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Radio-frequency treatment for stabilization of wheat germ: Dielectric properties and heating uniformity
Innovative Food Science and Emerging Technologies 48 (2018) 66–74
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Innovative Food Science and Emerging Technologies journal homepage: www.elsevier.com/locate/ifset
Radio-frequency treatment for stabilization of wheat germ: Dielectric properties and heating uniformity Bo Linga, James G. Lyngb, Shaojin Wanga,c,
T
⁎
a
Northwest A&F University, College of Mechanical and Electronic Engineering, Yangling, Shaanxi 712100, China Institute of Food and Health, University College Dublin, Belfield Dublin 4, Ireland c Department of Biological Systems Engineering, Washington State University, 213 L.J. Smith Hall, Pullman, WA 99164-6120, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Radio-frequency heating Wheat germ Thermal stabilization Dielectric properties Heating uniformity
Wheat germ (WG) is a valuable by-product of wheat milling but is highly susceptible to lipid rancidity induced by lipase (LA) activity. Therefore a stabilization step is required to reduce LA activity. Radio-frequency (RF) heating is an alternative thermal technology suitable for processing food and has great potential for WG stabilization. However, in order to develop effective RF stabilization treatments, a knowledge of the dielectric properties (DPs) (i.e. dielectric constant (ε″) and dielectric loss factor (ε″)) of WG at frequencies used for heating is required. Furthermore, an understanding of the influence of RF processing conditions on heating uniformity and LA inactivation is needed. Findings in the present study show that ε′ and ε″of WG increased with increasing temperature and moisture content (MC). Furthermore, the relationship between DPs and temperature/MC can be described by quadratic order models. RF heating uniformity in WG gradually improved with decreasing heating rate from 19.6 to 6.3 °C/min and MC from 15.96 to 7.05% (wet basis, w.b.). Placing the WG on a moving conveyor at 4 m/h slightly improved RF heating uniformity in the interior layers of the WG. Placing a pair of rectangular polyetherimide plates (12 × 8 cm) above and below the container during RF heating at 6.3 °C/min further improved temperature distribution leading to a more uniform LA inactivation in WG. Overall, the results obtained from this study are useful in computer simulation and optimizing process parameters for WG stabilization by RF heating.
1. Introduction Wheat germ (WG) is the embryo of wheat seeds which germinates to produce a plant but is also a by-product of commercial flour production. WG has traditionally been used as a cost-efficient ingredient for animal fodder or as raw material from which oil is extracted (Brandolini & Hidalgo, 2012). More recently, many studies have shown that WG can actually be used directly as an ingredient in bakery products, but also can be used as a source from which high quality protein isolates can be extracted (Zhu, Wang, & Guo, 2015). However, WG's very short shelf life due to its tendency towards lipase (LA) induced rancidity has limited its more widespread incorporation into food products. Processing methods which have been reported to reduce LA activity and stabilize WG include gamma irradiation (Jha, Kudachikar, & Kumar, 2013), sourdough fermentation (Marti et al., 2014) and conventional thermal treatments (Gili, Torrez Irigoyen, Penci, Giner, & Ribotta, 2018) with the latter being the most commonly employed.
However, WG has a flake like structure (which traps air) and also has a low moisture content (MC), which collectively give it a low thermal conductivity (Gili, Torrez Irigoyen, Penci, Giner, & Ribotta, 2017). Conventional hot air drying involves large volume samples which are exposed to long treatment times in dry air, but the duration of time at high temperatures can damage WG quality. Conventional steaming in moist air is also an effective method for enzyme inactivation, but the stabilized WG must subsequently be dried prior to storage. By contrast dielectric heating with microwave (MW) or radio-frequency (RF) energy, can rapidly raise food temperatures thereby significantly reducing heating time (Awuah, Ramaswamy, & Tang, 2015). RF and MW generate heat within the product volume by molecular frictions hence the term ‘volumetric heating’. However, RF heating has greater penetration depths and more uniform electromagnetic field patterns and for these reasons tends to produce more uniform heating. In the past five years, RF heating has been studied for various purposes in food and agricultural products, including disinfestation (Ling, Hou,
Abbreviations: WG, Wheat germ; LA, Lipase; MC, Moisture content; RF, Radio-frequency; MW, Microwave; DPs, Dielectric properties; PE, Polyethylene; PC, Polycarbonate; PEI, Polyetherimide ⁎ Corresponding author at: Northwest A&F University, College of Mechanical and Electronic Engineering, Yangling, Shaanxi 712100, China. E-mail address: [email protected] (S. Wang). https://doi.org/10.1016/j.ifset.2018.05.012 Received 14 January 2018; Received in revised form 13 May 2018; Accepted 14 May 2018 Available online 18 May 2018 1466-8564/ © 2018 Elsevier Ltd. All rights reserved.
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The selected MC levels were typical of the MC ranges of fresh WG reported by Brandolini and Hidalgo (2012). The average initial MC of WG was 11.33% w.b. To create samples with elevated MC, WG samples were kept over appropriate amount of deionized water in desiccators under ambient conditions for 2–4 days to allow them to absorb water. To obtain samples with reduced MC, the initial WG sample was held in a forced air oven at 50 °C for 4–6 h. Once the desired MC levels were attained the hydrated and dehydrated samples were stored in ziplocked PE bags at 4 °C until required for analysis.
Li, & Wang, 2016; Zhou, Guo, & Wang, 2017), drying (Zhou, Gao, Mitcham, & Wang, 2018), pasteurization (Li, Kou, Cheng, Zheng, & Wang, 2017; Nagaraj et al., 2016; Zheng, Zhang, & Wang, 2017) and thawing (Bedane, Chen, Marra, & Wang, 2017). Although available studies suggest that RF heating could be used to replace or supplement conventional heating in food processing, its application for WG stabilization has not achieved its true potential, which is partly due a lack of comprehensive knowledge and data. In order to develop an effective RF stabilization treatment for WG, it is essential to have a good knowledge of its dielectric properties (DPs) as these are the major factors describing the interaction between RF and WG. The dielectric constant (ε′) and loss factor (ε″) are two important parameters for dielectric heating treatments, with ε′ providing an index of a foods ability to store electromagnetic energy while ε″ indicating absorption of electromagnetic energy and its conversion to heat (Ling, Guo, Hou, Li, & Wang, 2015; Sosa-Morales, Valerio-Junco, López-Malo, & García, 2010). Therefore, a quantification of WG DPs is important for the design, optimization and control of RF energy based stabilization processes. In addition, the potential for non-uniform RF heating is still a major obstacle to RF adoption by the food industry. Non-uniform heating during RF application, occurs at the corners and edges of rectangular slabs which commonly absorb more energy compared to other parts of a container, which is mainly caused by differing DPs of the foodstuff and its surrounding medium, which leads resulting in an uneven distribution of the electric field (Marra, Zhang, & Lyng, 2009). The local high temperatures caused by non-uniform heating may result in severe quality loss, such as lipid oxidation or non-enzymatic browning reactions in foodstuffs (Alfaifi et al., 2014; Wang, Birla, Tang, & Hansen, 2006). Therefore, RF heating uniformity needs to be improved before industrial scale applications. Various methods have been adopted to improve the RF heating uniformity. These include replacing the surrounding media (air) with water or plastic material, examples including immersion of fruits (Wang et al., 2006) and meat products (Nagaraj et al., 2016) in water or covering the foodstuff with a plastic material which has a similar ε′ (Jiao, Tang, & Wang, 2014). Another approach for solids such as fruits (Pegna et al., 2017) and nuts (Ling et al., 2016) is to combine RF heating with product movement/rotating, adding forced hot air, sample mixing and moving on a conveyor belt. Although there are many reports on RF heating uniformity of different foodstuffs, studies related to heating uniformity of WG during RF stabilization are still not available. Based on the issues identified above, the objectives of this study were: (1) to determine the DPs of WG at three moisture levels and three frequencies measured at temperatures ranging from 25 to 85 °C, (2) to provide empirical models describing WG's DPs as a function of MC and temperature at the three frequencies, (3) to study WG RF heating uniformity under different heating conditions and (4) to evaluate the LA inactivation in WG treated using a range of RF heating conditions.
2.3. Determination of dielectric properties The DPs of WG were measured using an open-ended coaxial probe system (85070E-020), an impedance analyzer (E4991B-300) and a computer with the 85070E dielectric software installed (Keysight Technologies Co. Ltd., Palo Alto, California, USA). Samples were measured while being held in a custom-built sample test holder which produced close contact with the flat tip of the probe. According to the method of Gili et al. (2017), the flake density of WG flake with a MC of 7.05, 11.33, and 15.96% w.b. were 0.983 ± 0.010, 1.115 ± 0.008, and 1.327 ± 0.010 g/cm3, respectively. Known weights of WG flour (50 mesh) calculated form flake density of WG were placed into an airtight sample test holder, to maintain the sample MC constant during the measurement. To match the sample density samples were placed in contact with the probe and a screw knob at the bottom of the sample holder was rotated to a specific position to ensure the required volume/ sample density (Bedane et al., 2017). DPs of WG were measured between 25 and 85 °C in 15 °C increments at 13.56, 27.12 and 40.68 MHz. 2.4. Determination of heating uniformity of WG during RF treatment 2.4.1. RF treatment and temperature measurements RF heating was performed using a 6 kW, 27.12 MHz pilot-scale free running oscillator system (SO6B, Strayfield International, Wokingham, U.K.). This equipment allowed for adjustment of the parallel plate electrode separation from 9 to 19 cm using the movable top electrode (40 cm × 83 cm) so as to regulate the RF power, with conveyor belt speed also adjustable from 1 to 60 m/h. To evaluate the heating uniformity under different RF heating conditions, WG samples with 7 cm depth were put in a rectangular polycarbonate (PC) container (inner dimensions of 21 cm × 14 cm × 7.5 cm) (Fig. 1A). During RF heating, the container was placed at the center of the bottom electrode from 25 to 90 °C. This range was selected based on the temperature required to inactivate LA in WG (Kapranchikov, Zherebtsov, & Popova, 2004). During RF heating, a fiber-optic temperature sensor connected to a data logger (HQ-FTS- D120, Heqi Technologies Inc., Xian, China) was inserted through a hole at the side wall of the container to measure the temperature at the geometric center of the WG samples. The RF system was turned off when the geometric center of the sample reached 90 °C. Samples were then immediately removed from the RF oven and the temperatures throughout the entire container were measured to evaluate RF heating uniformity. The temperatures of WG after RF heating were measured using two methods, the first mapping the top surface temperature (Fig. 1A) using an infrared camera (A 300, FLIR Systems, Inc., North Billerica, MA, USA) while the second collected interior temperatures at 13 representative points in the central horizontal plane of the sample. The latter utilized a custom-designed temperature measurement platform (Fig. 1C) which consisted of a temperature recorder (RDXL12SD, OMEGA Engineering Inc., Taiwan, China) connected to 13 K-type thermocouples (OMEGA Engineering Inc., Stamford, CT, USA) which were mounted on a wooden board to ensure temperature measurements were simultaneously taken at a depth of 3.5 cm from the sample surface as shown in Fig. 1B. The entire temperature measurement procedure was completed within 20 s.
