. Increasing temperatures decreased the dielectric values for starch solutions (high moisture content) but increased the dielectric values for granular starches (low moisture content). Quadratic equations were developed to show the relationship between the changes in dielectric properties with temperature for starch species. There were signi®cant dierences in dielectric properties of dierent types of starches. # 1998 Canadian Institute of Food Science and Technology. Published by Elsevier Science Ltd. All rights reserved Keywords: dielectric properties, dielectric constant, loss factor, starch.
INTRODUCTION
of insucient gelatinization of starch during microwave baking of starch based products will be minimized. Dielectric properties depend on moisture content and temperature of the food (Calay et al., 1995). Dielectric constant and loss factor increase with moisture content are common (Roebuck et al., 1972; Nelson et al., 1991; Calay et al., 1995). The rate of change of dielectric constant and loss factor with temperature depends on the free and bound water content of the material (Calay et al., 1995). Nelson et al. (1991) showed that dielectric properties of potato starch increased with moisture content and temperature. Dielectric properties of potato starch in both gelatinized and granular form were compared at 25 C for solutions having 40±100% water and gelatinized starch was shown to have higher E00 values than the granular form (Roebuck et al., 1972). RyynaÈnen et al. (1995) compared dielectric properties of potato, wheat, corn and waxy corn starch solutions at 2750 MHz and at low concentrations of starch (5±30%) and no signi®cant dierence in dielectric properties was detected between dierent kinds of starch. The important factor which in¯uenced the dielectric properties was found to be the water content.
Dielectric properties are the key factors in the understanding of the interactions of microwaves with food materials. Dielectric constant (E0 ) is a measure of the ability of a material to store electromagnetic radiation and dielectric loss factor (E00 ) is a measure of the ability of a material to dissipate electrical energy into heat. Penetration depth shows how far a wave will penetrate before it reduces to 1/e of its intensity (Mudgett, 1986). Although many foods are produced by microwave heating; less satisfactory results are obtained in baked products in the microwave oven. This is due to the short heating time that is not enough for complete starch gelatinization. In order to improve quality of a microwaveable baked product, in the formulation it is necessary to use a starch type that can be quickly heated and gelatinized in the microwave oven. Dielectric properties of starch will give an idea which type of starch will be heated in a microwave oven quickly. Thus, the problem
*To whom correspondance should be addressed. Fax: +90312-2101270; e-mail: [email protected] 43
44
M. K. Ndife et al.
The objective of this study was to compare the dielectric properties of starches from dierent plant origins in solution form and granular form. This will be helpful to ®nd out the type of starch that will be heated in microwave oven faster which can be further used in the development of high quality microwave baked products. In order to predict thermal eects of the microwave heating process, equations that correlate the eects of variations in moisture content and temperature on dielectric constant and loss factor are determined in this study.
