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Dielectric and thermophysical properties of meat batters over


MEAT SCIENCE
Meat Science 68 (2004) 173–184 www.elsevier.com/locate/meatsci

Dielectric and thermophysical properties of meat batters over a temperature range of 5–85 °C
L. Zhang, J.G. Lyng *, N. Brunton, D. Morgan, B. McKenna
Department of Food Science, Faculty of Agriculture, University College Dublin, Bel?eld, Dublin 4, Ireland Received 29 July 2003; received in revised form 9 January 2004; accepted 9 February 2004

Abstract Dielectric (dielectric constant (e0 ) and loss factor (e00 )) and thermal (heat capacity (c), thermal conductivity (k) and thermal di?usivity (a?)) properties of two meat batters (pork luncheon roll (PLR) and white pudding (WP)) were measured between 5 and 85 °C. Radio frequency (RF) and microwave (MW) e00 values varied across 5–85 °C (P < 0:05). MW e0 and e00 values for WP tended to peak at 45 °C and decrease thereafter, whereas for PLR, e0 and e00 peaked at 65 °C which appeared to match potato starch gelatinisation within this product. WP and PLR had signi?cantly higher c values at 25 °C, which corresponded to the MP of pork fat. For PLR, an additional c peak was noted at 65 °C, which appeared to correspond to potato starch gelatinisation. At 85 °C, k values were higher (P < 0:05) than at 5, 25 and 45 °C but were not higher than values at 65 °C. a values increased with temperature (P < 0:05). ? 2004 Elsevier Ltd. All rights reserved.
Keywords: Dielectric and thermal properties; Microwave and radio frequency waves; Meat batters

1. Introduction Food professionals require data on the physical properties of foods for various purposes such as the design, installation, operation and control of processes, plant and equipment associated with food. Knowledge of such properties is necessary not only because they are of importance in their own right but also because they a?ect physical treatments received during processing. Dielectric properties are two such physical properties, which govern the interaction between RF or MW radiation and foods. Once interaction has occurred, thermal properties such as c and k determine how the food product heats. RF and MW have the ability to penetrate foodstu?s and considerably reduced cooking times. Indeed RF technology has been proposed as a promising technique for the continuous cooking of meats (Houben, Van Roon, Krol, & Jansen, 1990). However, these technologies have yet to be applied to the cooking of
Corresponding author. Tel.: +353-1-716-7710: fax: +353-1-7161149. E-mail address: james.lyng@ucd.ie (J.G. Lyng). 0309-1740/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.meatsci.2004.02.009
*

meats on a wide scale commercial basis either alone or in the form of multimedia cooking systems. With recent renewed interest in electroheating technologies, particularly RF heating (Laycock, Piyasena, & Mittal, 2003), the availability of the relevant physical properties will be central to the development and industrial application of these technologies. The RF region of electromagnetic radiation (EMR) refers to the region from 1 to 300 MHz and can be distinguished from the MW region, which extends from 300 MHz to 3 GHz (Risman, 1991). At RF frequencies, the primary heating mechanism is that associated with rapid ionic polarisation of dissolved ions (Ryynnen, 1995) as opposed to MW frequencies in a which heating occurs primarily due to the rapid motion of the dipolar water molecules in the alternating ?eld (Bu?er, 1993). The key dielectric properties of a foodstu? are e0 , which expresses the ability of the material to store energy, and e00 , which expresses the capacity of the material to dissipate energy (Engelder & Bu?er, 1991). A high e00 indicates an increased ability of the food to absorb energy. A number of factors have been shown to a?ect the dielectric properties of

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foodstu?s. These include chemical composition (Goedeken, Tong, & Virtanen, 1997; Kent & Anderson, 1996; Lyng, Scully, & McKenna, 2002), temperature (Pace, Westphall, Goldblith, & Van Dyke, 1968; Tran & Stuchly, 1987) and frequency of incident radiation (Engelder & Bu?er, 1991). Whilst a number of studies have examined the e?ect of temperature on dielectric properties of meats at MW frequencies (Bengtsson, Melin, Remi, & Sderlind, 1963; Bircan & o Barringer, 2002; Tran & Stuchly, 1987), no information is available on the latter at RF frequencies. A principal objective of the present study was to examine the e?ect of temperature on the dielectric properties of two porkbased meat batters at both RF and MW frequencies. Thermal properties (including k, a and c) of foods a?ect heat transfer within a product from absorbed MW and RF energy. During RF and MW cooking, heat will transfer from elevated temperature regions produced in areas of high RF or MW absorption to lower temperature regions in other parts of the product, which have not absorbed RF or MW energy to the same extent. A complete understanding of the heating process thus requires knowledge of the e?ect of temperature on these thermophysical properties. A secondary objective of the present study is the measurement of k, a and c of meat batters as a function of temperature. k is a measure of the rate of heat transfer, c indicates how much energy is required to increase the temperature of a known quantity of a material, whilst a combines k and c to give a measure of how quickly the temperature of a food will change when it is heated (Sanz, Alonso, & Masceroni, 1987).

