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Effect of pulse electric field applied to carrot prior to its heating
Journal of Food Engineering 278 (2020) 109936
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Journal of Food Engineering journal homepage: http://www.elsevier.com/locate/jfoodeng
Effect of pulse electric field applied to carrot prior to its heating Ji�rí Blahovec *, Pavel Kou�rím Department of Physics, Faculty of Engineering, Czech University of Life Sciences, 16521, Prague 6, Suchdol, Czech Republic
A R T I C L E I N F O
A B S T R A C T
Keywords: Critical temperature Carrot Impedance Permittivity Loss tangent Cell membranes
The tissue of carrot (variety Jereda) was tested using dynamic mechanical analysis (DMA) combined with dielectric thermal analysis (DETA), in air of 90% humidity in temperature scans between 30 and 90 � C. Tem perature plots of loss tangent were determined. In DETA, based on alternated current of frequency 20 kHz, both components of impedance were continually determined. Pulsed electric field (PEF) was applied prior to the DMA and DETA tests. Four sets of experiments were performed, without PEF (b), with a single 10 ms long pulse given an alternated field of 500 V/cm (c), with two similar pulses at 0.1 s interval (d) and with two similar pulse at 1 s interval (e). The impedance was recalculated giving parameters of a single model represented by parallel connection of a resistor Re and a capacitor with capacitance ωC. Three stages of the range of temperature, namely subcritical (30–70 � C), critical (70–80 � C), and supercritical (80–90 � C) were considered. For set b, in subcritical stage, the parameter, Re decreased whereas capacitance was nearly constant. Both of these parameters decreased slightly in the supercritical stage but whereas Re is in supercritical stage much lower than in the subcritical stage, the capacitance is in supercritical stage rather higher than it is in subcritical stage. In critical stage, Re sharply decreased and the capacitance showed a sharp peak, both indicating collapse of the cellular membranes. Application of PEF led to little reduction of the capacitance peak and the reduction was more effective after the repeated pulse application (that is in sets d and e). These results at carrot were compared with the previous research at potato and it was shown that PEF effects in critical and supercritical stages significantly differ in the both plant tissues.
1. Introduction
conductivity σ d:
Textural properties of cellular products can be modified either by thermal processing and/or with the application of electric pulses. Whereas the stationary electric fields are usually used as a mean for nondestructive determination of food quality (e.g. Jha and Matsuoka, 2004; Jha et al., 2011) the Pulsed Electric Fields (PEF) are stated in reference to the application of short electric pulses of high potential (Blahovec et al., 2017). It is known that the application of external pulsed electric field can damage the integrity of cellular membranes (e. g.Tsong and Su, 1999; De Vito et al., 2008; Vorobiev and Lebovka, 2010). Previously we used a special Z parameter defined by Lebovka et al. (2002) and repeatedly used by De Vito et al. (2008), this parameter is usually termed as disintegration parameter and it serves to description of the degree of such a damage. The parameter depends on the actual state of the material’s electrical conductivity σ and varies between 0, for materials in the initial untreated state with conductivity σ u, and 1 for materials with totally disintegrated cellular membranes with
Z¼
σ σd
σu σu
(1)
In practical cases, σ d is estimated by direct measurement of the tissue conductivity after long and high electric pulse processing: De Vito et al. (2008) used 0.1 s long pulses with 1 kV/cm intensity for apples. The effect of the PEF application in a defined pulse protocol is usually detected by similar means as those that are usually used in thermal analysis (e.g. Asavasanti et al., 2011, 2012, Barba et al., 2015). Temperature and time are the main parameters of any cooking process and they has to serve also as the fundamental technological parameters in the cooking of most biological products. Unfortunately, the details of the cooking process are only partially known, especially in cellular products after application of PEF. There is need further infor mation on parallel processes occurring in living cells and tissues during heating (Vilgis, 2015). For this purpose, indirect methods have to be used. The methods of thermal analysis (Haines, 2002) are suitable, provided that the drying of specimens due to increasing temperature is
* Corresponding author. E-mail address: [email protected] (J. Blahovec). https://doi.org/10.1016/j.jfoodeng.2020.109936 Received 16 May 2019; Received in revised form 8 November 2019; Accepted 21 January 2020 Available online 24 January 2020 0260-8774/© 2020 Elsevier Ltd. All rights reserved.
J. Blahovec and P. Kou�rím
Journal of Food Engineering 278 (2020) 109936
prevented (Blahovec et al., 2012; Xu and Li, 2014). Among the different methods of thermal analysis known, mainly differential scanning calo rimetry (DSC) is dedicated to the detection of phase transformations in the analysed products. In our studies of PEF role, two methods of thermal analysis, namely dynamic mechanical analysis (DMA) and dielectric thermal analysis (DETA) (Haines, 2002) are suitable – newly applied in combination (Blahovec and Kou�rím, 2019). Both methods are based on detection of typical changes in characteristic complex quantities of the specimen in temperature scans. In the case of DMA this role is played by the modulus of elasticity and in DETA a similar role is played by some electrical quantities, such as the capacitance and/or resistance of a heated spec imen that is detected usually by application of a low alternated electric field (Haines, 2002). A combination of both DMA and DETA was used in our previous work (Blahovec and Kou�rím, 2019) to study changes caused by electric pulses in potato. It was found that application of PEF causes nontrivial changes in the tested specimen, in the entire tested temperature range. Even if some changes caused by PEF could be clas sified as “additive” to the changes caused by thermal effects, the time development of some of them can be classified as rather strange (Imai zumi et al., 2015; Vorobiev and Lebovka, 2010) and dependent on de tails of the pulse protocol. Potato cells, in comparison to carrot, contain lots of starch grains with large amount of starch which, after contact with water at tem peratures higher than approximately 60 � C, changes its structure in a process termed gelatinization (Ratnayake and Jackson, 2007, 2009). Gelatinization is a very complicated process requiring, even in the simplest cases, destruction of membranes on both starch grains and vacuoles. PEF treatment causes damage of some membranes (Tsong and Su, 1999) and could also be involved in the gelatinization process, with possible roles in PEF effects observed in heated potatoes (Blahovec and Kou�rím, 2019). In this paper, we use the same PEF protocols as were used in the previous paper for a similar study with potato in a DMA and DETA study of the thermal stimulated changes in carrots after application of PEF. The aim of the paper is to compare the application of PEF in potatoes and carrots and to detect the main differences in the responses to PEF application in these two products. The obtained information could be useful in potential “cooking” of vegetables by PEF.
component, R0 and imaginary component, X0. The DMA instrument was arranged so that the electric properties of the tested specimen could be continuously measured as a real conductor described by the complex impedance: An RLC meter (Hameg 8118 with effective voltage 1 V, frequency 20 kHz and 3 samplings per min) was used for this purpose. Each specimen was carefully mechanically secured at two points so that its longitudinal axis was perpendicular to the fixing jaws (Blahovec et al., 2015). The free length of the specimen between the jaws was 10.8 mm. The height of the fixed specimen was appr. 3.8 mm. One set of the specimens (5 repetitions) was used for a standard DMA/DETA test (Blahovec and Kou�rím, 2019) – this set was denoted as the basic, or ‘b’. One of the jaws was fixed and the other was moving up and down with constant amplitude of 0.5 mm and a frequency of 0.2 Hz in the dynamic cantilever test. The force necessary for the oscillation was recorded, being the basis for the determination of the complex modulus. Every experiment was started at 30 � C; the loss tangent (LT) values were calculated as the ratio of the loss and storage module (for details, see Blahovec et al., 2012) in every time point where both modules were determined. The parallel DETA test is based on RLC measurement by Hameg 8118 at the same conditions described above. The humidity of air in the test chamber (90%) was kept constant during the whole experiment. The control of the air humidity in the test chamber was based on direct humidity measurement by a special hy grometer and water vapour ejection into the chamber. The temperature scan proceeded up to 90 � C with a rate of 1 K/min. The results of the DETA test were analysed on temperature plots of the impedance components: real Rr and imaginary Xr that were measured on the tested specimens at the same time as the mechanical module. We preferred to present the results of our experiments as rela tive results: R ¼ Rr/R0, and X ¼ Xr/R0, where R0 is the initial value of the real component of the specimen at 20 � C. This recalculation helps to reduce potential dimensional and surface variations in the prepared specimens. Similarly, as in our previous paper (Blahovec and Kou�rím, 2019), we used the simple electrical model of the tested tissues as a parallel connection of a resistor Re and a capacitor C given by the following formulas: Re ¼
R2 þ X 2 X ; ωC ¼ 2 R þ X2 R
(2)
where ω is the circular frequency of the electric current.