Statistical analysis
MATERIALS AND METHODS
Dielectric properties of starch solutions were shown to be dependent on water content (Tables 1 and 2). The increase in starch±water ratio from 1:1 to 1:2 increased dielectric constant and loss factor of starch solutions. This was expected since when there was much free water in the system, electric ®eld would see more polarizable dipole moments per unit volume of the sample. Dielectric constant and loss factor of starch in granular form also increased with increasing moisture content. A representative ®gure of this is given by wheat (Fig. 1) and corn starches (Fig. 2) for dielectric constant and loss factor respectively. Variation of tapioca, rice, waxymaize and amylomaize starches with temperature are given in Table 3. In granular starches, water is adsorbed on the starch and refered as irrotationally bound. The heating of granular starch can be explained by the bound water relaxation mechanism. The bound water relaxation phenomena result from the interaction between surface charges and centers of partial positive and negative charge on water dipoles (Mudgett, 1995). Although the mechanism of heating of starch solutions is dierent from that of starch granules the eects of moisture on dielectric properties are the same (Tables 1±3 and Figs 1 and 2). The change of dielectric constant and loss factor of dry granular samples (moisture content: 1%) with temperature was linear (Figs 1 and 2). The temperature dependence of both dielectric constant and loss factor increased, when the water activity of the granular starch was increased to 0.6 (moisture content:13%) (Figs 1 and 2). Nelson et al. (1991) also showed that temperature dependence increased markedly with increasing moisture content in various hydrocolloids, potato starch, locust bean gum, gum arabic, carrageenan and carboxy-metyl cellulose in granular form. Regression analysis was performed to relate the dielectric responses of wheat, rice, corn, tapioca, waxymaize and amylomaize solutions to temperatures between 30 and 95 C using the least square ®t. Dielectric constant and loss factor of all starches were found to vary quadratically with temperature. Equations are in the form of:
Materials Wheat starch (Midsol 50, Midwest Grain Products, Inc., Atchison, KS), tapioca starch (Actobind 2000, Advance Food Systems Inc., NJ), corn (amaizo, American Maize Products Company, Hammond, Indiana), waxy maize (amioca, American Maize Products Company, Hammond, Indiana), amylomaize (amylomaize VII, American Maize Products Company, Hammond, Indiana) and rice starch (Remy Neutral Dr, A and B Ingredients, Fair®eld, NJ) were used. Methods For preparation of starch solutions, starch was mixed with dierent levels of water to obtain a starch±water ratio of 1:1, 1:1.5 and 1:2 (w/w) which are within the typical concentrations used in baked products. To prepare granular starches at dierent water activities starch was placed in small containers and kept in a desiccator at 25 C. The bottom of the desiccator was ®lled with saturated sodium bromide solution to obtain relative humidity of 60%. Water activities of the samples were adjusted in the desiccator for 2 weeks before dielectric properties were measured. Low moisture level was obtained by oven drying of starch for 1 h at 130 C (Nelson et al., 1991). Water activity of the samples was measured by using a water activity meter (Aqualab-CX2, Decagon Devices Inc., Pullman, Washington). Dielectric properties were determined by using a dielectric probe and a network analyzer at 2450 MHz (HP 85070, Santa Rosa, CA). Dielectric measurements were taken in the temperature range of 30±95 C at 5 C intervals. Bulk density of the powdered samples were determined at 30 C before dielectric properties were determined. Weight of the samples were recorded. The reading of sample length from sample height gage and cross sectional area of sample holder provided sample volume and bulk density was calculated as g cmÿ3 (Nelson et al., 1991).