2. Materials and methods 2.1. Materials Fresh WG flakes from hard red winter wheat were obtained from a local flourmill (Shaanxi Bull Flour Co., Ltd., Meixian, Shaanxi, China). These flakes were packed in polyethylene (PE) bags and stored at −20 °C until required for analysis. The WG flakes had 11.33 ± 0.20 g moisture /100 g, 29.77 ± 0.31 g protein/100 g, 11.56 ± 0.40 g fat/ 100 g and 5.60 ± 0.06 g ash/100 g sample (all on a fresh weight basis) (AOAC, 2005). 2.2. Sample preparation Three WG samples with MC of 7.05, 11.33, and 15.96% w.b. were prepared to study the effect of MC on DPs and RF heating uniformity. 67
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Fig. 1. Rectangular polycarbonate container filled with wheat germ flakes for (A) surface temperature measurement, (B) top view of 13 different interior temperature measurement points and 5 different locations for lipase activity determination, and (C) the custom-designed temperature measurement platform (all dimensions are in cm).
Fig. 2. Rectangular polycarbonate container filled with wheat germ flakes with (A) surrounded on all surfaces by polyetherimide (PEI) plates (B) only side wall surrounded by rectangular PEI plates and (C) only bottom and top surfaces covered with rectangular PEI plates of differing sizes.
2.4.3. Effect of moisture content and bulk density on RF heating uniformity To evaluate the effect of MC and bulk density on RF heating uniformity, 650 g WG flakes with three MCs (7.05, 11.33, and 15.96% w.b.) and filled to a depth of 7 cm with three bulk density levels (0.32, 0.40 and 0.48 g/cm3). All samples had an initial MC of 11.33% w.b. and were heated in the PC container while stationary with an electrode separation of 13 cm.
2.4.2. Effect of heating rate and conveyor belt movement on RF heating uniformity The effect of heating rates on RF heating uniformity was initially assessed with a stationary sample. The PC container was filled with 650 g of WG flakes (11.33% w.b.) which had a bulk density of 0.32 g/ cm3 was heated by the RF system over a range of electrode separations from 9 to 13 cm at 1 cm intervals. Experiments were subsequently performed with moving samples with belt speeds calculated (from stationary studies) to ensure the samples remained underneath the top electrode until they had attained the target temperature. For example, based on a top electrode length of 0.83 m and a container length of 0.14 m and a heating time of 10.25 min at an electrode separation of 13 cm under stationary conditions, the belt speed was calculated and set at 4.0 m/h to evaluate the influence of moving conveyor belt on the RF heating uniformity.
2.4.4. Effect of surrounding material on RF heating uniformity Based on recent studies, the electromagnetic field uniformity during RF heating could be improved in a foodstuff if it was surrounded or partially covered with plastic material that has similar ε′ to the foodstuff and a negligible ε″ (Jiao et al., 2014; Jiao, Shi, Tang, Li, & Wang, 2015). Therefore, polyetherimide (PEI) with similar ε′ (3.2 at 1 MHz) to the WG (3.5 at 27 MHz and 11.33% w.b.) was selected to evaluate the effect of PEI plate (7.5 mm thickness) on RF heating uniformity. The PC 68
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Table 1 Dielectric properties of wheat germ samples at three moisture contents (MCs), five temperatures and three frequencies. MCs (% w.b.)
7.05
11.33
15.96
Temp. (°C)
25 40 55 70 85 25 40 55 70 85 25 40 55 70 85
ε′ at frequency (MHz)
ε″ at frequency (MHz)
13.56
27.12
2.78 ± 0.10 3.04 ± 0.30 3.46 ± 0.25 4.33 ± 0.51 5.39 ± 0.46 3.72 ± 0.21 4.68 ± 0.29 6.25 ± 0.34 8.00 ± 0.64 10.21 ± 0.64 4.87 ± 0.25 6.09 ± 0.31 8.08 ± 0.39 9.74 ± 0.88 11.00 ± 1.01
2.64 2.91 3.37 4.11 5.14 3.55 4.42 5.76 7.25 9.27 4.56 5.56 6.84 8.10 9.75
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
40.68 0.19 0.19 0.11 0.47 0.38 0.31 0.27 0.15 0.51 1.01 0.42 0.24 0.41 0.76 1.11
2.58 2.84 3.34 4.02 4.98 3.48 4.30 5.56 6.92 8.60 4.50 5.41 6.20 7.93 9.11
0.30 0.36 0.43 0.54 0.68 0.40 0.50 0.68 0.84 1.14 0.75 0.92 1.20 1.58 2.22
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.02 0.03 0.07 0.04 0.08 0.05 0.07 0.05 0.14 0.12 0.04 0.11 0.42 0.31 0.14
0.29 0.34 0.39 0.49 0.62 0.37 0.47 0.60 0.75 0.90 0.66 0.83 1.06 1.46 2.18
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
40.68 0.04 0.03 0.04 0.06 0.09 0.04 0.09 0.11 0.09 0.04 0.03 0.18 0.27 0.27 0.26
0.27 0.32 0.36 0.44 0.53 0.33 0.44 0.56 0.70 0.86 0.59 0.73 0.94 1.26 1.94
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.02 0.02 0.02 0.06 0.11 0.02 0.04 0.12 0.12 0.06 0.08 0.08 0.33 0.18 0.17
ε′/ ε′ ′ = α 0 + α1 MC + α2 T + α12 MCT + α11 MC 2 + α22 T 2 + α112 MC 2T + α122 MCT 2 + α111 MC 3 + α222 T 3
(3)
where, α0, α1, α2, α12, α11, α22, α112, α122, α111, and α222 are regression coefficients. The order of the polynomial, significant terms, significance of the model, and coefficient of the determination (R2) of the model to fit the experimental data were automatically given by the software.
2.4.5. Heating uniformity evaluation Temperature difference (ΔT = Tmax − Tmin), average temperature (Tavg) and heating uniformity index (λ) were used to compare the surface and interior heating uniformity of RF treatments. The λ value is defined as the ratio of the rise in standard deviation (Δσ, °C) of sample temperature to the rise in average sample temperature (Δμ, °C) over the treatment time and can be calculated from the following equation (Ling et al., 2016):
Δσ Δμ
0.18 0.17 0.28 0.28 0.38 0.23 0.27 0.15 0.28 0.48 0.38 0.35 0.67 0.85 1.01
27.12
were analyzed by response surface method (RSM). The model fitting and regression analysis were performed using Design-expert 8.0.6 (StatEase, Inc. Minneapolis, MN, USA). The general form of the polynomial model was as follows:
container filled with 650 g initial WG flakes (11.33% w.b.) with bulk density of 0.32 g/cm3 was surrounded by PEI plate in three different configurations: (1) all surrounded (Fig. 2A), (2) surrounded only on side walls (Fig. 2B), and (3) the center of both top and bottom surfaces covered with rectangular PEI plate of different sizes (6 × 4, 12 × 8 or 21 × 14 cm) (Fig. 2C). In general, surrounding or partially covering a tray with PEI should help to concentrate more electric energy in the whole sample or the covered area, thereby improving electromagnetic field uniformity.
λ=
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
13.56
2.7. Statistical analysis Data were expressed as mean ± standard deviations of triplicate measurements. Significant differences (p < 0.05) within means were subjected to analysis of variance and Tukey's honestly significant difference (HSD) test in the statistical software SPSS 16.0 version (SPSS Inc., Chicago, IL, USA).
(1)
Smaller λ values indicate better heating uniformity.
3. Results and discussion
2.5. Lipase activity determination in RF treated WG
3.1. Moisture and temperature dependent dielectric properties of WG
To evaluate the effect of RF heating on LA inactivation, initial WG sample (11.33% w.b.) with bulk density of 0.32 g/cm3 was heated from 25 to 90 °C using conditions which produce the best RF heating uniformity as described in sections of 2.4.2 and 2.4.4. RF heating was stopped when the geometric center temperature of samples achieved 90 °C. WG flakes from the five different locations throughout the PC container (Fig. 1B) were immediately collected using a stainless steel grain sampler (length of 20 cm and inner diameter of 1.8 cm), then packed in zip-lock bags and immersed into ice water to cool down the samples to ambient temperature. LA activity values in WG samples were immediately determined following the method reported by Rose and Pike (2006) and the results were expressed as percentage of enzyme activity in the unheated control as below:
The impact of moisture content, temperature and frequency on DPs of WG flakes are summarized in Table 1. Both ε′ and ε″ increased with increasing MC and temperatures, but decreased with increasing frequency. The DPs of foodstuffs mainly depend on the content of free water, as the MC raises the free water content in the WG, and the increases of temperature result in further increases of the ionic mobility (Calay, Newborough, Probert, & Calay, 1994). Thus the increase in DPs with an increase in the MC and temperature could be due to the increased polarization of molecules and ionic conductivity.