ground though a 10-mm plate using a mechanical mincer (Model TS8E, Tritacarne, Omas, Itali). The ground tissue was then placed in polyethylene bags, vacuumpackaged using a Webomatic vacuum packaging system (Model no. 021ODC681, Webomatic, Bochum, Germany) and stored at )18 °C until required for product manufacture. Suitable amounts of the frozen muscle and fat were tempered at 5 °C in a chill for 24 h prior to batter preparation. 2.2. Meat batter preparation and processing Two sample meat batters were prepared (WP and PLR). The ingredients and suppliers used in the preparation of these recipes are listed in Table 1. The procedure used in preparation of the batters is given in Table 2. Processing of the batters involved blending the thawed minced meat and fat with the remaining ingredients in a Manica bowl chopper (Model No. CM22, Equipaimentos Carnicos, Barcelona, Spain). A temperature probe (Model 2046T, Digitron, Sifam Instruments Limited, Torquay, England) was used to monitor the temperature of the emulsion, which was maintained between 10 and 12 °C during batter preparation. After blending, the batters were packed into 140-ml plastic cups (King Ireland, Dublin, Ireland), covered with cling ?lm and stored at 4 °C for a maximum of 24 h prior to analysis. Three 12-kg batches of each batter were prepared. 2.3. Proximate analysis Moisture content was determined by drying triplicate portions of each sample (48 h in an air oven at 105 °C) and determining the weight loss. Protein was determined by automatic Kjeldahl (Soderberg, 1995). Fat was determined by a solvent extraction method as described by Soderberg (1995) using the soxtec system (Model No. HT6, Tecator AB, Hoganas, Sweden). Ash was determined by heating in a furnace at 400 °C for 4 h and

2. Materials and methods 2.1. Meat handling Lean pork shoulder and pork back fat was obtained from a local producer (Galtee meats, Cork, Ireland) and

Table 1 Ingredients and suppliers used in the manufacture of white pudding and pork luncheon roll meat batters Ingredient Lean pork Pork fat Kibbled onion White pudding seasoning (55.08% salt) Pork luncheon roll seasoning (44.8% salt) Iced water Rusk meal Super?ne rusk Potato starch Cure solutiona 500E Protein concentrate Total
a

White pudding (kg) 5.0 2.4 0.3 0.3 2.3 1.1

Pork luncheon roll (kg) 4.4 2.6

Supplier Galtee meats, Cork, Ireland Galtee meats National Food Ingredients, Limerick, Ireland William Blake?s Ltd., Dublin, Ireland William Blake?s Ltd. William Blake?s Ltd. William Blake?s Ltd. William Blake?s Ltd. William Blake?s Ltd.

0.3 3.1 0.6 0.6 0.2 0.2 12.0

0.6 12.0

Cure solution consisted of water (81%), salt (12.5%), STPP (2.0%), sodium ascorbate (0.25%) and sodium nitrite (0.1%).

L. Zhang et al. / Meat Science 68 (2004) 173–184 Table 2 Procedure used in the preparation of white pudding and pork luncheon roll meat batters Step No. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. White pudding All meats placed in bowl and cure solution added Seasoning followed by water added Chopped for 120 s at knife and bowl speed 1 Kibbled onion and rusk meal added Chopped for 120 s at knife and bowl speed 2 Packed into plastic cups (King Ireland) Stored at )20 °C Pork luncheon roll Lean pork (2.8 kg) and pork fat (1.9 kg) placed in bowl 500E evenly distributed into bowl Half the water added (1.55 kg) Cure solution added Chopped for 90 s at knife and bowl speed 1 Remainder of lean pork (1.6 kg) and pork fat (0.7 kg) added Super?ne rusk, seasoning, potato starch and the remainder of water (1.55 kg) added Chopped for 30 s at knife and bowl speed 1 Chopped for a further 90 s at knife and bowl speed 2 Packed into plastic cups (King Ireland) Stored at )20 °C

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weighing the residue. NaCl was determined by titration of the ash sample from the previous method, using the method of Kirk and Sawyer (1991). 2.4. Manufacture and design of temperature control vessel A specially designed temperature controlled sample holder was constructed in order to ensure that the meat batters could be held at the speci?ed measurement temperatures (5, 25, 45, 65 and 85 °C), while dielectric and k measurements were made. The vessel consisted of two parts, a jacketed sample cup holder (in which the sample in a 140-ml plastic cup was placed and allowed to equilibrate) and a jacketed lid (to prevent heat loss from the surface of the sample). The sample holder section was manufactured from aluminium, which was precision engineered to ensure that the 140-ml plastic cups used in this work ?tted precisely inside giving good contact between the cup and the aluminium. The sample holder and lid sections were jacketed and water at the measurement temperatures was circulated from a water bath (Model No. LTD 20, Grant Instruments Ltd., Barrington, Cambridge CB2 5QZ, England). The lid was also manufactured from aluminium which had an opening designed to allow measurements to be taken from samples using dielectric or thermal conductivity probes while they were held at constant temperatures. To prevent surface cooling while sample temperatures were equilibrating, an aluminium stopper was inserted into the opening and removed immediately prior to making measurements. An illustration of the temperature control vessel is included in Fig. 1(a). In order to con?rm that the close proximity of dielectrically active aluminium to the probe did not interfere with dielectric readings at MW and RF frequencies, a comparison was made between dielectric measurements on meat batter samples from batch 1, which were measured at 25 °C in the presence and absence of the temperaturecontrolled vessel.