2. Materials and methods
2.3. PEF application
2.1. Test material
The specimens included in the sets for further testing (5 repetitions in every case) were removed from the DMA instrument after measurement of the initial impedance (see above) and prepared for PEF loading. It was done between two parallel steel electrodes 2 cm in diameter, which were located in the central part of the specimen perpendicularly to their 5.1 � 35 mm2 sides. Pulse loadings were performed using a special equipment (Blahovec et al., 2015): the basic AC sinusoidal signal with a frequency of 20 kHz was modulated into a nearly rectangular form of 10 ms pulse with height corresponding to the field intensity of 500 V/cm into the tested spec imen. This field intensity is the same as the field intensity used in the previous research on potato tubers (Blahovec and Kou�rím, 2019) and the test sets were organized identically: b/the basic set without PEF loading, c/the set of specimens previously loaded by one pulse, d/the set of specimens previously loaded by two sequential pulses with an interval of 100 ms between them, e/the set loaded by two pulses with an interval of 1 s. Specimens included into sets c, d, and e were tested just after pulsing procedure in the same DMA/DETA combined test as the specimens of the basic test (b).
‘Jereda’ variety of carrots, cultivated in the university garden of the Czech University of Life Sciences, Prague-Suchdol using standard farm technology, was used for this study. Harvested roots (August 2018) were carefully selected and damage-free roots (their diameters in the upper parts being 3–3.5 cm) were obtained stored few weeks in a refrigerator (at 2 � C and appr. 100% relative humidity). Roots prepared for experi ment were kept at room temperature in a box with 100% humidity for testing the next day. 2.2. Specimens and basic DMA/DETA test Rectangular specimens (5.1 mm - width � 3.8 mm - thickness � 35 mm - length) with their long axis parallel to the direction of root growth were cut from the external parts of carrots’ roots. For the cutting with a knife a special cutting jigs, keeping constant the dimensions and the rectangular shape of the specimens, were used. Inclusion of central root parts of different physical properties in the specimens was carefully avoided. About eight specimens were prepared from a single root. The procedures described below were performed on every specimen. For each specimen, the initial impedance at a temperature 20 � C was measured after being fixed to the DMA tester (see further). This mea surement gave the initial values of the specimen impedance with a real 2
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Journal of Food Engineering 278 (2020) 109936
2.4. Data analysis
et al., 2015). The applied sampling (3 per min), the rate of heating (1 K/min) and 5 repetitions of the measurement led to the final statistics of at least 15 measurement for one point/K in the analysed temperature scale.
The results obtained in at least five replications were analysed using the standard laboratory software Origin®, OriginPro Ver. 7 (OriginLab, Northampton, MA, USA). The data obtained for a specimen set were unified into a group of data. This group of data was classified according to temperature and analysed so that the results of all measurements falling into interval of one centigrade increase were evaluated statisti cally as one state and the corresponding mean values and standard de viations were calculated by a special FORTRAN programme (Blahovec
3. Results The electric data are presented in Fig. 1 in the plots of the model components (Re and ωC, see Eq. (2)) versus temperature. Significant differences were observed mainly among the values obtained in
Fig. 1. The basic results: Parameters of the model represented by Eq. (2) in the DETA test for different PEF pre-treatments of the tested carrot specimens. The points represent mean values measured in the set; the bars denote standard errors for illustration (only for one fourth of the points to prevent overlaps). a/Relative model resistance, b/Relative model capacitance. 3
J. Blahovec and P. Kou�rím
Journal of Food Engineering 278 (2020) 109936
different specimens belonging to set b and the specimens of the other sets. With increasing temperature, the relative model resistance (Re) decreases. In the initial state (set b), there exist two temperature ranges (50–60 � C and 70–80 � C) where this decrease is sharper than at the other temperatures. PEF treatment causes more regular Re decrease in the whole temperature range including of the Re decrease at the starting temperature. Two pulses caused bigger initial decrease of Re than a single pulse only; the influence of different inter-pulse intervals on the Re values that was observed in potato tubers (Blahovec and Kou�rím, 2019) was not observed in carrot. For the relative capacitance in Fig. 1b, we can find that the previous application of PEF is followed by: 1/increase of the product capacitance at temperatures lower than the area of the capacitance peak at temperatures between 70 and 85 � C, and 2/decrease of the relative height of the above mentioned peak. The capacitance increase at 1/can be divided into two parts: 1a/the initial increase at initial temperature, and 1b/the increase during the specimen heating between 30 and 40 � C. The increase 1b/is comparable in all PEF pro tocols that were used. The area of temperatures where the capacity peak is observed (70–80 � C in our case) will be termed “critical” (Blahovec and Kou�rím, 2019); in this area also, the biggest reduction of resistance was observed (Fig. 1a). Using this definition we will denote tempera tures lower than critical as “subcritical” and temperatures higher than critical as “supercritical”. The DMA results for different specimen sets are shown in Fig. 2. Similarly as in Fig. 1 for capacitance, relatively stable values were observed for the measured parameter (loss tangent) in the basic set for the temperature range 60–70 � C. Also in the case of loss tangent, a peak of the results was observed at critical temperatures above 70 � C. In contrast to the capacitance, this peak is not well ended at high tem peratures. At temperatures below 70 � C, an initial decrease of loss tangent with increasing temperature is observed. This trend is different in the basic set where an increasing trend starts at approximately 45 � C. In the sets with PEF pulsing, the loss tangent decreases with increasing temperatures in the whole temperature range up to ~70 � C. The dif ferences of the peak parts among the different carrot sets in Fig. 2 are relatively small and difficult to detect due to a high level of variability. It seems that higher level of PEF pulsing (repeating of pulses and higher prolongation of breaks between pulses) tends to lower values of the peak.
Re of carrot decreases with increasing temperature almost linearly and only in the final part, close and above 60 � C, some nonlinearities are observed. An exclusion from this rule was observed only in the basic test (b) without any PEF application. Comparisons of these results with the previous research on potatoes (Blahovec and Kou�rím, 2019, see also Fig. 3 a,b) lead to the following conclusions: i/the relative resistance in carrot is more sensitive to PEF treatment – all protocols used were fol lowed by initial resistance decreases but the differences in the effects of different PEF protocols are lower than in potato; ii/the thermal effects on relative resistance in carrot are lower than the corresponding effects observed in potatoes; iii/the thermal effects in carrot are contrary to potatoes nearly the same in all protocols. These results can be caused by lower internal porosity of potato comparing to carrot (Lozano et al., 1983). The internal pores representing by intracellular spaces after application of PEF are filled by conductive cellular liquid and the resulting resistance of the tissue then decreases. This effect is more pronounced in carrot with higher initial porosity. The increasing tem perature leads to increase of the intracellular spaces and, in this manner, to decrease of the tissue resistance; this effect is more pronounced in smaller spaces (i.e. in potatoes). At subcritical temperatures, higher values of relative capacitance were observed in the specimens processed by PEF compared to the basic ones (see also Fig. 4). This increase was also observed in potato (Bla hovec and Kou�rím, 2019) even though the levels of the increase were a little different and probably more regular than in carrot. The increase seems to be the result of the PEF initiated damage of cellular and vacuole membranes (Asavasanti et al., 2011, 2012). The “holes” formed in the membranes due to PEF treatment enable mixing of cellular electrolytes and their penetration into the intercellular space forming, in this way, additional “intercellular” capacitors of different capacity and also changing the capacity of the previous “intracellular” capacitors. This process seems to be peaked at the critical temperatures with standard thermally activated mechanisms. Thermal destructions of cellular membranes start to operate either in specimens without the previous PEF treatment (set b) or in specimens with PEF treatment (sets c, d, e). The differences among different PEF treatments lie in the form and level of cell damages prior to thermal tests (Fig. 4). All specimens in the su percritical state seem to be in a similar state of their cell stability (the thermal destruction of the cellular membranes in potatoes at tempera tures higher than �70 � C directly observed Imaizumi et al., 2015). The differences in the observed capacitance (see Figs. 1b and 4) of different specimens cannot usually be proved due to a high level of errors in the area of the supercritical states. Fig. 4 shows that increasing level of PEF treatment leads to increasing values of the carrot capacitance in the subcritical stage so that the initial difference between capacitance in subcritical and supercritical stages in the basic state is reduced after PEF treatment. The effect of PEF pulsing on the specimen capacitance can be given by a difference in the relative capacitance between processed and basic sets. This is shown also in Fig. 5 for the set e and the corresponding test with potato (Blahovec and Kou�rím, 2019). This figure demonstrates that the PEF stimulated increase of the capacitance in the subcritical stage is higher in carrot than in potato. This increase can be caused by the above mentioned PEF stimulated redistribution of the cellular liquids (see also Asavasanti et al., 2011, 2012). Fig. 5 also shows the different capaci tance behavior of carrot and potato in the critical and the supercritical temperatures. At supercritical temperatures, the obtained values of the difference have different sign. The quantitative evaluation of the dif ferences at the critical and supercritical temperatures is difficult to perform due to more complicated processes that accompany these stages in potato and that will be discussed in paragraph 4.4.