Relative enzyme activity (%) =
residual enzyme activity × 100 enzyme activity of control
3.2. Predictive models for dielectric properties of WG at three radio frequencies Table 2 shows the coefficients of the polynomial order models with their significant terms describing both ε′ and ε″ of WG as influenced by MC and temperature. A quadratic order model was the best fit for ε′ and ε″ at each frequency in the tested MC and temperature range. The linear (MC and T) terms had strong influence on the model at all three frequencies (p < 0.0001) both for ε′ and ε″. The interaction (MCT) term influenced strongly (p < 0.0001) ε″ only at 13.56 MHz. Each model provided a good fit to the ε′ and ε″ at the significance level of 0.0001
(2)
2.6. Model analysis To study the MC and temperature dependent DPs, the correlations of MC and temperature (T, °C) with DPs of WG at three frequencies 69
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Table 2 Regression coefficients of the polynomial model (Eq. (3)) to predict the dielectric properties of wheat germ as a function of moisture content (7–16% w.b.), temperature (25–85 °C) at three radio frequencies. Regression coefficients in Eq. (2)
α0 α1 α2 α12 α11 α22 R2 ⁎
13.56 MHz
27.12 MHz
40.68 MHz
ε′
ε″
ε′
ε″
ε′
ε″
−3.04 0.99⁎ −0.05⁎ 6.93 × 10−3 −0.04 4.74 × 10−4 0.977
1.65 −0.21⁎ −0.03⁎ 2.00 × 10−3⁎ 8.52 × 10−3 1.71 × 10−4 0.989
−3.11 1.05⁎ −0.05⁎ 4.93 × 10−3 −0.04 5.85 × 10−4 0.977
1.99 −0.26⁎ −0.03⁎ 2.16 × 10−3 9.99 × 10−3 1.69 × 10−4 0.968
−2.84 0.97⁎ −0.04⁎ 4.23 × 10−3 −0.04 5.16 × 10−4 0.979
1.64 −0.20⁎ −0.03⁎ 1.95 × 10−3 7.37 × 10−3 1.51 × 10−4 0.969
Indicated model terms significantly influence the model at 0.0001 probability levels.
sample surface and interior. However, the trends of ΔT and λ values in sample surface and interior were consistent. Under stationary heating conditions, the RF heating uniformity was gradually improved with the decreasing heating rate, as reflected by reducing ΔT and λ values. This indicated that larger electrode separations with slower heating may reduce over-heating during RF treatment. Furthermore, longer treatment times may allow heat conduction to improve the heating uniformity within the samples. When heating a moving sample with belt speeds of up to 4 m/h, there was no significant (p > 0.05) differences in surface ΔT value, while the interior ΔT value decreased significantly (p < 0.05) from 20.5 to 17.1 °C and the λ value also decreased from 0.10 to 0.07. Similarly, Chen, Huang, Wang, Li, and Wang (2016) also reported that RF heating uniformity in the middle of a layer of wheat seeds could be improved by conveyor belt movement.
with R2 value > 0.968 for different frequencies. The temperature and MC dependent DPs could change the electromagnetic power absorption, penetration depth and temperature distribution in foodstuffs (SosaMorales et al., 2010). Therefore, the models obtained from this study for predicting DPs of WG would be useful in a computer model for simulating temperature histories of WG during RF heating and in designing the dielectric heating stabilization system employed with RF energy. 3.3. Influence of heating rate and conveyor movement on heating uniformity Fig. 3 shows the influence of electrode separation on heating rates at the center of 7 cm deep WG samples during RF heating. The temperatures increased linearly with heating time under each electrode separation. About 3.3, 4.4, 6.3, 9.3 and 10.25 min were required to heat 650 g WG from 25 to 90 °C with heating rates of 19.6, 14.8, 10.3, 7.0, and 6.3 °C/min for electrode separations of 9, 10, 11, 12, and 13 cm, respectively. Fast heating methods (5–20 °C/min) based on RF energies have been proposed to replace conventional thermal treatments in food processing (Marra et al., 2009). Therefore, the electrode separations of 9, 11, and 13 cm were selected for evaluating the effect of heating rate (20, 10, and 6 °C/min) on RF heating uniformity. Fig. 4 shows the top surface temperature distribution in WG with different RF heating rates. All the plots showed that the cold spot was located at the center, while hot spots were located at the corners and edges. Temperature uniformity parameters of WG with different treatments are listed in Table 3. The Tavg of interior was higher than that on top surface, while the ΔT and λ values were lower than the top surface. This could be due to the heat loss to ambient air on the top surface and also the difference between temperature measurement methods for
3.4. Influence of MC and bulk density on heating uniformity As shown in Table 4, heating time decreased with increasing MC, while ΔT and λ values increased with increasing MC both for surface and interior, which indicate that the higher MC resulted in higher heating rate and lower temperature uniformity under given conditions. Similar trends were also found in other RF treated foodstuffs with different MC (Li et al., 2015; Ozturk, Kong, Singh, Kuzy, & Li, 2017). It is generally considered that higher MC results in a larger loss factor and increased RF heating rates. Over-heating and thermal run away phenomenon is more likely to appear during fast heating. Therefore, RF heating rates should be adjusted to a reasonable rate during stabilization of WG with different MC. On the other hand, there was no significant (p > 0.05) difference in temperature uniformity parameters between surface of WG with different bulk density, while the ΔT and λ values for interior decreased significantly (p < 0.05) with increasing bulk density from 0.32 to 0.48 g/cm3. It may be explained by densitydependent thermal properties of foodstuff, with a higher density at the interior layer leading to an increase in heating conduction for improving the heating uniformity within the WG. 3.5. Influence of surrounding material on the heating uniformity As shown in Fig. 5, the cold (center) and hot (corner and edge) spots still remained at the same locations when the WG were surrounded by PEI in a variety of configurations, indicating that surrounding the samples or selected sample surfaces with PEI did not completely change the RF heating patterns. Compared to WG without surrounding PEI, the ΔT and λ values both for sample surface and interior increased significantly (p < 0.05) in WG all surrounded by PEI (Table 5). RF treatment time also decreased from 10.3 to 9.3 min, which is probably because the PEI plates lead to an increased absorption of electrical energy into the sample. On the contrary, no significant (p > 0.05) differences were observed in ΔT and λ values between WG that had only sides surrounded and not surrounded by PEI both for sample surface
Fig. 3. Temperature-time profile of wheat germ with 11.33%, w.b. moisture content and bulk density of 0.32 g/cm3 in the center of the polycarbonate container with differing electrode separations. 70
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Fig. 4. Top surface temperature distribution of wheat germ with moisture content of 11.33% w.b. and bulk density of 0.32 g/cm3 in a rectangular polycarbonate container for radio-frequency heating in stationary case with different heating rates (A–C) and moving on conveyor belt at 4 m/h (D).
holding did not completely inactivate LA in WG. Kapranchikov et al. (2004) also reported that the LA in WG is heat-stable with over 20% its original activity retained after holding at 70 °C for up to 1 h. Although the relative LA activity at five different locations differed among RF treatments, LA values in location V was always the highest since the central area in each treatment was always coldest. Heating a moving case did not seem to improve the uniformity of LA inactivation. When covering the both top and bottom surfaces with rectangular PEI plates (12 × 8 cm), the relative LA activity in location V decreased significantly (p < 0.05) as compared to other treatments. This might be because the PEI plates caused an increased temperature in the cold spot, thus improving the RF heating uniformity and reducing the difference of LA activities between hot and cold spots in the WG sample.
and interior. When covering both sides (upper and lower surfaces) of container with PEI plates of different sizes, the average ΔT and λ values decreased both for surface and interior as shown in Table 5, which coincided with the surface temperature distributions as shown in Fig. 5. An obvious temperature increase (5–10 °C) could be observed in the center area of WG that was covered with PEI (Fig. 5 D–F) as compared to the WG that was not surrounded by PEI plates (Fig. 5 A). Similarly, Jiao et al. (2015) also reported that the ΔT and λ values on the surface of peanut butter could be reduced by covering cold spots with PEI blocks. However, they did not perform a statistical analysis of these differences. As shown in our study, a significant (p < 0.05) improvement in RF heating uniformity could be only observed when using 12 × 8 cm PEI plates. Under these conditions, the average ΔT values decreased from 33.0 to 24.6 °C in top surface and 20.5 to 15.6 °C in the interior, respectively, and the corresponding average λ values were also significantly (p < 0.05) smaller than those in the WG that was not surrounded.
