Conclusiones

As explained in the last section, dielectric measurements can provide important infor- mation during industrial processes due to the relationships between food properties and electromagnetic parameters. Complex permittivity can be correlated with structural as well as physical and chemical properties such as humidity, soluble solids content, porosity, characteristics of solid matrix, and density. The changes in these properties are usually related to the food treatments applied throughout the industrial process; for instance, losses of water in drying processes or losses of salts in desalting processes (De los Reyes et al. 2005c). Also, structural changes in macromolecules, such as protein denaturalization, can occur during processing, leading to a modification of the dielectric properties (Bircan and Barringer 2002). For all these reasons, the measurement of dielectric properties can be used as a tool for on-line food process control.

Low-power microwaves change their parameters (amplitude, phase) according to the food properties, and this change can be measured. This is the basic principle on which food-quality microwave sensors are based. As sensors use low-power microwaves, there are no permanent effects on the food. Also, some microwave sensors, such as coaxial

Table 3.2. Dielectric data of some commercial fats and oils.

300 MHz 1000 MHz 3000 MHz Sample 25°C 48°C 82°C 25°C 48°C 82°C 25°C 48°C 82°C Soybean ε′ 2.853 2.879 2.862 2.612 2.705 2.715 2.506 2.590 2.594 salad oil ε″ 0.159 0.138 0.092 0.168 0.174 0.140 0.138 0.168 0.160 Corn oil ε′ 2.829 2.868 2.861 2.638 2.703 2.713 2.526 2.567 2.587 ε″ 0.174 0.134 0.103 0.175 0.174 0.146 0.143 0.166 0.163 Lard ε′ 2.718 2.779 2.770 2.584 2.651 2.656 2.486 2.527 2.541 ε″ 0.153 0.137 0.109 0.158 0.159 0.137 0.127 0.154 0.148 Tallow ε′ 2.603 2.772 2.765 2.531 2.568 2.610 2.430 2.454 2.492 ε″ 0.126 0.141 0.105 0.147 0.146 0.134 0.118 0.143 0.144 Hydrogenated ε′ 2.683 2.777 2.772 2.530 2.654 2.665 2.420 2.534 2.550 vegetable ε″ 0.141 0.140 0.103 0.147 0.153 0.137 0.117 0.146 0.146 shortening Bacon fat ε′ 2.748 2.798 2.770 2.608 2.655 2.649 2.493 2.538 2.536 ε″ 0.165 0.139 0.099 0.163 0.161 0.144 0.130 0.152 0.149

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probes, waveguides, or striplines, can be used in nondestructive measurement methods. There are also noncontact techniques, such as free space, which have advantages as on- line sensors.

The aim of this section is to provide an overview of the most important microwave applications as techniques in food control.

Determination of Moisture Content

Water is the main component in foods influenced by microwave energy, and there- fore nowadays most methods of determining moisture content are based on electrical properties.

The determination of moisture content based on electromagnetic parameters has been used in agriculture for at least 90 years and has been in common use for about 50 years (Nelson 1977, 1991, 1999).

A lot of studies have been carried out relating the dielectric constant and loss factor with moisture content in foods (Bengtsson and Risman 1971; Roebuck and Goldblith 1972; Nelson 1978; Nelson et al. 1991; Ndife et al. 1998).

Further research in this field has occurred during recent years. Trabelsi and Nelson (2005c) studied a method of moisture sensing in grains and seeds by measurement of their dielectric properties. The reliability of the method was tested for soybeans, corn, wheat, sorghum, and barley. The frequency used was 7 GHz with the free-space technique. In the same year, the authors used the same technique at 2–18 GHz to determine the dielectric properties of cereal grains and oilseeds in order to predict the moisture content by micro- wave measurements (Trabelsi and Nelson 2005a). Funk et al. (2005) also studied moisture in grain. This paper presents a unified grain moisture algorithm, based on measurements of the real part of the complex permittivity of grain at 149 MHz using the transmission line method. Trabelsi and Nelson (2005b) reported the moisture content in unshelled and shelled peanuts using the free-space method at a frequency of 8 GHz.