Analysis of variance was used to determine signi®cant dierences between starch treatments. Treatment means were compared by using Duncan's Multiple range comparison test. Three samples per treatment were used in each analysis. Correlations were obtained to relate dielectric constant and loss factor to temperature for each starch and concentration (SAS, 1988). RESULTS AND DISCUSSION
Temperature ( C) 30 35 40 45 50 55 60 65 70 75 80 85 90 95
Tapioca
Corn
Wheat
Rice
Waxymaize
Amylomaize
Starch±water ratio (w/w) 1:1c* 1:1.5b 1:2a
Starch±water ratio (w/w) 1:1b 1:1.5b 1:2a
Starch water ratio (w/w) 1:1b 1:1.5b 1:2a
Starch water ratio (w/w) 1:1c 1:1.5b 1:2a
Starch water ratio (w/w) 1:1.5c 1:2b
Starch water ratio (w/w) 1:1.5c 1:2b
42.7 41.0 39.0 36.5 32.4 30.1 27.5 25.0 23.4 20.9 21.0 23.9 25.2 27.4
52.0 51.1 49.8 45.6 40.0 36.0 35.0 34.5 34.0 32.8 23.3 21.6 21.6 24.8
54.3 54.0 52.2 51.9 41.7 40.9 36.6 33.8 33.6 33.6 32.4 31.5 34.0 36.5
45.6 45.8 45.9 46.5 47.1 48.2 48.2 47.4 40.0 37.2 36.3 36.1 37.0 37.0
52.8 54.9 54.0 53.5 51.7 51.2 49.2 43.3 43.0 41.0 35.2 33.9 35.2 35.5
53.4 53.1 49.8 41.3 38.7 36.3 34.5 31.4 25.1 22.1 20.8 21.8 31.5 31.6
52.2 51.9 51.2 48.0 40.8 36.3 34.7 34.2 33.0 30.2 31.2 32.2 34.5 39.4
64.1 63.4 60.4 59.1 47.2 43.7 35.7 32.9 29.3 24.6 25.5 26.6 28.8 36.0
65.2 62.3 61.5 61.0 60.0 54.0 47.4 43.5 41.2 36.1 36.1 32.6 34.2 36.1
41.0 41.7 42.4 42.9 43.4 43.3 43.3 43.2 43.0 42.8 42.4 42.4 43.0 43.2
48.8 49.2 49.3 49.2 48.8 48.3 47.2 41.5 41.8 42.8 43.5 43.4 42.5 40.5
58.4 58.4 57.3 57.0 56.7 55.7 54.8 54.4 51.5 51.8 52.0 52.1 51.7 51.2
59.1 53.2 46.9 43.8 38.4 34.1 33.0 31.0 30.4 29.3 24.8 21.5 22.3 25.5
54.2 54.8 56.0 51.0 50.8 44.0 35.9 34.5 31.9 27.9 24.9 24.5 25.4 30.0
54.2 54.7 54.1 51.7 51.0 50.2 49.1 47.9 46.5 41.3 41.4 41.3 41.1 41.0
*Means water levels with dierent letters (a, b, c) have dielectric values signi®cantly dierent from the others. (Each measurement is mean of three replications).
63.1 63.4 64.0 64.4 64.7 65.3 64.6 64.7 63.0 61.6 59.0 59.7 61.1 60.8
Dielectric properties of six dierent species of starch at 2450 MHz
Table 1. The eects of temperature on dielectric constant of dierent starches at dierent water levels
45
46
Table 2. The eects of temperature on loss factor of dierent starches at dierent water levels Tapioca
Corn
Wheat
Rice
Waxymaize
Amylomaize
Temperature Starch±water ratio (w/w) Starch±water ratio (w/w) Starch±water ratio (w/w) Starch±water ratio (w/w) Starch water ratio (w/w) Starch water ratio (w/w) ( C) 1:1c 1:1.5b 1:2a 1:1c* 1:1.5b 1:2a 1:1c 1:1.5b 1:2a 1:1c 1:1.5b 1:2a 1:1.5b 1:2a 1:1.5b 1:2a 8.7 8.6 8.3 7.2 5.7 5.6 5.6 4.1 3.7 3.5 3.2 3.0 4.0 4.4
9.1 8.8 8.3 7.7 6.8 6.3 5.8 5.6 5.5 5.4 5.4 4.3 3.0 3.9
10.4 10.1 9.7 8.8 7.8 7.0 6.6 6.4 6.2 5.8 5.3 5.5 5.7 5.9
9.1 8.7 7.7 6.3 6.1 5.6 5.3 4.7 3.8 3.3 4.0 4.3 4.7 5.0
8.1 8.0 7.8 7.7 7.2 7.1 7.0 6.5 6.5 6.1 6.0 6.0 6.0 5.6
9.4 9.4 8.9 8.6 8.3 8.0 7.8 7.3 7.2 7.0 6.8 6.5 6.2 6.2
10.7 9.6 9.0 8.4 8.0 6.4 6.0 5.8 5.9 6.3 6.4 6.4 6.2 6.2
10.4 9.5 9.4 8.9 8.2 7.6 6.8 6.0 5.6 5.5 6.4 6.9 7.2 7.5
10.4 10.4 10.3 9.4 8.9 8.2 7.1 6.3 6.1 5.6 5.6 5.8 5.9 6.2
8.7 8.2 7.6 7.2 7.0 6.7 6.6 6.4 6.3 6.1 5.9 5.8 5.6 5.4
10.0 9.6 9.4 9.1 8.8 8.5 8.2 7.5 7.4 7.3 7.2 7.0 6.6 6.2
10.0 9.5 9.2 9.2 9.0 8.8 8.6 8.5 8.3 8.2 8.0 7.9 7.7 7.3
8.4 7.4 6.6 6.1 5.1 4.5 4.3 3.9 3.7 3.4 2.9 2.7 2.7 3.2
8.4 8.1 7.9 7.0 6.3 5.4 5.1 4.8 4.3 4.2 3.6 3.2 3.3 3.4
7.2 6.5 6.2 5.5 4.9 4.5 4.2 3.8 4.7 4.7 4.5 4.5 4.0 3.8
*Means water levels with dierent letters (a, b, c) have loss factor values signi®cantly dierent from the others, (each measurement is the mean of three replications).