4. Conclusion The DPs of WG were greatly influenced by MC and temperature at three frequencies (i.e. 13.56, 27.12, and 40.68 MHz). Both ε′ and ε″ of WG samples decreased with increasing frequency or decreasing temperature and MC. The MC and temperature dependent DPs at three frequencies could be described by quadratic order models and each model provided a good fit to the DPs at the significance level of 0.0001 with R2 value > 0.968. Corner and edge heating could be observed
3.6. Influence of different RF treatments on lipase activity Table 6 shows the LA activities in WG at five different locations, which include hot (I to IV) and cold (V) spots in the PC container treated by different RF heating conditions. RF heating to 90 °C without
Table 3 Temperature and heating uniformity index of wheat germ with moisture content of 11.33% w.b. and bulk density of 0.32 g/cm3 treated by radio-frequency heating while stationary condition (with differing heating rates) and when moving on a conveyor belt with at 4 m/h. Heating condition
Stationary heating at different heating rate 20 °C/min
ΔT (°C) Tavg (°C) λ
#
Top surface Interior layer Top surface Interior layer Top surface Interior layer
51.7 31.3 64.6 88.4 0.22 0.19
± ± ± ± ± ±
5.3a# 2.7a 4.9a 3.5a 0.03a 0.03a
Moving on a conveyor belt at 4 m/h at a heating rate of
10 °C/min
6 °C/min
6 °C/min
42.7 25.3 66.3 85.2 0.17 0.13
33.0 20.5 63.3 84.1 0.13 0.10
28.9 17.1 61.3 81.8 0.10 0.07
± ± ± ± ± ±
3.5b 2.3b 3.2a 2.5a 0.02b 0.02b
± ± ± ± ± ±
2.8c 1.7c 2.5a 2.1a 0.01c 0.01b
Means within a row followed by the same letter are not significantly different at the 5% probability level. 71
± ± ± ± ± ±
1.8c 1.2d 1.2a 1.6a 0.02c 0.01c
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Table 4 Temperature and heating uniformity index of wheat germ (WG) with three moisture contents and bulk density levels treated by radio-frequency heating at electrode separation of 13 cm. WG properties
7.05 Heating time (min)a ΔT (°C) Tavg (°C) λ
a ⁎
Bulk density (g/cm3)
Moisture content (%, w.b.)
Top surface Interior layer Top surface Interior layer Top surface Interior layer
15.5 25.7 16.7 62.2 82.7 0.09 0.07
± ± ± ± ± ±
⁎
1.6c 2.0c 2.2a 2.0c 0.01c 0.02bc
11.33
15.96
0.32
0.40
0.48
10.3
5.8
10.3
10.4
10.4
33.0 20.5 63.3 84.1 0.13 0.10
± ± ± ± ± ±
2.8b 1.7b 2.5a 2.1bc 0.01b 0.01b
40.8 29.9 67.9 88.9 0.17 0.15
± ± ± ± ± ±
3.7a 2.0a 3.4a 3.5ab 0.02a 0.03a
33.0 20.5 63.3 84.1 0.13 0.10
± ± ± ± ± ±
2.8b 1.7b 2.5a 2.1bc 0.01b 0.01b
31.9 19.0 61.4 86.9 0.13 0.09
± ± ± ± ± ±
2.4b 1.6bc 2.0a 1.1b 0.02ab 0.02bc
32.2 16.8 62.4 90.1 0.12 0.06
± ± ± ± ± ±
2.1b 1.4c 2.5a 1.5a 0.02bc 0.02c
Values indicate the mean times (min) required for the geometry center temperature to be increased from 25 to 90 °C. Means within a row followed by the same letter are not significantly different at the 5% probability level.
to 90 °C without a holding treatment did not completely inactivate LA in WG. Thus, future research is still needed to investigate a holding process, such as hot air holding, which may be used in combination with RF treatment to further improve the heating uniformity and efficiency of LA inactivation to ensure the WG quality.
under all RF heating conditions. RF heating uniformity was improved with decreasing heating rate or increasing bulk density of WG, but decreased with increasing MC of WG. RF heating uniformity was also improved by covering cold spots with rectangular PEI plates (12 × 8 cm) both top and bottom surfaces of the container, and the uniformity of LA inactivation was also improved. However, RF heating
Fig. 5. Top surface temperature distribution of wheat germ with moisture content of 11.33% w.b. and bulk density of 0.32 g/cm3 in a polycarbonate container surrounded by polyetherimide (PEI) plates treated by radio-frequency heating at a fixed electrode separation of 13 cm (with three PEI plates options, either (1) 6 × 4; (2) 12 × 8; or (3) 21 × 14 cm covering both top and bottom surfaces).
72
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Table 5 Temperature and heating uniformity index of wheat germ with a moisture content of 11.33% w.b. and a bulk density of 0.32 g/cm3 in a polycarbonate container with differing polyetherimide (PEI) plates placements while undergoing radio-frequency heating with an electrode separation of 13 cm. PEI plate placement
Heating time (min)a
ΔT (°C) Top surface
None All Sides only wall Top and bottom in Mode Ab Top and bottom in Mode Bc Top and bottom in Mode Cd a b c d ⁎
10.3 9.3 10.5 10.1 9.8 9.7
33.0 39.8 35.5 28.2 24.6 27.8
± ± ± ± ± ±
λ
Tavg (°C)
2.8b⁎ 2.6a 2.3ab 2.1bc 2.6c 2.3bc
Interior layer
Top surface
Interior layer
Top surface
Interior layer
20.5 28.5 22.6 18.8 15.6 17.0
63.3 70.1 60.1 62.4 61.6 60.9
84.1 88.3 80.5 83.0 80.1 80.4
0.13 0.16 0.14 0.11 0.08 0.09
0.10 0.14 0.11 0.08 0.06 0.07
± ± ± ± ± ±
1.7b 1.5a 2.2b 2.0bc 1.4c 1.9bc
± ± ± ± ± ±
2.5b 3.0a 2.4b 2.6b 2.2b 2.7b
± ± ± ± ± ±
2.1ab 1.9a 2.3b 1.5b 2.9b 1.7b
± ± ± ± ± ±
0.01b 0.01a 0.02ab 0.02bc 0.01c 0.02c
± ± ± ± ± ±
0.01b 0.02a 0.02ab 0.01bc 0.02c 0.02bc
Values indicate the mean time (min) required for the geometry center temperature to be increased from 25 to 90 °C. Mode A: Two 6 × 4 cm PEI plates covering both top and bottom sample surfaces. Mode B: Two 12 × 8 cm PEI plates covering both top and bottom sample surfaces. Mode C: Two 21 × 14 cm PEI plates covering both top and bottom sample surfaces. Means within a column followed by the same letter are not significantly different at the 5% probability level.
Table 6 Lipase activities of wheat germ at five different locations in polycarbonate container as influenced by different radio-frequency heating conditions under fixed heating rates (6 °C/min). Heating conditions Relative lipase activity (%)
I II III IV V
Control
Stationary heating
Moving on a conveyor belt at 4 m/h
Stationary heating with surrounding PEI platesa
100Aa# 100Aa 100Aa 100Aa 100Aa
72.2 67.9 69.5 65.4 87.6
71.5 75.6 76.1 73.1 90.4
72.6 70.1 68.9 71.6 78.1
± ± ± ± ±
3.1Bb 2.0Cbc 2.3Cbc 2.0Cc 2.5Ba
± ± ± ± ±
2.5Bb 1.9Bb 2.4Bb 2.1Bb 3.6Ba
± ± ± ± ±
1.7Bb 2.2Cb 2.3Cb 2.5Bb 2.0Ca
# Means within a column among the different locations followed by same lower case letters and means within a row among the different heating conditions followed by same capital letters are not significantly different at the 5% probability level. a Two 12 cm × 8 cm PEI plates covering both top and bottom sample surfaces.
Symbols ε′ ε″ ΔT Tavg λ R2
Gili, R. D., Torrez Irigoyen, R. M., Penci, M. C., Giner, S. A., & Ribotta, P. D. (2017). Physical characterization and fluidization design parameters of wheat germ. Journal of Food Engineering, 212, 29–37. Gili, R. D., Torrez Irigoyen, R. M., Penci, M. C., Giner, S. A., & Ribotta, P. D. (2018). Wheat germ thermal treatment in fluidised bed. Experimental study and mathematical modelling of the heat and mass transfer. Journal of Food Engineering, 221, 11–19. Jha, P. K., Kudachikar, V. B., & Kumar, S. (2013). Lipase inactivation in wheat germ by gamma irradiation. Radiation Physics and Chemistry, 86(6), 136–139. Jiao, Y., Tang, J., & Wang, S. (2015). A new strategy to improve heating uniformity of low moisture foods in radio frequency treatment for pathogen control. Journal of Food Engineering, 141C, 128–138. Jiao, Y., Shi, H., Tang, J., Li, F., & Wang, S. (2015). Improvement of radio frequency (RF) heating uniformity on low moisture foods with Polyetherimide (PEI) blocks. Food Research International, 74, 106–114. Kapranchikov, V. S., Zherebtsov, N. A., & Popova, T. N. (2004). Purification and characterization of lipase from wheat (Triticum aestivum L.) germ. Applied Biochemistry and Microbiology, 40(1), 84–88. Li, R., Kou, X., Cheng, T., Zheng, A., & Wang, S. (2017). Verification of radio frequency pasteurization process for in-shell almonds. Journal of Food Engineering, 192, 103–110. Li, Y., Zhang, Y., Lei, Y., Fu, H., Chen, X., & Wang, Y. (2015). Pilot-scale radio frequency pasteurisation of chili powder: heating uniformity and heating model. Journal of the Science of Food and Agriculture, 96(11), 3853–3859. Ling, B., Guo, W., Hou, L., Li, R., & Wang, S. (2015). Dielectric properties of pistachio kernels as influenced by frequency, temperature, moisture and salt content. Food and Bioprocess Technology, 8(2), 420–430. Ling, B., Hou, L., Li, R., & Wang, S. (2016). Storage stability of pistachios as influenced by radio frequency treatments for postharvest disinfestations. Innovative Food Science & Emerging Technologies, 33, 357–364. Marra, F., Zhang, L., & Lyng, J. G. (2009). Radio frequency treatment of foods: Review of recent advances. Journal of Food Engineering, 91(4), 497–508. Marti, A., Torri, L., Casiraghi, M. C., Franzetti, L., Limbo, S., Morandin, F., ... Pagani, M. A. (2014). Wheat germ stabilization by heat-treatment or sourdough fermentation: Effects on dough rheology and bread properties. LWT- Food Science and Technology, 59(2), 1100–1106. Nagaraj, G., Purohit, A., Harrison, M., Singh, R., Hung, Y.-C., & Mohan, A. (2016). Radiofrequency pasteurization of inoculated ground beef homogenate. Food Control, 59, 59–67. Ozturk, S., Kong, F., Singh, R. K., Kuzy, J. D., & Li, C. (2017). Radio frequency heating of corn flour: Heating rate and uniformity. Innovative Food Science & Emerging Technologies, 44, 191–201. Pegna, F. G., Sacchetti, P., Canuti, V., Trapani, S., Bergesio, C., Belcari, A., ... Meggiolaro, F. (2017). Radio frequency irradiation treatment of dates in a single layer to control Carpophilus hemipterus. Biosystems Engineering, 155, 1–11.