Since most efforts have been directed to the moisture determination of different mate- rials, commercial meters for on-line moisture measurements have already been devel- oped. These moisture meters are based on automatic on-line calculations of the reflected wave and dielectric permittivity, yielding physico-chemical properties, such as moisture content, composition, and density, without affecting the product. For instance, Keam Holdem® Industry (Auckland, New Zealand) provides on-line moisture testing and ana- lyzing systems. This manufacturer provides devices for measuring moisture in processed cheese, moisture and salt in butter, moisture and density in dried lumber and whole kernel grain, and fat-to-lean ratio in pork middles.

Another interesting application for on-line moisture measurement is a sensor for green tea developed by Okamura and Tsukamoto (2005), which can measure moisture content as high as 160 to 300 percent on a dry basis by use of microwaves at 3 GHz with a micro- stripline (fig. 3.15).

A microwave moisture meter has also been developed for continuous control of mois- ture in grains, sugar, and dry milk in technological processes (Lisovsky 2005). A con- sortium of companies from different countries, Microradar®, produces a commercial microwave moisture meter for measuring moisture in fluids, solids, and bulk materials based on this method.

In 2005, K. Joshi reported a technique for on-line, time domain, nondestructive micro- wave aquametry (United States Patent nos. 6,204,670 and 6,407,555). The paper reports a novel technique for determining moisture levels in substances such as seeds, soil, soap, tissue paper, and milk powder.

A Guided Microwave Spectometer® (Thermo Electron Corporation, United States) has been developed for on-line measurements of multiphase products (fig. 3.16). This guide is used to measure moisture in raw materials such as corn, rice, and soybeans and in processed materials such as tomato paste and ground meat. It can also measure Brix, pH, viscosity, and acid in orange juice, soft drinks, mayonnaise, and tomato products; fat

Fig. 3.15. Schema of a microstripline used for moisture measurement of tea leaves (Okamura and Tsukamoto 2005).

Transmitter

Microwave Signal

Receiver Product Flow

Fig. 3.16. Guided Microwave Spectometer® (Thermo Electron Corporation, United States) and its operation schema.

68 Advances in Food Diagnostics

in ground meats, peanut butter, and milk and other dairy products; salt in mashed potatoes and most vegetable products; and, lastly, alcohol in beverages.

Quality Control of Fish, Seafood, and Meat

The determination of quality parameters in biological tissues based on electromagnetic parameters is still a complex topic due to its heterogeneous composition and complex matrix. The permittivity and conductivity parameters are the properties that determine the propagation of an electromagnetic wave in biological tissue. Gabriel et al. (1996) described these parameters in detail. It is important to point out that the limitation of most dielectric probes is the volume of the sample that interacts with the field. The volume has to be representative of the whole piece of meat or fish, due to the fact that the electromagnetic parameters in this kind of tissue vary in a heterogeneous way.

The dielectric properties of various meat products under different conditions and using different methods at microwave frequencies have been measured (Bengtsson and Risman 1971; Ohlsson et al. 1974; To et al. 1974). The dielectric properties of turkey meat were measured at 915 and 2450 MHz (Sipahioglu et al. 2003a). The authors developed a number of equations to correlate the real and imaginary part of permittivity with temperature, mois- ture, water activity, and ash. Other equations were developed to model the dielectric prop- erties of ham as a function of temperature and composition (Sipahioglu et al. 2003b). The interaction between microwave radiation and meat products was also studied by Zhang et al. (2004). A complete study of the dielectric properties of meats and ingredients used in meat products at microwave and radio frequencies was also reported recently (Lyng et al. 2005). Some of the results given in this paper are shown in table 3.3.

The dielectric properties of fish products were also measured at microwave frequencies by different authors (Bengtsson et al. 1963; Kent 1972, 1977; Wu et al. 1988; Zheng et al. 1998).

Further investigations gave different control applications of dielectric measurements. As explained earlier, working in a frequency range (dielectric spectra) gives more infor- mation than measuring at a single frequency due to the fact that different effects can be

Species (anatomical Moisture Protein Fat Ash Salt

location) Type (%) (%) (%) (%) (%) ε′ ε″

Beef Lean 71.5 21.3 6.1 0.83 0.11 70.5 418.7

(forequarter trimmings)

Lamb (leg) Lean 73.0 21.9 3.6 1.48 0.14 77.9 387.2

Pork Lean 73.9 20.1 4.4 1.13 0.08 69.6 392.0 (shoulder) Chicken Lean 73.6 24.3 1.2 0.86 0.13 75.0 480.8 (breast) Turkey Lean 74.5 24.1 0.4 0.98 0.08 73.5 458.4 (breast)

Pork (back) Fat 19.0 3.9 76.1 0.20 0.07 12.5 13.1

Table 3.3. Composition of meat.

observed when dielectric spectra are being analyzed (see the temperature and frequency effects sections).