4. Discussion 4.1. Electric model of carrot The relative resistance can be interpreted as a measure of resistance of intracellular space; Figs. 1a and 3 a,b show that in the subcritical stage
4.2. DMA information Fig. 2. Loss tangent in the DMA test of the tested sets. The points represent mean values of the tested sets. The information on inset variation is given by bars representing standard errors, similarly as in Fig. 1
DMA data are represented in our paper by loss tangent (Fig. 2). The plots in Fig. 2 have similar character as the DETA data: the basic stage 4
J. Blahovec and P. Kou�rím
Journal of Food Engineering 278 (2020) 109936
Fig. 3. Parameters of linear approximation of data in Fig. 1a for carrot and the same data for potato variety Agria and Dali (Blahovec and Kou�rím, 2019). The plot is described by the following equation Re ¼ R40 þ S(t - 40), where R40 is relative Re at 40 � C (Fig. 3a), t temperature and S slope of the temperature plot (Fig. 3b). The data were obtained by liner regression analysis of the data in the temperature ranges 35–50 � C (set b) and 40–65 � C (other cases) with R2 higher than 0.9.
and supercritical one can be easily identified. The problem appears with identification of the exact border between the critical and supercritical stages due to high level of statistical dispersion and the lack of a char acteristic change in the loss tangent decrease that was usually observed at the specimens’ capacitance or resistance in Fig. 1. The loss tangent increases when the response to flow in the speci men’s motion decreases less than the corresponding response to defor mation and vice versa. In our case, the development of the loss tangent in the subcritical stage is given i/by PEF treatment that makes cell liq uids more movable outside the cells, ii/by the role of temperature in the composition, e.g. in viscosity of the cellular liquids and in the elastic response of the specimen. Fig. 2 shows that in the specimens without PEF treatment (set b) the liquid response decreases with increasing temperature less than the corresponding decrease of the elastic response. For the specimens with PEF pretreatment the opposite trend was observed but any differences among the sets c, d, e cannot be proved.
4.3. Disintegration parameter Our results were used to calculate the disintegration parameter Z for every experiment similarly as in the previous paper (Blahovec and Kou�rím, 2019): the calculation of Z is based on Eq. (1), where σu is the initial tissue conductivity, σ d the conductivity of the tissue with totally disintegrated cellular membranes and σ the actual conductivity of the tissue. We replaced the totally destroyed state by the data at which the highest Z value was observed for the set e. The conductivity σ was calculated as the reciprocal value of the specimen’s impedance (Blaho vec and Kou�rím, 2019): 1 R þX
σ ¼ pffiffiffi2ffiffiffiffiffiffiffiffiffiffiffi2ffiffi
(3)
The obtained results are given in Fig. 6. This figure well illustrates the division of the temperature scale into subcritical, critical and su percritical stages in which temperature increases of the parameter Z are 5
J. Blahovec and P. Kou�rím
Journal of Food Engineering 278 (2020) 109936
approximately linear. The approximation of data really gave constant temperature slopes for Z in the individual stages with R2 usually higher than 0.99. The obtained slopes are given in Table 1. Cross sections of the approximations in the individual stages can be used for estimation of the stages borders; the results are given in Table 2. The temperature ranges A, B, and C in Tables 1 and 2 can be understood respectively as a mark of subcritical, critical, and supercritical stages of the test. The subcritical stage in our case starts with some initial disintegration caused by the previous PEF treatment (detected by the initial values of Z at 30 � C). Table 1 shows that in the subcritical stage the disintegration parameter increases more steeply with increasing level of PEF pulsing (sets b→c→d→e): the slope in set e is about 5.5 times higher than in the basic set b. The temperature corresponding to the border between the subcritical and critical stages is a little higher than 70 � C and slightly moves to higher values with increasing level of PEF pulsing (Table 2). In critical and supercritical stages, the increasing level of PEF pulsing leads to opposite changes than those obtained at the subcritical temperatures, the slopes in Table 1 rather decrease with increasing level of PEF. This observation confirms the fact that at critical temperatures, and also above them, the main mechanisms of cell disintegration are controlled by temperature. Table 2 also shows that the supercritical stage starts at temperatures which are a little higher than 80 � C (at 81.8 � C in the basic state) but development of this temperature with changing level of PEF is not clear. In the supercritical stage, almost the same values of temper ature slope were obtained for all sets with PEF pulsing.
Fig. 4. Mean relative capacitances in stages A (subcritical, represented by mean values in range 55–65 � C) and CC (supercritical, represented by mean values in range 85–89 � C). Mean values at the peak maxima in critical stage B are added for comparison. The bars denote standard errors of all measured values in the temperature ranges.
4.4. Comparing with potato In this part, we would like to summarize the most important differ ences between carrot and potato. In the subcritical area there are mainly differences in the decrease of Re with increasing temperature. In carrot, Re decreased at low temperatures just after PEF treatment (this is well described by approximated value at 40 � C in Fig. 3a) and its decrease in the subcritical stage of heating, compared to potato, is relatively small and not dependent so much on the PEF protocol details (Fig. 3b, sets c, d). Similar changes of Re, that were typical in carrot after PEF applica tion, were reached in potato only in the set e. It means that PEF stim ulated destruction of the cells is more difficult in potato than in carrot and it needs in case of potato the repeated application of electric pulses with longer breaks between them (see also Asavasanti et al., 2011; 2012 for onion tissue). The Re for carrot in the basic state has a typical two step decrease at the end of the subcritical area; such a step was not observed in potato (Blahovec and Kou�rím, 2019) as well as in carrot after application of PEF (Fig. 1b). The capacitance in the subcritical stage increases due to aplication of PEF. This behavior was observed in both carrot and potato (Figs. 1b, 4 and 5). Fig. 5 shows that in a big amount of the subcritical stage (up to about 55 � C) the previous PEF application causes bigger increase of the carrot capacity than it is done under the same conditions in potato. In the final part of the subcritical stage this relation is changed: whereas the capacitance difference of carrot decreases with increasing temperature starting from about 45 � C, the potato’s capacitance difference at the same conditions increases up to the critical stage. These changes can be caused by redistributions of the cellular liquids after PEF applications (Asavasanti et al., 2011; 2012). In critical stage, the capacitance developments in carrot and potato start to differ substantially. In case of potato, the capacitance peak
Fig. 5. Relative difference between mean values of capacitances in set e (specimens after two pulses with interval of 1 s) and set b (the basic set) in carrot (this research) and in potato, variety Dali (Blahovec and Kou�rím, 2019). Relative difference is mathematically expressed by the following formula: (Ce – Cb)/Cb, where Cb and Ce are the mean values of capacities measured in the set b and e, respectively. The bars denote standard errors of the relative difference. Some bars were excluded due to potential overlapping.
Table 1 Slopes of linear parts in Fig. 6 Fig. 6. Plot of disintegration parameter Z versus temperature in the DMA/ DETA tests. The bars denote standard errors for illustration; there are used only for one fourth of the points to prevent overlaps.
6
Set
30–65 � C A
74–81 � C B
84–90 � CCC
b c d e
0.0012 0.0030 0.0061 0.0065
0.0742 0.0567 0.0457 0.0428
0.0348 0.0229 0.0219 0.0206
J. Blahovec and P. Kou�rím
Journal of Food Engineering 278 (2020) 109936
capacitance in the critical stage. In carrot, after application of PEF treatment, the capacitance in the supercritical stage is approximately of the same level as in the subcritical stage; in potato, the capacitance in supercritical stage is significantly lower than that in the subcritical stage. The DMA loss tangent reaches a flat peak in the critical stage without any clear finish. The capacitance decrease in supercritical stage, so different in carrot and potato, needs further research.
