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Effect of short time heating on the mechanical fracture and electrical impedance properties of spinach (Spinacia oleracea L.)
Journal of Food Engineering 194 (2017) 9e14
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of short time heating on the mechanical fracture and electrical impedance properties of spinach (Spinacia oleracea L.) Takashi Watanabe a, *, Yasumasa Ando a, Takahiro Orikasa b, Takeo Shiina c, Kaoru Kohyama a a b c
Food Research Institute, NARO, 2-1-12, Kannondai, Tsukuba, Ibaraki 305-8642, Japan Faculty of Agriculture, Iwate University, 3-18-8 Ueda, Morioka, Iwate 020-8550, Japan Graduate School of Horticulture, Chiba University, 648 Matsudo, Matsudo, Chiba 271-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2016 Received in revised form 15 August 2016 Accepted 2 September 2016 Available online 4 September 2016
We evaluated the mechanical properties and electrical impedance data of spinach after short time heating. The mechanical fracture properties of samples heated for 40 s significantly differed from those of the non-treated samples. A circular arc of the Cole-Cole plot obtained from the impedance data shrunk after heating for 10 s. Similarity of the circular arcs, i.e. non-uniformity in the electrical characteristics, changed after heating for 30 s. After heating, extracellular resistance values were decreased from 650 to 16 kU, and cell membrane capacitance values were decreased from 13 to 0.28 nF. These changes could be evaluated simply by using the length of the coordinate at the top of the circular arc from the origin, and these occurred earlier and were greater in magnitude than those in the fracture properties were. These electrical properties could be an index for the early detection of changes in the fracture properties of samples. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Spinach Short time heating Mechanical properties Electrical impedance Equivalent circuit model
1. Introduction Horticultural plants are very important for human diet as sources of vitamins, minerals and dietary fiber and they become a significant part of human life due to their medicinal and environmental uses as well as aesthetics and economic values. The stem, leaf, flowers, roots and the fruits of vegetables and fruit crops have the highest potential of export (Park, 1987; Mlcek et al., 2015; Nemli et al., 2015; Tsou et al., 2016). Short time heating for fresh agricultural products, which is called blanching, is commonly conducted to inhibit changes in the quality of fresh products that may occur during secondary processing and storage (Xiao et al., 2014). However, short time heating has also been known to affect the quality of final products in many cases (Selman, 1994). Therefore, several studies have focused on changes in the quality of agricultural products that occur during short time heating. These studies have mainly involved the investigation of characteristics such as nutrient composition, color, microstructure, and texture (Lin and Schyvens, 1995; Neri et al.,
* Corresponding author. E-mail address: [email protected] (T. Watanabe). http://dx.doi.org/10.1016/j.jfoodeng.2016.09.001 0260-8774/© 2016 Elsevier Ltd. All rights reserved.
2011; Jaiswal et al., 2012; Imaizumi et al., 2015) to determine the optimum heating conditions that can help minimize the degradation of these products. Spinach (Spinacia oleracea L.) is widely consumed all over the world. However, fresh spinach has a very short shelf life. Therefore, methods to extend the storage time of spinach, such as drying and freezing, have been widely studied (Park, 1987; Ozkan et al., 2007). Mechanical properties of agricultural products are an important factor that determines food texture. Changes in the mechanical properties of agricultural products that occur during short time heating affect the quality of the final products (Selman, 1994; Buggenhout et al., 2006; Nisha et al., 2006). For using short time heating for spinach products, it is necessary to study the effect of such heating on the mechanical properties of spinach in order to determine the optimum short time heating conditions. The mechanical properties of vegetables are anisotropic with the fiber direction. Therefore, it is important to consider the fiber direction when the mechanical properties are evaluated. A tensile test is a classic method used for the determination of the mechanical properties of leafy vegetables. This method can provide information about the mechanical properties of vegetables in specific fiber directions. The investigation of the mechanical properties of vegetables using tensile tests has been conducted for several
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vegetables such as cabbage (Kohyama et al., 2008), lettuce (Toole et al., 2000), and Solanaceae (Aranwela et al., 1999). However, the mechanical properties of spinach during short time heating have not been studied using tensile tests. In addition to mechanical properties, electrical impedance properties are important for the determination of the optimum heating conditions for agricultural products, and are used to obtain information about the cell structures of plants (Ohnishi et al., 2004). Impedance properties are often described on a complex plane such as a Cole-Cole plot (Cole, 1932). Our previous study showed that short time heating could damage the cell membrane of spinach, as indicated by the linear decrease in the coordinate at the top of the circular arc of the Cole-Cole plot, and the linearity was used as an index of the physiological activity during hot-air drying (Watanabe et al., 2016). Because the coordinate at the top of the circular arc of the Cole-Cole plots moved linearly, the circular arcs before and after the shrinkage may have similar shapes. The similarity was determined by non-uniformity in the electrical characteristics of the connected cells of agricultural products. In addition, there is a possibility that the non-uniformity is an index for the destruction of cell membranes during short time heating. However, the similarity of the circular arc after short time heating has not been examined so far. For agricultural products, electrical impedance data have been analyzed using equivalent circuit models based on cellular structure (Zhang and Willison, 1993). Information about each part of the cells in plant tissues, including the extra- and intra-cellular fluid and the cell membranes, is quantified using equivalent circuit analysis, which has helped understand how the state of cells affects the mechanical properties of vegetables (Fuentes et al., 2014; Imaizumi et al., 2015; Ando et al., 2016). However, the relationship between changes in the cellular structure and the mechanical properties of spinach during short time heating is not clear. The investigation of such a relationship is important for simple and rapid detection of the changes in the mechanical properties of spinach during heating, based on electrical impedance analysis. This knowledge is beneficial for determining the optimum conditions for the processing of agricultural products with good mechanical properties such as texture. In this study, changes in the mechanical properties of spinach were measured using the tensile test, and the electrical impedance data after the short time heating were analyzed using coordinates at the top of the circular arc on Cole-Cole plots and equivalent circuit models for plant cellular tissues. The similarity in the shapes of the circular arcs was examined to investigate non-uniformity in the electrical characteristics of the connected cells of the samples during heating. In addition, the relationship between the length of the coordinate at the top of the circular arc on Cole-Cole plots from the origin and the electrical parameters obtained from equivalent circuit model analysis was reported. Moreover, the relationship between the mechanical fracture properties and electrical impedance properties of spinach was discussed. 2. Materials and methods