Studies by Miura et al. concluded that spectra analysis is a very useful tool for quality control of foodstuffs. Specifically, the authors studied the differences between raw, frozen, and boiled chicken at 25°C. They also studied the dielectric spectra of fish, vegetables, eggs, dairy products, and beverages (Miura et al. 2003).

It has been reported that it is possible to predict the fat composition in fish or minced meat using electromagnetic measurements (Kent 1990; Kent et al. 1993; Borgaard et al. 2003). The fat content in these foods is clearly related to the water content of the product, so that if one is known the other can be determined. A microwave instrument that mainly consists of a microstripline is currently being marketed (Distell Company, West Lothian, Scotland). This compact and nondestructive meter can measure the lipid content of certain kinds of fish, meat, and poultry products.

Another important application of microwaves in foods is to analyze fish and meat freshness. After death, muscle is not able to utilize energy by the respiratory system. Glycolysis is a method of creating energy by converting glycogen to lactate. Postmortem changes lead to a temporary rigidity of muscles. Otherwise, glycolysis lowers the pH, bringing it closer to its isoelectric point and decreasing the water-holding capacity (Hull- berg 2004). The level of glycogen stored in the animal at the time of slaughter affects the texture of the future marketed meat. For all these reasons, during rigor mortis the dielectric properties are expected to change (Datta et al. 2005). A microwave polarimetric method was used to follow the changes in muscle structure during bovine meat aging (Clerjon and Damez 2005). Promising studies have been carried out to evaluate the freshness of fish products. Haddock muscle showed significant changes of its dielectric properties during rigor mortis at radio frequencies between 1 Hz and 100 KHz (Martisen et al. 2000). Kent et al. (2004b) studied the effect of storage time and temperature on the dielectric properties of thawed frozen cod (Gadus morhua) in order to estimate the quality of this product. The same year, Kent et al. (2004a) developed a combination of dielectric spectroscopy and multivariate analysis to determine the quality of chilled Baltic cod (G. morhua). These studies yielded a prototype developed by SEQUID (Seafood Quality Identification) (Knöchel et al. 2004; Kent el at 2005b) for measuring and analyz- ing the quality of different seafoods. The SEQUID project concentrated on the measure- ment of the dielectric properties of fish tissue as a function of time both in frozen and chilled storage. This project has shown that it is possible, using a combination of time domain reflectometry and multivariate analysis, to predict certain quality-related vari- ables, both sensory and biochemical, with an accuracy comparable to existing methods. Kent et al. (2005a) have also reported a way to determine the quality of frozen hake (Merluccius capensis) by analyzing its changes in microwave dielectric properties. A sensor for measuring freshness in fish is already on the market (Distell Company®, West Lothian, Scotland).

The determination of added water in fish, fish products, and meat using microwave dielectric spectra was widely studied by Kent et al. (2000, 2001, 2002). The added glaze on frozen foods such as cooked and peeled prawns was determined by measuring changes in the dielectric properties (Kent and Stroud 1999).

De los Reyes et al. (2005c) verified the viability of an on-line measurement system using low-power microwaves to determine the desalting point of salted cod. Dielectric spectroscopy was performed on cod samples at different desalting stages and on its

εʺ 0 50 150 250 100 300 200

1,00E+08 1,00E+09 1,00E+10 1,00E+11

Frequency (Hz) 16% NaCI

Salt solutions

Salted cod during desalting process; dry salted cod

1% NaCI 0.5% NaCI 0% NaCI t = 0 t 0,200 GHz 0,300 GHz 0,915 GHz 2,450 GHz 10 GHz 20 GHz

Fig. 3.17. Dielectric spectroscopy on cod samples at different desalting stages (gray continuous lines) and on salt solutions (black dotted lines). Discontinuous vertical lines mark some single frequencies, and t indicates desalting time, which increases in the arrow direction. The frequency is given in logarithm scale.