Dielectric constant Dielectric loss factor Temperature difference Average temperature Heating uniformity index Coefficient of determination
Acknowledgments This research was conducted in the College of Mechanical and Electronic Engineering in Northwest A&F University, and supported by Fundamental Research Funds for the Central Universities (2452017126), China Postdoctoral Science Foundation funded project (2016M600816) and National Key Research and Development Program of China (2016YFD0401004). References Alfaifi, B., Tang, J., Jiao, Y., Wang, S., Rasco, B., Jiao, S., & Sablani, S. (2014). Radio frequency disinfestation treatments for dried fruit: Model development and validation. Journal of Food Engineering, 120, 268–276. AOAC (2005). Official methods of analysis (16th ed.). Washington, DC: Association of Official Analytical Chemists. Awuah, G. B., Ramaswamy, H. S., & Tang, J. (2015). Radio frequency heating in food processing: principles and applications. CRC Press. Bedane, T. F., Chen, L., Marra, F., & Wang, S. (2017). Experimental study of radio frequency (RF) thawing of foods with movement on conveyor belt. Journal of Food Engineering, 201, 17–25. Brandolini, A., & Hidalgo, A. (2012). Wheat germ: Not only a by-product. International Journal of Food Sciences and Nutrition, 63(Suppl 1(S1)), 71–74. Calay, R. K., Newborough, M., Probert, D., & Calay, P. S. (1994). Predictive equations for the dielectric properties of foods. International Journal of Food Science and Technology, 29(6), 699–713. Chen, L., Huang, Z., Wang, K., Li, W., & Wang, S. (2016). Simulation and validation of radio frequency heating with conveyor movement. Journal of Electromagnetic Waves & Applications, 1–19.
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Rose, D. J., & Pike, O. A. (2006). A simple method to measure lipase activity in wheat and wheat bran as an estimation of storage quality. Journal of the American Oil Chemists' Society, 83(5), 415–419. Sosa-Morales, M. E., Valerio-Junco, L., López-Malo, A., & García, H. S. (2010). Dielectric properties of foods: Reported data in the 21st century and their potential applications. LWT- Food Science and Technology, 43(8), 1169–1179. Wang, S., Birla, S. L., Tang, J., & Hansen, J. D. (2006). Postharvest treatment to control codling moth in fresh apples using water assisted radio frequency heating. Postharvest Biology and Technology, 40(1), 89–96. Zheng, A., Zhang, L., & Wang, S. (2017). Verification of radio frequency pasteurization treatment for controlling Aspergillus parasiticus on corn grains. International Journal of
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Innovative Food Science and Emerging Technologies journal homepage: www.elsevier.com/locate/ifset
Radio-frequency treatment for stabilization of wheat germ: Dielectric properties and heating uniformity Bo Linga, James G. Lyngb, Shaojin Wanga,c,
T
⁎
a
Northwest A&F University, College of Mechanical and Electronic Engineering, Yangling, Shaanxi 712100, China Institute of Food and Health, University College Dublin, Belfield Dublin 4, Ireland c Department of Biological Systems Engineering, Washington State University, 213 L.J. Smith Hall, Pullman, WA 99164-6120, USA b
A R T I C LE I N FO
A B S T R A C T
Keywords: Radio-frequency heating Wheat germ Thermal stabilization Dielectric properties Heating uniformity
Wheat germ (WG) is a valuable by-product of wheat milling but is highly susceptible to lipid rancidity induced by lipase (LA) activity. Therefore a stabilization step is required to reduce LA activity. Radio-frequency (RF) heating is an alternative thermal technology suitable for processing food and has great potential for WG stabilization. However, in order to develop effective RF stabilization treatments, a knowledge of the dielectric properties (DPs) (i.e. dielectric constant (ε″) and dielectric loss factor (ε″)) of WG at frequencies used for heating is required. Furthermore, an understanding of the influence of RF processing conditions on heating uniformity and LA inactivation is needed. Findings in the present study show that ε′ and ε″of WG increased with increasing temperature and moisture content (MC). Furthermore, the relationship between DPs and temperature/MC can be described by quadratic order models. RF heating uniformity in WG gradually improved with decreasing heating rate from 19.6 to 6.3 °C/min and MC from 15.96 to 7.05% (wet basis, w.b.). Placing the WG on a moving conveyor at 4 m/h slightly improved RF heating uniformity in the interior layers of the WG. Placing a pair of rectangular polyetherimide plates (12 × 8 cm) above and below the container during RF heating at 6.3 °C/min further improved temperature distribution leading to a more uniform LA inactivation in WG. Overall, the results obtained from this study are useful in computer simulation and optimizing process parameters for WG stabilization by RF heating.
1. Introduction Wheat germ (WG) is the embryo of wheat seeds which germinates to produce a plant but is also a by-product of commercial flour production. WG has traditionally been used as a cost-efficient ingredient for animal fodder or as raw material from which oil is extracted (Brandolini & Hidalgo, 2012). More recently, many studies have shown that WG can actually be used directly as an ingredient in bakery products, but also can be used as a source from which high quality protein isolates can be extracted (Zhu, Wang, & Guo, 2015). However, WG's very short shelf life due to its tendency towards lipase (LA) induced rancidity has limited its more widespread incorporation into food products. Processing methods which have been reported to reduce LA activity and stabilize WG include gamma irradiation (Jha, Kudachikar, & Kumar, 2013), sourdough fermentation (Marti et al., 2014) and conventional thermal treatments (Gili, Torrez Irigoyen, Penci, Giner, & Ribotta, 2018) with the latter being the most commonly employed.
However, WG has a flake like structure (which traps air) and also has a low moisture content (MC), which collectively give it a low thermal conductivity (Gili, Torrez Irigoyen, Penci, Giner, & Ribotta, 2017). Conventional hot air drying involves large volume samples which are exposed to long treatment times in dry air, but the duration of time at high temperatures can damage WG quality. Conventional steaming in moist air is also an effective method for enzyme inactivation, but the stabilized WG must subsequently be dried prior to storage. By contrast dielectric heating with microwave (MW) or radio-frequency (RF) energy, can rapidly raise food temperatures thereby significantly reducing heating time (Awuah, Ramaswamy, & Tang, 2015). RF and MW generate heat within the product volume by molecular frictions hence the term ‘volumetric heating’. However, RF heating has greater penetration depths and more uniform electromagnetic field patterns and for these reasons tends to produce more uniform heating. In the past five years, RF heating has been studied for various purposes in food and agricultural products, including disinfestation (Ling, Hou,
Abbreviations: WG, Wheat germ; LA, Lipase; MC, Moisture content; RF, Radio-frequency; MW, Microwave; DPs, Dielectric properties; PE, Polyethylene; PC, Polycarbonate; PEI, Polyetherimide ⁎ Corresponding author at: Northwest A&F University, College of Mechanical and Electronic Engineering, Yangling, Shaanxi 712100, China. E-mail address: [email protected] (S. Wang). https://doi.org/10.1016/j.ifset.2018.05.012 Received 14 January 2018; Received in revised form 13 May 2018; Accepted 14 May 2018 Available online 18 May 2018 1466-8564/ © 2018 Elsevier Ltd. All rights reserved.
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The selected MC levels were typical of the MC ranges of fresh WG reported by Brandolini and Hidalgo (2012). The average initial MC of WG was 11.33% w.b. To create samples with elevated MC, WG samples were kept over appropriate amount of deionized water in desiccators under ambient conditions for 2–4 days to allow them to absorb water. To obtain samples with reduced MC, the initial WG sample was held in a forced air oven at 50 °C for 4–6 h. Once the desired MC levels were attained the hydrated and dehydrated samples were stored in ziplocked PE bags at 4 °C until required for analysis.
Li, & Wang, 2016; Zhou, Guo, & Wang, 2017), drying (Zhou, Gao, Mitcham, & Wang, 2018), pasteurization (Li, Kou, Cheng, Zheng, & Wang, 2017; Nagaraj et al., 2016; Zheng, Zhang, & Wang, 2017) and thawing (Bedane, Chen, Marra, & Wang, 2017). Although available studies suggest that RF heating could be used to replace or supplement conventional heating in food processing, its application for WG stabilization has not achieved its true potential, which is partly due a lack of comprehensive knowledge and data. In order to develop an effective RF stabilization treatment for WG, it is essential to have a good knowledge of its dielectric properties (DPs) as these are the major factors describing the interaction between RF and WG. The dielectric constant (ε′) and loss factor (ε″) are two important parameters for dielectric heating treatments, with ε′ providing an index of a foods ability to store electromagnetic energy while ε″ indicating absorption of electromagnetic energy and its conversion to heat (Ling, Guo, Hou, Li, & Wang, 2015; Sosa-Morales, Valerio-Junco, López-Malo, & García, 2010). Therefore, a quantification of WG DPs is important for the design, optimization and control of RF energy based stabilization processes. In addition, the potential for non-uniform RF heating is still a major obstacle to RF adoption by the food industry. Non-uniform heating during RF application, occurs at the corners and edges of rectangular slabs which commonly absorb more energy compared to other parts of a container, which is mainly caused by differing DPs of the foodstuff and its surrounding medium, which leads resulting in an uneven distribution of the electric field (Marra, Zhang, & Lyng, 2009). The local high temperatures caused by non-uniform heating may result in severe quality loss, such as lipid oxidation or non-enzymatic browning reactions in foodstuffs (Alfaifi et al., 2014; Wang, Birla, Tang, & Hansen, 2006). Therefore, RF heating uniformity needs to be improved before industrial scale applications. Various methods have been adopted to improve the RF heating uniformity. These include replacing the surrounding media (air) with water or plastic material, examples including immersion of fruits (Wang et al., 2006) and meat products (Nagaraj et al., 2016) in water or covering the foodstuff with a plastic material which has a similar ε′ (Jiao, Tang, & Wang, 2014). Another approach for solids such as fruits (Pegna et al., 2017) and nuts (Ling et al., 2016) is to combine RF heating with product movement/rotating, adding forced hot air, sample mixing and moving on a conveyor belt. Although there are many reports on RF heating uniformity of different foodstuffs, studies related to heating uniformity of WG during RF stabilization are still not available. Based on the issues identified above, the objectives of this study were: (1) to determine the DPs of WG at three moisture levels and three frequencies measured at temperatures ranging from 25 to 85 °C, (2) to provide empirical models describing WG's DPs as a function of MC and temperature at the three frequencies, (3) to study WG RF heating uniformity under different heating conditions and (4) to evaluate the LA inactivation in WG treated using a range of RF heating conditions.
