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Effects of solution spraying during hot air drying on drying rate, surface hardening and browning of fresh-cut Japanese pear
Engineering in Agriculture, Environment and Food xxx (2015) 1e6
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Research paper
Effects of solution spraying during hot air drying on drying rate, surface hardening and browning of fresh-cut Japanese pear Teppei Imaizumi a, Takahiro Orikasa b, *, Sonoko Morifusa c, Lam Van Man c, Yoshiki Muramatsu d, Shoji Koide b, Toshitaka Uchino e, Fumihiko Tanaka e, Daisuke Hamanaka e, Akio Tagawa c a
Graduate School of Bioresource and Bioenvironmental Sciences, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan Faculty of Agriculture, Iwate University, 3-18-8, Ueda, Morioka, Iwate, 020-8550, Japan Graduate School of Horticulture, Chiba University, 648, Matsudo, Matsudo, Chiba, 271-8510, Japan d Faculty of Regional Environment Science, 1-1-1, Sakuraoka, Setagaya-ku, Tokyo, 156-8502, Japan e Graduate School of Agriculture, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan b c
a r t i c l e i n f o
a b s t r a c t
Article history: Available online xxx
Three types of solutions (distilled water, aqueous ascorbic acid solution and aqueous citric acid solution) were sprayed periodically on the surface of fresh-cut Japanese pear during hot air drying at 40, 50 and 60 C. The effects of solution spraying on the drying kinetics, the surface hardening, and the sample discoloration were investigated. The drying rates of the sprayed samples increased 58e63, 16e19 and 20 e22% in comparison with those of the non-sprayed samples at 40, 50 and 60 C, respectively. The hardness of the sprayed samples decreased 12e24% and 2e9% compared to that of the non-sprayed samples, and the total color difference (DE) was 27e44 and 20e33% less compared the non-sprayed samples at 40 and 50 C, respectively. © 2015, Asian Agricultural and Biological Engineering Association. Published by Elsevier B.V. All rights reserved.
Keywords: Solution spraying Japanese pear Hot air drying Drying rate Surface color
1. Introduction In Japan, the domestic consumption of Japanese pears is the fourth highest consumption after banana, orange and apple (Ministry of Agriculture, Forestry and Fisheries of Japan (MAFF), 2008). This fruit contains a substantial amount of dietary fiber and many minerals, such as potassium (140 mg/100 g) and phosphorus (11 mg/100 g) (Science and Technology Agency in Japan, 2002). Because the Japanese pear is mostly consumed fresh (MAFF, 2009), it is also necessary to establish the processing techniques for Japanese pears to expand the consumption of this fruit. Dehydration is one of the most important methods for longterm food preservation. Dehydration can be accomplished by several methods, including hot air drying (Maskan, 2001), vacuum drying (Wu et al., 2007) and superheated steam drying (Sotome et al., 2005); the hot air drying method has been mainly used in the drying of fruits and vegetables. Raquel (2008) reported the
* Corresponding author. E-mail address: [email protected] (T. Orikasa).
drying characteristics and the change in sugar content of pears during the drying process. Mrad et al. (2012) and Raquel (2006) also studied the influence of the drying method on the physical properties and chemical compositions of pears, such as density, porosity and ascorbic acid. This information can improve the drying equipment, the process design and the quality and nutritive value of the dried product. Little research has been performed on the drying of Japanese pears; in particular, the relationships between the drying conditions and the changes in the physical and chemical properties of Japanese pears, such as surface hardening and shrinkage, have not been elucidated. It was reported that surface hardening causes a decrease in the drying rate of kiwifruits (Maskan, 2001; Orikasa et al., 2008), sweet potato (Orikasa et al., 2005), apple (Morifusa et al., 2012) and persimmon (Hayashi, 1989). Orikasa et al. (2008) and Morifusa et al. (2012) reported that the surface hardening and the drying rate could be controlled by periodically wetting the surface of kiwifruits and apples during hot air drying. The discoloration of fruits and vegetables during drying also causes their quality to deteriorate (Heaton and Alejandro, 1996; Maharaj and Sankat, 1996). Although sulfur dioxide and sulfite
http://dx.doi.org/10.1016/j.eaef.2015.01.001 1881-8366/© 2015, Asian Agricultural and Biological Engineering Association. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Imaizumi T, et al., Effects of solution spraying during hot air drying on drying rate, surface hardening and browning of fresh-cut Japanese pear, Engineering in Agriculture, Environment and Food (2015), http://dx.doi.org/10.1016/j.eaef.2015.01.001
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have been used to prevent discoloration (Fujimaki, 1976), some reports have indicated that using sulfur dioxide and sulfite causes some allergic disorders, such as adult bronchial asthma (Murata, 1998). Thus, alternative methods to prevent discoloration have been tried; for example, citric acid and ascorbic acid solutions are used as a substitute (Arai et al., 1998). Morifusa et al. (2012) examined the difference of the discoloration rate of cut apples by periodically spraying ascorbic acid, citric acid and saline solutions and distilled water on the material surface during hot air drying. The drying rate during hot air with spraying might increase because the sample surface is covered with the water of condensation same as superheated drying process. In this study, the effects of periodically spraying citric and ascorbic solutions were examined, and these operations were applied to Japanese pear drying to establish the optimal conditions for the drying of this fruit. In addition, a spraying method that maximized the drying rate and concurrently controlled the discoloration was evaluated. The objectives of this study were as follows: - Investigate the effects of spraying on the relationship between the surface hardening and the drying rate; - Describe the hot air drying kinetics of sliced Japanese pear, including volume change; - Examine the effects of spraying solutions on the surface hardening, drying rate and discoloration of sliced Japanese pear during hot air drying.