Table 2 Temperatures at intersections of liner parts from Table 1. Set
A-B
B-CC
b c d e
70.5 71.0 71.1 72.6
81.8 82.0 83.1 81.8
Acknowledgements
missed in some PEF protocols whereas in case of carrot, this peak was always observed (see Fig. 1b). Moreover, in the supercritical area the capacitance of carrot maintains approximately the same level as in the subcritical part or it is higher, well above 0.6 (Fig. 4) whereas in potato, the capacitance decreases significantly to values well below 0.3 (Bla hovec and Kou�rím, 2019). The differences between pulsed and basic products in supercritical stage (e.g. Fig. 1b) can be omitted in compar ison to this fundamental difference. In our opinion, the potato behavior is strongly influenced by starch (Ratnayake and Jackson, 2007, 2009) and especially by its gelatinization that needs, for this process, destruction of three types of membranes: cellular, vacuoles and starch grains. Whereas the first two membranes could be easily damaged by PEF, this process is more difficult in case of the starch grains due to the higher electrical resistivity of starch (very crude comparing of data for starch by Chaiwanichsiri et al., 2001 and data for water by Hayashi, 2004 indicate two order difference between their resistivity). It means that concentration of the same electric potential on a grain membrane needs about 100 times higher external electric field than on the cellular or vacuole membranes of the same dimensions. The starch membrane should be damaged thermally in connection with starch gelatinization in the critical stage and this is why that most changes detected by different methods in potato during its heating are concentrated in this stage (Imaizumi et al., 2015; Fuentes et al., 2014). Starch gelatinization is accompanied by changes of volume (Ratnayake and Jackson, 2007, 2009) in process that is termed as starch swelling (Li and Yeh, 2001). The starch swelling could lead to destruction of cellular walls and for mation of qualitatively new structures in thermally processed potatoes and in this manner it could lead to reduction of space sources of their tissue capacitance. Similar process like the starch swelling in potato is missing in carrot. In carrot the cell wall structure can be at least partly conserved even after the membrane destruction and the cell walls could form the basis of the higher capacitances observed at carrot in the su percritical stage.
The authors thank Dr. J. Vacek for the experimental material and Dr S. Yanniotis for valuable comments. The Research Intention MSM 6046070905 (Czech Republic) supported the paper. References Asavasanti, S., Ristenpart, W., Stroeve, P., Barrett, D.M., 2011. Permeabilization of plant tissues by monopolar pulsed electric fields: effect of frequency. J. Food Sci. 76, E98–E111. Asavasanti, S., Stroeve, P., Barrett, D.M., Jernstedt, J.A., Ristenpart, W., 2012. Enhanced electroporation in plant tissues via low frequency pulsed electric fields: influence of cytoplasmic streaming. Biotechnol. Prog. 28, 445–453. Barba, F., Parniakov, O., Pereira, S., Wiktor, A., Grimi, N., Boussetta, N., Saraiva, J., Raso, J., Martin-Belloso, O., Witrowa-Rajchert, D., Lebovka, N., Vorobiev, E., 2015. Current applications and new opportunities for the use of pulsed electric fields in food science and industry. Food Res. Int. 77, 773–798. Blahovec, J., Kou�rím, P., 2019. Pulsed electric stimulated changes in potatoes during their cooking: DMA and DETA analysis. J. Food Eng. 240, 183–190. Blahovec, J., Lahodov� a, M., Z� ame�cník, J., 2012. Potato DMA analysis in area of starch gelatinization. Food Bioprocess Technol. 5, 929–938. Blahovec, J., Kou�rím, P., Kindl, M., 2015. Low temperature carrot cooking supported by pulsed electric field - DMA and DETA thermal analysis. Food Bioprocess Technol. 8, 2027–2035. Blahovec, J., Lebovka, N., Vorobiev, E., 2017. Pulsed electric fields pretreatments for the cooking of foods. Food Eng. Rev. 9, 226–236. Chaiwanichsiri, S., Ohnishi, S., Suzuki, T., Takai, R., Miyawaki, O., 2001. Measurement of electrical conductivity, differential scanning calorimetry and viscosity of starch and flour suspensions during gelatinisation process. J. Sci. Food Agric. 81, 1586–1591. De Vito, F., Ferrari, G., Lebovka, N.I., Shynkaryk, N.V., Vorobiev, E., 2008. Pulse duration and efficiency of soft cellular tissue disintegration by pulsed electric fields. Food Bioprocess Technol. 1, 307–313. Fuentes, A., Vazquez, J.L., Perez-Gago, M.B., Vonasek, E., Nitin, N., Barrett, D.M., 2014. Application of non-destructive impedance spectroscopy to determination of the effect of temperature on potato microstructure and texture. J. Food Eng. 133, 16–22. Haines, P.J., 2002. Principles of Thermal Analysis and Calorimetry. The Royal Society of Chemistry, Cambridge. Hayashi, M., 2004. Temperature–electrical conductivity relation of water for environmental and geophysical data inversion. Environ. Monit. Assess. 96, 119–128. Imaizumi, T., Tanaka, F., Hamanaka, D., Sato, Y., Uchino, T., 2015. Effects of hot water treatment on electrical properties, cell membrane structure and texture of potato tubers. J. Food Eng. 162, 56–62. Jha, S.N., Matsuoka, T., 2004. Changes in electrical resistance of eggplant with gloss, weight and storage period. Biosyst. Eng. 87, 119–123. Jha, S.N., Narsaiah, K., Basediya, A.L., Sharma, R., Jaiswal, P., Kumar, R., Bhardwaj, R., 2011. Measurement techniques and application of electrical properties for nondestructive quality evaluation of foods—a review. J. Food Sci. Technol. 48, 387–411. Lebovka, N.I., Bazhal, M.I., Vorobiev, E., 2002. Estimation of characteristic damage time of food materials in pulsed-electric fields. J. Food Eng. 54, 337–346. Li, J.-Y., Yeh, A.-Y., 2001. Relationships between thermal, rheological, characteristics and swelling power for various starches. J. Food Eng. 50, 141–148. Lozano, J.E., Rotstein, E., Urbicain, M.J., 1983. Shrinkage, porosity and bulk density of foodstuffs at changing moisture contents. J. Food Sci. 48, 1497. Ratnayake, W.S., Jackson, D.S., 2007. A new insight into the gelatinization process of native starches. Carbohydr. Polym. 67, 511–529. Ratnayake, W.S., Jackson, D.S., 2009. Starch gelatinization. In: Advances in Food and Nutrition Research, vol. 55. Elsevier Inc, pp. 221–268. Tsong, T.Y., Su, Z.-D., 1999. Biological effects of electric shock and heat denaturation and oxidation of molecules, membranes, and cellular functions. Ann. N. Y. Acad. Sci. 888, 211–232. Vilgis, T.A., 2015. Soft matter food physics—the physics of food and cooking. Rep. Prog. Phys. 78, 124602. Vorobiev, E., Lebovka, N., 2010. Enhanced extraction from solid foods and biosuspensions by pulsed electric energy. Food Eng. Rev. 2, 95–108. Xu, C., Li, Y., 2014. Development of carrot parenchyma softening during heating detected in vivo by dynamic mechanical analysis. Food Contr. 44, 214–219.
4.5. Practical consequences to PEF cooking Pulsed electric field applied to cooking (Blahovec et al., 2017) per tains to prospective cooking technologies. Our results show that appli cation of PEF methods to cooking is rather complicated process and the successful cooking protocols have to respect the individual structure of the cooked products even if their structures seem to be simple. 5. Conclusions The DETA effects in PEF processed carrot are well described by a simple model: the parallel connection of a resistor Re and a capacitor ωC. In the basic state the thermal scale can be divided, similarly as with potato, into three stages: A (subcritical), B (critical), and C (supercriti cal), where stage B (70–80 � C) is characterized by the destruction of cell’s membranes and also by starch gelatinization in potato. Starch gelatinization probably needs disruption of cell membranes and direct contact of water from vacuoles with starch. PEF of middle intensity (in our case 500 V/cm) applied to carrot causes significant lowering of Re at the beginning and then further regular temperature dependent decrease in the whole subcritical temperature range. At the same conditions capacitance of the specimen increases. The PEF protocols that were applied to the carrot specimens, contrary to potato, did not eliminate 7
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GlossarySymbol Unit Description
Re ( ): Relative resistance of the model (parallel connection of a resistor and a capacitor) Rr Ω: Real component of impedance R0 Ω: Real component of impedance of a specimen before test at temperature 20 � C X ( ): Relative imaginary component of impedance X ¼ Xr/R0 Xr Ω: Imaginary component of impedance Z ( ): Disintegration parameter, see Eq. (1) σ ( ): Tissue conductivity (expressed in dimensionless form) σd ( ): Initial tissue conductivity in the untreated state (in dimensionless form) σu ( ): Conductivity of the tissue in disintegrated state (in dimensionless form) in our case it was the maximum conductivity observed at the set e ω s 1: Circular frequency of the alternated current used in DETA test ωC ( ): Relative capacitance of the model (parallel connection of a resistor and a capacitor)
A: Subcritical range B: Critical range b: Set of DMA/DETA test (basic test – without pulse application) C F: Specimen capacity CC: Supercritical range c: Set of DMA/DETA test (application of one pulse) DETA: Dielectric Thermal Analysis DMA: Dynamic Mechanical Analysis d: Set of DMA/DETA test (application of two pulses with a break of 100 ms) e: Set of DMA/DETA test (application of two pulses with a break of 1 s) PEF: Pulse Electric Field R ( ): Relative real component of impedance R ¼ Rr/R0
8
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Journal of Food Engineering journal homepage: http://www.elsevier.com/locate/jfoodeng
Effect of pulse electric field applied to carrot prior to its heating Ji�rí Blahovec *, Pavel Kou�rím Department of Physics, Faculty of Engineering, Czech University of Life Sciences, 16521, Prague 6, Suchdol, Czech Republic
A R T I C L E I N F O
A B S T R A C T
Keywords: Critical temperature Carrot Impedance Permittivity Loss tangent Cell membranes
The tissue of carrot (variety Jereda) was tested using dynamic mechanical analysis (DMA) combined with dielectric thermal analysis (DETA), in air of 90% humidity in temperature scans between 30 and 90 � C. Tem perature plots of loss tangent were determined. In DETA, based on alternated current of frequency 20 kHz, both components of impedance were continually determined. Pulsed electric field (PEF) was applied prior to the DMA and DETA tests. Four sets of experiments were performed, without PEF (b), with a single 10 ms long pulse given an alternated field of 500 V/cm (c), with two similar pulses at 0.1 s interval (d) and with two similar pulse at 1 s interval (e). The impedance was recalculated giving parameters of a single model represented by parallel connection of a resistor Re and a capacitor with capacitance ωC. Three stages of the range of temperature, namely subcritical (30–70 � C), critical (70–80 � C), and supercritical (80–90 � C) were considered. For set b, in subcritical stage, the parameter, Re decreased whereas capacitance was nearly constant. Both of these parameters decreased slightly in the supercritical stage but whereas Re is in supercritical stage much lower than in the subcritical stage, the capacitance is in supercritical stage rather higher than it is in subcritical stage. In critical stage, Re sharply decreased and the capacitance showed a sharp peak, both indicating collapse of the cellular membranes. Application of PEF led to little reduction of the capacitance peak and the reduction was more effective after the repeated pulse application (that is in sets d and e). These results at carrot were compared with the previous research at potato and it was shown that PEF effects in critical and supercritical stages significantly differ in the both plant tissues.