2.5. Analysis of dielectric properties 2.5.1. Measurement of dielectric properties at RF and MW frequencies e0 and e00 of the meat batters at RF frequencies were measured using an open-ended co-axial probe (CapenHurst Technologies, Chester, UK) connected to a Hewlett Packard network analyser (Model No. 8714ET, Agilent Technologies, California, USA). Fig. 1(a) and (b) provides illustrations of the measurement system used. Measurements were carried out in the temperature range 5–85 °C at 20 °C intervals. Prior to analysis, samples in plastic cups covered with cling-?lm were equilibrated in a water bath (Model No. OLS 200, Grant Instruments Ltd.) set at the appropriate temperature. The surface temperature of the samples was checked immediately prior to each measurement using a right-angled surface thermocouple (Model No. TP107, Eurolec Instrumentation, Dundalk, Ireland). Following equilibration, samples were removed and immediately transferred to the temperature control vessel. Calibration of the probe at 27.12 MHz was carried out prior to analysis by the sequential attachment of a 50 X load and a shorting block. Samples were placed on a plastic laboratory jack (Lennox Laboratory Supplies, Dublin, Ireland) and raised to place the surface of the meat batter in contact with the probe so as not to e?ect the calibration. e0 and e00 were calculated by inputting real and imaginary impedance values from the Smith chart displayed on the network analyser into a previously con?gured Excel worksheet (CapenHurst Technologies). Real and imaginary impedance values for air and water at 25 °C were also required for the calculation. Measurements at MW frequencies were carried out in an identical manner to those at RF frequencies apart from the following exceptions. An Agilent Technologies open-ended co-axial probe (Model No. 85070C) with a frequency range of 0.3–3.0 GHz was used for the dielectric measurements and calculations of e0 and e00 at 896, 915 and 2450 MHz were carried out with aid of the 85070C software package (Version C1-02, Agilent technologies).

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Fig. 1. (a,b) Illustration of apparatus used to hold samples at a constant temperature during dielectric and thermal conductivity measurements.

Four measurements at MW and RF frequencies were carried out on three cups at each temperature for each batch. 2.5.2. Calculation of dielectric properties e0 and e00 measurements were used to calculate power re?ected (Pr ), power transmitted (Pt ), tan delta (tan d) and penetration depth (dp ). These properties give an additional insight into how the food product interacts

with the incident EMR. Table 3 lists the equations used to calculate these properties together with the literature source for each equation. 2.6. k measurement k (W m?1 °C?1 ) measurements were carried out using a probe based on the design of Sweat (1986) as described

L. Zhang et al. / Meat Science 68 (2004) 173–184 Table 3 Equations used to calculate derived dielectric parameters Equation no. 1 2 3 4 Calculated parameter Power re?ected (Pr) Power transmitted (Pt) Tan Delta (Tand) Penetration Depth (dp ) Equation p??? 2 e0 ? 1 Pr ? p??? e0 ? 1 Pt ? 1 ? Pr e00 tan d ? 0 8 2s????????????????????? 39?1  00 2 pe? < ?? = 2 k 2 e0 4 1 ? e dp ? ? 15 2p : ; e0 Units % % Dimensionless cm Source Bu?er (1993) Bu?er (1993) Engelder and Bu?er (1991) Bu?er (1993)

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by Lyng et al. (2002). The probe was calibrated using agar and glycerol (99% Analar, BDH laboratory Supplies, BH15 1TD, UK) as described by Sweat (1986) and Balaban and Pigott (1992). Prior to analysis, samples were equilibrated at the appropriate temperature in a temperature-control vessel as described in Section 2.4. The k probe was inserted into the geometric centre of the meat batter and allowed to equilibrate prior to commencing a measurement. Calculation of k was performed using the method of Sweat (1986). Four measurements at MW and RF frequencies were carried out on three cups at each temperature for each batch. 2.7. c measurement c measurements were carried out using a TA Instruments di?erential scanning calorimeter (DSC) (Model No. DSC 2010, TA Instruments Inc., Newcastle, USA) and the method of Murphy, Marks, and Marcy (1998). The instrument was calibrated with indium (melting point 156.6 °C). Prior to analysis, meat batters for each recipe were homogenised to a consistent paste using a Moulinex blender (Model No. MO531, Moulinex Swan, Birmingham, UK). Samples (15–20 mg) were weighed into coated aluminium pans (TA instruments) and hermetically sealed. A hermetically sealed empty pan was used as the reference pan. After cooling with liquid nitrogen, samples were equilibrated at )5 °C and then heated from 5 to 85 °C at heating rate of 10 °C min?1 . c values were calculated using the following equation (Sanz et al., 1987):     60 ? E DH Ctemp x ? ; ?5? ? HR m where Ctemp x is the heat capacity of sample at temperature x (kJ kg?1 C?1 ), E is a calibration factor calculated using a sapphire calibrant in an identical heating program as outlined above, HR is the heating rate (°C min?1 ), m is the sample mass (mg) and DH is the di?erence in y-axis displacement between a blank run (i.e., using an empty sample pan as both the reference and the sample) and sapphire heating runs (mW). Within each batch, triplicate measurements (expressed in kJ kg?1 °C?1 ) were taken at each temperature from each of the three cups.

