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Effects of mild heating treatment on texture degradation and peroxidase inactivation of carrot under pasteurization conditions
Journal of Food Engineering 257 (2019) 19–25
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Effects of mild heating treatment on texture degradation and peroxidase inactivation of carrot under pasteurization conditions
T
Teppei Imaizumia,∗, Fumihiko Tanakab, Toshitaka Uchinob a b
Faculty of Applied Biological Sciences, Gifu University, 1-1 Yanagido, Gifu, 501-1193, Japan Faculty of Agriculture, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Mild heating Peroxidase Food processing Pectin
Mild heating is an attractive method to improve the texture of vegetables. In this study, we applied mild heating treatment with various temperatures and durations to carrot roots, and we evaluated the effects on texture degradation and peroxidase inactivation during subsequent heating treatment to mimic pasteurization or blanching. Mild heating, especially at 60 °C for 60 min, inhibited softening during subsequent heating treatment. Activation energy required for the softening was 153.3 and 185.2 kJ mol−1 for untreated and mild-heated (60 °C, 60 min) samples, respectively. To evaluate tissue condition, we applied electrical impedance analysis. During mild heating, the cell membranes were damaged and electrolyte flow was activated. We assumed that mild heating increased calcium levels in the cell wall, leading to the crosslinking of pectin, thus improving the cell structure of carrot samples. Although mild heating at 60 °C did not inactivate all of the peroxidase, mild heating at 70 °C inactivated the heat-resistant fraction of peroxidases. The results of this study can contribute to designing processes for improving the texture of processed vegetables.
1. Introduction Carrots are widely consumed around the world and are important in providing nutrition and health benefits. They are rich in β-carotene and a good source of carbohydrates and minerals such as calcium, phosphorus, iron, and magnesium (Sharma et al., 2012). In recent years, the consumption of carrots and related products has increased steadily due to its recognition as a rich source of β-carotene (Hiranvarachat et al., 2011). Carrots are used in various processed foods such as frozen vegetables and retort pouch foods. In Japan, ready-to-eat food products containing carrot as an ingredient (e.g. salad, soup, Nimono [Japanese simmered dish], and bento lunch boxes) are popular and easy to buy in supermarkets and convenience stores. Fruits and vegetables often undergo thermal treatment such as pasteurization or blanching. For example, in cooking at schools, nursing homes, hospitals, and restaurants, it is recommended to conduct sufficient heating under thermal pasteurization conditions for food safety. In addition, blanching is conducted at over 80 °C to inactivate enzymes that cause the deterioration of vegetable products during subsequent processes and storage. However, high-temperature heating usually results in the degradation of tissue structure, leading to undesirable food texture (Abu-Ghannam and Crowley, 2006; Peng et al., 2014). Vegetables are often required to keep their structure and texture even if ∗
heated for a long time. Because texture is an important quality attribute considered by consumers, the inhibition or limitation of such degradation is important to improve food value. To inhibit or block tissue degradation, mild heating is generally carried out within the range of 50–80 °C for several types of vegetable, including potato (Canet et al., 2005), sweet potato (Chhe et al., 2017; Walter et al., 2003), pumpkin (de Souza Silva et al., 2011), and carrot (Lee et al., 1979). At mild temperatures, pectin methyl esterase (PME) induces the demethylation of pectin molecules. Pectin molecules exist as binding substances between cells. Demethylated pectin molecules are not easily decomposed by β-elimination and promote crosslinking via divalent ions such as Ca2+, thus contributing to strengthening tissue structure (Imaizumi et al., 2017a). Tissue degradation is inhibited by mild heating of vegetables during subsequent processing and cooking. In this study, we focused particularly on the optimum conditions for mild heating for the inhibition of softening during high-temperature heating, as occurs in pasteurization or blanching. These treatments are often conducted for vegetables at 80–95 °C (Imaizumi et al., 2017a; Peng et al., 2017, 2014). The purpose of our research was to clarify the influence of mild temperature and time on texture degradation during pasteurization (80–95 °C), and also to consider changes in structure presumed from electrical properties. The kinetics of texture change enabled us to understand the inhibiting effect of mild heating on
Corresponding author. E-mail address: [email protected] (T. Imaizumi).
https://doi.org/10.1016/j.jfoodeng.2019.03.024 Received 3 November 2018; Received in revised form 29 March 2019; Accepted 30 March 2019 Available online 02 April 2019 0260-8774/ © 2019 Published by Elsevier Ltd.
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Fig. 1. Outline of research procedures.
2.3. Electrical parameters
softening during subsequent high-temperature heating. Additionally, electrical properties obtained by impedance analysis are useful for evaluating tissue conditions such as cell membrane damage and porosity (Blahovec and Kouřím, 2019; Imaizumi et al., 2017b, 2018; Watanabe et al., 2017). Based on these aspects, we evaluated the effective mild heating conditions for the treatment of carrots. Furthermore, we assessed blanching processes such as high-temperature heating and evaluated their contribution to the determination of processing time. We focused on peroxidase (POD), which is often used as an indicator enzyme of blanching completion (Imaizumi et al., 2017c). The results obtained from our study can contribute to the design and development of vegetable processing steps in the food industry.
Electrical evaluation was conducted to determine tissue condition, just after mild heating treatment, based on Imaizumi et al. (2015). Two needle electrodes (diameter, 0.25 mm), connected to an LCR tester (3532-50, HIOKI), were inserted 9 mm deep from the top surface of the sample. Resistance and reactance of the sample were measured at 200 points over the frequency range from 50 Hz to 5 MHz, in which β-dispersion due to cell structure appears. The measurements were conducted at a measuring voltage of 1 V, and automatically recorded using a computer for analysis. Relationships between impedance and the values are expressed as shown below:
Z = R + jX 2. Materials and methods
(1)
where Z is impedance (Ω), R is resistance (Ω), X is reactance (Ω), and j is an imaginary unit. Equivalent circuit analysis was conducted by fitting the CPE model (Ando et al., 2014) described as equation (2) to the measured values:
2.1. Sample preparation and overview of experiment Carrot roots (Daucus carota subsp. sativus) were used in this study. An outline of the research procedure is shown in Fig. 1. Carrots were cut into cylinders (15 mm diameter and 10 mm height) using a cork borer and a knife. Each cylindrical sample was immersed in 200 mL of distilled water, whose temperature was maintained at 50 °C, 60 °C, 70 °C, or 80 °C for 20, 40, or 60 min as a mild heating treatment. Using distilled water for immersion, the chemical influences of dissolved components in the heating medium were removed. Peng et al. (2014) showed that the osmotic pressure of the immersion liquid did not contribute to the texture change behavior of carrot during high-temperature heating. After treatment, the sample was immediately cooled in iced water for 2 min to remove excess heat inside. Then, the sample was kept at room temperature for over 30 min wrapped in film to reach a constant temperature. Each sample that had undergone mild heating was then subjected to high-temperature heating in 500 mL of distilled water at 95 °C for up to 60 min and cooled again. Kinetic analysis was conducted on texture degradation during high-temperature heating. Samples that had been treated with mild heating at 60 °C for 60 min and a fresh sample (as control) were processed by high-temperature heating at 80 °C, 85 °C, 90 °C, and 95 °C, and the temperature dependence of the kinetics were investigated. The temperature setting was determined in consideration of past reports (Peng et al., 2014; Vu et al., 2004) POD activity of the samples was measured for every sample as described below, and enzyme inactivation during high-temperature heating at 95 °C was investigated for samples treated with mild heating at 60 °C for 60 min and controls.