7.8 7.4 7.0 6.8 6.4 5.9 5.3 4.8 4.2 4.1 4.0 3.4 2.7 3.5
M. K. Ndife et al.
30 35 40 45 50 55 60 65 70 75 80 85 90 95
Dielectric properties of six dierent species of starch at 2450 MHz
47
Fig. 1. Variation of dielectric constant of granular wheat starch at dierent moistures (~): dry, (^): aw=0.6, (Ð):model. Three replications were used to generate each data point.
Fig. 2. Variation of loss factor of corn starch solutions. (~): dry, (^): aw=0.6, (Ð):model. Three replications were used to generate each data point.
0 a bT cT2
1
00 a bT cT2
2
where E0 is dielectric constant, E00 is loss factor, a, b and c are constants, T is temperature. These quadratic equations are used to determine the dielectric constant and loss factor of starch species at
the desired temperature. Moreover, when experimental data are inadequate, or in order to avoid developing temperature dependent equations for starch based products, the quadratic equations (eqns 1 and 2) may be used by incorporating them within a microwave-heating model. Equations together with experimental data were shown in Figs 1±5. Constants and coecients of the equations of starch species are given in Table 4.
48
M. K. Ndife et al. Table 3. Variation of dielectric properties of granular starches with moisture content
Temperature ( C) Dielectric property Dielectric constant
Starch type Tapioca Corn Wheat Rice Waxymaize Amylomaize
Loss factor
Tapioca Corn Wheat Rice Waxymaize Amylomaize
30
35
40
45
50
55
60
65
70
75
Dry aw=0.6b Drya aw=0.6b Drya aw=0. 6b Drya aw=0.6b Drya aw=0.6b Drya aw=0.6b
2.25 3.11 2.74 2.25 2.42 4.43 1.25 2.50 2.25 3.06 2.42 3.23
2.18 3.15 2.56 2.26 2.49 4.40 1.20 2.52 2.26 3.02 2.33 3.24
2.14 3.21 2.58 2.68 2.52 4.39 1.18 2.54 2.68 3.00 2.30 3.29
2.06 3.28 2.61 2.69 2.55 4.40 1.22 2.60 2.69 3.20 2.30 3.42
2.03 3.33 2.69 2.77 2.60 4.43 1.28 2.65 2.77 3.49 2.44 3.76
2.06 3.41 2.73 2.87 2.74 4.75 1.35 2.57 2.87 3.68 2.52 3.87
2.24 3.49 2.75 2.85 2.8 4.86 1.37 2.60 2.85 4.00 2.58 3.96
2.30 3.59 2.81 2.84 2.83 4.97 1.39 2.70 2.84 3.94 2.67 4.08
2.38 3.79 2.80 2.86 2.91 5.09 1.46 3.04 2.86 4.20 2.77 4.2
2.49 4.12 2.86 2.86 2.99 5.39 1.53 4.40 2.84 6.72 2.86 4.93
2.55 2.66 2.79 5.07 6.46 9.00 2.90 2.92 2.96 3.50 4.99 5.80 3.11 3.21 3.33 6.33 6.98 7.89 1.61 1.81 2.66 5.07 6.20 8.00 2.91 2.99 3.04 8.25 10.70 12.00 2.93 2.98 3.09 6.91 10.23 12.66