with weighing 1.5 ± 0.5 g were selected and used for the short time heating experiment. Unheated samples were used as controls in this study. 2.2. Short time heating Short time heating was performed using the conditions described by Watanabe et al. (2016). In brief, three samples placed on a wire netting were heated using steam at 100 C in a water oven (AX-XP100, Sharp, Osaka, Japan) for 10, 20, 30, 40, and 60 s. After heating, the samples were cooled immediately in ice water for 30 s. 2.3. Tensile test The mechanical properties of the spinach samples were measured by a tensile test using a universal testing machine (Model 5542, Instron, Massachusetts, USA) with a load cell of 50 N at 20 C (Kohyama et al., 2008). Rectangular specimens (10 50 mm) cut out along the side of the major vein on the area around on the top of a spinach leaf were used. Major veins were avoided during the preparation of the specimens, because they could add complexities to the results (Aranwela et al., 1999). Both the ends of each specimen (17.5 mm each) were clamped using screw grips (CAT#2710004, Instron, Massachusetts, USA), and were tensed at a constant speed of 1 mm/s until fracture. Fracture was determined as the point where the maximum load was detected. The thickness and width of each fractured specimen was measured using calipers (CD15CPX, Mitsutoyo, Kanagawa, Japan). In this study, we focused on the fracture point in the tensile test and discussed changes in the mechanical properties after short time heating. Fracture stress was derived as a ratio of the maximum load to the cross sectional area (thickness width) of each fractured specimens. Fracture strain was calculated as the ratio of deformation at the fracture to the initial length of the samples between the clamps (15 mm). Fracture modulus was defined as the ratio of fracture stress to fracture strain. Fracture energy was calculated as the area under the loaddeformation curve until fracture. The mechanical properties of the heated samples were measured 10e15 min after cooling. The measurements were repeated 6e17 times. 2.4. Electrical impedance spectroscopy The electrical impedance data of the samples were measured using an impedance analyzer (E4990A, Keysight Technology, California, USA) with steel needle electrodes spaced 17 mm apart at 20 C. The impedance data were obtained along the major vein at points distanced approximately 5 mm apart from the main vein on the top half area of a spinach leaf, similar to the conditions of the mechanical test. The electrical impedance data were derived at 81 frequency points with a logarithmic interval from 42 Hz to 5 MHz at a measuring voltage of 1.0 V. The electrical impedance data of the heated samples were measured after cooling within 10e30 s. The measurements were repeated 6e18 times. The complex impedance, Z, (U) was defined as:
2.1. Materials
Z ¼ R þ jX;
We used the spinach cultivar SC0-114. After the harvest season of the cultivar, we offered other cultivars that had almost the same mechanical and electrical properties as SC0-114. The spinach was harvested and packaged during June 19eJuly 1, 2015, in Ibaraki, Japan. The spinach was then transported to the Food Research Institute, NARO, Ibaraki, Japan, and stored immediately in a refrigerator at 4 C until use. All the tests were performed within 3 days from the day of harvest. Only the leaves of the spinach plants
where j is the imaginary unit, R is the real part (resistance (U)), and X is the imaginary part of the impedance (reactance (U)). The relationship between R and X is presented as a Cole-Cole plot. The length of the coordinate at the top of the circular arc of Cole-Cole plots from the origin (Watanabe et al., 2016) was calculated. As noted below, the Cole-Cole plot of biological tissues is distorted due to the non-uniform electrical properties of cells in the tissues. We defined a ratio of the reactance X to the resistance R (jXj/
(1)
T. Watanabe et al. / Journal of Food Engineering 194 (2017) 9e14
R) at the top of the circular arcs as a new index of the similarity of the circular arcs and the non-uniformity of the electrical impedance properties of the cells in the heated spinach samples. Equivalent circuit models were used to analyze the electrical impedance data of the samples. The Hayden model (Fig. 1 (a)), which consists of cell membrane resistance, Rm, intracellular resistance, Ri, extracellular resistance, Re and cell membrane capacitance, Cm, is well known and frequently used to analyze the impedance properties of agricultural products (Hayden et al., 1969). The Hayden model represents the structure of a cell and describes an exact semicircle as a Cole-Cole plot. However, biological tissues are composed of a number of cells and produce a time constant distribution, because their electrical properties are nonhomogeneous. As a result, a Cole-Cole plot of the tissues is distorted in the direction of the real axis. To fit the model to the impedance properties of biological tissues, Ando et al. (2014) modified the Hayden model (Fig. 1 (b)) by eliminating Rm and inserting a constant phase element, CPE (Zoltowski, 1998), instead of Cm to adjust the non-uniformity of the electrical properties among cells in these tissues. The impedance of the CPE, ZCPE, is defined as:
1 ; ðjuÞp T
(2)
where u is angular frequency (rad/s), p is CPE exponent, and T is the CPE coefficient (F/s(1p)). The modified model has been used previously for examining cell structures on processed vegetables such as potatoes during hot air drying (Ando et al., 2014) and hot water treatment (Imaizumi et al., 2015), and on carrots after dehydrofreezing (Ando et al., 2016). In this study, the modified Hayden model was used to analyze the electrical impedance data of the spinach samples. The complex impedance of the modified model can be represented by Eq. (3):
h n i Re 1 þ up T ð2Ri þ Re Þ$ cos p2 p þ up TRi ðRe þ Ri Þg Z¼ fup TðRe þ Ri Þg2 þ 2up TðRe þ Ri Þ$ cos p2 p þ 1 up TR2e $ sin p2 p j : fup TðRe þ Ri Þg2 þ 2up TðRe þ Ri Þ$ cos p2 p þ 1
(3)
The equivalent circuit parameters were estimated by complex nonlinear least squares curve fitting (Macdonald, 1992). Under the condition that the relaxation angular frequency (the angular frequency at which the imaginary part of the impedance is minimal) is
(b)
Cm
Rm
Ri
Re
1
Cm ¼ T p ðRe þ Ri Þ
1p p
:
(4) 2
We evaluated coefficients of determination (r ) to investigate the adaptability of the equivalent circuit models for spinach samples using Eq. (5) described as Imaizumi et al. (2015):
r2 ¼ 1
P 2 i ð Z i Z calc Þ ; 2 P i Z i Z
(5)
avg
where jZjcalc is the value of jZj calculated using the equivalent circuit models, and jZjavg is the average value of jZj. 2.5. Statistics Dunnett's test with a significance level of P < 0.05 was conducted using the SPSS package (SPSS ver.17.0, SPSS, Chicago, USA) for statistical analyses. 3. Results and discussion 3.1. Mechanical properties Fig. 2 shows the load-deformation curves observed during the tensile tests of the samples heated for different periods. For all the samples, the load increased nearly linearly as the deformation proceeded to the fracture point, and then drastically dropped to zero. The mechanical fracture properties calculated from the loaddeformation curves of the tensile tests are shown in Table 1. No significant differences in the fracture load and energy of the samples were observed after short time heating for 0e60 s. However, significant increase in the fracture strain, stress, and modulus were observed for the samples heated for 40e60 s. In addition, the values of the cross sectional area at the fracture point were significantly decreased for the samples heated for 40e60 s. We assumed that the decrease in the cross sectional area was chiefly due to escaping of the air and the water transpiration caused in the samples by the heat treatment. The decrease in the cross sectional area suggested a concentration of the cell walls, which is an intrinsic mechanical characteristic (Chanliaud et al., 2002). Therefore, the decrease in the cross sectional area caused by the heating could be a primary factor responsible for the significant increase in the fracture stress and modulus. The increase in the fracture strain caused by short time heating could be explained by the loss of cell turgor (Chiralt et al., 2001). At low turgor pressure, cell debonding is more likely to occur than cell rupture during the fracture test. The lower the turgor pressure of the samples, the higher was the fracture strain.