2.8. Measurement of thermal transition properties of pork fat and potato starch Thermal transition properties of pork fat were obtained using DSC and an identical heating program as described in 2.7 above. An empty aluminium pan was used as the reference. Potato starch thermal transition properties were determined by DSC on 1:1 starch:distilled water mixtures using the method of Davis, Grider, and Gordon (1986). Using this method, samples were equilibrated at 30 °C and then heated to 90 °C at 5 °C min?1 . 2.9. a measurement An a apparatus similar in design to that of Dickerson (1965) and Magee and Bransburg (1995) was constructed. The tubes were ?lled with the meat batters using a mechanical ?ller (Model No. EM-12, Equipaimentos Carnicos) and sealed. The tube was then allowed to equilibrate at 5 °C in a Lauda water bath (Model No. C12 CP, Lauda Knigshofen, Germany). The water o bath was then heated at rate of 0.3 °C min?1 to a temperature of 25 °C. After the core of the sample had reached this temperature, the water bath was again heated at 0.3 °C min?1 to 45 °C. This procedure was then repeated at 20 °C intervals until the core of the sample had reached a temperature of 85 °C. Temperatures were monitored every 30 s using a Grant squirrel temperature logger (Model No. 1250, Grant Instruments Ltd., Cambridge, UK), which measured and displayed temperatures to ?0.1 °C. a values were calculated as described by Dickerson (1965) and expressed as m2 s?1 ? 10?7 . For each batch and recipe, measurements were made in triplicate. 2.10. Statistical analyses Analysis of variance (ANOVA) was used to test the e?ect of batch, frequency and temperature on the thermophysical properties measured using the SAS software package (Version 8.2, Statistical Analysis Systems, Cary, NC). When signi?cant di?erences were indicated by ANOVA (P < 0:05), Tukey pairwise comparisons were preformed to indicate where the di?erences between treatments existed.

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3. Results 3.1. Proximate analysis of meat batters Mean values for proximate analysis of three batches of WP and PLR samples are presented in Table 4. No signi?cant di?erence between batches of either recipe (P P 0:05) was found. The WP recipe was found to have a signi?cantly higher moisture (P < 0:01), ash (P < 0:001) and salt content (P < 0:001), and a signi?cantly lower carbohydrate (P < 0:01) and protein content (P < 0:05) than the PLR formulation. No signi?cant di?erence was detected between formulations in fat content (P P 0:05). 3.2. Dielectric properties of meat batters at MW and RF frequencies 3.2.1. E?ect of temperature ANOVA revealed no signi?cant di?erence (P P 0:05) in e0 and e00 values of batch 1 samples (measured at 25 °C) in the presence and absence of the temperature control vessel indicating that use of the vessel did not in?uence the values measured. At RF frequencies, e0 for both formulations exhibited no signi?cant variation (P P 0:05) across the measured temperature range (Table 5). In contrast, signi?cant variations in e00 at 27.12 MHz were noted with values tending to increase from 5 to 65 °C (Table 6). Although an average e00 decrease was observed from 65 to 85 °C at RF frequencies, Tukey pairwise comparison of the means found no signi?cant di?erence (P P 0:05) between e00 values at these temperatures in either formulation.

Signi?cant variations in e00 at MW frequencies were noted for both recipes although the situation regarding e0 was less clear. For WP samples, ANOVA indicated that at MW frequencies, temperature signi?cantly a?ected e0 values. Within WP samples e0 values between 5 and 45 °C appeared to plateau with a possible decline from 65 to 85 °C though Tukey pairwise comparison of the means indicated that this decline was not clearly signi?cant at all frequencies. In contrast, no such trend was evident in either the means or Tukey pairwise comparison of the PLR e0 values (Table 5). For WP recipes at MW frequencies, e00 tended to increase between 5 and 45 °C, while for PLR this increase occurred between 5 and 65 °C. However, Tukey pairwise comparison of the means (Table 5) revealed no signi?cant di?erence (P P 0:05) between 45 and 65 °C measurements at MW frequencies in either product. In both WP and PLR there appeared to be a possible decrease in e00 at 85 °C, though statistically this was only evident in PLR at MW frequencies. Values for Pr for WP and PLR ranged from 0.46% to 0.54% at RF and MW frequencies but no major trends across the temperature range 5–85 °C were evident. Pt values ranged from 0.46% to 0.55% and again no trends were evident. Values for dp for both recipes ranged from 0.92 to 11.38 cm (Table 7) and showed a signi?cant variation across the temperature range examined at all frequencies for both batters. In general for both products at RF and MW frequencies, dp values tended to decrease with increasing temperature. However, whilst values at 5–25 °C were generally signi?cantly higher than values from 45 to 85 °C (P < 0:05), no signi?cant decrease

Table 4 White pudding and pork luncheon roll proximate analysis (% dry weight) Product White pudding Pork luncheon roll P value
a;b
A

Moisture 61.2 60.4b 0.003
a

Fat 15.8 15.1a NS
a

Protein 10.8 11.3b 0.015
a

CarbohydrateA 9.1 11.4b 0.003
a

Ash 3.1 1.8b 0.000
a

Salt 2.3a 1.2b 0.000

Means in the same column with unlike letters are di?erent (P < 0:05). Carbohydrate content was calculated as 100% (moisture% + fat% + protein% + ash%).

Table 5 The e?ect of temperature on e0 of white pudding and pork luncheon roll meat batters Temperature (°C) 5 25 45 65 85 P value
a–c

White pudding 27.12 MHz 44.24 42.63 42.03 41.71 47.85 NS 896 MHz 37.48 36.72bc 38.83c 35.67ab 33.84a 0.001
bc

Pork luncheon roll 915 MHz 37.38 36.57b 38.72b 35.56a 33.72a 0.001
b

2450 MHz 33.00 32.83ab 34.64b 31.45ab 29.97a 0.023
ab

27.12 MHz 46.60 46.59 47.24 50.22 50.19 NS

896 MHz 40.13 39.71a 36.49a 39.60a 34.93a NS
a

915 MHz 40.03 39.61a 36.39a 39.49a 34.82a NS
a

2450 MHz 36.08a 35.90a 33.01a 35.63a 31.16a NS

Means in the same column with unlike letters are di?erent (P < 0:05).