{
}
( )
π R e ⎡1 + ω pT (2Ri + R e)cos 2 p + ω pTRi (R e + Ri ) ⎤ ⎣ ⎦ Z= π {ω pT (R e + Ri )} 2 + 2ω pT (R e + Ri )cos 2 p + 1
( )
−j
ω pTRe2 sin
( p) π 2
{ω pT (R e + Ri )} 2 + 2ω pT (R e + Ri )cos
( p) + 1 π 2
−1
(2)
), p is a where ω is angular frequency (rad s ), T is a constant (F s CPE exponent (0 ≤ p ≤ 1), Re is extracellular resistance (Ω), and Ri is intracellular resistance (Ω). The equivalent circuit parameters T, p, Re, and Ri were determined using complex nonlinear least squares curve fitting (Macdonald, 1992). Additionally, the cell membrane capacitance Cm (F) was defined using the following equation (Ando et al., 2014): 1
Cm = T p (R e + Ri )
1−p p
(p−1)
(3)
2.4. Peroxidase assay POD activity was measured based on the methods described by Agüero et al. (2008) and Neves et al. (2012), with a minor modification. Briefly, the carrot sample was homogenized with 10 mL of distilled water. The homogenized liquids were centrifuged at 1670×g for 15 min, and the supernatants were filtered. The substrate solution was prepared by mixing guaiacol, 30% H2O2, and 0.1 M potassium phosphate buffer (pH 6.5) in a volumetric ratio of 1:1:998. The sample solution (0.12 mL) and the substrate solution (3.48 mL) were mixed and absorbance at 470 nm was measured using a spectrophotometer (V-530, JASCO). Enzymatic activity was determined from the slope of the initial linear portion in the absorbance, and divided by the initial activity, which was obtained from the fresh sample. Measurements were replicated 3 times.
2.2. Firmness Firmness was measured in accordance with the method described by Ando et al. (2016). In this measurement, we used a creep meter (RE3305, YAMADEN), equipped with a wedge-shape plunger (P-29, YAMADEN), and the sample stage was moved at 1 mm s−1. The probe was inserted into the center of the upper surface of a carrot cylinder (15 mm diameter and 10 mm height). Peak force at yield point on the forcedisplacement curve was determined as firmness (N). The measurements were replicated 6 times.
3. Results and discussion 3.1. Kinetic evaluation of effect of mild heating on firmness Fig. 2 shows the changes in firmness of each sample after mild heating during high-temperature heating at 95 °C. Although the 20
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Fig. 2. Changes in firmness of carrot samples after mild heating during subsequent high temperature heating at 95 °C. Mild heating treatments were performed at 50 °C (A), 60 °C (B), 70 °C (C), and 80 °C (D) for 20, 40, or 60 min. Bar: SE, Curves: First-order kinetic model.
firmness decreased with high-temperature heating time, mild heating inhibited this progression. To evaluate the inhibiting effect of mild heating on softening, kinetic analysis was conducted. Here, the firstorder kinetic equation is described below:
dC = −kC dt
Table 1 Rate constants of the softening process by high temperature heating at 95 °C obtained from samples after mild heating. (Value ± 95% confidence interval).
(4)
where C is a quality index or concentration of a chemical compound, t is the reaction time, and k is the rate constant. The texture property is presented as a fraction of texture change C, which provides an accurate way to determine the extent of quality change at any time t and can be expressed as the equation below (Peng et al., 2014):
C=
Ft − Fe F0 − Fe
Mild heating condition
k (min−1)
Fe (N)
Control 50 °C
0.158 0.151 0.085 0.072 0.061 0.031 0.029 0.039 0.057 0.049 0.075 0.065 0.056
3.50 4.19 2.81 5.01 4.29 0.46 2.88 1.53 6.31 6.92 3.14 3.89 3.54
60 °C
70 °C
80 °C
(5)
where, F is firmness (N), subscript t, 0, and e indicate value at time t (min) and 0 and equilibrium value, respectively. In this study, the kinetic equation was fitted to changes in firmness with high-temperature heating time, and the k value (min−1) was determined. The model fitted well to the measured values, as shown in Fig. 2 (curves). Fitting quality was also evaluated by r2 (see Table 1) and the value was found to be high enough in every condition. The obtained k and Fe values are also described in Table 1. Samples after mild heating treatment at 50 °C for 20 min softened similarly to control samples, suggesting that this condition did not have any delaying effect on softening. When the mild heating treatment temperature or time was increased, the k values for firmness significantly decreased to less than half the values of the control sample. However, the initial firmness after mild heating treatment at 80 °C was lower than that of control. This is likely because softening could have progressed during mild heating at 80 °C. Thus, we propose that mild heating should be conducted at less than 80 °C, and
20 min 40 min 60 min 20 min 40 min 60 min 20 min 40 min 60 min 20 min 40 min 60 min
± ± ± ± ± ± ± ± ± ± ± ± ±
0.042 0.038 0.049 0.010 0.033 0.024 0.020 0.008 0.041 0.016 0.031 0.034 0.046
± ± ± ± ± ± ± ± ± ± ± ± ±
r2 2.12 1.82 5.38 1.33 6.48 14.46 9.74 3.41 9.16 10.46 3.08 3.25 3.40
0.991 0.991 0.958 0.998 0.971 0.973 0.953 0.981 0.952 0.948 0.980 0.971 0.940
based on the k value of firmness, 60 °C and 70 °C seem to be better conditions. Although the optimum temperature of carrot PME activity was reported to be around 50 °C (Duvetter et al., 2009), the result of our experiment did not support this fact. As mentioned above, in order to crosslink the pectin molecule, not only the optimum temperature but also a sufficient calcium concentration is necessary. Imaizumi et al. (2015) indicated that the cell membranes of potato tubers were destroyed by heating at over 60 °C. It is known that calcium is accumulated in cells, especially cell vacuoles, and destruction of membranes by heating leads to the release of calcium outside of the cell membrane. In our treatment at 60 °C and 70 °C, intracellular calcium could have been sufficiently supplied to the cell wall region, leading to increased crosslinking of pectin and higher structural stability. Additionally, 21
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Fig. 3. Changes in firmness of carrot samples without pretreatment (A) and after mild heating at 60 °C for 60 min (B) during subsequent high temperature heating at 80 °C, 85 °C, 90 °C, and 95 °C. Bar: SE, Curves: First-order kinetic model.