1.2
CPE
(a)
invariant, the capacitance of the cell membranes (Cm) was calculated using Eq. (4):
Re
Ri
Control 10 s 20 s 30 s 40 s 60 s
Fracture point
1 Load (N)
ZCPE ¼
11
0.8
0.6 0.4 0.2 0 0
Fig. 1. Equivalent circuit models. (a) Hayden model, (b) Modified Hayden model. Rm: cell membrane resistance, Ri: intracellular resistance, Re: extracellular resistance, Cm: cell membrane capacitance, CPE: constant phase element.
2
4 Deformation (mm)
6
8
Fig. 2. Load-deformation curves of spinach heated for different periods.
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T. Watanabe et al. / Journal of Food Engineering 194 (2017) 9e14
Table 1 Mechanical fracture properties of spinach heated for different periods. Heating periods (s)
Cross sectional area (mm2)
Control 10 20 30 40 60
2.20 2.11 1.90 1.80 0.84 0.97
± ± ± ± ± ±
0.13 0.13 0.19 0.20 0.06* 0.10*
Load (N) 0.86 0.93 0.66 0.73 0.68 0.76
± ± ± ± ± ±
0.07 0.03 0.11 0.07 0.05 0.05
Strain () 0.19 0.27 0.18 0.26 0.23 0.31
± ± ± ± ± ±
Stress (MPa)
0.02 0.02 0.03 0.03 0.03 0.02*
0.41 0.45 0.36 0.43 0.74 0.82
± ± ± ± ± ±
Modulus (MPa)
0.04 0.04 0.07 0.06 0.08* 0.09*
2.30 1.71 1.91 1.62 3.33 2.76
± ± ± ± ± ±
Energy (mJ)
0.21 0.17 0.23 0.10 0.45* 0.43
1.6 2.2 1.1 1.7 1.3 1.8
± ± ± ± ± ±
0.28 0.22 0.24 0.30 0.21 0.22
Each value represents the mean values ± S.E. (n ¼ 6e17). Initial length of specimens was 15 mm. *Indicates significant differences compared to the control (non-heated samples) with P < 0.05. Specimens (10 50 mm) were tensed at a constant rate of 1 mm/s at 20 C. Strain is calculated as ratios of deformation to initial length of samples between clamps. Stress is derived as ratios of the maximum load by the cross sectional area (thickness width). Modulus is defined as the ratio of fracture stress to fracture strain. Energy is calculated as the area under the load-deformation curve until fracture.
Therefore, we assumed that the increase in fracture strain was due to the loss of cell turgor pressure caused by the damage to cell membranes during short time heating. 3.2. Electrical impedance characteristics 3.2.1. Coordinates at the top of the circular arc on Cole-Cole plots Fig. 3 shows the Cole-Cole plots for the samples heated for different periods. The linear part in the low frequency domain is based on the polarization at the electrode surface (Pliquett, 2010) and is not related to the characteristics of the cellular structure. Therefore, the corresponding parts were eliminated for examination in this study. The circular arc shrunk with increase in the heating duration. Fig. 4 (a) shows the coordinate at the top of the circular arc of the Cole-Cole plots for the different durations for which the samples were heated in all the experiments in this study. All the data have been plotted in alignment. In addition, Fig. 4 (b) shows the linear path traced by the coordinate at the top of the circular arc of the Cole-Cole plots (r2 > 0.99), and the inclination of a linear equation yielded a value similar to that obtained in a previous report (Watanabe et al., 2016). 3.2.2. Similarity of circular arcs of Cole-Cole plots Fig. 5 shows the jXj/R at the coordinate at the top of the circular arc for samples heated for different periods. The jXj/R of the control sample and those heated for 10 s and 20 s have same values. Thus, the similarity between the control sample and the samples heated for 10 s and 20 s was clear. The jXj/R of the samples heated for 30e60 s showed a decrease. The results indicated that the circular arc of the Cole-Cole plots for the samples heated for 30e60 s was deformed in the downward direction. Given that the shape of a Cole-Cole plot was determined by non-uniformity in the electrical characteristics of the connected cells (i.e. cell structures and cell sizes) of the samples, the changes in the shape may suggest commencement of the heterogeneous breakdown of the cell
Fig. 4. Coordinates at the top of the circular arc on Cole-Cole plots of spinach heated for different periods. (a) Measured values in all experiments, (b) Averaged value at each heating periods (Control, 10, 20, 30, 40, 60 s). * Indicates significant differences compared to the control (non-heated samples) with P < 0.05.
structure, such as membrane destruction in each cell. We assumed that jXj/R could be an index of the destruction of the cell membranes of samples during heating. The destruction of the cell membranes was a factor for leaking of the water-soluble component from the agricultural products during processing. The index 0.6 0.5
250
High
Frequency
Low
0.4 |X|/R (-)
|X| (kΩ)
200 150
* *
0.3
*
0.2
100
Control 10 s 20 s
50 0
0
200
400 R (kΩ)
600
800
Fig. 3. Cole-Cole plots of spinach heated for different periods. Closed symbols mean the top of the circular arcs.
0.1 0 0
10
20
30 40 50 Heating periods (s)
60
70
Fig. 5. The ratio of reactance to resistance (jXj/R) at the coordinates at the top of the circular arc on Cole-Cole plots of spinach heated for different periods. Parameters represent the mean values ± S.E. (n ¼ 6e17). * Indicates significant differences compared to the control (non-heated samples) with P < 0.05.
T. Watanabe et al. / Journal of Food Engineering 194 (2017) 9e14
using the Hayden model and the actual values. In contrast, the calculated values obtained using the modified Hayden model agreed well with the actual values (r2 > 0.99). The modified model could describe the impedance characteristics of the inhomogeneous tissues of the samples as the CPE allowed the phase angles of Cm to be changed flexibly (Ando et al., 2014). This confirmed the adaptability of the modified Hayden model for spinach during short time heating. The changes in the parameters obtained from the equivalent circuit analysis are shown in Fig. 7 (a)e(c). The Ri increased and Re and Cm decreased as the heating periods increased. This trend potentially indicates that intracellular fluids with low resistance leaked to the outside of the cells due to changes in the cell structure, such as cell membrane degradation (Ando et al., 2014; Imaizumi et al., 2015). Changes in the length of the coordinate at the top of the circular arc of the Cole-Cole plots from the origin and the values of Re and Cm normalized to the values observed in the controls are shown in Fig. 7 (d). These normalized values showed the same trends of decreases during short time heating. The cell membrane has high electrical impedance in the low frequency domain. Under the experimental conditions used in this study, at the point of minimum frequency on ideal Cole-Cole plots for plant cells, the real part values of the complex electrical impedance were close to Re. In contrast, at the point of maximum frequency, the values were close to the parallel resistances, Ri and Re, because the electrical impedance of the cell membrane was decreased at this point. Thus, the approximate diameter (f) on the real part axis of the circular arc can be represented using Eq. (6):
1200 R Measured data of R 1000
X Measured data of X Calc R Calculated data of R
R, |X| (kΩ)
800
Calc X Calculated data of X
600 400
(a)
200 0
100
1000
10000
100000
1000000
10000000
Frequency (Hz) 1200 R Measured data of R 1000
X Measured data of X Calc R Calculated data of R
R, |X| (kΩ)
800
Calc X Calculated data of X
600 400
(b)
200 0 100
1000
10000
100000
1000000
13
10000000
Frequency (Hz) Fig. 6. Adaptability of the equivalent circuit models for spinach. (a) Hayden model, (b) Modified Hayden model.