1. Introduction
conductivity σ d:
Textural properties of cellular products can be modified either by thermal processing and/or with the application of electric pulses. Whereas the stationary electric fields are usually used as a mean for nondestructive determination of food quality (e.g. Jha and Matsuoka, 2004; Jha et al., 2011) the Pulsed Electric Fields (PEF) are stated in reference to the application of short electric pulses of high potential (Blahovec et al., 2017). It is known that the application of external pulsed electric field can damage the integrity of cellular membranes (e. g.Tsong and Su, 1999; De Vito et al., 2008; Vorobiev and Lebovka, 2010). Previously we used a special Z parameter defined by Lebovka et al. (2002) and repeatedly used by De Vito et al. (2008), this parameter is usually termed as disintegration parameter and it serves to description of the degree of such a damage. The parameter depends on the actual state of the material’s electrical conductivity σ and varies between 0, for materials in the initial untreated state with conductivity σ u, and 1 for materials with totally disintegrated cellular membranes with
Z¼
σ σd
σu σu
(1)
In practical cases, σ d is estimated by direct measurement of the tissue conductivity after long and high electric pulse processing: De Vito et al. (2008) used 0.1 s long pulses with 1 kV/cm intensity for apples. The effect of the PEF application in a defined pulse protocol is usually detected by similar means as those that are usually used in thermal analysis (e.g. Asavasanti et al., 2011, 2012, Barba et al., 2015). Temperature and time are the main parameters of any cooking process and they has to serve also as the fundamental technological parameters in the cooking of most biological products. Unfortunately, the details of the cooking process are only partially known, especially in cellular products after application of PEF. There is need further infor mation on parallel processes occurring in living cells and tissues during heating (Vilgis, 2015). For this purpose, indirect methods have to be used. The methods of thermal analysis (Haines, 2002) are suitable, provided that the drying of specimens due to increasing temperature is
* Corresponding author. E-mail address: [email protected] (J. Blahovec). https://doi.org/10.1016/j.jfoodeng.2020.109936 Received 16 May 2019; Received in revised form 8 November 2019; Accepted 21 January 2020 Available online 24 January 2020 0260-8774/© 2020 Elsevier Ltd. All rights reserved.
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Journal of Food Engineering 278 (2020) 109936
prevented (Blahovec et al., 2012; Xu and Li, 2014). Among the different methods of thermal analysis known, mainly differential scanning calo rimetry (DSC) is dedicated to the detection of phase transformations in the analysed products. In our studies of PEF role, two methods of thermal analysis, namely dynamic mechanical analysis (DMA) and dielectric thermal analysis (DETA) (Haines, 2002) are suitable – newly applied in combination (Blahovec and Kou�rím, 2019). Both methods are based on detection of typical changes in characteristic complex quantities of the specimen in temperature scans. In the case of DMA this role is played by the modulus of elasticity and in DETA a similar role is played by some electrical quantities, such as the capacitance and/or resistance of a heated spec imen that is detected usually by application of a low alternated electric field (Haines, 2002). A combination of both DMA and DETA was used in our previous work (Blahovec and Kou�rím, 2019) to study changes caused by electric pulses in potato. It was found that application of PEF causes nontrivial changes in the tested specimen, in the entire tested temperature range. Even if some changes caused by PEF could be clas sified as “additive” to the changes caused by thermal effects, the time development of some of them can be classified as rather strange (Imai zumi et al., 2015; Vorobiev and Lebovka, 2010) and dependent on de tails of the pulse protocol. Potato cells, in comparison to carrot, contain lots of starch grains with large amount of starch which, after contact with water at tem peratures higher than approximately 60 � C, changes its structure in a process termed gelatinization (Ratnayake and Jackson, 2007, 2009). Gelatinization is a very complicated process requiring, even in the simplest cases, destruction of membranes on both starch grains and vacuoles. PEF treatment causes damage of some membranes (Tsong and Su, 1999) and could also be involved in the gelatinization process, with possible roles in PEF effects observed in heated potatoes (Blahovec and Kou�rím, 2019). In this paper, we use the same PEF protocols as were used in the previous paper for a similar study with potato in a DMA and DETA study of the thermal stimulated changes in carrots after application of PEF. The aim of the paper is to compare the application of PEF in potatoes and carrots and to detect the main differences in the responses to PEF application in these two products. The obtained information could be useful in potential “cooking” of vegetables by PEF.
component, R0 and imaginary component, X0. The DMA instrument was arranged so that the electric properties of the tested specimen could be continuously measured as a real conductor described by the complex impedance: An RLC meter (Hameg 8118 with effective voltage 1 V, frequency 20 kHz and 3 samplings per min) was used for this purpose. Each specimen was carefully mechanically secured at two points so that its longitudinal axis was perpendicular to the fixing jaws (Blahovec et al., 2015). The free length of the specimen between the jaws was 10.8 mm. The height of the fixed specimen was appr. 3.8 mm. One set of the specimens (5 repetitions) was used for a standard DMA/DETA test (Blahovec and Kou�rím, 2019) – this set was denoted as the basic, or ‘b’. One of the jaws was fixed and the other was moving up and down with constant amplitude of 0.5 mm and a frequency of 0.2 Hz in the dynamic cantilever test. The force necessary for the oscillation was recorded, being the basis for the determination of the complex modulus. Every experiment was started at 30 � C; the loss tangent (LT) values were calculated as the ratio of the loss and storage module (for details, see Blahovec et al., 2012) in every time point where both modules were determined. The parallel DETA test is based on RLC measurement by Hameg 8118 at the same conditions described above. The humidity of air in the test chamber (90%) was kept constant during the whole experiment. The control of the air humidity in the test chamber was based on direct humidity measurement by a special hy grometer and water vapour ejection into the chamber. The temperature scan proceeded up to 90 � C with a rate of 1 K/min. The results of the DETA test were analysed on temperature plots of the impedance components: real Rr and imaginary Xr that were measured on the tested specimens at the same time as the mechanical module. We preferred to present the results of our experiments as rela tive results: R ¼ Rr/R0, and X ¼ Xr/R0, where R0 is the initial value of the real component of the specimen at 20 � C. This recalculation helps to reduce potential dimensional and surface variations in the prepared specimens. Similarly, as in our previous paper (Blahovec and Kou�rím, 2019), we used the simple electrical model of the tested tissues as a parallel connection of a resistor Re and a capacitor C given by the following formulas: Re ¼
R2 þ X 2 X ; ωC ¼ 2 R þ X2 R
(2)
where ω is the circular frequency of the electric current.