2. Materials and methods 2.1. Materials The fully ripe Japanese pear fruits were purchased from a local market and were immediately used for the experiments. The skin of the fruit was peeled off manually using a knife. The flesh then was cut into crescent-shaped pieces and its thickness was 10.5 ± 0.1 mm. The initial moisture content of the pear was measured by the method of using film with diatomite (Japan Food Research Laboratories, 1973) and was determined to be 6.4 ± 0.1 kgwater/kg-solid (86.5%(w.b.)). This method can dry completely high moisture content sample by using a diatomite as dehydrate additive, which prevent hardening of the sample surface and accelerate moving moisture in the dried sample (Tsutsumi, 1996; Orikasa et al., 2014). Tsutsumi (1996) reported that the value by the method using film with diatomite was almost the same as that by AOAC method (AOAC, 1995).
Fig. 1. Schematic diagram of the experimental hot air drying system.
study, to examine the ameliorating on surface hardening and discoloration during hot air drying, distilled water, 0.2% citric acid and 0.2% ascorbic acid solutions were sprayed on the sample surface. The sprayings were performed from the top to the bottom of the sample at intervals of 30 min using an atomizer (material: PET, capacity: 50 ml). The sprayings did not substantially increase the moisture content of the sample because the amount of liquid delivered in each spraying was less than 0.01 g. The solutionsprayed sample (sample “W”) was dried until the moisture content reached 2.0 kg-water/kg-solid (66.7%(w.b.)). 2.3. Volume The volume of sample D was measured by the water substitution method (Sjoholm and Gekas, 1995). The volume change (SV) was calculated using the following equation:
SV ¼
V V0
(1)
where V0 is the initial volume of the sample before drying (m3), and V is the volume of the dried sample (m3).
2.2. Hot air drying
2.4. Hardness
A schematic diagram of the experimental hot air drying system is shown in Fig. 1. The apparatus consisted of a forced convection drying oven (WFO-400, Tokyo Science Machine, Japan) for maintaining the desired drying temperatures, an acrylic pipe (diameter 15 cm, length 31 cm), a fan (San Ace 120, Sanyo Electric Machine, Japan), a DC power source (AD-8724D, A & D, Japan) for controlling air velocity (1.0 m/s), a timer (Lab. clock ELT-2, As One, Japan) and a digital balance connected to a personal computer. The sample mass was measured at 10 min intervals using a digital balance and automatically recorded on the personal computer. The fan was stopped 1 min before measuring the sample weight and was turned back on after the measurement. The changes in the moisture content of the sample were measured until the final moisture content reached 1.0 kg-water/kg-solid (50%(w.b.)) by using the non-sprayed sample (sample “D”) at temperatures of 40, 50 and 60 C. In this
The hardness of all the samples was measured using a hardness tester (GS-754G, TECLOCK, Japan) before and after the drying test. The hardness was measured at three points on the sample surface and then converted into the unit of force (N), according to the ASTM-D-2240 (ASTM, 2003) standard. Measurements of each sample were taken in triplicate. 2.5. Color The L* (lightness: L* ¼ 0 for black, L* ¼ 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) indexes of the CIEclairage, L*, a*, b*) LAB (Commission Internationale de l'e colorimetric system were used to evaluate the color change of the samples. The L*, a* and b* values of all the samples were measured
Please cite this article in press as: Imaizumi T, et al., Effects of solution spraying during hot air drying on drying rate, surface hardening and browning of fresh-cut Japanese pear, Engineering in Agriculture, Environment and Food (2015), http://dx.doi.org/10.1016/j.eaef.2015.01.001
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using a chromameter (CR-200b, MINOLTA, Japan) before and after the drying test. Measurements of each sample were taken in triplicate. The color difference DE after drying was calculated using the following equation:
DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðL0 L*Þ2 þ ða0 a*Þ2 þ ðb0 b*Þ2
(2)
where L0, a0, and b0 are the values of the fresh sample. 3. Results and discussion 3.1. Drying shrinkage It is evident that the loss of water during the drying process of fruits and vegetables causes a change in their volume. This volume change is expressed as shrinkage (Sjoholm and Gekas, 1995; Wu et al., 2007). Drying shrinkage affects the product quality, the drying process and the rehydration capability of the dried food materials (Karathanos et al., 1993; Maskan, 2001; McMinn and Magee, 1997a, 1997b). In this study, the volume of sample D was measured, and the volume change SV was calculated using Eq. (1). Fig. 2 shows the relationship between SV and the moisture content at three different drying temperatures. The SV was observed to decrease linearly as the moisture content decreased. Therefore, the SV of Japanese pears during hot air drying was expressed using the following equation:
SV ¼
V ¼ nM þ l V0
(3)