900 800 700 600 500 400 300 200 100 0 0 0.02 0.04 0.06 0.08 0.1 0.12 0.14

XNaCI (gNaCI/gtotal)

ε″ 0,2 GHz 0,3 GHz 0,915 GHz 2,45 GHz 20 GHz 10 GHz

Fig. 3.18. Loss factor values versus sodium chloride content for each single frequency marked in figure 3.17.

desalting solutions in order to find an appropriate measurement frequency (or frequency range) (fig. 3.17).

Optimum frequencies were selected and dielectric properties data were related to other physico-chemical properties of cod samples measured at the same desalting stages, such as moisture and salt content. Good correlations (approximately R2_{= 0,99) were found}

between salt content in cod samples and its loss factor values at 200 and 300 MHz. These results indicated the viability of developing an on-line control system for the cod desalt- ing process (fig. 3.18).

Applications of Dielectric Properties in Fruits and Vegetables

The dielectric properties of various fruits and vegetables have been reported many times (Tran et al. 1984; Nelson 1982; Seaman and Seals 1991; Nelson et al. 1993, 1994; Kuang and Nelson 1997; Sipahioglu and Barringer 2003).

A recent study on the frequency and temperature dependence of the permittivity of fresh fruits and vegetables was reported in ISEMA 2005 (Nelson 2005). Dielectric properties were also measured in fruits such as apples or citric fruits and related to process variables for a posterior implementation of an on-line quality control system (Romero et al. 2004; De los Reyes et al. 2005a, 2005b). A summary of dielectric properties of different fruits is shown in table 3.4.

Other studies were recently carried out to correlate dielectric properties with some interesting variables of vacuum-impregnated squash. Complex permittivity of fresh squash samples, sucrose and sodium chloride solutions, and squash samples vacuum impregnated with these solutions were measured with a coaxial probe (HP85070E) connected to a net- works analyzer (E8362B) by De los Reyes et al. (2005a). ε′ and ε″ values of sucrose and sodium chloride solutions were correlated with water activity. The authors found that the same equations can also be used to correlate permittivity and water activity of vacuum- impregnated squash samples. Structural and physico–chemical properties of these samples were also determined and qualitatively related to dielectric properties.

De los Reyes et al. (2005b) studied the suitability of using low-power microwaves for on-line nondestructive measurement of dielectric properties of citric fruits. The authors tried to relate the dielectric properties to process variables. The dielectric properties of citric fruits were measured using a coaxial probe and a network analyzer in the range of 800 MHz to 10 GHz frequencies. The process variables measured were water content, density, Brix, ash content, and water activity. Spectral analysis revealed good correlation among the different measuring frequencies with certain process properties, especially free salts and free water.

Dairy Products

The dielectric properties of dairy products have hardly been studied. Rzepecka and Pereira (1974) studied the dielectric properties of whey and skimmed-milk powders. Mudgett et al. (1974, 1980) modeled the dielectric properties of aqueous solutions of nonfat dried milk. Representative dielectric properties of milk and its constituents are given in table 3.5.

Other studies on dielectric properties of butter have been reported (Sone et al. 1970; Rzepecka and Pereira 1974). The permittivity value decreased rapidly at temperatures

72 Advances in Food Diagnostics

below freezing point due to the fact that free water crystallizes. On the other hand, above 30°C the permittivity increase might be due to the disintegration of the emulsion (Venkatesh and Raghavan 2004). A linear increase in dielectric constant with moisture content was shown by Prakash and Armstrong (1970) in a small range of moisture content. A method for evaluating the moisture and salt contents of salted butter was presented by Doi et al. (1991). The dielectric properties of salted water-in-oil (W/O) emulsion as a model of salted butter were studied by Shiinoki et al. (1998). The authors reported the relationship between the dielectric properties of a W/O emulsion and such microwave properties, and discussed the viability of the microwave transmission method for monitor- ing on-line the moisture and salt contents in a continuous salted-butter-making process. Dielectric properties of cheese were studied by some authors (Datta et al. 2005; Green 1997; Herve et al. 1998). It was shown that the dielectric properties of cheese are depend- ent on the composition. Dielectric properties showed an increase when moisture content

Table 3.4. Permittivities of fresh fruits and vegetables at 23°C.