2. Materials and methods
2.3. PEF application
2.1. Test material
The specimens included in the sets for further testing (5 repetitions in every case) were removed from the DMA instrument after measurement of the initial impedance (see above) and prepared for PEF loading. It was done between two parallel steel electrodes 2 cm in diameter, which were located in the central part of the specimen perpendicularly to their 5.1 � 35 mm2 sides. Pulse loadings were performed using a special equipment (Blahovec et al., 2015): the basic AC sinusoidal signal with a frequency of 20 kHz was modulated into a nearly rectangular form of 10 ms pulse with height corresponding to the field intensity of 500 V/cm into the tested spec imen. This field intensity is the same as the field intensity used in the previous research on potato tubers (Blahovec and Kou�rím, 2019) and the test sets were organized identically: b/the basic set without PEF loading, c/the set of specimens previously loaded by one pulse, d/the set of specimens previously loaded by two sequential pulses with an interval of 100 ms between them, e/the set loaded by two pulses with an interval of 1 s. Specimens included into sets c, d, and e were tested just after pulsing procedure in the same DMA/DETA combined test as the specimens of the basic test (b).
‘Jereda’ variety of carrots, cultivated in the university garden of the Czech University of Life Sciences, Prague-Suchdol using standard farm technology, was used for this study. Harvested roots (August 2018) were carefully selected and damage-free roots (their diameters in the upper parts being 3–3.5 cm) were obtained stored few weeks in a refrigerator (at 2 � C and appr. 100% relative humidity). Roots prepared for experi ment were kept at room temperature in a box with 100% humidity for testing the next day. 2.2. Specimens and basic DMA/DETA test Rectangular specimens (5.1 mm - width � 3.8 mm - thickness � 35 mm - length) with their long axis parallel to the direction of root growth were cut from the external parts of carrots’ roots. For the cutting with a knife a special cutting jigs, keeping constant the dimensions and the rectangular shape of the specimens, were used. Inclusion of central root parts of different physical properties in the specimens was carefully avoided. About eight specimens were prepared from a single root. The procedures described below were performed on every specimen. For each specimen, the initial impedance at a temperature 20 � C was measured after being fixed to the DMA tester (see further). This mea surement gave the initial values of the specimen impedance with a real 2
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Journal of Food Engineering 278 (2020) 109936
2.4. Data analysis
et al., 2015). The applied sampling (3 per min), the rate of heating (1 K/min) and 5 repetitions of the measurement led to the final statistics of at least 15 measurement for one point/K in the analysed temperature scale.
The results obtained in at least five replications were analysed using the standard laboratory software Origin®, OriginPro Ver. 7 (OriginLab, Northampton, MA, USA). The data obtained for a specimen set were unified into a group of data. This group of data was classified according to temperature and analysed so that the results of all measurements falling into interval of one centigrade increase were evaluated statisti cally as one state and the corresponding mean values and standard de viations were calculated by a special FORTRAN programme (Blahovec
3. Results The electric data are presented in Fig. 1 in the plots of the model components (Re and ωC, see Eq. (2)) versus temperature. Significant differences were observed mainly among the values obtained in
Fig. 1. The basic results: Parameters of the model represented by Eq. (2) in the DETA test for different PEF pre-treatments of the tested carrot specimens. The points represent mean values measured in the set; the bars denote standard errors for illustration (only for one fourth of the points to prevent overlaps). a/Relative model resistance, b/Relative model capacitance. 3
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Journal of Food Engineering 278 (2020) 109936
different specimens belonging to set b and the specimens of the other sets. With increasing temperature, the relative model resistance (Re) decreases. In the initial state (set b), there exist two temperature ranges (50–60 � C and 70–80 � C) where this decrease is sharper than at the other temperatures. PEF treatment causes more regular Re decrease in the whole temperature range including of the Re decrease at the starting temperature. Two pulses caused bigger initial decrease of Re than a single pulse only; the influence of different inter-pulse intervals on the Re values that was observed in potato tubers (Blahovec and Kou�rím, 2019) was not observed in carrot. For the relative capacitance in Fig. 1b, we can find that the previous application of PEF is followed by: 1/increase of the product capacitance at temperatures lower than the area of the capacitance peak at temperatures between 70 and 85 � C, and 2/decrease of the relative height of the above mentioned peak. The capacitance increase at 1/can be divided into two parts: 1a/the initial increase at initial temperature, and 1b/the increase during the specimen heating between 30 and 40 � C. The increase 1b/is comparable in all PEF pro tocols that were used. The area of temperatures where the capacity peak is observed (70–80 � C in our case) will be termed “critical” (Blahovec and Kou�rím, 2019); in this area also, the biggest reduction of resistance was observed (Fig. 1a). Using this definition we will denote tempera tures lower than critical as “subcritical” and temperatures higher than critical as “supercritical”. The DMA results for different specimen sets are shown in Fig. 2. Similarly as in Fig. 1 for capacitance, relatively stable values were observed for the measured parameter (loss tangent) in the basic set for the temperature range 60–70 � C. Also in the case of loss tangent, a peak of the results was observed at critical temperatures above 70 � C. In contrast to the capacitance, this peak is not well ended at high tem peratures. At temperatures below 70 � C, an initial decrease of loss tangent with increasing temperature is observed. This trend is different in the basic set where an increasing trend starts at approximately 45 � C. In the sets with PEF pulsing, the loss tangent decreases with increasing temperatures in the whole temperature range up to ~70 � C. The dif ferences of the peak parts among the different carrot sets in Fig. 2 are relatively small and difficult to detect due to a high level of variability. It seems that higher level of PEF pulsing (repeating of pulses and higher prolongation of breaks between pulses) tends to lower values of the peak.
Re of carrot decreases with increasing temperature almost linearly and only in the final part, close and above 60 � C, some nonlinearities are observed. An exclusion from this rule was observed only in the basic test (b) without any PEF application. Comparisons of these results with the previous research on potatoes (Blahovec and Kou�rím, 2019, see also Fig. 3 a,b) lead to the following conclusions: i/the relative resistance in carrot is more sensitive to PEF treatment – all protocols used were fol lowed by initial resistance decreases but the differences in the effects of different PEF protocols are lower than in potato; ii/the thermal effects on relative resistance in carrot are lower than the corresponding effects observed in potatoes; iii/the thermal effects in carrot are contrary to potatoes nearly the same in all protocols. These results can be caused by lower internal porosity of potato comparing to carrot (Lozano et al., 1983). The internal pores representing by intracellular spaces after application of PEF are filled by conductive cellular liquid and the resulting resistance of the tissue then decreases. This effect is more pronounced in carrot with higher initial porosity. The increasing tem perature leads to increase of the intracellular spaces and, in this manner, to decrease of the tissue resistance; this effect is more pronounced in smaller spaces (i.e. in potatoes). At subcritical temperatures, higher values of relative capacitance were observed in the specimens processed by PEF compared to the basic ones (see also Fig. 4). This increase was also observed in potato (Bla hovec and Kou�rím, 2019) even though the levels of the increase were a little different and probably more regular than in carrot. The increase seems to be the result of the PEF initiated damage of cellular and vacuole membranes (Asavasanti et al., 2011, 2012). The “holes” formed in the membranes due to PEF treatment enable mixing of cellular electrolytes and their penetration into the intercellular space forming, in this way, additional “intercellular” capacitors of different capacity and also changing the capacity of the previous “intracellular” capacitors. This process seems to be peaked at the critical temperatures with standard thermally activated mechanisms. Thermal destructions of cellular membranes start to operate either in specimens without the previous PEF treatment (set b) or in specimens with PEF treatment (sets c, d, e). The differences among different PEF treatments lie in the form and level of cell damages prior to thermal tests (Fig. 4). All specimens in the su percritical state seem to be in a similar state of their cell stability (the thermal destruction of the cellular membranes in potatoes at tempera tures higher than �70 � C directly observed Imaizumi et al., 2015). The differences in the observed capacitance (see Figs. 1b and 4) of different specimens cannot usually be proved due to a high level of errors in the area of the supercritical states. Fig. 4 shows that increasing level of PEF treatment leads to increasing values of the carrot capacitance in the subcritical stage so that the initial difference between capacitance in subcritical and supercritical stages in the basic state is reduced after PEF treatment. The effect of PEF pulsing on the specimen capacitance can be given by a difference in the relative capacitance between processed and basic sets. This is shown also in Fig. 5 for the set e and the corresponding test with potato (Blahovec and Kou�rím, 2019). This figure demonstrates that the PEF stimulated increase of the capacitance in the subcritical stage is higher in carrot than in potato. This increase can be caused by the above mentioned PEF stimulated redistribution of the cellular liquids (see also Asavasanti et al., 2011, 2012). Fig. 5 also shows the different capaci tance behavior of carrot and potato in the critical and the supercritical temperatures. At supercritical temperatures, the obtained values of the difference have different sign. The quantitative evaluation of the dif ferences at the critical and supercritical temperatures is difficult to perform due to more complicated processes that accompany these stages in potato and that will be discussed in paragraph 4.4.