f ¼ Re
(6)
The Ri values were extremely low compared to the Re values, which indicated that the diameter, that is size of the circular arc, was dependent on the Re values. The length of the coordinate at the top of the circular arc from the origin also represents the size of the arc. Thus, the normalized values had the same trends during short time heating. These results showed that changes in the capacitance of the cell membranes and in the extracellular resistance of the heated spinach samples could be evaluated simply by using the length of the coordinate at the top of the circular arc.
can also be applied to the selection of processing methods for agricultural products that will not easily destroy the cell membranes. 3.2.3. Equivalent circuit analysis Fig. 6 shows the electrical impedance values calculated using the Hayden model (a) and the modified Hayden model (b). An obvious inconsistency was observed between the values obtained 800
16
(a)
600
Cm (nF) C m
(kΩ) RRe e
*
200 *
0 0
10
(b)
12
*
400
8 4
*
20 30 40 50 Heating periods (s)
*
*
60
0
70
0
10
*
20 30 40 50 Heating periods (s)
*
60
70
1 *
12
Normalized value (-)
16
(kΩ) RRi i
Ri Re : Ri þ Re
(c)
8
(d)
0.1
4 0
LTO Re Re Cm Cm
0.01 0
10
20 30 40 50 Heating periods (s)
60
70
0
10
20 30 40 50 Heating peripds (s)
60
70
Fig. 7. Electrical impedance parameters of spinach heated for different periods. Ri: intracellular resistance, Re: extracellular resistance, Cm: cell membrane capacitance, LTO: length of top coordinates of the oval circular arc on Cole-Cole plots from the origin. Parameters represent the mean values ± S.E. (n ¼ 6e17). * Indicates significant differences compared to the control (non-heated samples) with P < 0.05.
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T. Watanabe et al. / Journal of Food Engineering 194 (2017) 9e14
3.3. Relationship between the mechanical fracture properties and electrical impedance properties As shown in Table 1, the mechanical fracture properties of the samples heated for 40 s and more were significantly different from those of the control. However, significant changes in the electrical impedance parameters were observed even in the samples heated for 10e40 s (Figs. 4 and 5). In addition, from Fig. 7 and Table 1, the changes in these electrical impedance parameters were found to be larger than those observed in each of the mechanical fracture properties. Therefore, it is clear that the changes in the electrical impedance parameters occurred earlier and were greater in magnitude than the changes in the mechanical fracture properties at the fracture point during short time heating of the samples. The damage to cell structure, indicated by the electrical impedance data, was used as an index of the physiological activity during the growth of plants and is very sensitive to multiple external stresses such as heat and osmotic and drought stresses (Marangoni et al., 1996; Kocheva et al., 2004; Balogh et al., 2013). Thus, we assumed that the electrical impedance properties of the samples had higher sensitivity to heat damage than the mechanical properties. Based on this, we believe that the electrical impedance properties analyzed based on the structure of the tissues can be an index for the early detection of changes in the mechanical fracture properties of samples during short time heating. This knowledge is beneficial for determining the optimum conditions to be used for the processing of agricultural products with good mechanical properties such as texture. The wide applicability of such an index during heat processing or storage will be investigated using other vegetable and fruit samples in our future studies. 4. Conclusion The mechanical properties determined using tensile fracture tests and the electrical impedance characteristics of spinach samples after short time heating by steam at 100 C were measured in this study. (1) The load-deformation curves obtained using the tensile tests indicated that the fracture properties of the samples significantly change after heating for 40e60 s. These changes may be caused by concentration of cell wall and loss of turgor pressure in the cells. (2) The similarity of the circular arc of Cole-Cole plots during heating was evaluated using jXj/R of the coordinate at the top of the circular arcs. The similarity, i.e. non-uniformity in the electrical characteristics of the connected cells of the samples, changed after heating for 30 s. This may suggest heterogeneous breakdown of the cell structure, such as membrane destruction in each cell. (3) Intracellular resistance increased and the extracellular resistance and cell membrane capacitance decreased with increasing heating periods. These phenomena indicated that intracellular fluids leaked to the outside of the cells during the degradation of the cell membranes under heat stress. In addition, the length of a coordinate at the top of the circular arc could easily express changes in the capacitance of the cell membrane and the extracellular resistance values. (4) The changes in the electrical impedance properties occurred earlier and were greater in magnitude than the changes in the mechanical fracture properties of the samples. We believe that the electrical impedance properties can be a good index for the early detection of changes in the mechanical properties of samples during short time heating.