Drya aw=0.6b Drya aw=0.6b Drya aw=0.6b Drya aw=0.6b Drya aw=0.6b Drya aw=0.6b
0.08 0.25 0.14 0.26 0.05 1.13 0.00 0.00 0.43 0.43 0.37 0.48
0.08 0.27 0.17 0.27 0.06 1.14 0.00 0.00 0.41 0.41 0.38 0.50
0.07 0.26 0.19 0.31 0.07 1.13 0.00 0.00 0.41 0.41 0.38 0.49
0.07 0.26 0.19 0.34 0.10 1.10 0.00 0.00 0.41 0.41 0.37 0.51
0.06 0.26 0.18 0.44 0.11 1.10 0.00 0.00 0.42 0.42 0.36 0.61
0.06 0.27 0.19 0.51 0.13 1.10 0.00 0.00 0.44 0.44 0.37 0.62
0.05 0.29 0.20 0.57 9.13 1.10 0.00 0.00 0.44 0.44 0.38 0.62
0.06 0.30 0.20 0.64 0.14 1.13 0.00 0.00 0.45 0.45 0.38 0.66
0.07 0.34 0.21 0.78 0.19 1.17 0.00 0.00 0.44 0.44 0.39 0.70
0.10 0.39 0.21 1.53 0.21 1.23 0.00 0.05 0.42 0.42 0.4 0.80
0.10 0.58 0.22 2.00 0.24 1.40 0.00 0.10 0.43 0.43 0.41 1.07
a*
80
85
0.13 0.88 0.24 2.30 0.26 1.50 0.05 0.25 0.45 0.45 0.42 1.60
90
0.16 1.55 0.23 2.60 0.25 1.63 0.08 0.50 0.45 0.45 0.44 2.05
95 2.80 11.94 3.00 9.00 3.32 10.19 3.11 10.00 3.10 14.00 3.21 14.33 0.16 3.18 0.24 3.20 0.26 1.98 0.10 1.00 0.46 0.45 0.46 2.20
*Means starches with dierent moisture contents have signi®cantly dierent dielectric values or loss factors. (Each measurement is the mean of three replications.)
Fig. 3. Variation of dielectric constant of starches with temperature for starch-water ratio of 1:2. (&): waxy maizee, (*):amylomaizea, (&):corne, (): wheatc, (*): tapiocad, (x):riceb, (Ð):model. Starches with dierent letters have signi®cantly dierent dielectric constant values. Three replications were used to generate each data point.
Dielectric properties of six dierent species of starch at 2450 MHz
49
Fig. 4. Variation of loss factor of starches with temperature for starch±water ratio of 1:2. (&): waxy maized, (*):amylomaized, (&): cornb, (): wheatb, (*): tapiocac, (x): ricea, (Ð):model. Starches with dierent letters have signi®cantly dierent loss factor values. Three replications were used to generate each data point.
Fig. 5. Variation of loss factor of dry starches with temperature. (&): waxy maizea, (*):amylomaizeb, (&): cornc, (): wheatd, (*): tapiocae, (x): ricef, (Ð):model. Starches with dierent letters have signi®cantly dierent loss factor values. Three replications were used to generate each data point.