2.3. Determination of dielectric properties The DPs of WG were measured using an open-ended coaxial probe system (85070E-020), an impedance analyzer (E4991B-300) and a computer with the 85070E dielectric software installed (Keysight Technologies Co. Ltd., Palo Alto, California, USA). Samples were measured while being held in a custom-built sample test holder which produced close contact with the flat tip of the probe. According to the method of Gili et al. (2017), the flake density of WG flake with a MC of 7.05, 11.33, and 15.96% w.b. were 0.983 ± 0.010, 1.115 ± 0.008, and 1.327 ± 0.010 g/cm3, respectively. Known weights of WG flour (50 mesh) calculated form flake density of WG were placed into an airtight sample test holder, to maintain the sample MC constant during the measurement. To match the sample density samples were placed in contact with the probe and a screw knob at the bottom of the sample holder was rotated to a specific position to ensure the required volume/ sample density (Bedane et al., 2017). DPs of WG were measured between 25 and 85 °C in 15 °C increments at 13.56, 27.12 and 40.68 MHz. 2.4. Determination of heating uniformity of WG during RF treatment 2.4.1. RF treatment and temperature measurements RF heating was performed using a 6 kW, 27.12 MHz pilot-scale free running oscillator system (SO6B, Strayfield International, Wokingham, U.K.). This equipment allowed for adjustment of the parallel plate electrode separation from 9 to 19 cm using the movable top electrode (40 cm × 83 cm) so as to regulate the RF power, with conveyor belt speed also adjustable from 1 to 60 m/h. To evaluate the heating uniformity under different RF heating conditions, WG samples with 7 cm depth were put in a rectangular polycarbonate (PC) container (inner dimensions of 21 cm × 14 cm × 7.5 cm) (Fig. 1A). During RF heating, the container was placed at the center of the bottom electrode from 25 to 90 °C. This range was selected based on the temperature required to inactivate LA in WG (Kapranchikov, Zherebtsov, & Popova, 2004). During RF heating, a fiber-optic temperature sensor connected to a data logger (HQ-FTS- D120, Heqi Technologies Inc., Xian, China) was inserted through a hole at the side wall of the container to measure the temperature at the geometric center of the WG samples. The RF system was turned off when the geometric center of the sample reached 90 °C. Samples were then immediately removed from the RF oven and the temperatures throughout the entire container were measured to evaluate RF heating uniformity. The temperatures of WG after RF heating were measured using two methods, the first mapping the top surface temperature (Fig. 1A) using an infrared camera (A 300, FLIR Systems, Inc., North Billerica, MA, USA) while the second collected interior temperatures at 13 representative points in the central horizontal plane of the sample. The latter utilized a custom-designed temperature measurement platform (Fig. 1C) which consisted of a temperature recorder (RDXL12SD, OMEGA Engineering Inc., Taiwan, China) connected to 13 K-type thermocouples (OMEGA Engineering Inc., Stamford, CT, USA) which were mounted on a wooden board to ensure temperature measurements were simultaneously taken at a depth of 3.5 cm from the sample surface as shown in Fig. 1B. The entire temperature measurement procedure was completed within 20 s.
2. Materials and methods 2.1. Materials Fresh WG flakes from hard red winter wheat were obtained from a local flourmill (Shaanxi Bull Flour Co., Ltd., Meixian, Shaanxi, China). These flakes were packed in polyethylene (PE) bags and stored at −20 °C until required for analysis. The WG flakes had 11.33 ± 0.20 g moisture /100 g, 29.77 ± 0.31 g protein/100 g, 11.56 ± 0.40 g fat/ 100 g and 5.60 ± 0.06 g ash/100 g sample (all on a fresh weight basis) (AOAC, 2005). 2.2. Sample preparation Three WG samples with MC of 7.05, 11.33, and 15.96% w.b. were prepared to study the effect of MC on DPs and RF heating uniformity. 67
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Fig. 1. Rectangular polycarbonate container filled with wheat germ flakes for (A) surface temperature measurement, (B) top view of 13 different interior temperature measurement points and 5 different locations for lipase activity determination, and (C) the custom-designed temperature measurement platform (all dimensions are in cm).
Fig. 2. Rectangular polycarbonate container filled with wheat germ flakes with (A) surrounded on all surfaces by polyetherimide (PEI) plates (B) only side wall surrounded by rectangular PEI plates and (C) only bottom and top surfaces covered with rectangular PEI plates of differing sizes.
2.4.3. Effect of moisture content and bulk density on RF heating uniformity To evaluate the effect of MC and bulk density on RF heating uniformity, 650 g WG flakes with three MCs (7.05, 11.33, and 15.96% w.b.) and filled to a depth of 7 cm with three bulk density levels (0.32, 0.40 and 0.48 g/cm3). All samples had an initial MC of 11.33% w.b. and were heated in the PC container while stationary with an electrode separation of 13 cm.
2.4.2. Effect of heating rate and conveyor belt movement on RF heating uniformity The effect of heating rates on RF heating uniformity was initially assessed with a stationary sample. The PC container was filled with 650 g of WG flakes (11.33% w.b.) which had a bulk density of 0.32 g/ cm3 was heated by the RF system over a range of electrode separations from 9 to 13 cm at 1 cm intervals. Experiments were subsequently performed with moving samples with belt speeds calculated (from stationary studies) to ensure the samples remained underneath the top electrode until they had attained the target temperature. For example, based on a top electrode length of 0.83 m and a container length of 0.14 m and a heating time of 10.25 min at an electrode separation of 13 cm under stationary conditions, the belt speed was calculated and set at 4.0 m/h to evaluate the influence of moving conveyor belt on the RF heating uniformity.
2.4.4. Effect of surrounding material on RF heating uniformity Based on recent studies, the electromagnetic field uniformity during RF heating could be improved in a foodstuff if it was surrounded or partially covered with plastic material that has similar ε′ to the foodstuff and a negligible ε″ (Jiao et al., 2014; Jiao, Shi, Tang, Li, & Wang, 2015). Therefore, polyetherimide (PEI) with similar ε′ (3.2 at 1 MHz) to the WG (3.5 at 27 MHz and 11.33% w.b.) was selected to evaluate the effect of PEI plate (7.5 mm thickness) on RF heating uniformity. The PC 68
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Table 1 Dielectric properties of wheat germ samples at three moisture contents (MCs), five temperatures and three frequencies. MCs (% w.b.)
7.05
11.33
15.96
Temp. (°C)
25 40 55 70 85 25 40 55 70 85 25 40 55 70 85
ε′ at frequency (MHz)
ε″ at frequency (MHz)
13.56
27.12
2.78 ± 0.10 3.04 ± 0.30 3.46 ± 0.25 4.33 ± 0.51 5.39 ± 0.46 3.72 ± 0.21 4.68 ± 0.29 6.25 ± 0.34 8.00 ± 0.64 10.21 ± 0.64 4.87 ± 0.25 6.09 ± 0.31 8.08 ± 0.39 9.74 ± 0.88 11.00 ± 1.01
2.64 2.91 3.37 4.11 5.14 3.55 4.42 5.76 7.25 9.27 4.56 5.56 6.84 8.10 9.75
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
40.68 0.19 0.19 0.11 0.47 0.38 0.31 0.27 0.15 0.51 1.01 0.42 0.24 0.41 0.76 1.11
2.58 2.84 3.34 4.02 4.98 3.48 4.30 5.56 6.92 8.60 4.50 5.41 6.20 7.93 9.11
0.30 0.36 0.43 0.54 0.68 0.40 0.50 0.68 0.84 1.14 0.75 0.92 1.20 1.58 2.22
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.02 0.03 0.07 0.04 0.08 0.05 0.07 0.05 0.14 0.12 0.04 0.11 0.42 0.31 0.14
0.29 0.34 0.39 0.49 0.62 0.37 0.47 0.60 0.75 0.90 0.66 0.83 1.06 1.46 2.18
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
40.68 0.04 0.03 0.04 0.06 0.09 0.04 0.09 0.11 0.09 0.04 0.03 0.18 0.27 0.27 0.26
0.27 0.32 0.36 0.44 0.53 0.33 0.44 0.56 0.70 0.86 0.59 0.73 0.94 1.26 1.94
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
0.02 0.02 0.02 0.06 0.11 0.02 0.04 0.12 0.12 0.06 0.08 0.08 0.33 0.18 0.17
ε′/ ε′ ′ = α 0 + α1 MC + α2 T + α12 MCT + α11 MC 2 + α22 T 2 + α112 MC 2T + α122 MCT 2 + α111 MC 3 + α222 T 3
(3)
where, α0, α1, α2, α12, α11, α22, α112, α122, α111, and α222 are regression coefficients. The order of the polynomial, significant terms, significance of the model, and coefficient of the determination (R2) of the model to fit the experimental data were automatically given by the software.
