2014 Volume 20 Issue 4 Pages 775-783
Changes in total chlorophyll and ascorbic acid (AsA) contents and total color difference (TCD) of blanched green bell peppers during hot air and microwave drying were investigated. The dehydration kinetics of unblanched and blanched green bell peppers at 60°C were determined. Total chlorophyll and AsA contents as well as TCD were determined in the range of 40 – 70°C for hot air drying and 500 – 800 W for microwave drying. The decomposition rates of total chlorophyll, AsA and TCD followed first order reaction kinetics. Each rate constant exhibited an Arrhenius-type temperature dependence. The activation energies (E) of total chlorophyll, AsA contents and TCD were determined for hot air drying.
The bell pepper (Capsicum annuum L.) is an important vegetable crop that can be grown in temperate or tropical regions. The color of ripe bell peppers can be white, red, yellow or orange, while unripe bell peppers are green in color (Simonne et al., 1997). Green bell peppers contain large amounts of vitamin C (76 mg/100 g) and minerals, such as potassium (190 mg/100 g), calcium (11 mg/100 g) and iron (0.4 mg/100 g) (STAJ, 2002). Green bell peppers are consumed raw or cooked as well as in dehydrated form.
The green bell pepper, like other vegetables, is highly perishable; thus, dehydration plays an important role in extending its shelf life. Dehydration can be accomplished by several methods, including hot air, microwave and vacuum drying. However, losses of chemical compounds and physical properties, such as chlorophyll and water-soluble vitamins (mainly ascorbic acid (AsA)) and total color difference (TCD) occur during vegetable drying (Guine and Barroca, 2012; Di Scala and Crapiste, 2008; Marfil et al., 2008; Ozkan et al., 2007; Maharaj and Sankat, 1996).
Chlorophyll pigments (mainly chlorophyll a) are responsible for the green color of the green bell pepper and are extremely sensitive to heat. Undesirable changes in the color of the green bell pepper during drying may decrease its quality. Boyer and Huff (i) reported that most vegetables should be blanched before drying. According to Fellows (2000), the addition of sodium carbonate (0.125% w/w) to the blanching water prevents the loss of chlorophyll and retains the color of green vegetables. The main function of blanching is to destroy the enzymatic activity of vegetables; blanching also reduces the drying time and kills many spoilage organisms (Boyer and Huff (i); Fellows, 2000). Peroxidase (POD) is the most heat-stable enzyme in fruits and vegetables, and the POD content is widely used as an index of blanching (Morales-Blancas, et al., 2002; Fellows, 2000; Negi and Roy, 2000; Gunes and Bayindirli, 1993). Okoli et al. (1988) reported that blanching spinach at 95°C for 1 min was sufficient to inactivate POD. Imaizumi et al. (2013) investigated the applicability of microwaving (MW) for blanching taro and yam. According to Gunes and Bayindirli (1993), a residual POD activity of between 3 and 10% after blanching was recommended for preservation by freezing.
Although maintaining the green color of the dried green bell pepper is crucial, little research has been conducted on the decomposition of chlorophyll during the drying process. In addition, few researchers have addressed the problem of AsA decomposition in green bell pepper during drying. These aspects of quality change during drying needed investigation to establish the optimal conditions for the green bell pepper. Thus, the aims of this study were to:
Sample preparation Green bell pepper fruits (cv., unknown; Kagoshima Prefecture, Japan) were purchased from a local market and stored in a refrigerator at 8°C for a maximum period of four days. For each experiment, ten fruits with a diameter of 5.2 ± 0.22 cm were selected randomly; the diameters were measured using a Vernier caliper. After being washed and drained, the fruits were cut into 40 × 40 mm pieces (the average weight of twenty pieces was 5.10 ± 0.19 g) and were used for the drying, blanching or chemical analytical experiments. The initial moisture content of the green bell pepper was obtained using the vacuum oven method (AOAC, 1984) and was determined to be 95.15% ± 0.0002 w.b. (19.62 ± 0.08 d.b. decimal).
Blanching For each experiment, approximately 20 to 35 g of sample was blanched using one of the following methods: (i) one liter of plain boiling water; (ii) boiling solutions containing 1, 5 or 10 g NaHCO3/L for 60 s; (iii) boiling solutions containing 1, 5 or 10 g NaCl/L for 60 s; (iv) microwave blanching: the sample was covered with a watch glass, placed in the center of the oven (MRO-DF6, 2008; Hitachi, Thailand), and heated at 500 W for 60 s or 600 W for 30 s. After blanching, all of the samples were immediately immersed in chilled water for 60 s, drained thoroughly and used for the drying experiment or to determine the residual POD activity, AsA content, TCD change and total chlorophyll content.