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Mlcek, J., Valsikova, M., Druzbikova, H., Ryant, P., Jurikova, T., Sochor, J., Borkovcova, M., 2015. The antioxidant capacity and macroelement content of several onion cultivars. Turk J. Agric. For. 39, 999e1004. Nemli, S., Kianoosh, T., Tanyolaç, M.B., 2015. Genetic diversity and population structure of common bean (Phaseolus vulgaris L.) accessions through retrotransposon-based interprimer binding sites (iPBSs) markers. Turk J. Agric. For. 39, 940e948. Neri, L., Hernando, I.H., Munuera, I.P., Sacchetti, G., Pittia, P., 2011. Effect of blanching in water and sugar solutions on texture and microstructure of sliced carrots. J. Food Sci. 76, e23ee30. Nisha, P., Singhal, R.S., Pandit, A.B., 2006. Kinetic modelling of texture development in potato cubes (Solanum tuberosum L.), green gram whole (Vigna radiate L.) and red gram splits (Cajanus cajan L.). J. Food Eng. 76, 524e530. Ohnishi, S., Shimiya, Y., Kumagai, H., Miyawaki, O., 2004. Effect of freezing on electrical and rheological properties of food materials. Food Sci. Technol. Res. 10, 453e459. Ozkan, I.A., Akbudak, B., Akbudak, N., 2007. Microwave drying characteristics of spinach. J. Food Eng. 78, 577e583. Park, Y.W., 1987. Effect of freezing, thawing, drying, and cooking on carotene retention in carrots, broccoli and spinach. J. Food Sci. 52, 1022e1025. Pliquett, U., 2010. Bioimpedance: a review for food processing. Food Eng. Rev. 2, 74e94. Selman, J.D., 1994. Vitamin retention during blanching of vegetables. Food Chem. 49, 137e147. Tsou, C.H., Li, L., Vijayan, K., 2016. The intra-familial relationships of Pentaphylacaceae as revealed by DNA sequence analysis. Biochem. Genet. 54, 270e282. Toole, G.A., Parker, M.L., Smith, A.C., Waldron, K.W., 2000. Mechanical properties of lettuce. J. Mater. Sci. 35, 3553e3559. Watanabe, T., Orikasa, T., Shono, H., Koide, S., Ando, Y., Shiina, T., Tagawa, A., 2016. The influence of inhibit avoid water defect responses by heat pretreatment on hot air drying rate of spinach. J. Food Eng. 168, 113e118. Xiao, H.W., Bai, J.W., Gao, Z.J., 2014. The application of superheated steam impingement blanching (SSIB) in agricultural products processing e a review. J. Food Eng. 132, 39e47. Zhang, M.I.N., Willison, J.H.M., 1993. Electrical impedance analysis in plant tissues: impedance measurement in leaves. J. Exp. Bot. 44, 1369e1375. Zoltowski, P., 1998. On the electrical capacitance of interfaces exhibiting constant phase element behavior. J. Electroanal. Chem. 443, 149e154.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effect of short time heating on the mechanical fracture and electrical impedance properties of spinach (Spinacia oleracea L.) Takashi Watanabe a, *, Yasumasa Ando a, Takahiro Orikasa b, Takeo Shiina c, Kaoru Kohyama a a b c
Food Research Institute, NARO, 2-1-12, Kannondai, Tsukuba, Ibaraki 305-8642, Japan Faculty of Agriculture, Iwate University, 3-18-8 Ueda, Morioka, Iwate 020-8550, Japan Graduate School of Horticulture, Chiba University, 648 Matsudo, Matsudo, Chiba 271-8510, Japan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2016 Received in revised form 15 August 2016 Accepted 2 September 2016 Available online 4 September 2016
We evaluated the mechanical properties and electrical impedance data of spinach after short time heating. The mechanical fracture properties of samples heated for 40 s significantly differed from those of the non-treated samples. A circular arc of the Cole-Cole plot obtained from the impedance data shrunk after heating for 10 s. Similarity of the circular arcs, i.e. non-uniformity in the electrical characteristics, changed after heating for 30 s. After heating, extracellular resistance values were decreased from 650 to 16 kU, and cell membrane capacitance values were decreased from 13 to 0.28 nF. These changes could be evaluated simply by using the length of the coordinate at the top of the circular arc from the origin, and these occurred earlier and were greater in magnitude than those in the fracture properties were. These electrical properties could be an index for the early detection of changes in the fracture properties of samples. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Spinach Short time heating Mechanical properties Electrical impedance Equivalent circuit model
1. Introduction Horticultural plants are very important for human diet as sources of vitamins, minerals and dietary fiber and they become a significant part of human life due to their medicinal and environmental uses as well as aesthetics and economic values. The stem, leaf, flowers, roots and the fruits of vegetables and fruit crops have the highest potential of export (Park, 1987; Mlcek et al., 2015; Nemli et al., 2015; Tsou et al., 2016). Short time heating for fresh agricultural products, which is called blanching, is commonly conducted to inhibit changes in the quality of fresh products that may occur during secondary processing and storage (Xiao et al., 2014). However, short time heating has also been known to affect the quality of final products in many cases (Selman, 1994). Therefore, several studies have focused on changes in the quality of agricultural products that occur during short time heating. These studies have mainly involved the investigation of characteristics such as nutrient composition, color, microstructure, and texture (Lin and Schyvens, 1995; Neri et al.,
* Corresponding author. E-mail address: [email protected] (T. Watanabe). http://dx.doi.org/10.1016/j.jfoodeng.2016.09.001 0260-8774/© 2016 Elsevier Ltd. All rights reserved.
2011; Jaiswal et al., 2012; Imaizumi et al., 2015) to determine the optimum heating conditions that can help minimize the degradation of these products. Spinach (Spinacia oleracea L.) is widely consumed all over the world. However, fresh spinach has a very short shelf life. Therefore, methods to extend the storage time of spinach, such as drying and freezing, have been widely studied (Park, 1987; Ozkan et al., 2007). Mechanical properties of agricultural products are an important factor that determines food texture. Changes in the mechanical properties of agricultural products that occur during short time heating affect the quality of the final products (Selman, 1994; Buggenhout et al., 2006; Nisha et al., 2006). For using short time heating for spinach products, it is necessary to study the effect of such heating on the mechanical properties of spinach in order to determine the optimum short time heating conditions. The mechanical properties of vegetables are anisotropic with the fiber direction. Therefore, it is important to consider the fiber direction when the mechanical properties are evaluated. A tensile test is a classic method used for the determination of the mechanical properties of leafy vegetables. This method can provide information about the mechanical properties of vegetables in specific fiber directions. The investigation of the mechanical properties of vegetables using tensile tests has been conducted for several
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vegetables such as cabbage (Kohyama et al., 2008), lettuce (Toole et al., 2000), and Solanaceae (Aranwela et al., 1999). However, the mechanical properties of spinach during short time heating have not been studied using tensile tests. In addition to mechanical properties, electrical impedance properties are important for the determination of the optimum heating conditions for agricultural products, and are used to obtain information about the cell structures of plants (Ohnishi et al., 2004). Impedance properties are often described on a complex plane such as a Cole-Cole plot (Cole, 1932). Our previous study showed that short time heating could damage the cell membrane of spinach, as indicated by the linear decrease in the coordinate at the top of the circular arc of the Cole-Cole plot, and the linearity was used as an index of the physiological activity during hot-air drying (Watanabe et al., 2016). Because the coordinate at the top of the circular arc of the Cole-Cole plots moved linearly, the circular arcs before and after the shrinkage may have similar shapes. The similarity was determined by non-uniformity in the electrical characteristics of the connected cells of agricultural products. In addition, there is a possibility that the non-uniformity is an index for the destruction of cell membranes during short time heating. However, the similarity of the circular arc after short time heating has not been examined so far. For agricultural products, electrical impedance data have been analyzed using equivalent circuit models based on cellular structure (Zhang and Willison, 1993). Information about each part of the cells in plant tissues, including the extra- and intra-cellular fluid and the cell membranes, is quantified using equivalent circuit analysis, which has helped understand how the state of cells affects the mechanical properties of vegetables (Fuentes et al., 2014; Imaizumi et al., 2015; Ando et al., 2016). However, the relationship between changes in the cellular structure and the mechanical properties of spinach during short time heating is not clear. The investigation of such a relationship is important for simple and rapid detection of the changes in the mechanical properties of spinach during heating, based on electrical impedance analysis. This knowledge is beneficial for determining the optimum conditions for the processing of agricultural products with good mechanical properties such as texture. In this study, changes in the mechanical properties of spinach were measured using the tensile test, and the electrical impedance data after the short time heating were analyzed using coordinates at the top of the circular arc on Cole-Cole plots and equivalent circuit models for plant cellular tissues. The similarity in the shapes of the circular arcs was examined to investigate non-uniformity in the electrical characteristics of the connected cells of the samples during heating. In addition, the relationship between the length of the coordinate at the top of the circular arc on Cole-Cole plots from the origin and the electrical parameters obtained from equivalent circuit model analysis was reported. Moreover, the relationship between the mechanical fracture properties and electrical impedance properties of spinach was discussed. 2. Materials and methods