L. Zhang et al. / Meat Science 68 (2004) 173–184 Table 6 The e?ect of temperature on e00 of white pudding and pork luncheon roll meat batters Temperature (°C) 5 25 45 65 85 P value
a–c

179

White pudding 27.12 MHz 880.41 1118.17ab 1457.18c 1631.39c 1350.15bc 0.000
a

Pork luncheon roll 896 MHz 42.09 48.31a 59.82b 58.59b 50.64ab 0.000
a

915 MHz 41.32 47.42a 58.68c 57.46bc 49.64ab 0.000
a

2450 MHz 21.92 23.09ab 26.85c 25.61bc 23.34ab 0.005
a

27.12 MHz 526.66 701.76b 855.51b 938.11c 933.21c 0.000
a

896 MHz 28.48 32.86a 34.49ab 41.22b 32.45a 0.004
a

915 MHz 28.01 32.27a 33.85ab 40.43b 31.85a 0.004
a

2450 MHz 17.14a 18.03ab 18.32ab 21.03b 16.24a 0.014

Means in the same column with unlike letters are di?erent (P < 0:05).

above 45 °C was noted (P P 0:05). Although not significant (P P 0:05) within any of the frequencies measured, looking at the data collectively suggests that an average increase in dp possibly occurs at 85 °C, though values at 5 °C were always highest of all temperatures. 3.2.2. E?ect of frequency and formulation on dielectric properties Mean values for both recipes from 5 to 85 °C for e0 , 00 e , tan d, dp and Pr tended to decrease with increasing frequency. However, values at 896 and 916 MHz were not signi?cantly di?erent from each other for any of the properties examined (P P 0:05). If Tables 4–6 are examined, WP generally has lower mean dp values and higher e00 values than PLR. 3.3. k results k values for WP and PLR in Table 8 ranged from 0.34 to 0.48 W m?1 °C?1 . ANOVA revealed that k of both recipes measured at 85 °C were signi?cantly higher than those at 5–45 °C (P < 0:05). However, values at 65 °C were not signi?cantly lower (P P 0:05) than values at 85 °C. ANOVA revealed no signi?cant di?erence between WP and PLR k values at any of the measurement temperatures (P P 0:05). 3.4. c results The c values of each recipe were not signi?cantly di?erent at 5, 45 and 85 °C. However, as shown in Table 9 peak values for the PLR emulsion occurred at

25 and 65 °C (P < 0:05), while for the WP recipe peak values occurred at 25 °C only (P < 0:05). Fig. 2 shows sample thermograms for the PLR and WP recipes. For the PLR, two speci?c endotherms were detected at 25 and 65 °C, which correspond with the peak values for c. By contrast, only one endotherm for the WP was detected. ANOVA revealed a signi?cant di?erence between mean c values of WP and PLR at all measurement temperatures ?P < 0:000?. 3.5. a results a values displayed in Table 10 were signi?cantly different at all measurement temperatures (P < 0:05) and increased linearly with increasing temperature for both recipes (WP r2 ? 0:985, PLR r2 ? 0:977). ANOVA revealed no signi?cant di?erence between a of WP and PLR at any of the measurement temperatures (P P 0:05).
Table 8 Mean thermal conductivity (k) (W m?1 °C?1 ) of white pudding and pork luncheon roll from 5 to 85 °C Temperature (°C) 5 25 45 65 85 P value
a;b

White pudding 0.34 0.35a 0.36a 0.41ab 0.48b 0.014
a

Pork luncheon roll 0.35a 0.36a 0.36a 0.38ab 0.45b 0.014

Means in the same column with unlike letters are di?erent (P < 0:05).

Table 7 The e?ect of temperature on penetration depth (dp ) (cm) of white pudding and pork luncheon roll meat batters Temperature (°C) 5 25 45 65 85 P value
a–c

White pudding 27.12 MHz 8.62 7.62b 6.63a 6.28a 7.04ab 0.000
c

Pork luncheon roll 896 MHz 1.75 1.55b 1.34a 1.34a 1.50ab 0.000
c

915 MHz 1.74 1.54b 1.33a 1.33a 1.49ab 0.000
c

2450 MHz 1.08 1.03bc 0.92a 0.93ab 1.00abc 0.002
c

27.12 MHz 11.38 9.74b 8.79a 8.41a 8.43a 0.000
c

896 MHz 2.52 2.20ab 2.04ab 1.81a 2.22ab 0.014
a

915 MHz 2.50 2.18ab 2.03ab 1.80a 2.20ab 0.014
b

2450 MHz 1.41ab 1.34a 1.28a 1.16a 1.42ab 0.047

Means in the same column with unlike letters are di?erent (P < 0:05).

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Table 9 Heat capacity (c) (kJ kg?1 °C?1 ) of white pudding and pork luncheon roll at temperatures from 5 to 85 °C Temperature (°C) 5 25 45 65 85 P value White pudding 2.956 3.331b 2.979a 3.003a 2.899a 0.000
a

Pork luncheon roll 3.324a 3.773b 3.336a 3.606b 3.335a 0.000

a;b Means in the same column with unlike letters are di?erent (P < 0:05).