Hot air drying A schematic diagram of the experimental apparatus used for hot air drying is shown in Fig. 1. The drying system was controlled using RsCom software (Ver. 2.40, A & D Co., Ltd., Japan). The drying chamber fan was stopped 1 min before measuring the sample weight and turned on after the measurement. The hot air drying experiments were conducted at temperatures of 40, 50, 60 or 70°C. The temperature in the center of a sample, which was measured using an inserted 0.1 mm diameter T-type thermocouple, asymptotically approached the dry bulb temperature of the drying air during the first 30 – 40 min of the hot air drying tests. The weight of the sample ranged from between approximately 20 and 30 g. The changes in mass during hot air drying were recorded at intervals of 0.5 h until the final moisture content reached 0.17 ± 0.002 d.b. decimal (14.20 ± 0.50% w.b.). The average air velocity, measured using an anemometer (V-01-AND2H; I Denshi Giken Co., Ltd., Japan), was 1.2 m·s−1. The relative humidity, which ranged from 2 to 20%, was determined using a midi logger with a humidity-to-voltage conversion module (GL200; Graphtec Corp., China). The experiments were replicated three times for each drying condition and the mean value was used for discussion.
Schematic diagram of the experimental apparatus used for hot air drying
Microwave drying The microwave drying experiments were conducted using a microwave oven (MRO-DF6, 2008; Hitachi, Japan) with 500 W, 600 W or 800 W output. A schematic diagram of the experimental apparatus used for the microwave drying is provided in Fig. 2. A glass petri dish (35 cm diameter and 2 cm deep) containing between 20 and 35 g of sample was placed in the center of the oven to allow good absorption of the microwave energy. The temperature at the center of a green bell pepper sample was measured using an inserted fluorescence-based fiber-optic thermometer with 0.1 mm diameter optical fiber between 10 s of radiation and 10 min of tempering. The change in mass during microwave drying was recorded by removing the sample and weighing it using a digital balance (GX-200; A & D Co., Ltd, Japan) at 1 min intervals until the moisture content reached 0.17 ± 0.002 d.b. decimal. The experiments were replicated three times for each drying condition, and the mean value was used for discussion.
Schematic diagram of the experimental apparatus used for microwave drying
Ascorbic acid (AsA) content The AsA content was determined using a reflectometer (RQ-flex-plus; Merck, Japan), as described by Merck KGaA (ii). A 5 g sample was placed in a beaker and approximately 100 mL of a 1% oxalic acid solution was added. The mixture was homogenized for 1 min and then centrifuged at 8000 rpm for 10 min. The supernatant was used for the determination of the AsA content. The AsA content was calculated using the following equation:
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Total color difference (TCD) The L* (lightness: L* = 0 for black, L* = 100 for white), a* (redness-greenness: a* < 0 for green, a* > 0 for red) and b* (yellowness-blueness: b* < 0 for blue, b* > 0 for yellow) indices of the CIELAB (Commission Internationale de L'éclairage, L*, a*, b*) colorimetric system were used to evaluate the difference between the color of the green bell pepper samples before and after blanching. The L*, a* and b* values were measured using a chromameter (CR-200b; Minolta, Japan) with a D65 light source at four different points on the surface of the green bell pepper samples at each predetermined moisture content level during drying. The change in the surface color of the sample, which was referred to as the TCD, was calculated according to the following equation:
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Determination of peroxidase (POD) activity using the pyrogallol method A 5 g sample was placed in a beaker and approximately 100 g of a 0.5 M cold sucrose solution (5°C) was added. The mixture was homogenized for 1 min and then centrifuged at 8000 rpm at 4°C for 10 min. The supernatant was used as the crude enzyme extract. The assay mixture contained 0.5 mL of buffer (0.1 M sodium dihydrogenphosphate and 0.1 M disodium hydrogen-phosphate, pH 6.0), 0.5 mL of 0.01 M pyrogallol, 0.1 mL of hydrogen peroxide (H2O2) and 1.4 mL of distilled water. The reaction was started by adding 0.5 mL of the crude enzyme extract. The POD activity was determined from the absorbance at 430 nm over a 1 min period, using a spectrophotometer (V-630Bio; Jasco, Japan). The POD activity was calculated using the following equation:
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Total chlorophyll content The chlorophyll was extracted from the green bell pepper samples using 80% acetone. A 1 g sample was placed in a mortar and ground with a pestle for a short time, then 80% acetone was gradually added with continued grinding until the tissue was finely ground. The mixture was homogenized for 1 min and then centrifuged at 8000 rpm for 10 min. The residue was returned to the mortar, more 80% acetone was added, and it was ground again. This procedure was repeated several times until the residue was colorless. The residue was ground at least once with pure acetone and then water was added to bring the acetone concentration to 80%. The extraction procedures were performed in dim light. The pooled supernatants were diluted to 100 mL with 80% acetone and the absorbance of the solution at 663.2 and 646.8 nm against the solvent blank (80% acetone) was read using a spectrophotometer (V-630Bio; Jasco); chlorophyll a was reported to show a maximum absorbance at 663.2 nm, chlorophyll b at 646.8 nm (Lichtenthaler, 1987). The total chlorophyll content was calculated using the following equation (Lichtenthaler, 1987):
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Statistical analysis The data are presented as the mean values ± standard deviation (SD). Comparisons between the mean values were examined using Tukey's test at a P < 0.05 significance level (R- Ver. 2.15.1 for Windows). The goodness-of-fit of the tested mathematical models to the experimental data was evaluated with the coefficient of determination (R2) and the root mean square error (RMSE). A high goodness-of-fit was characterized by a high R2 and low RMSE (Radhika et al., 2011; Orikasa et al., 2008). The RMSE was calculated as follows:
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Standardization of blanching process The blanching method that provided adequate POD inactivation, as well as the smallest changes in AsA content and TCD was selected for further drying experiments. The effects of different blanching conditions on POD activity, AsA content and TCD are presented in Table 1. The data indicated that the samples blanched using the microwave oven exhibited the highest AsA retention. However, we observed that the waxy skin of the microwave-blanched samples was swollen; thus, microwave blanching was not a suitable blanching method for green bell pepper. The results indicated that blanching in a solution containing 1 g/L of NaHCO3 provided adequate POD inactivation, the lowest TCD value and good AsA retention; it was therefore selected for the drying studies.
| Blanching condition | POD activity (unit/mL) | Retention of POD (%) | ΔE | AsA (mg/100 g dry matter) | |
|---|---|---|---|---|---|
| Fresh sample | 2.8625 ± 0.3197 | 100 | 0.00 | 1581.91 ± 9.50 | |
| Plain water (100°C) | 0.0114 ± 0.0004 | 0.39 | 4.99 ± 0.55 | 1329.47 ± 4.12 | |
| 500 W, 60 s | 0.0044 ± 0.0017 | 0.15 | 3.63 ± 0.55 | 1427.66 ± 14.48 | |
| 600 W, 30 s | 0.0091 ± 0.0030 | 0.32 | 3.66 ± 0.48 | 1417.96 ± 8.61 | |
| 1 | 0.0051 ± 0.0008 | 0.18 | 3.57 ± 0.49 | 1338.42 ± 12.18 | |
| NaCl (g/L) | 5 | 0.0061 ± 0.0004 | 0.21 | 4.15 ± 0.48 | 1311.09 ± 9.82 |
| 10 | 0.0080 ± 0.0012 | 0.28 | 3.91 ± 0.40 | 1301.80 ± 3.02 | |
| 1 | 0.0055 ± 0.0020 | 0.19 | 2.42 ± 0.46 | 1338.42 ± 11.89 | |
| NaHCO3 (g/L) | 5 | 0.0062 ± 0.0003 | 0.21 | 2.49 ± 0.42 | 1334.10 ± 10.29 |
| 10 | 0.0062 ± 0.0024 | 0.21 | 2.57 ± 0.49 | 1325.45 ± 17.61 |
Mean values ± SD
Kinetics of moisture content changes in unblanched and blanched samples during hot air drying In this study, the unblanched and blanched samples were dried at 60°C using hot air drying. Alterations in the moisture content of samples during hot air drying are presented in Fig. 3. The moisture content decreased gradually with increased drying time, exhibiting a gentle downward curve. It was also observed that the blanching treatment decreased the drying time compared to that of the unblanched sample. This result is consistent with previous studies, such as that of Boyer and Huff (i), who reported that blanching reduced the drying time of fruits and vegetables. Ertekin and Yaldiz (2004) found that pretreatment of eggplant by blanching reduced the drying time by approximately 43.14%.