Imaizumi et al. (2017c) showed that temperature rise of vegetable samples during high-temperature heating was slow because of its low thermal conductivity. Therefore, demethylation of pectin might progress adequately at these temperatures. Subsequently, cell membrane damage may be inhibited or may occur at slower rates in samples that underwent mild heating. Because mild heating at 60 °C for 60 min was significantly effective, the temperature dependence of the softening process was investigated. Samples that had undergone mild heating (60 °C, 60 min) and control samples were heated in hot water at 80 °C, 85 °C, 90 °C, and 95 °C had their firmness measured. As shown in Fig. 3, the equation fitted closely to the measured firmness (r2 = 0.950–0.994). The obtained Fe values of 80 °C, 85 °C, 90 °C, and 95 °C were 6.96, 4.01, 3.46, and 2.50 N for control samples and 0.00, 8.38, 7.75, and 3.61 N for samples after mild heating. Overall, samples after mild heating had higher Fe values. At 80 °C, because the softening of the sample was very gentle, an accurate equilibrium value could not be obtained. Thus, the softening inhibiting effect was discussed using rate constants. The obtained rate constants were plotted against the reciprocal of the immersion temperature (Fig. 4), and the Arrhenius equation (6) was fitted to the plots.
E k = A⋅exp ⎛− a ⎞ ⎝ RT ⎠
Table 2 Activation energy and frequency factor of the softening process by high temperature heating at 95 °C obtained from control samples and samples after mild heating (60 °C, 60 min).
Control Mild heating 60 ˚C-60 min
Ea (kJ mol−1)
A (min−1)
r2
153.3 185.2
8.7 × 1020 7.3 × 1024
0.989 0.948
8.7 × 1020 min−1 for controls and 185.2 kJ mol−1 and 7.3 × 1024 min−1 for samples after mild heating, respectively. Okazaki et al. (1997) suggested a close relationship between softening of Japanese radish and the decomposition of pectin, because they had a similar activation energy of 146 kJ mol−1 and 144 kJ mol−1, respectively. Although the activation energy of the control was close to this value, that of the sample that had undergone mild heating was higher. Because pectin molecules in carrot tissue were demethylated by mild heating, the probability of occurrence of β-elimination may be decreased. In addition, the frequency factor of the sample after mild heating was higher than that of the control sample. Some previous reports have shown that water soluble pectin is converted to pectin with stronger bonds by mild heating (Christiaens et al., 2011; Sila et al., 2006). One reason for this conversion may be due to cross-linkages of pectin molecules via divalent ions. Imaizumi et al. (2017a) observed small particles of water-soluble pectin using atomic force microscopy, and suggested that water-insoluble pectin, which had a chain-like shape or network, contributed to the texture more than water-soluble pectin. Thus, we hypothesized that the increase in the frequency factor was caused by an increase in the levels of water-insoluble pectin.
(6)
where, A is the frequency factor (min−1), Ea is the activation energy (J mol−1), R is the universal gas constant (J K−1 mol−1), and T is the absolute temperature (K). The A and Ea values were determined using a linear least-squares method (r2 = 0.948–0.989) (see Table 2). The activation energy and frequency factor were 153.3 kJ mol−1 and
3.2. Evaluation of tissue condition by using electrical parameters Imaizumi et al. (2015) suggested that intracellular resistance Ri increases due to electrolyte outflow from the cells. They suggested that the loss of ionic regulating capacity was caused by the denaturation of membrane proteins and a decline in electrolyte retention capacity due to cell membrane damage, resulting in electrolyte outflow. If the value is high, the opportunity to supply intracellular electrolyte to pectin molecules in the cell wall region will be high. Fig. 5 shows the electrical properties obtained from each sample. Treatment at over 60 °C for 60 min showed high Ri values, thus electrolyte outflow seemed to be active. For extracellular resistance Re, no significant difference was observed in samples treated at over 60 °C. At 50 °C, the values decreased with time. It has been shown that a change in Re during hot water heating was strongly related to the discharge of air in the tissue (Imaizumi et al., 2017a). Briefly, they showed that extracellular resistance declined with decreasing porosity of sweet potato. Thus, pores
Fig. 4. Arrhenius plots of the rate constant of softening by high temperature heating at 80 °C, 85 °C, 90 °C, and 95 °C. Control: values obtained from samples without preheating, MH: values obtained from samples treated at 60 °C for 60 min, Dashed line: Arrhenius equation. 22
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Fig. 5. Equivalent circuit parameters, intracellular resistance Ri, extracellular resistance Re, and cellular membrane capacitance Cm of carrot samples after mild heating. Numbers on the axis labels represent mild heating conditions (temperature-time). Bar: SE, Different letters indicate significant differences (p < 0.05).
3.3. Effect of mild heating on enzymatic activity
in the carrot sample were lost slowly at 50 °C and within 20 min at higher temperatures. Imaizumi et al. (2015) demonstrated that cell membrane capacitance Cm was decreased by cell damage. Additionally, they assumed that the cell membrane has capacitance because the lipid bilayer behaves as a capacitor. In our results, the value increased for the samples treated at 50 °C and 60 °C for 20 min. No increase in Cm value was observed during high-temperature heating. When the area of the lipid bilayer increases or the thickness reduces, the capacitance will increase. In this experiment, the sample was immersed in distilled water with low osmotic pressure, which would allow moisture to be taken into the cell. Therefore, we believe that the lipid bilayer became thinner and broader because turgor pressure (due to moisture uptake) increased and pressed against the cell membrane. After that, the value of Cm decreased likely due to cell membrane damage. In summary, membrane capacitance increased up to around 2 pF like in conditions of “50–20” with an increase in turgor pressure, and it can be considered that the cell membrane has been considerably damaged if it subsequently falls below 1 pF, as in conditions of “70–60”, “80–40”, and “80–60”.