2 0 -2 Heat flow (mW) -4 -6 -8 -10 -12 -15 5 25 45 Temp (?C) 65 85 PLR WP

Fig. 2. DSC thermograms of WP and PLR meat batters.

Table 10 Thermal di?usivity (a) (m2 s?1 ? 10?7 ) of white pudding and pork luncheon roll at temperatures from 5 to 85 °C Temperature range (°C) 5–25 25–45 45–65 65–85 P value White pudding 1.18a 1.28b 1.38c 1.53d 0.000 Pork luncheon roll 1.16a 1.26b 1.41c 1.53d 0.000

a–d Means in the same column with unlike letters are di?erent (P < 0:05).

4. Discussion Few studies to date have focussed on the dielectric properties of meats at 27.12 MHz. Tran and Stuchly (1987) using an open ended coaxial probe system similar to that of the present study showed that in the RF frequency range (100 MHz), both e0 and e00 of whole beef, beef liver, chicken and salmon increased with increasing temperatures between 1.2 and 64 °C. However, measurements above 64 °C were not presented. In regard to

e0 measurements in the 5–65 °C range, these ?ndings are in contrast with the present study where no clear pattern emerged in regard to the e?ect of temperature on e0 values of meat batters at either RF or MW frequencies. Structurally, the whole beef and chicken products examined by Tran and Stuchly (1987) would most resemble the products examined in this work. However, within these products it must be noted that Tran and Stuchly (1987) were examining entire meats, which had their myo?brillar and connective tissue structures largely intact. In contrast, the present study is focusing on comminuted meats where these proteins are disrupted by comminution. Indeed it is also notable that e0 differences were more evident in WP samples compared to PLR samples, which visual inspection suggests was less comminuted product. Furthermore, the products in this work have non-meat ingredients (including water binders) added. The ?ndings of Tran and Stuchly (1987) on e0 values could be in some way re?ect shrinkage in the myo?brillar proteins which would be expected in the temperature range 40–50 °C (Ledward, 1979; Sims & Bailey, 1992) and which could exert a greater in?uence on the entire meats. However, further work is needed to verify this. Similar to the ?ndings in the present study, other authors have reported less straightforward relationships between dielectric properties and temperature for meats and meat products. Tanaka, Mallikarjunan, Kim, and Hung (2000) demonstrated that while e0 decreased with increasing temperature for marinated chicken breast at both 915 and 2450 MHz, e00 increased with increasing temperature at 915 MHz, but at 2450 MHz it decreased from 0 to 35 °C and then increased from 35 to 70 °C. Tong and Lentz (1993) also reported that e00 decreased with increasing temperature between 0 and 50 °C but increased with increasing temperature between 50 and 90 °C for bentonite pastes at 2450 MHz. The authors postulated that di?ering trends within different temperature ranges ‘‘may be prevalent in salty foods in which ionic loss is important and increases with increasing temperature’’. From examination of the data in the present study, it could be suggested that changes such as myo?brillar and collagen denaturation, and phase changes in fat and starch (if present) may be responsible for these changes, which occur as products are heated through a temperature range from 5 to 85 °C. Suggestions of this nature have been made by Bircan and Barringer (2002), who found that e0 decreased with increasing temperature and e00 increased with increasing temperature at MW frequencies (915 and 2450 MHz), and both properties increased sharply at temperatures between 70 and 80 °C in whole beef, chicken breast and thigh, perch, cod and salmon. They postulated that because this sharp increase appeared to match the denaturation temperature of collagen, it could be due to a loss of water and ions from the denatured proteins. In contrast in the present study, e00 values at RF and MW

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frequencies suggested a possible decrease between 65 and 85 °C (Table 6). Furthermore, no signi?cant difference (P P 0:05) was noted between values for e0 at 65 and 85 °C for any of the frequencies measured. This could be related to the lower meat and thus meat protein content (Tables 1 and 4) of the emulsions used in the present study compared to the whole meats measured by Bircan and Barringer (2002). As indicated by Tables 1 and 4, PLR contained potato starch while WP did not. Miller, Gordon, and Davis (1991) have shown that there is a marked increase in e0 and e00 for potato starch:water mixes at temperatures between 62 and 75 °C, which correspond to the temperature range associated with starch gelatinisation. The authors state that this increase could be due to the release of naturally occurring phosphate groups from the starch granules during the gelatinisation process. Fig. 2 shows a DSC thermogram for potato starch:water mixture in the temperature range 30–80 °C, which shows an endotherm between 55 and 72 °C presumably that associated with starch gelatinisation. However, when dielectric properties were examined to verify if similar observations to Miller et al. (1991) could be found in the e0 and e00 of the PLR, a signi?cant increase (P < 0:05) in e00 between 45 and 65 °C at 27.12 MHz was observed, but this increase was not signi?cant (P P 0:05) at any other frequency. A more detailed speci?c study would be required to explore this e?ect. An increasing e00 is indicative of an increased ability to absorb MW and RF energy as demonstrated by the following equations (Brennan, Butters, Cowell, & Lilly, 1976): PAbs PAbs
RF aE 2

f e00
2

A ; d

?6a? ?6b?