Changes in the moisture content of unblanched and blanched green bell peppers during hot air drying
The drying rates were calculated using the data describing the changes in moisture content as follows:
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Figure 4 presents the relationship between the drying rate and the moisture content of the unblanched and blanched samples. The results indicate that blanching increased the drying rate. The figure also shows that for both unblanched and blanched samples, the drying process occurred only in the falling-rate period. These results concur with those previously reported for the drying curves of fruits and vegetables, such as red pepper (Doymaz and Pala, 2002), red bell pepper (Vega et al., 2007), tomatoes (Doymaz, 2007), and kiwifruit (Orikasa et al., 2008).
Relationship between the drying rate and the moisture content of unblanched and blanched green bell peppers during hot air drying
Many researchers have utilized an exponential or diffusion model as the mathematical model to analyze the data obtained from drying fruits and vegetables (Orikasa et al., 2008; Ertekin and Yaldiz, 2004; Maskan, 2001; Tagawa et al., 1996).
The diffusion equation is an infinite series (Crank, 1975) and can be expressed as follows:
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The infinite series on the right side of “Eq. 7” converges rapidly to the first term when the drying time t becomes large, giving:
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is called a drying constant. The B1 and K values are estimated by applying the experimental data to “Eq. 8” using a non-linear least squares method. In the current study, all values of B1 were determined to be 0.98, which is very close to 1. When the value of B1 is equal to 1, “Eq. 8” corresponds to an exponential model “Eq. 9”. Therefore, an exponential model was used to explain the moisture content change of samples during hot air drying:
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Under the initial condition t = 0, M = Mo, “Eq. 9” can be written as follows:
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The k0 value was determined by means of the non-linear least squares method with Me assumed to be zero. The drying constant k0 was 0.44 h−1 for unblanched samples and 0.70 h−1 for blanched samples (RMSE = 0.012 d.b. decimal and 0.09 and R2 = 0.99).
The solid lines in Fig. 3 present a comparison of the experimental results with the calculated results. As Fig. 3 demonstrates, the experimental results agreed well with the calculated results. Thus, pretreatment of green bell pepper using the blanching method obviously reduced the drying time.
Degradation of total chlorophyll Blanched samples (samples were blanched in a solution containing 1 g/L of NaHCO3) were utilized to study the kinetics of degradation of total chlorophyll during hot air and microwave drying. The total chlorophyll content of fresh green bell pepper samples was 444.58 mg/100 g of dry matter, which corresponds to 20.25 mg/100 g of fresh green pepper; after blanching, the total chlorophyll content of the samples was 438.38 mg/100 g of dry matter. It was observed that the drying conditions affected the loss of total chlorophyll. The relationship between the residual ratio of chlorophyll content and the moisture content under different drying conditions is shown in Fig. 5. The residual ratio of total chlorophyll is defined as the total chlorophyll content in the dried sample divided by the total chlorophyll content in the initial sample.
Relationship between the residual ratio of total chlorophyll and the moisture content during drying using different methods
Ha: hot air drying
MW: microwave drying
The kinetics of total chlorophyll decomposition is expressed in a first order reaction as follows (Stanley, 1978):
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Integrating under the initial condition of x = 0 to t = 0, “Eq. 11” can be rewritten as follows:
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The rate constants k1 were determined from the slopes of the curves obtained by plotting 
vs. time (t).
Table 2 presents the coefficients of the decomposition rate k1. The values of k1 ranged from 0.014 to 0.097 for hot air drying and 0.025 to 0.039 for microwave drying (RMSE = 0.47 – 0.99 mg/100 g of dry matter and R2 = 0.95 – 0.99). These data indicated that the value of k1 increased with increases in the drying temperature and the microwave energy. The statistical analysis demonstrated the significant influence of the drying conditions on the degradation of total chlorophyll (p < 0.05).
| Drying condition | k1 | k2 | k3 | ΔE*e |
|---|---|---|---|---|
| 40°C | 0.014 h−1 | 0.043 h−1 | 0.16 h−1 | 20.2 |
| 50°C | 0.020 h−1 | 0.062 h−1 | 0.19 h−1 | 23.2 |
| 60°C | 0.066 h−1 | 0.123 h−1 | 0.22 h−1 | 24.9 |
| 70°C | 0.097 h−1 | 0.174 h−1 | 0.28 h−1 | 28.3 |
| 500 W | 0.025 min−1 | 0.077 min−1 | 0.26 min−1 | 16.9 |
| 600 W | 0.030 min−1 | 0.082 min−1 | 0.35 min−1 | 18.1 |
| 800 W | 0.039 min−1 | 0.075 min−1 | 0.43 min−1 | 18.7 |
Degradation of ascorbic acid (AsA) AsA is a water-soluble vitamin, and the AsA content is widely used as an indicator of the change in food quality during fruit and vegetable processing (Chuah et al., 2008; Di Scala and Crapiste, 2008; Orikasa et al., 2008; Alibas et al., 2007; Negi and Roy, 2000). In this study, the retention of AsA in the blanched green bell pepper samples were examined under 7 different drying conditions. The relationships between the residual ratio of AsA content and the moisture content are shown in Fig. 6. The data demonstrated that AsA content decreased as moisture content decreased.