Enzymatic activity is important to determine blanching time for vegetables. Inactivating enzymes during mild heating as a pretreatment will influence subsequent processes. POD is often used as an index of blanching, because it is one of the most heat stable enzymes. Fig. 6 shows the residual POD activity of each sample after mild heating. Although POD inactivation proceeded at 50 °C and 60 °C, further inactivation stopped after residual POD activity decreased to 20%. On the other hand, the residual activity decreased to less than 5% at 70 °C and 80 °C. Neves et al. reported that POD has heat-labile and heat-resistant fractions (2012). In our results, we propose that the former fraction was inactivated at 60 °C, and inactivation of the latter fraction required higher temperatures of 70–80 °C. Therefore, for actual frozen vegetables, mild heating at 60 °C should be combined with a high-temperature treatment. Fig. 7 shows the changes in residual POD activity after mild heating (60 °C, 60 min) and control samples during subsequent high-temperature heating at 95 °C. Both samples after mild 23
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Fig. 6. Residual peroxidase (POD) activity of carrot samples after mild heating. Numbers on the axis labels represent mild heating conditions (temperature-time). Bar: SE, Different letters indicate significant differences (p < 0.05).
significantly contribute to designing processes for improving the texture of vegetables. Acknowledgement This work was supported by Japan Society for the Promotion of Science(JSPS) KAKENHI Grant Number 17H06727. References Abu-Ghannam, N., Crowley, H., 2006. The effect of low temperature blanching on the texture of whole processed new potatoes. J. Food Eng. 74, 335–344. https://doi.org/ 10.1016/j.jfoodeng.2005.03.025. Agüero, M.V., Ansorena, M.R., Roura, S.I., del Valle, C.E., 2008. Thermal inactivation of peroxidase during blanching of butternut squash. LWT - Food Sc. Technol. 41 (3), 401–407. https://doi.org/10.1016/j.lwt.2007.03.029. Ando, Y., Maeda, Y., Mizutani, K., Wakatsuki, N., Hagiwara, S., Nabetani, H., 2016. Effect of air-dehydration pretreatment before freezing on the electrical impedance characteristics and texture of carrots. J. Food Eng. 169, 114–121. https://doi.org/10. 1016/j.jfoodeng.2015.08.026. Ando, Y., Mizutani, K., Wakatsuki, N., 2014. Electrical impedance analysis of potato tissues during drying. J. Food Eng. 121, 24–31. https://doi.org/10.1016/j.jfoodeng. 2013.08.008. Blahovec, J., Kouřím, P., 2019. Pulsed electric stimulated changes in potatoes during their cooking: DMA and DETA analysis. J. Food Eng. 240, 183–190. https://doi.org/10. 1016/J.JFOODENG.2018.07.025. Canet, W., Alvarez, M.D., Fernández, C., 2005. Optimization of low-temperature blanching for retention of potato firmness: effect of previous storage time on compression properties. Eur. Food Res. Technol. 221, 423–433. https://doi.org/10.1007/ s00217-005-1195-3. Chhe, C., Imaizumi, T., Tanaka, F., Uchino, T., 2017. Effects of hot-water blanching on the biological and physicochemical properties of sweet potato slices. Eng. Agric. Environ. Food. https://doi.org/10.1016/j.eaef.2017.10.002. Christiaens, S., Van Buggenhout, S., Houben, K., Fraeye, I., Van Loey, A.M., Hendrickx, M.E., 2011. Towards a better understanding of the pectin structure–function relationship in broccoli during processing: Part I—macroscopic and molecular analyses. Food Res. Int. 44, 1604–1612. https://doi.org/10.1016/j.foodres.2011.04.029. de Souza Silva, K., Caetano, L.C., Garcia, C.C., Romero, J.T., Santos, A.B., Mauro, M.A., 2011. Osmotic dehydration process for low temperature blanched pumpkin. J. Food Eng. 105, 56–64. https://doi.org/10.1016/j.jfoodeng.2011.01.025. Duvetter, T., Sila, D.N., Van Buggenhout, S., Jolie, R., Van Loey, A., Hendrickx, M., 2009. Pectins in processed fruit and vegetables: Part I-Stability and catalytic activity of pectinases. Compr. Rev. Food Sci. Food Saf. 8, 75–85. https://doi.org/10.1111/j. 1541-4337.2009.00070.x. Hiranvarachat, B., Devahastin, S., Chiewchan, N., 2011. Effects of acid pretreatments on some physicochemical properties of carrot undergoing hot air drying. Food Bioprod. Process. 89, 116–127. https://doi.org/10.1016/j.fbp.2010.03.010. Imaizumi, T., Szymańska-Chargot, M., Pieczywek, P.M., Chylińska, M., Kozioł, A., Ganczarenko, D., Tanaka, F., Uchino, T., Zdunek, A., 2017a. Evaluation of pectin nanostructure by atomic force microscopy in blanched carrot. LWT - Food Sci. Technol. 84, 658–667. https://doi.org/10.1016/j.lwt.2017.06.038. LebensmittelWissenschaft -Technol. 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. https://doi.org/10.1016/j.jfoodeng.2015.04.003. Imaizumi, T., Tanaka, F., Sato, Y., Yoshida, Y., Uchino, T., 2017b. Evaluation of electrical and other physical properties of heated sweet potato. J. Food Process. Eng. 40,
Fig. 7. Changes in residual peroxidase (POD) activity of carrot samples without pretreatment (Control) and after mild heating at 60 °C for 60 min (MH) during subsequent high temperature heating at 95 °C. Bar: SE.
heating and control samples required 60 s of high-temperature heating to reduce the activity to below 5%. Thus, mild heating at 60 °C had little effect on the subsequent blanching time. 4. Conclusion In this study, we investigated mild heating effects on texture degradation of carrot during subsequent thermal treatment to mimic pasteurization and blanching. Softening during subsequent high-temperature heating was affected by the temperature and duration of mild heating. The temperatures at 60 °C and 70 °C was significantly effective for maintaining firmness. In addition, mild heating changed the activation energy and frequency factor of softening, possibly depending on the different states of pectin molecules in the tissue. Additionally, electrical properties seemed to reflect the condition of the cell membrane and porosity in the tissue. Although the texture of vegetables deteriorates due to thorough heating for food safety, especially in hospitals and schools, it was shown that the texture can be improved by appropriate pretreatment. We also investigated the effects of mild heating on enzymatic activity, which can provide useful information to design food-processing steps. Mild heating at 70 °C sufficiently inactivated POD, whereas mild heating at 60 °C was not effective for inactivating the heat-resistant fraction of POD. Thus, additional blanching is required to inactivate the enzyme fully. Future studies are required to further elucidate the relationships between physical and electrical properties and tissue conditions. Our current findings can 24
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e12490. https://doi.org/10.1111/jfpe.12490. Imaizumi, T., Tanaka, F., Uchino, T., 2017c. Numerical modeling of peroxidase inactivation in potato tubers. Trans. ASABE (Am. Soc. Agric. Biol. Eng.) 60, 545–550. https://doi.org/10.13031/trans.11955. Imaizumi, T., Yamauchi, M., Sekiya, M., Shimonishi, Y., Tanaka, F., 2018. Responses of phytonutrients and tissue condition in persimmon and cucumber to postharvest UV-C irradiation. Postharvest Biol. Technol. 145, 33–40. https://doi.org/10.1016/j. postharvbio.2018.06.003. Lee, C.Y., Bourne, M.C., Buren, J.P., 1979. Effect of blanching treatment on the firmness of carrots. J. Food Sci. 44, 615–616. https://doi.org/10.1111/j.1365-2621.1979. tb03848.x. Macdonald, J.R., 1992. Impedance spectroscopy. Ann. Biomed. Eng. 20, 289–305. https://doi.org/10.1007/BF02368532. Neves, F.I.G., Vieira, M.C., Silva, C.L.M., 2012. Inactivation kinetics of peroxidase in zucchini (Cucurbita pepo L.) by heat and UV-C radiation. Innov. Food Sci. Emerg. Technol. 13, 158–162. https://doi.org/10.1016/j.ifset.2011.10.013. Okazaki, T., Maeshige, S., Suzuki, K., 1997. Kinetic studies on softening of Japanese radish and β-elimination of citrus pectin during thermal processes. Nippon Shokuhin Kagaku Kogaku Kaishi 44, 647–652. https://doi.org/10.3136/nskkk.44.647. Peng, J., Tang, J., Barrett, D.M., Sablani, S.S., Anderson, N., Powers, J.R., 2017. Thermal pasteurization of ready-to-eat foods and vegetables: critical factors for process design and effects on quality. Crit. Rev. Food Sci. Nutr. 57, 2970–2995. https://doi.org/10.