2.4.5. Heating uniformity evaluation Temperature difference (ΔT = Tmax − Tmin), average temperature (Tavg) and heating uniformity index (λ) were used to compare the surface and interior heating uniformity of RF treatments. The λ value is defined as the ratio of the rise in standard deviation (Δσ, °C) of sample temperature to the rise in average sample temperature (Δμ, °C) over the treatment time and can be calculated from the following equation (Ling et al., 2016):
Δσ Δμ
0.18 0.17 0.28 0.28 0.38 0.23 0.27 0.15 0.28 0.48 0.38 0.35 0.67 0.85 1.01
27.12
were analyzed by response surface method (RSM). The model fitting and regression analysis were performed using Design-expert 8.0.6 (StatEase, Inc. Minneapolis, MN, USA). The general form of the polynomial model was as follows:
container filled with 650 g initial WG flakes (11.33% w.b.) with bulk density of 0.32 g/cm3 was surrounded by PEI plate in three different configurations: (1) all surrounded (Fig. 2A), (2) surrounded only on side walls (Fig. 2B), and (3) the center of both top and bottom surfaces covered with rectangular PEI plate of different sizes (6 × 4, 12 × 8 or 21 × 14 cm) (Fig. 2C). In general, surrounding or partially covering a tray with PEI should help to concentrate more electric energy in the whole sample or the covered area, thereby improving electromagnetic field uniformity.
λ=
± ± ± ± ± ± ± ± ± ± ± ± ± ± ±
13.56
2.7. Statistical analysis Data were expressed as mean ± standard deviations of triplicate measurements. Significant differences (p < 0.05) within means were subjected to analysis of variance and Tukey's honestly significant difference (HSD) test in the statistical software SPSS 16.0 version (SPSS Inc., Chicago, IL, USA).
(1)
Smaller λ values indicate better heating uniformity.
3. Results and discussion
2.5. Lipase activity determination in RF treated WG
3.1. Moisture and temperature dependent dielectric properties of WG
To evaluate the effect of RF heating on LA inactivation, initial WG sample (11.33% w.b.) with bulk density of 0.32 g/cm3 was heated from 25 to 90 °C using conditions which produce the best RF heating uniformity as described in sections of 2.4.2 and 2.4.4. RF heating was stopped when the geometric center temperature of samples achieved 90 °C. WG flakes from the five different locations throughout the PC container (Fig. 1B) were immediately collected using a stainless steel grain sampler (length of 20 cm and inner diameter of 1.8 cm), then packed in zip-lock bags and immersed into ice water to cool down the samples to ambient temperature. LA activity values in WG samples were immediately determined following the method reported by Rose and Pike (2006) and the results were expressed as percentage of enzyme activity in the unheated control as below:
The impact of moisture content, temperature and frequency on DPs of WG flakes are summarized in Table 1. Both ε′ and ε″ increased with increasing MC and temperatures, but decreased with increasing frequency. The DPs of foodstuffs mainly depend on the content of free water, as the MC raises the free water content in the WG, and the increases of temperature result in further increases of the ionic mobility (Calay, Newborough, Probert, & Calay, 1994). Thus the increase in DPs with an increase in the MC and temperature could be due to the increased polarization of molecules and ionic conductivity.
Relative enzyme activity (%) =
residual enzyme activity × 100 enzyme activity of control
3.2. Predictive models for dielectric properties of WG at three radio frequencies Table 2 shows the coefficients of the polynomial order models with their significant terms describing both ε′ and ε″ of WG as influenced by MC and temperature. A quadratic order model was the best fit for ε′ and ε″ at each frequency in the tested MC and temperature range. The linear (MC and T) terms had strong influence on the model at all three frequencies (p < 0.0001) both for ε′ and ε″. The interaction (MCT) term influenced strongly (p < 0.0001) ε″ only at 13.56 MHz. Each model provided a good fit to the ε′ and ε″ at the significance level of 0.0001
(2)
2.6. Model analysis To study the MC and temperature dependent DPs, the correlations of MC and temperature (T, °C) with DPs of WG at three frequencies 69
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Table 2 Regression coefficients of the polynomial model (Eq. (3)) to predict the dielectric properties of wheat germ as a function of moisture content (7–16% w.b.), temperature (25–85 °C) at three radio frequencies. Regression coefficients in Eq. (2)
α0 α1 α2 α12 α11 α22 R2 ⁎
13.56 MHz
27.12 MHz
40.68 MHz
ε′
ε″
ε′
ε″
ε′
ε″
−3.04 0.99⁎ −0.05⁎ 6.93 × 10−3 −0.04 4.74 × 10−4 0.977
1.65 −0.21⁎ −0.03⁎ 2.00 × 10−3⁎ 8.52 × 10−3 1.71 × 10−4 0.989
−3.11 1.05⁎ −0.05⁎ 4.93 × 10−3 −0.04 5.85 × 10−4 0.977
1.99 −0.26⁎ −0.03⁎ 2.16 × 10−3 9.99 × 10−3 1.69 × 10−4 0.968
−2.84 0.97⁎ −0.04⁎ 4.23 × 10−3 −0.04 5.16 × 10−4 0.979
1.64 −0.20⁎ −0.03⁎ 1.95 × 10−3 7.37 × 10−3 1.51 × 10−4 0.969
Indicated model terms significantly influence the model at 0.0001 probability levels.
sample surface and interior. However, the trends of ΔT and λ values in sample surface and interior were consistent. Under stationary heating conditions, the RF heating uniformity was gradually improved with the decreasing heating rate, as reflected by reducing ΔT and λ values. This indicated that larger electrode separations with slower heating may reduce over-heating during RF treatment. Furthermore, longer treatment times may allow heat conduction to improve the heating uniformity within the samples. When heating a moving sample with belt speeds of up to 4 m/h, there was no significant (p > 0.05) differences in surface ΔT value, while the interior ΔT value decreased significantly (p < 0.05) from 20.5 to 17.1 °C and the λ value also decreased from 0.10 to 0.07. Similarly, Chen, Huang, Wang, Li, and Wang (2016) also reported that RF heating uniformity in the middle of a layer of wheat seeds could be improved by conveyor belt movement.
with R2 value > 0.968 for different frequencies. The temperature and MC dependent DPs could change the electromagnetic power absorption, penetration depth and temperature distribution in foodstuffs (SosaMorales et al., 2010). Therefore, the models obtained from this study for predicting DPs of WG would be useful in a computer model for simulating temperature histories of WG during RF heating and in designing the dielectric heating stabilization system employed with RF energy. 3.3. Influence of heating rate and conveyor movement on heating uniformity Fig. 3 shows the influence of electrode separation on heating rates at the center of 7 cm deep WG samples during RF heating. The temperatures increased linearly with heating time under each electrode separation. About 3.3, 4.4, 6.3, 9.3 and 10.25 min were required to heat 650 g WG from 25 to 90 °C with heating rates of 19.6, 14.8, 10.3, 7.0, and 6.3 °C/min for electrode separations of 9, 10, 11, 12, and 13 cm, respectively. Fast heating methods (5–20 °C/min) based on RF energies have been proposed to replace conventional thermal treatments in food processing (Marra et al., 2009). Therefore, the electrode separations of 9, 11, and 13 cm were selected for evaluating the effect of heating rate (20, 10, and 6 °C/min) on RF heating uniformity. Fig. 4 shows the top surface temperature distribution in WG with different RF heating rates. All the plots showed that the cold spot was located at the center, while hot spots were located at the corners and edges. Temperature uniformity parameters of WG with different treatments are listed in Table 3. The Tavg of interior was higher than that on top surface, while the ΔT and λ values were lower than the top surface. This could be due to the heat loss to ambient air on the top surface and also the difference between temperature measurement methods for
3.4. Influence of MC and bulk density on heating uniformity As shown in Table 4, heating time decreased with increasing MC, while ΔT and λ values increased with increasing MC both for surface and interior, which indicate that the higher MC resulted in higher heating rate and lower temperature uniformity under given conditions. Similar trends were also found in other RF treated foodstuffs with different MC (Li et al., 2015; Ozturk, Kong, Singh, Kuzy, & Li, 2017). It is generally considered that higher MC results in a larger loss factor and increased RF heating rates. Over-heating and thermal run away phenomenon is more likely to appear during fast heating. Therefore, RF heating rates should be adjusted to a reasonable rate during stabilization of WG with different MC. On the other hand, there was no significant (p > 0.05) difference in temperature uniformity parameters between surface of WG with different bulk density, while the ΔT and λ values for interior decreased significantly (p < 0.05) with increasing bulk density from 0.32 to 0.48 g/cm3. It may be explained by densitydependent thermal properties of foodstuff, with a higher density at the interior layer leading to an increase in heating conduction for improving the heating uniformity within the WG. 3.5. Influence of surrounding material on the heating uniformity As shown in Fig. 5, the cold (center) and hot (corner and edge) spots still remained at the same locations when the WG were surrounded by PEI in a variety of configurations, indicating that surrounding the samples or selected sample surfaces with PEI did not completely change the RF heating patterns. Compared to WG without surrounding PEI, the ΔT and λ values both for sample surface and interior increased significantly (p < 0.05) in WG all surrounded by PEI (Table 5). RF treatment time also decreased from 10.3 to 9.3 min, which is probably because the PEI plates lead to an increased absorption of electrical energy into the sample. On the contrary, no significant (p > 0.05) differences were observed in ΔT and λ values between WG that had only sides surrounded and not surrounded by PEI both for sample surface
Fig. 3. Temperature-time profile of wheat germ with 11.33%, w.b. moisture content and bulk density of 0.32 g/cm3 in the center of the polycarbonate container with differing electrode separations. 70
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Fig. 4. Top surface temperature distribution of wheat germ with moisture content of 11.33% w.b. and bulk density of 0.32 g/cm3 in a rectangular polycarbonate container for radio-frequency heating in stationary case with different heating rates (A–C) and moving on conveyor belt at 4 m/h (D).