with weighing 1.5 ± 0.5 g were selected and used for the short time heating experiment. Unheated samples were used as controls in this study. 2.2. Short time heating Short time heating was performed using the conditions described by Watanabe et al. (2016). In brief, three samples placed on a wire netting were heated using steam at 100 C in a water oven (AX-XP100, Sharp, Osaka, Japan) for 10, 20, 30, 40, and 60 s. After heating, the samples were cooled immediately in ice water for 30 s. 2.3. Tensile test The mechanical properties of the spinach samples were measured by a tensile test using a universal testing machine (Model 5542, Instron, Massachusetts, USA) with a load cell of 50 N at 20 C (Kohyama et al., 2008). Rectangular specimens (10 50 mm) cut out along the side of the major vein on the area around on the top of a spinach leaf were used. Major veins were avoided during the preparation of the specimens, because they could add complexities to the results (Aranwela et al., 1999). Both the ends of each specimen (17.5 mm each) were clamped using screw grips (CAT#2710004, Instron, Massachusetts, USA), and were tensed at a constant speed of 1 mm/s until fracture. Fracture was determined as the point where the maximum load was detected. The thickness and width of each fractured specimen was measured using calipers (CD15CPX, Mitsutoyo, Kanagawa, Japan). In this study, we focused on the fracture point in the tensile test and discussed changes in the mechanical properties after short time heating. Fracture stress was derived as a ratio of the maximum load to the cross sectional area (thickness width) of each fractured specimens. Fracture strain was calculated as the ratio of deformation at the fracture to the initial length of the samples between the clamps (15 mm). Fracture modulus was defined as the ratio of fracture stress to fracture strain. Fracture energy was calculated as the area under the loaddeformation curve until fracture. The mechanical properties of the heated samples were measured 10e15 min after cooling. The measurements were repeated 6e17 times. 2.4. Electrical impedance spectroscopy The electrical impedance data of the samples were measured using an impedance analyzer (E4990A, Keysight Technology, California, USA) with steel needle electrodes spaced 17 mm apart at 20 C. The impedance data were obtained along the major vein at points distanced approximately 5 mm apart from the main vein on the top half area of a spinach leaf, similar to the conditions of the mechanical test. The electrical impedance data were derived at 81 frequency points with a logarithmic interval from 42 Hz to 5 MHz at a measuring voltage of 1.0 V. The electrical impedance data of the heated samples were measured after cooling within 10e30 s. The measurements were repeated 6e18 times. The complex impedance, Z, (U) was defined as:
2.1. Materials
Z ¼ R þ jX;
We used the spinach cultivar SC0-114. After the harvest season of the cultivar, we offered other cultivars that had almost the same mechanical and electrical properties as SC0-114. The spinach was harvested and packaged during June 19eJuly 1, 2015, in Ibaraki, Japan. The spinach was then transported to the Food Research Institute, NARO, Ibaraki, Japan, and stored immediately in a refrigerator at 4 C until use. All the tests were performed within 3 days from the day of harvest. Only the leaves of the spinach plants
where j is the imaginary unit, R is the real part (resistance (U)), and X is the imaginary part of the impedance (reactance (U)). The relationship between R and X is presented as a Cole-Cole plot. The length of the coordinate at the top of the circular arc of Cole-Cole plots from the origin (Watanabe et al., 2016) was calculated. As noted below, the Cole-Cole plot of biological tissues is distorted due to the non-uniform electrical properties of cells in the tissues. We defined a ratio of the reactance X to the resistance R (jXj/
(1)
T. Watanabe et al. / Journal of Food Engineering 194 (2017) 9e14
R) at the top of the circular arcs as a new index of the similarity of the circular arcs and the non-uniformity of the electrical impedance properties of the cells in the heated spinach samples. Equivalent circuit models were used to analyze the electrical impedance data of the samples. The Hayden model (Fig. 1 (a)), which consists of cell membrane resistance, Rm, intracellular resistance, Ri, extracellular resistance, Re and cell membrane capacitance, Cm, is well known and frequently used to analyze the impedance properties of agricultural products (Hayden et al., 1969). The Hayden model represents the structure of a cell and describes an exact semicircle as a Cole-Cole plot. However, biological tissues are composed of a number of cells and produce a time constant distribution, because their electrical properties are nonhomogeneous. As a result, a Cole-Cole plot of the tissues is distorted in the direction of the real axis. To fit the model to the impedance properties of biological tissues, Ando et al. (2014) modified the Hayden model (Fig. 1 (b)) by eliminating Rm and inserting a constant phase element, CPE (Zoltowski, 1998), instead of Cm to adjust the non-uniformity of the electrical properties among cells in these tissues. The impedance of the CPE, ZCPE, is defined as:
1 ; ðjuÞp T
(2)
where u is angular frequency (rad/s), p is CPE exponent, and T is the CPE coefficient (F/s(1p)). The modified model has been used previously for examining cell structures on processed vegetables such as potatoes during hot air drying (Ando et al., 2014) and hot water treatment (Imaizumi et al., 2015), and on carrots after dehydrofreezing (Ando et al., 2016). In this study, the modified Hayden model was used to analyze the electrical impedance data of the spinach samples. The complex impedance of the modified model can be represented by Eq. (3):
h n i Re 1 þ up T ð2Ri þ Re Þ$ cos p2 p þ up TRi ðRe þ Ri Þg Z¼ fup TðRe þ Ri Þg2 þ 2up TðRe þ Ri Þ$ cos p2 p þ 1 up TR2e $ sin p2 p j : fup TðRe þ Ri Þg2 þ 2up TðRe þ Ri Þ$ cos p2 p þ 1
(3)
The equivalent circuit parameters were estimated by complex nonlinear least squares curve fitting (Macdonald, 1992). Under the condition that the relaxation angular frequency (the angular frequency at which the imaginary part of the impedance is minimal) is
(b)
Cm
Rm
Ri
Re
1
Cm ¼ T p ðRe þ Ri Þ
1p p
:
(4) 2
We evaluated coefficients of determination (r ) to investigate the adaptability of the equivalent circuit models for spinach samples using Eq. (5) described as Imaizumi et al. (2015):
r2 ¼ 1
P 2 i ð Z i Z calc Þ ; 2 P i Z i Z
(5)
avg
where jZjcalc is the value of jZj calculated using the equivalent circuit models, and jZjavg is the average value of jZj. 2.5. Statistics Dunnett's test with a significance level of P < 0.05 was conducted using the SPSS package (SPSS ver.17.0, SPSS, Chicago, USA) for statistical analyses. 3. Results and discussion 3.1. Mechanical properties Fig. 2 shows the load-deformation curves observed during the tensile tests of the samples heated for different periods. For all the samples, the load increased nearly linearly as the deformation proceeded to the fracture point, and then drastically dropped to zero. The mechanical fracture properties calculated from the loaddeformation curves of the tensile tests are shown in Table 1. No significant differences in the fracture load and energy of the samples were observed after short time heating for 0e60 s. However, significant increase in the fracture strain, stress, and modulus were observed for the samples heated for 40e60 s. In addition, the values of the cross sectional area at the fracture point were significantly decreased for the samples heated for 40e60 s. We assumed that the decrease in the cross sectional area was chiefly due to escaping of the air and the water transpiration caused in the samples by the heat treatment. The decrease in the cross sectional area suggested a concentration of the cell walls, which is an intrinsic mechanical characteristic (Chanliaud et al., 2002). Therefore, the decrease in the cross sectional area caused by the heating could be a primary factor responsible for the significant increase in the fracture stress and modulus. The increase in the fracture strain caused by short time heating could be explained by the loss of cell turgor (Chiralt et al., 2001). At low turgor pressure, cell debonding is more likely to occur than cell rupture during the fracture test. The lower the turgor pressure of the samples, the higher was the fracture strain.