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Journal of Food Engineering journal homepage: www.elsevier.com/locate/jfoodeng
Effects of mild heating treatment on texture degradation and peroxidase inactivation of carrot under pasteurization conditions
T
Teppei Imaizumia,∗, Fumihiko Tanakab, Toshitaka Uchinob a b
Faculty of Applied Biological Sciences, Gifu University, 1-1 Yanagido, Gifu, 501-1193, Japan Faculty of Agriculture, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka, 819-0395, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Mild heating Peroxidase Food processing Pectin
Mild heating is an attractive method to improve the texture of vegetables. In this study, we applied mild heating treatment with various temperatures and durations to carrot roots, and we evaluated the effects on texture degradation and peroxidase inactivation during subsequent heating treatment to mimic pasteurization or blanching. Mild heating, especially at 60 °C for 60 min, inhibited softening during subsequent heating treatment. Activation energy required for the softening was 153.3 and 185.2 kJ mol−1 for untreated and mild-heated (60 °C, 60 min) samples, respectively. To evaluate tissue condition, we applied electrical impedance analysis. During mild heating, the cell membranes were damaged and electrolyte flow was activated. We assumed that mild heating increased calcium levels in the cell wall, leading to the crosslinking of pectin, thus improving the cell structure of carrot samples. Although mild heating at 60 °C did not inactivate all of the peroxidase, mild heating at 70 °C inactivated the heat-resistant fraction of peroxidases. The results of this study can contribute to designing processes for improving the texture of processed vegetables.
1. Introduction Carrots are widely consumed around the world and are important in providing nutrition and health benefits. They are rich in β-carotene and a good source of carbohydrates and minerals such as calcium, phosphorus, iron, and magnesium (Sharma et al., 2012). In recent years, the consumption of carrots and related products has increased steadily due to its recognition as a rich source of β-carotene (Hiranvarachat et al., 2011). Carrots are used in various processed foods such as frozen vegetables and retort pouch foods. In Japan, ready-to-eat food products containing carrot as an ingredient (e.g. salad, soup, Nimono [Japanese simmered dish], and bento lunch boxes) are popular and easy to buy in supermarkets and convenience stores. Fruits and vegetables often undergo thermal treatment such as pasteurization or blanching. For example, in cooking at schools, nursing homes, hospitals, and restaurants, it is recommended to conduct sufficient heating under thermal pasteurization conditions for food safety. In addition, blanching is conducted at over 80 °C to inactivate enzymes that cause the deterioration of vegetable products during subsequent processes and storage. However, high-temperature heating usually results in the degradation of tissue structure, leading to undesirable food texture (Abu-Ghannam and Crowley, 2006; Peng et al., 2014). Vegetables are often required to keep their structure and texture even if ∗
heated for a long time. Because texture is an important quality attribute considered by consumers, the inhibition or limitation of such degradation is important to improve food value. To inhibit or block tissue degradation, mild heating is generally carried out within the range of 50–80 °C for several types of vegetable, including potato (Canet et al., 2005), sweet potato (Chhe et al., 2017; Walter et al., 2003), pumpkin (de Souza Silva et al., 2011), and carrot (Lee et al., 1979). At mild temperatures, pectin methyl esterase (PME) induces the demethylation of pectin molecules. Pectin molecules exist as binding substances between cells. Demethylated pectin molecules are not easily decomposed by β-elimination and promote crosslinking via divalent ions such as Ca2+, thus contributing to strengthening tissue structure (Imaizumi et al., 2017a). Tissue degradation is inhibited by mild heating of vegetables during subsequent processing and cooking. In this study, we focused particularly on the optimum conditions for mild heating for the inhibition of softening during high-temperature heating, as occurs in pasteurization or blanching. These treatments are often conducted for vegetables at 80–95 °C (Imaizumi et al., 2017a; Peng et al., 2017, 2014). The purpose of our research was to clarify the influence of mild temperature and time on texture degradation during pasteurization (80–95 °C), and also to consider changes in structure presumed from electrical properties. The kinetics of texture change enabled us to understand the inhibiting effect of mild heating on
Corresponding author. E-mail address: [email protected] (T. Imaizumi).
https://doi.org/10.1016/j.jfoodeng.2019.03.024 Received 3 November 2018; Received in revised form 29 March 2019; Accepted 30 March 2019 Available online 02 April 2019 0260-8774/ © 2019 Published by Elsevier Ltd.
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Fig. 1. Outline of research procedures.
2.3. Electrical parameters
softening during subsequent high-temperature heating. Additionally, electrical properties obtained by impedance analysis are useful for evaluating tissue conditions such as cell membrane damage and porosity (Blahovec and Kouřím, 2019; Imaizumi et al., 2017b, 2018; Watanabe et al., 2017). Based on these aspects, we evaluated the effective mild heating conditions for the treatment of carrots. Furthermore, we assessed blanching processes such as high-temperature heating and evaluated their contribution to the determination of processing time. We focused on peroxidase (POD), which is often used as an indicator enzyme of blanching completion (Imaizumi et al., 2017c). The results obtained from our study can contribute to the design and development of vegetable processing steps in the food industry.