As temperature increased the dielectric constant and loss factor of all starch solutions decreased (Figs 3 and 4). This was expected since the free water content has a negative temperature coecient at microwave
frequencies (Calay et al., 1995). Dielectric constants of starches were signi®cantly dierent from each other (p 0.05) (Fig. 3). ANOVA results showed that loss factor of corn, wheat and rice starch were signi®cantly
50
M. K. Ndife et al. Table 4. Equation constants and coecients of quadratic equations for dierent starch species between temperatures 30±95 C Dielectric constant
Starch type Tapioca Tapioca Tapioca Corn Corn Corn Wheat Wheat Wheat Rice Rice Rice Waxy maize Waxy maize Amylomaize Amylomaize
Starch±water ratio
a
b c a bx cx2
1:1 1.0:1.5 1.0:2.0 1:1 1.0:1.5 1.0:2.0 1:1 1.0:1.5 1.0:2.0 1:1 1.0:1.5 1.0:2.0 1.0:1.5 1.0:2.0 1.0:1.5 1.0:2.0
73.30 76.72 95.68 38.15 59.98 107.4 89.19 90.54 94.00 36.81 54.98 65.24 90.54 99.40 63.02 59.24
ÿ1.193 ÿ0.847 ÿ1.520 0.410 ÿ0.072 ÿ2.030 ÿ1.408 ÿ1.160 ÿ0.950 0.193 ÿ0.160 ÿ0.228 ÿ1.160 ÿ1.164 ÿ0.254 0.211
0.0071 0.0028 0.0092 ÿ0.005 ÿ0.0024 0.0127 0.0087 0.0049 0.0031 ÿ0.0014 0.0001 0.0008 0.0049 0.009 0.000 ÿ0.0022
Table 5. Bulk density and dielectric properties of dry granular starches at 30 C Starch type
Bulk density (g cm3)
Dielectric constant
Loss factor
Corn Rice Tapioca Wheat Waxymaize Amylomaize
0.810b* 0.678a 0.808b 0.790b 0.902c 0.886c
2.74a 1.25b 2.25a 2.42b 2.81b 2.42b
0.14c 0.00a 0.08b 0.05b 0.43e 0.37d
*Means columns with dierent letters are signi®cantly dierent. Each measurement is the mean of three replications.
dierent than waxy, amylomaize and tapioca starch (p 0.05) (Fig. 4). The parameter that in¯uences microwave heatability the most is the dielectric loss factor (Chen et al., 1993). The higher the loss factor the faster will be the heating in microwave oven. Therefore, wheat, rice and corn starches are expected to be heated and gelatinize faster than the other starches in the microwave oven. It is logical to select these starches for microwave baked products. The variation of dielectric properties of starches with respect to temperature in granular form is totally different from the variation in solution forms. Although dielectric constant and loss factor of starches in solution forms decreased during heating, both of these properties increased for starches in granular form. Fig. 5 shows the change of loss factor of the dry powdered starches with respect to temperature. Calay et al. (1995) reported that the increase in loss factor for low moisture materials was due to the bound water relaxation mechanism where it was stated that bound water had a positive
Coefficient of determination r2 0.95 0.94 0.93 0.79 0.92 0.91 0.93 0.91 0.94 0.71 0.78 0.93 0.91 0.98 0.94 0.66
Loss factor a 17,950 12,946 17,237 18,901 9,679 11,723 19,294 19,372 18,137 11,445 12,277 11.64 15,272 14,143 11,479 11,447
b a bx cx2 ÿ0.339 ÿ0.139 ÿ0.26 ÿ0.384 ÿ0.0548 ÿ0.079 ÿ0.351 ÿ0.356 ÿ0.27 ÿ0.115 ÿ0.082 ÿ0.066 ÿ0.278 ÿ0.207 ÿ0.182 ÿ0.126
c
0.002 0.0004 0.00144 0.0025 0.00014 0.0002 0.00231 0.0024 0.00147 0.00056 0.000206 0.00025 0.00156 0.00096 0.0011 0.00038
Coefficient of determination r2 0.94 0.94 0.97 0.94 0.98 0.99 0.96 0.84 0.93 0.98 0.99 0.98 0.99 0.98 0.87 0.98
temperature coecient at the microwave frequencies. Dry starches exhibited small loss factor (Fig. 5). Rice starch had the smallest loss factor compared to other starches, and this is most likely associated with the bulk density (Table 5). It is known that loss factor of granular materials are dependent on bulk density (Nelson, 1983). At a given moisture content loss factor was shown to vary linearly with bulk density (Sokhansanj and Nelson, 1988). The reason why rice starch has the smallest dielectric loss factor may be due to the fact that it had the lowest bulk density. A correlation coecient of 0.90 was found between bulk density and loss factor while the correlation coecient showing the relation between bulk density and dielectric constant was determined as 0.85. Penetration depth of a material is a very important parameter in characterizing microwave heating. Penetration depth is important for microwave baking since it shows the depth at which heat is dissipated uniformly. The increase in penetration depth will increase the uniformity in microwave heating. Penetration depth for a material was calculated by using the following formula (Metaxas and Meredith, 1983). 8" 9 00 2 #0:5 =ÿ0:5 < l0 1 0 ÿ1 Dp ; 2 20 0:5 :
3
where Dp is the penetration depth (cm), l0 is the wave length in free space 12.237 cm at 2450 MHz, E0 is the dielectric constant and E00 is the loss factor. The change of penetration depth of dierent kinds of starch with respect to temperature are given in Fig. 6. The increase in temperature increased penetration depth of especially amylomaize and waxy maize starches.