holding did not completely inactivate LA in WG. Kapranchikov et al. (2004) also reported that the LA in WG is heat-stable with over 20% its original activity retained after holding at 70 °C for up to 1 h. Although the relative LA activity at five different locations differed among RF treatments, LA values in location V was always the highest since the central area in each treatment was always coldest. Heating a moving case did not seem to improve the uniformity of LA inactivation. When covering the both top and bottom surfaces with rectangular PEI plates (12 × 8 cm), the relative LA activity in location V decreased significantly (p < 0.05) as compared to other treatments. This might be because the PEI plates caused an increased temperature in the cold spot, thus improving the RF heating uniformity and reducing the difference of LA activities between hot and cold spots in the WG sample.
and interior. When covering both sides (upper and lower surfaces) of container with PEI plates of different sizes, the average ΔT and λ values decreased both for surface and interior as shown in Table 5, which coincided with the surface temperature distributions as shown in Fig. 5. An obvious temperature increase (5–10 °C) could be observed in the center area of WG that was covered with PEI (Fig. 5 D–F) as compared to the WG that was not surrounded by PEI plates (Fig. 5 A). Similarly, Jiao et al. (2015) also reported that the ΔT and λ values on the surface of peanut butter could be reduced by covering cold spots with PEI blocks. However, they did not perform a statistical analysis of these differences. As shown in our study, a significant (p < 0.05) improvement in RF heating uniformity could be only observed when using 12 × 8 cm PEI plates. Under these conditions, the average ΔT values decreased from 33.0 to 24.6 °C in top surface and 20.5 to 15.6 °C in the interior, respectively, and the corresponding average λ values were also significantly (p < 0.05) smaller than those in the WG that was not surrounded.
4. Conclusion The DPs of WG were greatly influenced by MC and temperature at three frequencies (i.e. 13.56, 27.12, and 40.68 MHz). Both ε′ and ε″ of WG samples decreased with increasing frequency or decreasing temperature and MC. The MC and temperature dependent DPs at three frequencies could be described by quadratic order models and each model provided a good fit to the DPs at the significance level of 0.0001 with R2 value > 0.968. Corner and edge heating could be observed
3.6. Influence of different RF treatments on lipase activity Table 6 shows the LA activities in WG at five different locations, which include hot (I to IV) and cold (V) spots in the PC container treated by different RF heating conditions. RF heating to 90 °C without
Table 3 Temperature and heating uniformity index of wheat germ with moisture content of 11.33% w.b. and bulk density of 0.32 g/cm3 treated by radio-frequency heating while stationary condition (with differing heating rates) and when moving on a conveyor belt with at 4 m/h. Heating condition
Stationary heating at different heating rate 20 °C/min
ΔT (°C) Tavg (°C) λ
#
Top surface Interior layer Top surface Interior layer Top surface Interior layer
51.7 31.3 64.6 88.4 0.22 0.19
± ± ± ± ± ±
5.3a# 2.7a 4.9a 3.5a 0.03a 0.03a
Moving on a conveyor belt at 4 m/h at a heating rate of
10 °C/min
6 °C/min
6 °C/min
42.7 25.3 66.3 85.2 0.17 0.13
33.0 20.5 63.3 84.1 0.13 0.10
28.9 17.1 61.3 81.8 0.10 0.07
± ± ± ± ± ±
3.5b 2.3b 3.2a 2.5a 0.02b 0.02b
± ± ± ± ± ±
2.8c 1.7c 2.5a 2.1a 0.01c 0.01b
Means within a row followed by the same letter are not significantly different at the 5% probability level. 71
± ± ± ± ± ±
1.8c 1.2d 1.2a 1.6a 0.02c 0.01c
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Table 4 Temperature and heating uniformity index of wheat germ (WG) with three moisture contents and bulk density levels treated by radio-frequency heating at electrode separation of 13 cm. WG properties
7.05 Heating time (min)a ΔT (°C) Tavg (°C) λ
a ⁎
Bulk density (g/cm3)
Moisture content (%, w.b.)
Top surface Interior layer Top surface Interior layer Top surface Interior layer
15.5 25.7 16.7 62.2 82.7 0.09 0.07
± ± ± ± ± ±
⁎
1.6c 2.0c 2.2a 2.0c 0.01c 0.02bc
11.33
15.96
0.32
0.40
0.48
10.3
5.8
10.3
10.4
10.4
33.0 20.5 63.3 84.1 0.13 0.10
± ± ± ± ± ±
2.8b 1.7b 2.5a 2.1bc 0.01b 0.01b
40.8 29.9 67.9 88.9 0.17 0.15
± ± ± ± ± ±
3.7a 2.0a 3.4a 3.5ab 0.02a 0.03a
33.0 20.5 63.3 84.1 0.13 0.10
± ± ± ± ± ±
2.8b 1.7b 2.5a 2.1bc 0.01b 0.01b
31.9 19.0 61.4 86.9 0.13 0.09
± ± ± ± ± ±
2.4b 1.6bc 2.0a 1.1b 0.02ab 0.02bc
32.2 16.8 62.4 90.1 0.12 0.06
± ± ± ± ± ±
2.1b 1.4c 2.5a 1.5a 0.02bc 0.02c
Values indicate the mean times (min) required for the geometry center temperature to be increased from 25 to 90 °C. Means within a row followed by the same letter are not significantly different at the 5% probability level.
to 90 °C without a holding treatment did not completely inactivate LA in WG. Thus, future research is still needed to investigate a holding process, such as hot air holding, which may be used in combination with RF treatment to further improve the heating uniformity and efficiency of LA inactivation to ensure the WG quality.
under all RF heating conditions. RF heating uniformity was improved with decreasing heating rate or increasing bulk density of WG, but decreased with increasing MC of WG. RF heating uniformity was also improved by covering cold spots with rectangular PEI plates (12 × 8 cm) both top and bottom surfaces of the container, and the uniformity of LA inactivation was also improved. However, RF heating
Fig. 5. Top surface temperature distribution of wheat germ with moisture content of 11.33% w.b. and bulk density of 0.32 g/cm3 in a polycarbonate container surrounded by polyetherimide (PEI) plates treated by radio-frequency heating at a fixed electrode separation of 13 cm (with three PEI plates options, either (1) 6 × 4; (2) 12 × 8; or (3) 21 × 14 cm covering both top and bottom surfaces).
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Table 5 Temperature and heating uniformity index of wheat germ with a moisture content of 11.33% w.b. and a bulk density of 0.32 g/cm3 in a polycarbonate container with differing polyetherimide (PEI) plates placements while undergoing radio-frequency heating with an electrode separation of 13 cm. PEI plate placement
Heating time (min)a
ΔT (°C) Top surface
None All Sides only wall Top and bottom in Mode Ab Top and bottom in Mode Bc Top and bottom in Mode Cd a b c d ⁎
10.3 9.3 10.5 10.1 9.8 9.7
33.0 39.8 35.5 28.2 24.6 27.8
± ± ± ± ± ±
λ
Tavg (°C)
2.8b⁎ 2.6a 2.3ab 2.1bc 2.6c 2.3bc
Interior layer
Top surface
Interior layer
Top surface
Interior layer
20.5 28.5 22.6 18.8 15.6 17.0
63.3 70.1 60.1 62.4 61.6 60.9
84.1 88.3 80.5 83.0 80.1 80.4
0.13 0.16 0.14 0.11 0.08 0.09
0.10 0.14 0.11 0.08 0.06 0.07
± ± ± ± ± ±
1.7b 1.5a 2.2b 2.0bc 1.4c 1.9bc
± ± ± ± ± ±
2.5b 3.0a 2.4b 2.6b 2.2b 2.7b
± ± ± ± ± ±
2.1ab 1.9a 2.3b 1.5b 2.9b 1.7b
± ± ± ± ± ±
0.01b 0.01a 0.02ab 0.02bc 0.01c 0.02c
± ± ± ± ± ±
0.01b 0.02a 0.02ab 0.01bc 0.02c 0.02bc
Values indicate the mean time (min) required for the geometry center temperature to be increased from 25 to 90 °C. Mode A: Two 6 × 4 cm PEI plates covering both top and bottom sample surfaces. Mode B: Two 12 × 8 cm PEI plates covering both top and bottom sample surfaces. Mode C: Two 21 × 14 cm PEI plates covering both top and bottom sample surfaces. Means within a column followed by the same letter are not significantly different at the 5% probability level.
Table 6 Lipase activities of wheat germ at five different locations in polycarbonate container as influenced by different radio-frequency heating conditions under fixed heating rates (6 °C/min). Heating conditions Relative lipase activity (%)
I II III IV V
Control
Stationary heating
Moving on a conveyor belt at 4 m/h
Stationary heating with surrounding PEI platesa
100Aa# 100Aa 100Aa 100Aa 100Aa
72.2 67.9 69.5 65.4 87.6
71.5 75.6 76.1 73.1 90.4
72.6 70.1 68.9 71.6 78.1
± ± ± ± ±
3.1Bb 2.0Cbc 2.3Cbc 2.0Cc 2.5Ba
± ± ± ± ±
2.5Bb 1.9Bb 2.4Bb 2.1Bb 3.6Ba
± ± ± ± ±
1.7Bb 2.2Cb 2.3Cb 2.5Bb 2.0Ca
# Means within a column among the different locations followed by same lower case letters and means within a row among the different heating conditions followed by same capital letters are not significantly different at the 5% probability level. a Two 12 cm × 8 cm PEI plates covering both top and bottom sample surfaces.
Symbols ε′ ε″ ΔT Tavg λ R2
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