Dielectric Dielectric Loss Moisture Tissue Constant (ε′) Factor (ε″) Content Density (frequency, MHz) (frequency, MHz) Fruit/Vegetable %(w.b.)a _{(g cm}−3_{) 915 2450 915 2450} Apple 88 0.76 57 54 8 10 Avocado 71 0.99 47 45 16 12 Banana 78 0.94 64 60 19 18 Cantaloupe 92 0.93 68 66 14 13 Carrot 87 0.99 59 56 18 15 Cucumber 97 0.85 71 69 11 12 Grape 82 1.10 69 65 15 17 Grapefruit 91 0.83 75 73 14 15 Honeydew 89 0.95 72 69 18 17 Kiwifruit 87 0.99 70 66 18 17 Lemon 91 0.88 73 71 15 14 Lime 90 0.97 72 70 18 15 Mandarin juice 88 0,96 68 65 12 14 Mango 86 0.96 64 61 13 14 Onion 92 0.97 61 64 12 14 Orange 87 0.92 73 69 14 16 Papaya 88 0.96 69 67 10 14 Peach 90 0.92 70 67 12 14 Pear 84 0.94 67 64 11 13 Potato 79 1.03 62 57 22 17 Radish 96 0.76 68 67 20 15 Squash 95 0.70 63 62 15 13 Strawberry 92 0.76 73 71 14 14 Sweet potato 80 0.95 55 52 16 14 Tomato 91 1.02 75 71 18 16 Turnip 92 0.89 63 61 13 12

Source: Adapted from Nelson et al. 1994; Betoret et al. 2004; De los Reyes et al. 2005a, 2005b, 2005d.

increased (Green 1997). Dielectric properties of cottage cheese were studied by Herve et al. (1998) in order to extend the shelf life of the product with microwave treatment. The authors concluded that the cheese with the highest fat content had the lowest dielectric constant. This might be due to the fact that fat content is related to the moisture content in a negative way, and a lower dielectric constant yields a lower dielectric constant. Datta et al. (2005) showed the dielectric constant and loss factor of processed cheese at different compositions for temperatures of 20°C and 70°C. The authors concluded that the dielectric properties of processed cheese are not generally temperature dependent.

Catalá Civera (1999) found different values for the dielectric properties of cured and soft cheese, suggesting that microwave sensors can be a good method of controlling curing processes. In figure 3.19 dielectric constant and loss factor spectra of cured and soft cheese

Description Fat (%) Protein (%) Lactose (%) Moisture (%) ε′ ε″

1% Milk 0.94 3.31 4.93 90.11 70.60 17.60

3.25% Milk 3.17 3.25 4.79 88.13 68.00 17.60

Water+ lactose I 0.00 0.00 4.00 96.00 78.20 13.80 Water+ lactose II 0.00 0.00 7.00 93.00 77.30 14.40 Water+ lactose III 0.00 0.00 10.00 90.00 76.30 14.90 Water+ sodium caseinate I 0.00 3.33 0.00 96.67 74.60 15.50 Water+ sodium caseinate II 0.00 6.48 0.00 93.62 73.00 15.70 Water+ sodium caseinate III 0.00 8.71 0.00 91.29 71.40 15.90 Lactose (solid) 0.00 0.00 100.00 0.00 1.90 0.00 Sodium caseinate (solid) 0.00 100.00 0.00 0.00 1.60 0.00 Milk fat (solid) 100.00 0.00 0.00 0.00 2.60 0.20 Water (distilled) 0.00 0.00 0.00 100.00 78.00 13.40

Table 3.5. Dielectric properties of milk and its constituents at 2.45 GHz and 20°C.

Source: Adapted from Kudra et al. 1992.

Cured cheese Soft cheese 0 2 4 6 8 0 10 20 30 40 50 60 70 80 90 20 30 40 50 60 70 80 10 ×109 Frequency (Hz) (a) ε′ Cured cheese Soft cheese 0 2 4 6 8 10 ×109 Frequency (Hz) (b) ε″

Fig. 3.19. Dielectric constant (a) and loss factor (b) versus frequency of soft and cured cheese (adapted from Catalá Civera 1999).

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are shown. Dielectric values of soft cheese are higher than cured ones since soft cheese has higher moisture content.