Dielectric properties of six dierent species of starch at 2450 MHz
51
Fig. 6. Variation of penetration depth starches for starch-water ratio of 1.0:1.5. ( ): ricea, (&): wheata, (&): corna, ( ): waxy maizeb, ( ), tapiocaa ( ): amylomaizec. Starches with dierent letters have signi®cantly dierent loss factor values. Three replications were used to generate each data point..
Amylomaize and waxy maize starches were found to have signi®cantly higher penetration depth than the other starches where as no signi®cant dierence was detected between the other starches (p 0.05). This was expected since with materials showing high loss factors microwave energy does not penetrate deeply (Buer, 1993). This shows that a microwave will penetrate deeper when amylomaize or waxymaize starches are used in formulations. CONCLUSIONS Dielectric constant and loss factor of starches were shown to be dependent on moisture content, temperature and starch type. Dielectric values were directly related with moisture content. Dielectric constant and loss factor of granular starches (low moisture content) increased while those of starch solutions (high moisture content) decreased with temperature increase. Wheat, corn and rice starches may be selected to be used in microwave cake formulations since loss factor of these starches were found to be high compared to others. ACKNOWLEDGEMENTS Financial support was provided by The Ohio Agricultural Research and Development Center and by
NATO A-2 Fellowship of The Scienti®c and Technical Research Council of Turkey (TUBITAK). REFERENCES Buer, C. (1993) Microwave Cooking and Processing: Engineering Fundamentals for the Food Scientist, pp. 157±158. Avi Book, New York. Calay, R. K., Newborough, M., Probert, D. and Calay, P. S. (1995) Predictive Equations for dielectric properties of foods. International Journal of Food Science and Technology 29, 699±713. Chen, M., Siochi, E. J., Ward, T. C. and Mcgrath, J. E. (1993) Basic Ideas of Microwave processing of polymers. Polymer Engineering and Science 33, 1092±1109. Metaxas, A. C. and Meredith, R. J. (1983) Industrial Microwave Heating, Peter Peregrinus, London. Mudgett, R. E. (1986) Microwave properties and heating characteristics of foods. Food Technology 40, 84±93. Mudgett, R. E. (1995) In Engineering Properties of Foods, Rao, M. A. and Rizvi, S. S. H. pp. 389±455. Marcel Dekker Inc., New York. Nelson, S. (1983) Density dependence of dielectric properties of particulate materials. Transactions of the ASAE 26, 1823± 1825, 1829. Nelson, S., Prakash, A. and Lawrence, K. (1991) Moisture and temperature dependence of the permitivities of some Hydrocolloids at 2.45 GHz. The Journal of Microwave Power and Electromagnetic Energy 26, 178±185. Roebuck, B. D., Goldblith, S. A. and Westphal, W. B. (1972) Dielectric properties of carbohydrate-water mixtures at microwave frequencies. Journal of Food Science 37, 199±204.
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(Received 30 September 1997; accepted 29 July 1998)