Electrical evaluation was conducted to determine tissue condition, just after mild heating treatment, based on Imaizumi et al. (2015). Two needle electrodes (diameter, 0.25 mm), connected to an LCR tester (3532-50, HIOKI), were inserted 9 mm deep from the top surface of the sample. Resistance and reactance of the sample were measured at 200 points over the frequency range from 50 Hz to 5 MHz, in which β-dispersion due to cell structure appears. The measurements were conducted at a measuring voltage of 1 V, and automatically recorded using a computer for analysis. Relationships between impedance and the values are expressed as shown below:
Z = R + jX 2. Materials and methods
(1)
where Z is impedance (Ω), R is resistance (Ω), X is reactance (Ω), and j is an imaginary unit. Equivalent circuit analysis was conducted by fitting the CPE model (Ando et al., 2014) described as equation (2) to the measured values:
2.1. Sample preparation and overview of experiment Carrot roots (Daucus carota subsp. sativus) were used in this study. An outline of the research procedure is shown in Fig. 1. Carrots were cut into cylinders (15 mm diameter and 10 mm height) using a cork borer and a knife. Each cylindrical sample was immersed in 200 mL of distilled water, whose temperature was maintained at 50 °C, 60 °C, 70 °C, or 80 °C for 20, 40, or 60 min as a mild heating treatment. Using distilled water for immersion, the chemical influences of dissolved components in the heating medium were removed. Peng et al. (2014) showed that the osmotic pressure of the immersion liquid did not contribute to the texture change behavior of carrot during high-temperature heating. After treatment, the sample was immediately cooled in iced water for 2 min to remove excess heat inside. Then, the sample was kept at room temperature for over 30 min wrapped in film to reach a constant temperature. Each sample that had undergone mild heating was then subjected to high-temperature heating in 500 mL of distilled water at 95 °C for up to 60 min and cooled again. Kinetic analysis was conducted on texture degradation during high-temperature heating. Samples that had been treated with mild heating at 60 °C for 60 min and a fresh sample (as control) were processed by high-temperature heating at 80 °C, 85 °C, 90 °C, and 95 °C, and the temperature dependence of the kinetics were investigated. The temperature setting was determined in consideration of past reports (Peng et al., 2014; Vu et al., 2004) POD activity of the samples was measured for every sample as described below, and enzyme inactivation during high-temperature heating at 95 °C was investigated for samples treated with mild heating at 60 °C for 60 min and controls.
{
}
( )
π R e ⎡1 + ω pT (2Ri + R e)cos 2 p + ω pTRi (R e + Ri ) ⎤ ⎣ ⎦ Z= π {ω pT (R e + Ri )} 2 + 2ω pT (R e + Ri )cos 2 p + 1
( )
−j
ω pTRe2 sin
( p) π 2
{ω pT (R e + Ri )} 2 + 2ω pT (R e + Ri )cos
( p) + 1 π 2
−1
(2)
), p is a where ω is angular frequency (rad s ), T is a constant (F s CPE exponent (0 ≤ p ≤ 1), Re is extracellular resistance (Ω), and Ri is intracellular resistance (Ω). The equivalent circuit parameters T, p, Re, and Ri were determined using complex nonlinear least squares curve fitting (Macdonald, 1992). Additionally, the cell membrane capacitance Cm (F) was defined using the following equation (Ando et al., 2014): 1
Cm = T p (R e + Ri )
1−p p
(p−1)
(3)
2.4. Peroxidase assay POD activity was measured based on the methods described by Agüero et al. (2008) and Neves et al. (2012), with a minor modification. Briefly, the carrot sample was homogenized with 10 mL of distilled water. The homogenized liquids were centrifuged at 1670×g for 15 min, and the supernatants were filtered. The substrate solution was prepared by mixing guaiacol, 30% H2O2, and 0.1 M potassium phosphate buffer (pH 6.5) in a volumetric ratio of 1:1:998. The sample solution (0.12 mL) and the substrate solution (3.48 mL) were mixed and absorbance at 470 nm was measured using a spectrophotometer (V-530, JASCO). Enzymatic activity was determined from the slope of the initial linear portion in the absorbance, and divided by the initial activity, which was obtained from the fresh sample. Measurements were replicated 3 times.
2.2. Firmness Firmness was measured in accordance with the method described by Ando et al. (2016). In this measurement, we used a creep meter (RE3305, YAMADEN), equipped with a wedge-shape plunger (P-29, YAMADEN), and the sample stage was moved at 1 mm s−1. The probe was inserted into the center of the upper surface of a carrot cylinder (15 mm diameter and 10 mm height). Peak force at yield point on the forcedisplacement curve was determined as firmness (N). The measurements were replicated 6 times.
3. Results and discussion 3.1. Kinetic evaluation of effect of mild heating on firmness Fig. 2 shows the changes in firmness of each sample after mild heating during high-temperature heating at 95 °C. Although the 20
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Fig. 2. Changes in firmness of carrot samples after mild heating during subsequent high temperature heating at 95 °C. Mild heating treatments were performed at 50 °C (A), 60 °C (B), 70 °C (C), and 80 °C (D) for 20, 40, or 60 min. Bar: SE, Curves: First-order kinetic model.
firmness decreased with high-temperature heating time, mild heating inhibited this progression. To evaluate the inhibiting effect of mild heating on softening, kinetic analysis was conducted. Here, the firstorder kinetic equation is described below:
dC = −kC dt
Table 1 Rate constants of the softening process by high temperature heating at 95 °C obtained from samples after mild heating. (Value ± 95% confidence interval).
(4)
where C is a quality index or concentration of a chemical compound, t is the reaction time, and k is the rate constant. The texture property is presented as a fraction of texture change C, which provides an accurate way to determine the extent of quality change at any time t and can be expressed as the equation below (Peng et al., 2014):
C=
Ft − Fe F0 − Fe
Mild heating condition
k (min−1)
Fe (N)
Control 50 °C
0.158 0.151 0.085 0.072 0.061 0.031 0.029 0.039 0.057 0.049 0.075 0.065 0.056
3.50 4.19 2.81 5.01 4.29 0.46 2.88 1.53 6.31 6.92 3.14 3.89 3.54
60 °C
70 °C
80 °C
(5)
where, F is firmness (N), subscript t, 0, and e indicate value at time t (min) and 0 and equilibrium value, respectively. In this study, the kinetic equation was fitted to changes in firmness with high-temperature heating time, and the k value (min−1) was determined. The model fitted well to the measured values, as shown in Fig. 2 (curves). Fitting quality was also evaluated by r2 (see Table 1) and the value was found to be high enough in every condition. The obtained k and Fe values are also described in Table 1. Samples after mild heating treatment at 50 °C for 20 min softened similarly to control samples, suggesting that this condition did not have any delaying effect on softening. When the mild heating treatment temperature or time was increased, the k values for firmness significantly decreased to less than half the values of the control sample. However, the initial firmness after mild heating treatment at 80 °C was lower than that of control. This is likely because softening could have progressed during mild heating at 80 °C. Thus, we propose that mild heating should be conducted at less than 80 °C, and
20 min 40 min 60 min 20 min 40 min 60 min 20 min 40 min 60 min 20 min 40 min 60 min
± ± ± ± ± ± ± ± ± ± ± ± ±
0.042 0.038 0.049 0.010 0.033 0.024 0.020 0.008 0.041 0.016 0.031 0.034 0.046
± ± ± ± ± ± ± ± ± ± ± ± ±
r2 2.12 1.82 5.38 1.33 6.48 14.46 9.74 3.41 9.16 10.46 3.08 3.25 3.40
0.991 0.991 0.958 0.998 0.971 0.973 0.953 0.981 0.952 0.948 0.980 0.971 0.940
based on the k value of firmness, 60 °C and 70 °C seem to be better conditions. Although the optimum temperature of carrot PME activity was reported to be around 50 °C (Duvetter et al., 2009), the result of our experiment did not support this fact. As mentioned above, in order to crosslink the pectin molecule, not only the optimum temperature but also a sufficient calcium concentration is necessary. Imaizumi et al. (2015) indicated that the cell membranes of potato tubers were destroyed by heating at over 60 °C. It is known that calcium is accumulated in cells, especially cell vacuoles, and destruction of membranes by heating leads to the release of calcium outside of the cell membrane. In our treatment at 60 °C and 70 °C, intracellular calcium could have been sufficiently supplied to the cell wall region, leading to increased crosslinking of pectin and higher structural stability. Additionally, 21
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Fig. 3. Changes in firmness of carrot samples without pretreatment (A) and after mild heating at 60 °C for 60 min (B) during subsequent high temperature heating at 80 °C, 85 °C, 90 °C, and 95 °C. Bar: SE, Curves: First-order kinetic model.