4. Discussion 4.1. Electric model of carrot The relative resistance can be interpreted as a measure of resistance of intracellular space; Figs. 1a and 3 a,b show that in the subcritical stage
4.2. DMA information Fig. 2. Loss tangent in the DMA test of the tested sets. The points represent mean values of the tested sets. The information on inset variation is given by bars representing standard errors, similarly as in Fig. 1
DMA data are represented in our paper by loss tangent (Fig. 2). The plots in Fig. 2 have similar character as the DETA data: the basic stage 4
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Journal of Food Engineering 278 (2020) 109936
Fig. 3. Parameters of linear approximation of data in Fig. 1a for carrot and the same data for potato variety Agria and Dali (Blahovec and Kou�rím, 2019). The plot is described by the following equation Re ¼ R40 þ S(t - 40), where R40 is relative Re at 40 � C (Fig. 3a), t temperature and S slope of the temperature plot (Fig. 3b). The data were obtained by liner regression analysis of the data in the temperature ranges 35–50 � C (set b) and 40–65 � C (other cases) with R2 higher than 0.9.
and supercritical one can be easily identified. The problem appears with identification of the exact border between the critical and supercritical stages due to high level of statistical dispersion and the lack of a char acteristic change in the loss tangent decrease that was usually observed at the specimens’ capacitance or resistance in Fig. 1. The loss tangent increases when the response to flow in the speci men’s motion decreases less than the corresponding response to defor mation and vice versa. In our case, the development of the loss tangent in the subcritical stage is given i/by PEF treatment that makes cell liq uids more movable outside the cells, ii/by the role of temperature in the composition, e.g. in viscosity of the cellular liquids and in the elastic response of the specimen. Fig. 2 shows that in the specimens without PEF treatment (set b) the liquid response decreases with increasing temperature less than the corresponding decrease of the elastic response. For the specimens with PEF pretreatment the opposite trend was observed but any differences among the sets c, d, e cannot be proved.
4.3. Disintegration parameter Our results were used to calculate the disintegration parameter Z for every experiment similarly as in the previous paper (Blahovec and Kou�rím, 2019): the calculation of Z is based on Eq. (1), where σu is the initial tissue conductivity, σ d the conductivity of the tissue with totally disintegrated cellular membranes and σ the actual conductivity of the tissue. We replaced the totally destroyed state by the data at which the highest Z value was observed for the set e. The conductivity σ was calculated as the reciprocal value of the specimen’s impedance (Blaho vec and Kou�rím, 2019): 1 R þX
σ ¼ pffiffiffi2ffiffiffiffiffiffiffiffiffiffiffi2ffiffi
(3)
The obtained results are given in Fig. 6. This figure well illustrates the division of the temperature scale into subcritical, critical and su percritical stages in which temperature increases of the parameter Z are 5
J. Blahovec and P. Kou�rím
Journal of Food Engineering 278 (2020) 109936
approximately linear. The approximation of data really gave constant temperature slopes for Z in the individual stages with R2 usually higher than 0.99. The obtained slopes are given in Table 1. Cross sections of the approximations in the individual stages can be used for estimation of the stages borders; the results are given in Table 2. The temperature ranges A, B, and C in Tables 1 and 2 can be understood respectively as a mark of subcritical, critical, and supercritical stages of the test. The subcritical stage in our case starts with some initial disintegration caused by the previous PEF treatment (detected by the initial values of Z at 30 � C). Table 1 shows that in the subcritical stage the disintegration parameter increases more steeply with increasing level of PEF pulsing (sets b→c→d→e): the slope in set e is about 5.5 times higher than in the basic set b. The temperature corresponding to the border between the subcritical and critical stages is a little higher than 70 � C and slightly moves to higher values with increasing level of PEF pulsing (Table 2). In critical and supercritical stages, the increasing level of PEF pulsing leads to opposite changes than those obtained at the subcritical temperatures, the slopes in Table 1 rather decrease with increasing level of PEF. This observation confirms the fact that at critical temperatures, and also above them, the main mechanisms of cell disintegration are controlled by temperature. Table 2 also shows that the supercritical stage starts at temperatures which are a little higher than 80 � C (at 81.8 � C in the basic state) but development of this temperature with changing level of PEF is not clear. In the supercritical stage, almost the same values of temper ature slope were obtained for all sets with PEF pulsing.
Fig. 4. Mean relative capacitances in stages A (subcritical, represented by mean values in range 55–65 � C) and CC (supercritical, represented by mean values in range 85–89 � C). Mean values at the peak maxima in critical stage B are added for comparison. The bars denote standard errors of all measured values in the temperature ranges.
4.4. Comparing with potato In this part, we would like to summarize the most important differ ences between carrot and potato. In the subcritical area there are mainly differences in the decrease of Re with increasing temperature. In carrot, Re decreased at low temperatures just after PEF treatment (this is well described by approximated value at 40 � C in Fig. 3a) and its decrease in the subcritical stage of heating, compared to potato, is relatively small and not dependent so much on the PEF protocol details (Fig. 3b, sets c, d). Similar changes of Re, that were typical in carrot after PEF applica tion, were reached in potato only in the set e. It means that PEF stim ulated destruction of the cells is more difficult in potato than in carrot and it needs in case of potato the repeated application of electric pulses with longer breaks between them (see also Asavasanti et al., 2011; 2012 for onion tissue). The Re for carrot in the basic state has a typical two step decrease at the end of the subcritical area; such a step was not observed in potato (Blahovec and Kou�rím, 2019) as well as in carrot after application of PEF (Fig. 1b). The capacitance in the subcritical stage increases due to aplication of PEF. This behavior was observed in both carrot and potato (Figs. 1b, 4 and 5). Fig. 5 shows that in a big amount of the subcritical stage (up to about 55 � C) the previous PEF application causes bigger increase of the carrot capacity than it is done under the same conditions in potato. In the final part of the subcritical stage this relation is changed: whereas the capacitance difference of carrot decreases with increasing temperature starting from about 45 � C, the potato’s capacitance difference at the same conditions increases up to the critical stage. These changes can be caused by redistributions of the cellular liquids after PEF applications (Asavasanti et al., 2011; 2012). In critical stage, the capacitance developments in carrot and potato start to differ substantially. In case of potato, the capacitance peak
Fig. 5. Relative difference between mean values of capacitances in set e (specimens after two pulses with interval of 1 s) and set b (the basic set) in carrot (this research) and in potato, variety Dali (Blahovec and Kou�rím, 2019). Relative difference is mathematically expressed by the following formula: (Ce – Cb)/Cb, where Cb and Ce are the mean values of capacities measured in the set b and e, respectively. The bars denote standard errors of the relative difference. Some bars were excluded due to potential overlapping.
Table 1 Slopes of linear parts in Fig. 6 Fig. 6. Plot of disintegration parameter Z versus temperature in the DMA/ DETA tests. The bars denote standard errors for illustration; there are used only for one fourth of the points to prevent overlaps.
6
Set
30–65 � C A
74–81 � C B
84–90 � CCC
b c d e
0.0012 0.0030 0.0061 0.0065
0.0742 0.0567 0.0457 0.0428
0.0348 0.0229 0.0219 0.0206
J. Blahovec and P. Kou�rím
Journal of Food Engineering 278 (2020) 109936
capacitance in the critical stage. In carrot, after application of PEF treatment, the capacitance in the supercritical stage is approximately of the same level as in the subcritical stage; in potato, the capacitance in supercritical stage is significantly lower than that in the subcritical stage. The DMA loss tangent reaches a flat peak in the critical stage without any clear finish. The capacitance decrease in supercritical stage, so different in carrot and potato, needs further research.