Relationship between the residual ratio of ascorbic acid and the moisture content during drying using different methods
The kinetics of AsA decomposition in a first order reaction was expressed as follows (Hosaka, 1972):
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The rate constants k2 were determined from the slopes of the curves obtained by plotting on the y axis and time (t) on the x axis. The values of k2 under the 7 drying conditions are provided in Table 2. The k2 values ranged from 0.043 to 0.174 (RMSE = 0.50 – 0.99 mg/100 g of dry matter and R2 = 0.95 – 0.99); raising the temperature or increasing the microwave energy increased the value of k2. The statistical analysis revealed that there was significant influence of the drying conditions on the degradation of AsA (p < 0.05).
Total color difference (TCD) A small amount of color change during drying, i.e., maintaining a color close to that of the fresh sample, is preferable for a dried product because the color indicates its quality. The change in the surface color of a sample is referred to as the TCD, which was calculated using “Eq. 2”, in which the L*, a* and b* values were observed at 1 h intervals during hot air drying and vacuum drying, and at 1 s intervals during microwave drying. The L0*, a0* and b0* values of the blanched green bell pepper samples, which were 38.9, −13.6 and 15.5, respectively, were used as the reference values. Figure 7 presents the changes in the surface color of the blanched samples under 7 different drying conditions. The data demonstrated that raising the drying temperature or increasing the microwave energy increased the TCD of the samples.
Changes in the TCD of blanched green bell pepper during different drying methods Bars: Standard error
The color decomposition of the blanched green bell pepper samples during hot air drying and microwave drying was described using the fractional conversion model (Ochoa et al., 2001; Avila and Silva, 1999; Levenspiel, 1972), as follows:
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The temperature-dependency of reaction rate constants An Arrhenius-type equation was applied to describe the temperature-dependency of reaction rate constants of total chlorophyll (k1), AsA (k2) and TCD (k3) for hot air drying as follows:
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The constant d1 and the activation energy E1 of total chlorophyll were determined by a least squares method using the coefficient of decomposition rate k1 in Table 2. The constant d1 was 3.0 h−1. The activation energy E1 was 62.4 kJ.mol−1.
The constant d2 and the activation energy E2 of AsA were determined using the value of k2 in Table 2. The constant d2 was 2.6 h−1. The activation energy E2 was 43.5 kJ.mol−1.
The constant d3 and the activation energy E3 for TCD were also determined using the value of k3 in Table 2. The activation energy E3 was 16.2 kJ.mol−1. The constant d3 value was 1.4 h−1.
These values are comparable to those previously reported for the activation energies of other food materials, i.e., 42.8 kJ/mol for red pepper (Bursa var.) slices in a fluidized bed dryer (Kaymak-Ertekin, 2002); 44 kJ.mol-1 for whole red pepper (Jaranda var.) during hot air drying with a through-flow air velocity of 0.5 m. s-1 at 50, 55, 60 and 70°C (Sanjuan et al., 2003); 39.7 kJ.mol-1 for red bell pepper pieces (1 cm cubes) during hot air drying at different temperatures (50, 60, 70 and 80°C) and using an air velocity of 2.5 m.s-1 (Vega et al., 2007); and 26.9 kJ.mol-1 for sliced red pepper (2 × 2 × 5 cm) in a cross-flow dryer at 50, 60 and 70°C, air velocity of 1.2 m.s-1 (Di Scala and Crapiste, 2008).
In this study, the impact of drying methods on the quality attributes of blanched green bell pepper was investigated. Blanching in a solution containing 1 g/L of NaHCO3 provided adequate POD inactivation, the smallest loss of AsA content and low TCD. The decomposition of total chlorophyll, AsA and TCD followed first order reaction kinetics, and the experimental results were in accord with the calculated results. An Arrhenius-type equation was used to calculate the temperature dependency of hot air drying. The activation energies of total chlorophyll, AsA and TCD were 62.4, 43.5 and 16.2 kJ/mol, respectively.
Acknowledgements This research was supported in part by a scientific fund from the Ministry of Education, Culture, Sports, Science, and Technology, Japan (MEXT). The authors would like to thank Vegetech Co., Ltd, Japan for providing the experimental materials.