MW aE

f e00 ;

where PAbs is the power absorbed by the product at RF (Eq. (6a)) or MW (Eq. (6b)) frequencies, E is the electrical ?eld strength, f is the frequency, A is the area of the dielectric and d is the thickness of the dielectric. Pace et al. (1968) suggested that as the temperature of the product increases, so does its ability to absorb EMR energy. The ?ndings of this work suggest that this may hold true for comminuted meat products up to a temperature of 45–65 °C but that beyond this temperature, a plateau or decline may occur. In addition to the temperature e?ect, the mean e00 was signi?cantly higher for the WP as compared to the PLR formulation (P < 0:05). As outlined in Section 1, the higher e00 of the WP could be explained by its higher salt content as indicated by the proximate analysis (Table 4). The e?ect of increasing e00 and reducing dp at higher salt concentrations has previously been observed at MW frequencies (2450 MHz) for beef burgers with higher salt contents (Lyng et al., 2002) and chicken breasts mari-

nated in salt solutions of di?erent concentrations (Tanaka et al., 2000). For the two meat batters dp s at microwave frequencies ranged from 0.92 to 2.52 cm (Table 7). This range is in line with that reported by Ohlsson, Bengtsson, and Risman (1974), who found that dp s for raw beef samples ranged from 0.75 to 3.5 cm at frequencies from 434 to 2450 MHz. The authors also reported that at temperatures from 3 to 60 °C, at 434 and 915 MHz, dp s decreased with increasing temperature but did not report values above 60 °C. This is in agreement with the general trend for the e?ect of temperature between 5 and 65 °C on dp of both meat batters in the present study (Table 7), though it must be noted that not all the decreases in this study were statistically signi?cant. In contrast with their other ?ndings, Ohlsson et al. (1974) also reported that at 2450 MHz, dp increased with increasing temperature from 3 to 60 °C, though no explanation for this di?erence was given. In the present study, at 2450 MHz dp decreased with increasing temperature from 5 to 65 °C, although the degree of variation was much less than at other frequencies (2.31 cm for WP at 27.12 MHz versus 0.15 cm for WP at 2450 MHz). It is also worth noting that dp s at RF were substantially higher than at MW frequencies and also the fact that the PLR samples had higher dp s when corresponding values were examined (Table 7). The latter observation is most likely due to the higher salt content in the WP samples (Table 4). Mittal, Wang, and Usborne (1989) reported k values for a beef/pork fat-based meat batter with a similar moisture content to the batter in the present study (60%) of 0.404–0.453 W m?1 °C?1 . In addition, Timbers, Randall, and Raymond (1982) reported values of 0.355– 0.456 W m?1 °C?1 for a range of beef pork-based emulsions of varying compositions measured at 25 °C. Furthermore, the range of k values quoted in the present study is in agreement with those for a range of whole meats including restructured beef products (Shahedi Baghe-Khandan & Okos, 1981; Tsai, Unklesbay, Unklesbay, & Clarke, 1998), fresh turkey (Chang, Carpenter, & Toledo, 1998), whole beef (Perez & Calvelo, 1984), fresh lamb (Pham & Willix, 1989) and chicken breast meat (Sweat, Haugh, & Stadelman, 1973). Similar to the ?ndings of this study, Mittal et al. (1989) reported that on cooking, the k of a beef/pork fat-based emulsion increased by 12%. Shahedi Baghe-Khandan and Okos (1981) also found that as whole and ground beef sample temperatures increased from 30 to 70 °C, an increase in k occurred. However, these workers also reported that between temperatures of 70 and 120 °C, a reduction in k occurred and attributed this to loss of water from the products during heating. In contrast, no decrease was observed between 65 and 85 °C (Table 8) in the present study, and in fact k continued to increase in this temperature range. However, emulsion products such as WP and PLR are generally encased (which was broadly

182

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simulated at all stages during sample equilibration in the present study) during industrial cooking, and while yield results are not presented in the present study, preliminary work on these products indicated that cook loss from these emulsion products was under 1%. The signi?cant increase in k values with temperature indicates that an increase in temperature in the product results in an increase in the ability of the meat batters to conduct heat from regions of high temperature to areas of low temperature. The fact that no signi?cant di?erences in k between the two recipes was detected is not surprising given the overriding in?uence of moisture content on k (Muzilla, Unkelsbay, Unkelsbay, & Helsel, 1990; Sweat et al., 1973). In the present study, statistical di?erences exist between the moisture contents of the WP and PLR (P < 0:01), but it must be emphasized that the magnitude of this di?erence is small (0.8%) (Table 4). c values for the WP meat batter varied from 2.899 to 3.311 kJ kg?1 °C?1 , whereas values for the PLR meat batter varied from 3.324 to 3.773 kJ kg?1 °C?1 over the temperature range 5–85 °C. These values are broadly in agreement with those determined by Mittal et al. (1989), who reported that c varied from 2.65 to 3.83 kJ kg?1 °C?1 in beef/pork-based emulsion-type sausages. The authors also noted that c for sausages cooked at 45 °C was 3.00 kJ kg?1 °C?1 , and that this value decreased to 2.85 kJ kg?1 °C?1 for sausages cooked at 65 °C. In both cases, the sausages had a similar moisture content to that of the meat batters examined in the present work (60%). This is signi?cant as c has been found to be dependent on moisture content (Tocci & Masceroni, 1998). It should be noted also that the values for c determined by Mittal et al. (1989) were determined by adiabatic calorimetry which some authors have reported to be more accurate than the DSC method used in the present work as the sample size is much larger (Ohlsson, 1991). However, in the present case, the accuracy of the DSC method appeared to be adequate as relative standard deviations (rsd) for duplicate measurements ranged from 1.42% to 5.97% for PLR, while for the WP recipe values ranged from 4.61% to 6.40%. This lack of a large variation for duplicate measurements may have been due to the fact that samples were thoroughly pre-blended prior to each measurement to improve homogeneity. The slightly greater variation in the rsd of the WP recipe is probably attributable to the fact that the prior to pre-blending, the WP is a coarsely comminuted meat batter, while the PLR could be classed as a ?nely comminuted meat batter. Other workers have used the DSC method for the determination of c in meats including Tocci, Flores, and Masceroni (1997), who used the method for determining the c and enthalpy of boneless mutton, and Karanakar, Mishra, and Bandyopadhyay (1998) who used the method for shrimp meat. In both cases the authors found the accuracy of the DSC method to be su?cient for the products examined.