Table 2 Temperatures at intersections of liner parts from Table 1. Set
A-B
B-CC
b c d e
70.5 71.0 71.1 72.6
81.8 82.0 83.1 81.8
Acknowledgements
missed in some PEF protocols whereas in case of carrot, this peak was always observed (see Fig. 1b). Moreover, in the supercritical area the capacitance of carrot maintains approximately the same level as in the subcritical part or it is higher, well above 0.6 (Fig. 4) whereas in potato, the capacitance decreases significantly to values well below 0.3 (Bla hovec and Kou�rím, 2019). The differences between pulsed and basic products in supercritical stage (e.g. Fig. 1b) can be omitted in compar ison to this fundamental difference. In our opinion, the potato behavior is strongly influenced by starch (Ratnayake and Jackson, 2007, 2009) and especially by its gelatinization that needs, for this process, destruction of three types of membranes: cellular, vacuoles and starch grains. Whereas the first two membranes could be easily damaged by PEF, this process is more difficult in case of the starch grains due to the higher electrical resistivity of starch (very crude comparing of data for starch by Chaiwanichsiri et al., 2001 and data for water by Hayashi, 2004 indicate two order difference between their resistivity). It means that concentration of the same electric potential on a grain membrane needs about 100 times higher external electric field than on the cellular or vacuole membranes of the same dimensions. The starch membrane should be damaged thermally in connection with starch gelatinization in the critical stage and this is why that most changes detected by different methods in potato during its heating are concentrated in this stage (Imaizumi et al., 2015; Fuentes et al., 2014). Starch gelatinization is accompanied by changes of volume (Ratnayake and Jackson, 2007, 2009) in process that is termed as starch swelling (Li and Yeh, 2001). The starch swelling could lead to destruction of cellular walls and for mation of qualitatively new structures in thermally processed potatoes and in this manner it could lead to reduction of space sources of their tissue capacitance. Similar process like the starch swelling in potato is missing in carrot. In carrot the cell wall structure can be at least partly conserved even after the membrane destruction and the cell walls could form the basis of the higher capacitances observed at carrot in the su percritical stage.
The authors thank Dr. J. Vacek for the experimental material and Dr S. Yanniotis for valuable comments. The Research Intention MSM 6046070905 (Czech Republic) supported the paper. References Asavasanti, S., Ristenpart, W., Stroeve, P., Barrett, D.M., 2011. Permeabilization of plant tissues by monopolar pulsed electric fields: effect of frequency. J. Food Sci. 76, E98–E111. Asavasanti, S., Stroeve, P., Barrett, D.M., Jernstedt, J.A., Ristenpart, W., 2012. Enhanced electroporation in plant tissues via low frequency pulsed electric fields: influence of cytoplasmic streaming. Biotechnol. Prog. 28, 445–453. Barba, F., Parniakov, O., Pereira, S., Wiktor, A., Grimi, N., Boussetta, N., Saraiva, J., Raso, J., Martin-Belloso, O., Witrowa-Rajchert, D., Lebovka, N., Vorobiev, E., 2015. Current applications and new opportunities for the use of pulsed electric fields in food science and industry. Food Res. Int. 77, 773–798. Blahovec, J., Kou�rím, P., 2019. Pulsed electric stimulated changes in potatoes during their cooking: DMA and DETA analysis. J. Food Eng. 240, 183–190. Blahovec, J., Lahodov� a, M., Z� ame�cník, J., 2012. Potato DMA analysis in area of starch gelatinization. Food Bioprocess Technol. 5, 929–938. Blahovec, J., Kou�rím, P., Kindl, M., 2015. Low temperature carrot cooking supported by pulsed electric field - DMA and DETA thermal analysis. Food Bioprocess Technol. 8, 2027–2035. Blahovec, J., Lebovka, N., Vorobiev, E., 2017. Pulsed electric fields pretreatments for the cooking of foods. Food Eng. Rev. 9, 226–236. Chaiwanichsiri, S., Ohnishi, S., Suzuki, T., Takai, R., Miyawaki, O., 2001. Measurement of electrical conductivity, differential scanning calorimetry and viscosity of starch and flour suspensions during gelatinisation process. J. Sci. Food Agric. 81, 1586–1591. De Vito, F., Ferrari, G., Lebovka, N.I., Shynkaryk, N.V., Vorobiev, E., 2008. Pulse duration and efficiency of soft cellular tissue disintegration by pulsed electric fields. Food Bioprocess Technol. 1, 307–313. Fuentes, A., Vazquez, J.L., Perez-Gago, M.B., Vonasek, E., Nitin, N., Barrett, D.M., 2014. Application of non-destructive impedance spectroscopy to determination of the effect of temperature on potato microstructure and texture. J. Food Eng. 133, 16–22. Haines, P.J., 2002. Principles of Thermal Analysis and Calorimetry. The Royal Society of Chemistry, Cambridge. Hayashi, M., 2004. Temperature–electrical conductivity relation of water for environmental and geophysical data inversion. Environ. Monit. Assess. 96, 119–128. Imaizumi, T., Tanaka, F., Hamanaka, D., Sato, Y., Uchino, T., 2015. Effects of hot water treatment on electrical properties, cell membrane structure and texture of potato tubers. J. Food Eng. 162, 56–62. Jha, S.N., Matsuoka, T., 2004. Changes in electrical resistance of eggplant with gloss, weight and storage period. Biosyst. Eng. 87, 119–123. Jha, S.N., Narsaiah, K., Basediya, A.L., Sharma, R., Jaiswal, P., Kumar, R., Bhardwaj, R., 2011. Measurement techniques and application of electrical properties for nondestructive quality evaluation of foods—a review. J. Food Sci. Technol. 48, 387–411. Lebovka, N.I., Bazhal, M.I., Vorobiev, E., 2002. Estimation of characteristic damage time of food materials in pulsed-electric fields. J. Food Eng. 54, 337–346. Li, J.-Y., Yeh, A.-Y., 2001. Relationships between thermal, rheological, characteristics and swelling power for various starches. J. Food Eng. 50, 141–148. Lozano, J.E., Rotstein, E., Urbicain, M.J., 1983. Shrinkage, porosity and bulk density of foodstuffs at changing moisture contents. J. Food Sci. 48, 1497. Ratnayake, W.S., Jackson, D.S., 2007. A new insight into the gelatinization process of native starches. Carbohydr. Polym. 67, 511–529. Ratnayake, W.S., Jackson, D.S., 2009. Starch gelatinization. In: Advances in Food and Nutrition Research, vol. 55. Elsevier Inc, pp. 221–268. Tsong, T.Y., Su, Z.-D., 1999. Biological effects of electric shock and heat denaturation and oxidation of molecules, membranes, and cellular functions. Ann. N. Y. Acad. Sci. 888, 211–232. Vilgis, T.A., 2015. Soft matter food physics—the physics of food and cooking. Rep. Prog. Phys. 78, 124602. Vorobiev, E., Lebovka, N., 2010. Enhanced extraction from solid foods and biosuspensions by pulsed electric energy. Food Eng. Rev. 2, 95–108. Xu, C., Li, Y., 2014. Development of carrot parenchyma softening during heating detected in vivo by dynamic mechanical analysis. Food Contr. 44, 214–219.
4.5. Practical consequences to PEF cooking Pulsed electric field applied to cooking (Blahovec et al., 2017) per tains to prospective cooking technologies. Our results show that appli cation of PEF methods to cooking is rather complicated process and the successful cooking protocols have to respect the individual structure of the cooked products even if their structures seem to be simple. 5. Conclusions The DETA effects in PEF processed carrot are well described by a simple model: the parallel connection of a resistor Re and a capacitor ωC. In the basic state the thermal scale can be divided, similarly as with potato, into three stages: A (subcritical), B (critical), and C (supercriti cal), where stage B (70–80 � C) is characterized by the destruction of cell’s membranes and also by starch gelatinization in potato. Starch gelatinization probably needs disruption of cell membranes and direct contact of water from vacuoles with starch. PEF of middle intensity (in our case 500 V/cm) applied to carrot causes significant lowering of Re at the beginning and then further regular temperature dependent decrease in the whole subcritical temperature range. At the same conditions capacitance of the specimen increases. The PEF protocols that were applied to the carrot specimens, contrary to potato, did not eliminate 7
J. Blahovec and P. Kou�rím
Journal of Food Engineering 278 (2020) 109936
GlossarySymbol Unit Description
Re ( ): Relative resistance of the model (parallel connection of a resistor and a capacitor) Rr Ω: Real component of impedance R0 Ω: Real component of impedance of a specimen before test at temperature 20 � C X ( ): Relative imaginary component of impedance X ¼ Xr/R0 Xr Ω: Imaginary component of impedance Z ( ): Disintegration parameter, see Eq. (1) σ ( ): Tissue conductivity (expressed in dimensionless form) σd ( ): Initial tissue conductivity in the untreated state (in dimensionless form) σu ( ): Conductivity of the tissue in disintegrated state (in dimensionless form) in our case it was the maximum conductivity observed at the set e ω s 1: Circular frequency of the alternated current used in DETA test ωC ( ): Relative capacitance of the model (parallel connection of a resistor and a capacitor)
A: Subcritical range B: Critical range b: Set of DMA/DETA test (basic test – without pulse application) C F: Specimen capacity CC: Supercritical range c: Set of DMA/DETA test (application of one pulse) DETA: Dielectric Thermal Analysis DMA: Dynamic Mechanical Analysis d: Set of DMA/DETA test (application of two pulses with a break of 100 ms) e: Set of DMA/DETA test (application of two pulses with a break of 1 s) PEF: Pulse Electric Field R ( ): Relative real component of impedance R ¼ Rr/R0
8