Imaizumi et al. (2017c) showed that temperature rise of vegetable samples during high-temperature heating was slow because of its low thermal conductivity. Therefore, demethylation of pectin might progress adequately at these temperatures. Subsequently, cell membrane damage may be inhibited or may occur at slower rates in samples that underwent mild heating. Because mild heating at 60 °C for 60 min was significantly effective, the temperature dependence of the softening process was investigated. Samples that had undergone mild heating (60 °C, 60 min) and control samples were heated in hot water at 80 °C, 85 °C, 90 °C, and 95 °C had their firmness measured. As shown in Fig. 3, the equation fitted closely to the measured firmness (r2 = 0.950–0.994). The obtained Fe values of 80 °C, 85 °C, 90 °C, and 95 °C were 6.96, 4.01, 3.46, and 2.50 N for control samples and 0.00, 8.38, 7.75, and 3.61 N for samples after mild heating. Overall, samples after mild heating had higher Fe values. At 80 °C, because the softening of the sample was very gentle, an accurate equilibrium value could not be obtained. Thus, the softening inhibiting effect was discussed using rate constants. The obtained rate constants were plotted against the reciprocal of the immersion temperature (Fig. 4), and the Arrhenius equation (6) was fitted to the plots.
E k = A⋅exp ⎛− a ⎞ ⎝ RT ⎠
Table 2 Activation energy and frequency factor of the softening process by high temperature heating at 95 °C obtained from control samples and samples after mild heating (60 °C, 60 min).
Control Mild heating 60 ˚C-60 min
Ea (kJ mol−1)
A (min−1)
r2
153.3 185.2
8.7 × 1020 7.3 × 1024
0.989 0.948
8.7 × 1020 min−1 for controls and 185.2 kJ mol−1 and 7.3 × 1024 min−1 for samples after mild heating, respectively. Okazaki et al. (1997) suggested a close relationship between softening of Japanese radish and the decomposition of pectin, because they had a similar activation energy of 146 kJ mol−1 and 144 kJ mol−1, respectively. Although the activation energy of the control was close to this value, that of the sample that had undergone mild heating was higher. Because pectin molecules in carrot tissue were demethylated by mild heating, the probability of occurrence of β-elimination may be decreased. In addition, the frequency factor of the sample after mild heating was higher than that of the control sample. Some previous reports have shown that water soluble pectin is converted to pectin with stronger bonds by mild heating (Christiaens et al., 2011; Sila et al., 2006). One reason for this conversion may be due to cross-linkages of pectin molecules via divalent ions. Imaizumi et al. (2017a) observed small particles of water-soluble pectin using atomic force microscopy, and suggested that water-insoluble pectin, which had a chain-like shape or network, contributed to the texture more than water-soluble pectin. Thus, we hypothesized that the increase in the frequency factor was caused by an increase in the levels of water-insoluble pectin.
(6)
where, A is the frequency factor (min−1), Ea is the activation energy (J mol−1), R is the universal gas constant (J K−1 mol−1), and T is the absolute temperature (K). The A and Ea values were determined using a linear least-squares method (r2 = 0.948–0.989) (see Table 2). The activation energy and frequency factor were 153.3 kJ mol−1 and
3.2. Evaluation of tissue condition by using electrical parameters Imaizumi et al. (2015) suggested that intracellular resistance Ri increases due to electrolyte outflow from the cells. They suggested that the loss of ionic regulating capacity was caused by the denaturation of membrane proteins and a decline in electrolyte retention capacity due to cell membrane damage, resulting in electrolyte outflow. If the value is high, the opportunity to supply intracellular electrolyte to pectin molecules in the cell wall region will be high. Fig. 5 shows the electrical properties obtained from each sample. Treatment at over 60 °C for 60 min showed high Ri values, thus electrolyte outflow seemed to be active. For extracellular resistance Re, no significant difference was observed in samples treated at over 60 °C. At 50 °C, the values decreased with time. It has been shown that a change in Re during hot water heating was strongly related to the discharge of air in the tissue (Imaizumi et al., 2017a). Briefly, they showed that extracellular resistance declined with decreasing porosity of sweet potato. Thus, pores
Fig. 4. Arrhenius plots of the rate constant of softening by high temperature heating at 80 °C, 85 °C, 90 °C, and 95 °C. Control: values obtained from samples without preheating, MH: values obtained from samples treated at 60 °C for 60 min, Dashed line: Arrhenius equation. 22
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Fig. 5. Equivalent circuit parameters, intracellular resistance Ri, extracellular resistance Re, and cellular membrane capacitance Cm of carrot samples after mild heating. Numbers on the axis labels represent mild heating conditions (temperature-time). Bar: SE, Different letters indicate significant differences (p < 0.05).
3.3. Effect of mild heating on enzymatic activity
in the carrot sample were lost slowly at 50 °C and within 20 min at higher temperatures. Imaizumi et al. (2015) demonstrated that cell membrane capacitance Cm was decreased by cell damage. Additionally, they assumed that the cell membrane has capacitance because the lipid bilayer behaves as a capacitor. In our results, the value increased for the samples treated at 50 °C and 60 °C for 20 min. No increase in Cm value was observed during high-temperature heating. When the area of the lipid bilayer increases or the thickness reduces, the capacitance will increase. In this experiment, the sample was immersed in distilled water with low osmotic pressure, which would allow moisture to be taken into the cell. Therefore, we believe that the lipid bilayer became thinner and broader because turgor pressure (due to moisture uptake) increased and pressed against the cell membrane. After that, the value of Cm decreased likely due to cell membrane damage. In summary, membrane capacitance increased up to around 2 pF like in conditions of “50–20” with an increase in turgor pressure, and it can be considered that the cell membrane has been considerably damaged if it subsequently falls below 1 pF, as in conditions of “70–60”, “80–40”, and “80–60”.