The endotherm which occurs at 25 °C in DSC thermograms of both recipes (Fig. 2) most likely corresponds to the melting temperature of the emulsi?ed pork fat as thermograms of pork fat alone yield a sharp endotherm at this point (Fig. 3). This would broadly correlate with the observations of Barbut and Mittal (1991), who noted a slight decrease in modulus of rigidity in poultry meat emulsions at temperatures up to 32 °C, which was attributed to the melting of fat. Previous authors have demonstrated, using DSC measurements, that peak temperatures for potato starch gelatinisation endotherms occur between 58 and 72 °C depending on the potato variety and growth conditions (Miller et al., 1991; Svegmark et al., 2002; Tester, Debon, Davies, & Gidley, 1999). The endotherm present in the PLR thermograms in the temperature range (60–72 °C) most likely corresponds to the gelatinisation of potato starch which formed part of the formulation for this recipe (Table 1) and is re?ected in the higher carbohydrate content of this formulation as presented in Table 4 (P < 0:05). In addition, DSC thermograms of a potatostarch:water mixture yields a single endotherm between 58 and 72 °C (Fig. 3). However, it is also worth noting that Barbut and Mittal (1991) reported increases in the modulus of rigidity of poultry meat emulsions at temperatures 55 °C which they attributed to myosin denaturation. A further increase in the modulus of rigidity was also noted at temperatures of up to 68 °C, which these workers attributed to the denaturation of collagen and sarcoplasmic proteins. While such transitions almost certainly occurred in the present samples, no evidence of these phase transitions occurring in the WP samples (which did not contain potato starch) was observed (Fig. 2). a values recorded in the present study ranged from 1.15 to 1.53 m2 s?1 ? 10?7 over a temperature range of 5–85 °C and were of a similar order of magnitude to those recorded by Timbers et al. (1982) for pork-based meat batters (1.165–1.1325 m2 s?1 ? 10?7 ) at 15–75 °C.
Pork Fat Potato Starch

1 -1 Heat flow (mW) -3 -5 -7 -9 -11 -13 -15 5 25 45 Temp (?C) 65

85

Fig. 3. DSC thermograms of pork fat (PF) and a potato starch (PS) water mix (1:1).

L. Zhang et al. / Meat Science 68 (2004) 173–184

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Mittal et al. (1989) reported a values of 1.06 m2 s?1 ? 10?7 for a beef/pork fat-based meat emulsion, while Mittal and Blaisdell (1984) reported a values of 1.168–1.245 m2 s?1 ? 10?7 for meat emulsions of various fat and protein ratios. The authors also reported that this value increased to 1.09 m2 s?1 ? 10?7 after cooking at 65 °C. The linear relationship between product temperature and a is in agreement with the results of Chul, Sang, Hyun, and Bong (1993), who also found that di?usivities of pork meat products increased linearly with heating temperature. Results from the present study indicate that temperature has a marked e?ect on a and that heat transfer within the meat batters increases as the temperature of the product increases. As indicated by Eq. (7), a combines k and c and also density q (kg m?3 ) a? k : qc ?7?

Di?erences in c were observed at 25 and 65 °C for PLR and 25 °C only in WP. These were most likely due to melting of pork fat and pork starch gelatinisation (Fig. 3) but were not re?ected in the a values reported in Table 10. However, since a is measured across 20 °C temperature intervals, any phase change occurring in relatively narrow temperature ranges may not have been emerged in the a values.

5. Conclusion This work con?rms previous ?ndings with regard to the marked e?ect of frequency on dielectric properties (e.g. e0 , e00 and dp ) and the in?uence of key ingredients on dielectric (salt vs. e00 ) properties. However, other factors such as starch inclusion, level of comminution and changes occurring within meat products (e.g., melting of fat, myo?brillar and connective tissue denaturation, starch gelatinisation) may also a?ect selected dielectric (e.g., myo?brillar and connective tissue denaturation vs. e00 or dp ) and thermal (e.g., myo?brillar and connective tissue denaturation vs. k; fat melting and starch gelatinisation vs. c) properties which in turn may alter the interaction between meats and RF and MW radiation during the cooking process. The work also suggests that highly comminuted meat batters may behave di?erently with respect to certain dielectric and thermal properties as compared to whole meats. However, with the limited range of products in this study, further work and a wider range of meats will need to be studied to con?rm this.

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