1.2
CPE
(a)
invariant, the capacitance of the cell membranes (Cm) was calculated using Eq. (4):
Re
Ri
Control 10 s 20 s 30 s 40 s 60 s
Fracture point
1 Load (N)
ZCPE ¼
11
0.8
0.6 0.4 0.2 0 0
Fig. 1. Equivalent circuit models. (a) Hayden model, (b) Modified Hayden model. Rm: cell membrane resistance, Ri: intracellular resistance, Re: extracellular resistance, Cm: cell membrane capacitance, CPE: constant phase element.
2
4 Deformation (mm)
6
8
Fig. 2. Load-deformation curves of spinach heated for different periods.
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T. Watanabe et al. / Journal of Food Engineering 194 (2017) 9e14
Table 1 Mechanical fracture properties of spinach heated for different periods. Heating periods (s)
Cross sectional area (mm2)
Control 10 20 30 40 60
2.20 2.11 1.90 1.80 0.84 0.97
± ± ± ± ± ±
0.13 0.13 0.19 0.20 0.06* 0.10*
Load (N) 0.86 0.93 0.66 0.73 0.68 0.76
± ± ± ± ± ±
0.07 0.03 0.11 0.07 0.05 0.05
Strain () 0.19 0.27 0.18 0.26 0.23 0.31
± ± ± ± ± ±
Stress (MPa)
0.02 0.02 0.03 0.03 0.03 0.02*
0.41 0.45 0.36 0.43 0.74 0.82
± ± ± ± ± ±
Modulus (MPa)
0.04 0.04 0.07 0.06 0.08* 0.09*
2.30 1.71 1.91 1.62 3.33 2.76
± ± ± ± ± ±
Energy (mJ)
0.21 0.17 0.23 0.10 0.45* 0.43
1.6 2.2 1.1 1.7 1.3 1.8
± ± ± ± ± ±
0.28 0.22 0.24 0.30 0.21 0.22
Each value represents the mean values ± S.E. (n ¼ 6e17). Initial length of specimens was 15 mm. *Indicates significant differences compared to the control (non-heated samples) with P < 0.05. Specimens (10 50 mm) were tensed at a constant rate of 1 mm/s at 20 C. Strain is calculated as ratios of deformation to initial length of samples between clamps. Stress is derived as ratios of the maximum load by the cross sectional area (thickness width). Modulus is defined as the ratio of fracture stress to fracture strain. Energy is calculated as the area under the load-deformation curve until fracture.
Therefore, we assumed that the increase in fracture strain was due to the loss of cell turgor pressure caused by the damage to cell membranes during short time heating. 3.2. Electrical impedance characteristics 3.2.1. Coordinates at the top of the circular arc on Cole-Cole plots Fig. 3 shows the Cole-Cole plots for the samples heated for different periods. The linear part in the low frequency domain is based on the polarization at the electrode surface (Pliquett, 2010) and is not related to the characteristics of the cellular structure. Therefore, the corresponding parts were eliminated for examination in this study. The circular arc shrunk with increase in the heating duration. Fig. 4 (a) shows the coordinate at the top of the circular arc of the Cole-Cole plots for the different durations for which the samples were heated in all the experiments in this study. All the data have been plotted in alignment. In addition, Fig. 4 (b) shows the linear path traced by the coordinate at the top of the circular arc of the Cole-Cole plots (r2 > 0.99), and the inclination of a linear equation yielded a value similar to that obtained in a previous report (Watanabe et al., 2016). 3.2.2. Similarity of circular arcs of Cole-Cole plots Fig. 5 shows the jXj/R at the coordinate at the top of the circular arc for samples heated for different periods. The jXj/R of the control sample and those heated for 10 s and 20 s have same values. Thus, the similarity between the control sample and the samples heated for 10 s and 20 s was clear. The jXj/R of the samples heated for 30e60 s showed a decrease. The results indicated that the circular arc of the Cole-Cole plots for the samples heated for 30e60 s was deformed in the downward direction. Given that the shape of a Cole-Cole plot was determined by non-uniformity in the electrical characteristics of the connected cells (i.e. cell structures and cell sizes) of the samples, the changes in the shape may suggest commencement of the heterogeneous breakdown of the cell
Fig. 4. Coordinates at the top of the circular arc on Cole-Cole plots of spinach heated for different periods. (a) Measured values in all experiments, (b) Averaged value at each heating periods (Control, 10, 20, 30, 40, 60 s). * Indicates significant differences compared to the control (non-heated samples) with P < 0.05.
structure, such as membrane destruction in each cell. We assumed that jXj/R could be an index of the destruction of the cell membranes of samples during heating. The destruction of the cell membranes was a factor for leaking of the water-soluble component from the agricultural products during processing. The index 0.6 0.5
250
High
Frequency
Low
0.4 |X|/R (-)
|X| (kΩ)
200 150
* *
0.3
*
0.2
100
Control 10 s 20 s
50 0
0
200
400 R (kΩ)
600
800
Fig. 3. Cole-Cole plots of spinach heated for different periods. Closed symbols mean the top of the circular arcs.