Enzymatic activity is important to determine blanching time for vegetables. Inactivating enzymes during mild heating as a pretreatment will influence subsequent processes. POD is often used as an index of blanching, because it is one of the most heat stable enzymes. Fig. 6 shows the residual POD activity of each sample after mild heating. Although POD inactivation proceeded at 50 °C and 60 °C, further inactivation stopped after residual POD activity decreased to 20%. On the other hand, the residual activity decreased to less than 5% at 70 °C and 80 °C. Neves et al. reported that POD has heat-labile and heat-resistant fractions (2012). In our results, we propose that the former fraction was inactivated at 60 °C, and inactivation of the latter fraction required higher temperatures of 70–80 °C. Therefore, for actual frozen vegetables, mild heating at 60 °C should be combined with a high-temperature treatment. Fig. 7 shows the changes in residual POD activity after mild heating (60 °C, 60 min) and control samples during subsequent high-temperature heating at 95 °C. Both samples after mild 23
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Fig. 6. Residual peroxidase (POD) activity of carrot samples after mild heating. Numbers on the axis labels represent mild heating conditions (temperature-time). Bar: SE, Different letters indicate significant differences (p < 0.05).
significantly contribute to designing processes for improving the texture of vegetables. Acknowledgement This work was supported by Japan Society for the Promotion of Science(JSPS) KAKENHI Grant Number 17H06727. References Abu-Ghannam, N., Crowley, H., 2006. The effect of low temperature blanching on the texture of whole processed new potatoes. J. Food Eng. 74, 335–344. https://doi.org/ 10.1016/j.jfoodeng.2005.03.025. Agüero, M.V., Ansorena, M.R., Roura, S.I., del Valle, C.E., 2008. Thermal inactivation of peroxidase during blanching of butternut squash. LWT - Food Sc. Technol. 41 (3), 401–407. https://doi.org/10.1016/j.lwt.2007.03.029. Ando, Y., Maeda, Y., Mizutani, K., Wakatsuki, N., Hagiwara, S., Nabetani, H., 2016. Effect of air-dehydration pretreatment before freezing on the electrical impedance characteristics and texture of carrots. J. Food Eng. 169, 114–121. https://doi.org/10. 1016/j.jfoodeng.2015.08.026. Ando, Y., Mizutani, K., Wakatsuki, N., 2014. Electrical impedance analysis of potato tissues during drying. J. Food Eng. 121, 24–31. https://doi.org/10.1016/j.jfoodeng. 2013.08.008. Blahovec, J., Kouřím, P., 2019. Pulsed electric stimulated changes in potatoes during their cooking: DMA and DETA analysis. J. Food Eng. 240, 183–190. https://doi.org/10. 1016/J.JFOODENG.2018.07.025. Canet, W., Alvarez, M.D., Fernández, C., 2005. Optimization of low-temperature blanching for retention of potato firmness: effect of previous storage time on compression properties. Eur. Food Res. Technol. 221, 423–433. https://doi.org/10.1007/ s00217-005-1195-3. Chhe, C., Imaizumi, T., Tanaka, F., Uchino, T., 2017. Effects of hot-water blanching on the biological and physicochemical properties of sweet potato slices. Eng. Agric. Environ. Food. https://doi.org/10.1016/j.eaef.2017.10.002. Christiaens, S., Van Buggenhout, S., Houben, K., Fraeye, I., Van Loey, A.M., Hendrickx, M.E., 2011. Towards a better understanding of the pectin structure–function relationship in broccoli during processing: Part I—macroscopic and molecular analyses. Food Res. Int. 44, 1604–1612. https://doi.org/10.1016/j.foodres.2011.04.029. de Souza Silva, K., Caetano, L.C., Garcia, C.C., Romero, J.T., Santos, A.B., Mauro, M.A., 2011. Osmotic dehydration process for low temperature blanched pumpkin. J. Food Eng. 105, 56–64. https://doi.org/10.1016/j.jfoodeng.2011.01.025. Duvetter, T., Sila, D.N., Van Buggenhout, S., Jolie, R., Van Loey, A., Hendrickx, M., 2009. Pectins in processed fruit and vegetables: Part I-Stability and catalytic activity of pectinases. Compr. Rev. Food Sci. Food Saf. 8, 75–85. https://doi.org/10.1111/j. 1541-4337.2009.00070.x. Hiranvarachat, B., Devahastin, S., Chiewchan, N., 2011. Effects of acid pretreatments on some physicochemical properties of carrot undergoing hot air drying. Food Bioprod. Process. 89, 116–127. https://doi.org/10.1016/j.fbp.2010.03.010. Imaizumi, T., Szymańska-Chargot, M., Pieczywek, P.M., Chylińska, M., Kozioł, A., Ganczarenko, D., Tanaka, F., Uchino, T., Zdunek, A., 2017a. Evaluation of pectin nanostructure by atomic force microscopy in blanched carrot. LWT - Food Sci. Technol. 84, 658–667. https://doi.org/10.1016/j.lwt.2017.06.038. LebensmittelWissenschaft -Technol. 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. https://doi.org/10.1016/j.jfoodeng.2015.04.003. Imaizumi, T., Tanaka, F., Sato, Y., Yoshida, Y., Uchino, T., 2017b. Evaluation of electrical and other physical properties of heated sweet potato. J. Food Process. Eng. 40,
Fig. 7. Changes in residual peroxidase (POD) activity of carrot samples without pretreatment (Control) and after mild heating at 60 °C for 60 min (MH) during subsequent high temperature heating at 95 °C. Bar: SE.
heating and control samples required 60 s of high-temperature heating to reduce the activity to below 5%. Thus, mild heating at 60 °C had little effect on the subsequent blanching time. 4. Conclusion In this study, we investigated mild heating effects on texture degradation of carrot during subsequent thermal treatment to mimic pasteurization and blanching. Softening during subsequent high-temperature heating was affected by the temperature and duration of mild heating. The temperatures at 60 °C and 70 °C was significantly effective for maintaining firmness. In addition, mild heating changed the activation energy and frequency factor of softening, possibly depending on the different states of pectin molecules in the tissue. Additionally, electrical properties seemed to reflect the condition of the cell membrane and porosity in the tissue. Although the texture of vegetables deteriorates due to thorough heating for food safety, especially in hospitals and schools, it was shown that the texture can be improved by appropriate pretreatment. We also investigated the effects of mild heating on enzymatic activity, which can provide useful information to design food-processing steps. Mild heating at 70 °C sufficiently inactivated POD, whereas mild heating at 60 °C was not effective for inactivating the heat-resistant fraction of POD. Thus, additional blanching is required to inactivate the enzyme fully. Future studies are required to further elucidate the relationships between physical and electrical properties and tissue conditions. Our current findings can 24
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