0.1 0 0
10
20
30 40 50 Heating periods (s)
60
70
Fig. 5. The ratio of reactance to resistance (jXj/R) at the coordinates at the top of the circular arc on Cole-Cole plots of spinach heated for different periods. Parameters represent the mean values ± S.E. (n ¼ 6e17). * Indicates significant differences compared to the control (non-heated samples) with P < 0.05.
T. Watanabe et al. / Journal of Food Engineering 194 (2017) 9e14
using the Hayden model and the actual values. In contrast, the calculated values obtained using the modified Hayden model agreed well with the actual values (r2 > 0.99). The modified model could describe the impedance characteristics of the inhomogeneous tissues of the samples as the CPE allowed the phase angles of Cm to be changed flexibly (Ando et al., 2014). This confirmed the adaptability of the modified Hayden model for spinach during short time heating. The changes in the parameters obtained from the equivalent circuit analysis are shown in Fig. 7 (a)e(c). The Ri increased and Re and Cm decreased as the heating periods increased. This trend potentially indicates that intracellular fluids with low resistance leaked to the outside of the cells due to changes in the cell structure, such as cell membrane degradation (Ando et al., 2014; Imaizumi et al., 2015). Changes in the length of the coordinate at the top of the circular arc of the Cole-Cole plots from the origin and the values of Re and Cm normalized to the values observed in the controls are shown in Fig. 7 (d). These normalized values showed the same trends of decreases during short time heating. The cell membrane has high electrical impedance in the low frequency domain. Under the experimental conditions used in this study, at the point of minimum frequency on ideal Cole-Cole plots for plant cells, the real part values of the complex electrical impedance were close to Re. In contrast, at the point of maximum frequency, the values were close to the parallel resistances, Ri and Re, because the electrical impedance of the cell membrane was decreased at this point. Thus, the approximate diameter (f) on the real part axis of the circular arc can be represented using Eq. (6):
1200 R Measured data of R 1000
X Measured data of X Calc R Calculated data of R
R, |X| (kΩ)
800
Calc X Calculated data of X
600 400
(a)
200 0
100
1000
10000
100000
1000000
10000000
Frequency (Hz) 1200 R Measured data of R 1000
X Measured data of X Calc R Calculated data of R
R, |X| (kΩ)
800
Calc X Calculated data of X
600 400
(b)
200 0 100
1000
10000
100000
1000000
13
10000000
Frequency (Hz) Fig. 6. Adaptability of the equivalent circuit models for spinach. (a) Hayden model, (b) Modified Hayden model.
f ¼ Re
(6)
The Ri values were extremely low compared to the Re values, which indicated that the diameter, that is size of the circular arc, was dependent on the Re values. The length of the coordinate at the top of the circular arc from the origin also represents the size of the arc. Thus, the normalized values had the same trends during short time heating. These results showed that changes in the capacitance of the cell membranes and in the extracellular resistance of the heated spinach samples could be evaluated simply by using the length of the coordinate at the top of the circular arc.
can also be applied to the selection of processing methods for agricultural products that will not easily destroy the cell membranes. 3.2.3. Equivalent circuit analysis Fig. 6 shows the electrical impedance values calculated using the Hayden model (a) and the modified Hayden model (b). An obvious inconsistency was observed between the values obtained 800
16
(a)
600
Cm (nF) C m
(kΩ) RRe e
*
200 *
0 0
10
(b)
12
*
400
8 4
*
20 30 40 50 Heating periods (s)
*
*
60
0
70
0
10
*
20 30 40 50 Heating periods (s)
*
60
70
1 *
12
Normalized value (-)
16
(kΩ) RRi i
Ri Re : Ri þ Re
(c)
8
(d)
0.1
4 0
LTO Re Re Cm Cm
0.01 0
10
20 30 40 50 Heating periods (s)
60
70
0
10
20 30 40 50 Heating peripds (s)
60
70
Fig. 7. Electrical impedance parameters of spinach heated for different periods. Ri: intracellular resistance, Re: extracellular resistance, Cm: cell membrane capacitance, LTO: length of top coordinates of the oval circular arc on Cole-Cole plots from the origin. Parameters represent the mean values ± S.E. (n ¼ 6e17). * Indicates significant differences compared to the control (non-heated samples) with P < 0.05.
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T. Watanabe et al. / Journal of Food Engineering 194 (2017) 9e14
3.3. Relationship between the mechanical fracture properties and electrical impedance properties As shown in Table 1, the mechanical fracture properties of the samples heated for 40 s and more were significantly different from those of the control. However, significant changes in the electrical impedance parameters were observed even in the samples heated for 10e40 s (Figs. 4 and 5). In addition, from Fig. 7 and Table 1, the changes in these electrical impedance parameters were found to be larger than those observed in each of the mechanical fracture properties. Therefore, it is clear that the changes in the electrical impedance parameters occurred earlier and were greater in magnitude than the changes in the mechanical fracture properties at the fracture point during short time heating of the samples. The damage to cell structure, indicated by the electrical impedance data, was used as an index of the physiological activity during the growth of plants and is very sensitive to multiple external stresses such as heat and osmotic and drought stresses (Marangoni et al., 1996; Kocheva et al., 2004; Balogh et al., 2013). Thus, we assumed that the electrical impedance properties of the samples had higher sensitivity to heat damage than the mechanical properties. Based on this, we believe that the electrical impedance properties analyzed based on the structure of the tissues can be an index for the early detection of changes in the mechanical fracture properties of samples during short time heating. This knowledge is beneficial for determining the optimum conditions to be used for the processing of agricultural products with good mechanical properties such as texture. The wide applicability of such an index during heat processing or storage will be investigated using other vegetable and fruit samples in our future studies. 4. Conclusion The mechanical properties determined using tensile fracture tests and the electrical impedance characteristics of spinach samples after short time heating by steam at 100 C were measured in this study. (1) The load-deformation curves obtained using the tensile tests indicated that the fracture properties of the samples significantly change after heating for 40e60 s. These changes may be caused by concentration of cell wall and loss of turgor pressure in the cells. (2) The similarity of the circular arc of Cole-Cole plots during heating was evaluated using jXj/R of the coordinate at the top of the circular arcs. The similarity, i.e. non-uniformity in the electrical characteristics of the connected cells of the samples, changed after heating for 30 s. This may suggest heterogeneous breakdown of the cell structure, such as membrane destruction in each cell. (3) Intracellular resistance increased and the extracellular resistance and cell membrane capacitance decreased with increasing heating periods. These phenomena indicated that intracellular fluids leaked to the outside of the cells during the degradation of the cell membranes under heat stress. In addition, the length of a coordinate at the top of the circular arc could easily express changes in the capacitance of the cell membrane and the extracellular resistance values. (4) The changes in the electrical impedance properties occurred earlier and were greater in magnitude than the changes in the mechanical fracture properties of the samples. We believe that the electrical impedance properties can be a good index for the early detection of changes in the mechanical properties of samples during short time heating.
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