where M is the moisture content (kg-water/kg-solid), and n and l are constants. The solid lines in Fig. 2 represent the results that were calculated from Eq. (3). As shown in Fig. 2, the measured results agreed well with the calculated results. Thus, the volume of the sample at the specific moisture content could be predicted using Eq. (3). The values of n and l at each temperature are presented in Fig. 2. There was no significant difference in the temperature range of 40e60 C in the values of n and l. Therefore, the volume change was not significantly dependent on the drying temperature for the temperature and moisture content ranges tested in this study, and SV could be approximated by the linear equation of the moisture content using the mean values of n and l (n ¼ 0.13, l ¼ 0.13). The calculated value of SV using the mean values of n and l is shown with the short dashed line in Fig. 2. It is necessary to consider the shrinkage of the surface area to understand the drying characteristics. Murata et al., 1993 reported that the surface area shrinkage ratio SA of agricultural products during drying can be expressed according to the following equation:
SA ¼
A ¼ A0
V V0
2 3
2
¼ ðnM þ lÞ3
Fig. 2. Volume change SV as a function of moisture content of samples during hot air drying (sample D). Solid line: Calculated value of a non least square method. Short dashes line: Calculated value of the Eq (3)
(4)
where A (m2) is the surface area of the sample in the corresponding moisture content and A0 (m2) is the initial surface area. Thus, the surface area at a specific moisture content could be predicted using A0. 3.2. Drying kinetics of sample D The drying characteristics of sample D were investigated at temperatures of 40, 50 and 60 C with an air velocity of 1.0 m s1. Fig. 3 shows the changes in the moisture content during hot air
Fig. 3. Changes in moisture content during hot air drying of Japanese pear (sample D).
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drying of sample D. The moisture content decreases gradually as the drying time increases, and the drying time was shortened when the temperature was high. Because shrinkage of fruits and vegetables is caused by hot air drying (Maskan, 2001; Orikasa et al., 2005), the changes in the volume and the surface area must be determined to analyze the drying characteristics of Japanese pears. In the experiment, the volume change was measured to predict the surface area of the sample. As shown in Fig. 2, the volume of the sample decreased linearly during drying. The drying rate of the samples that shrank during drying is shown by the following equation (Murata et al., 1993):
R¼
Wd dM , A dt
(5)
where R is the drying rate (kg$m2$h1), Wd is the dry mass(kg) of the sample, and A is the surface area (m2) at the specific moisture content calculated from Eq. (4). Fig. 4 shows the relationship between the drying rate R calculated using Eq. (5) and the moisture content M at each drying temperature. The drying rates decreased linearly as the moisture content decreased. Thus, the drying of sample D assumed to be the first falling rate period. The changes in the moisture content during the first falling rate period can be defined by the following equation:
dM ¼ kf ðM Me Þ dt
drying were discussed. The sprayings were performed at intervals of 30 min. The sprayed sample (sample “W”) was dried until the moisture content reached 2.0 kg-water/kg-solid (66.7%(w.b.)). Fig. 5 shows the changes in the moisture content of each drying sample. The drying time of sample W was 0.5e1.0 h less than that of sample D at all temperatures. Fig. 6 shows the changes in the drying rate R of sample D and sample W during hot air drying. According to these results, the drying rate of sample W was greater than that of sample D at any temperature. Morifusa et al. (2012) and Orikasa et al. (2008) reported that drying rate of fruit and vegetables are decreased by the surface hardening. Thus, these results in this study indicated the possibility that the surface hardening was controlled by the solution sprayings. The drying rate is obtained using Eqs. (4) and (5).
RC ¼
Wd dM , 2 A0 ðnM þ lÞ3 dt
(8)
The equation is integrated with respect to t with the initial condition (M ¼ M0). Thus, the moisture content can be obtained using following equation:
M¼
1 n
1 n A R , 0 C ,t þ ðnM0 þ lÞ3 3 Wd
3
l
(9)
(6)
where kf is the drying rate constant (h1) and Me is the equilibrium moisture content (kg-water/kg-solid). Eq. (7) can be obtained by solving Eq. (6) under the initial conditions (M ¼ M0 and t ¼ 0).
M Me ¼ exp kf t M0 Me
(7)
The parameters kf and Me were determined using a nonlinear least squares method and the measured moisture content of the sample. The calculated results are represented as curves in Fig. 3. The measured results agreed well with the calculated results (RMSE ¼ 0.06e0.11 kg-water/kg-solid. According to these results, the drying process of sample D from the initial moisture content to 1.0 kg-water/kg-solid (50%(w.b.)) could be estimated using the exponential model (7).
3.3. Drying kinetics of sample W In this study, distilled water, 0.2% citric acid and 0.2% ascorbic acid solutions were sprayed on the sample surface, and the effect of the spraying process on the drying rate of the samples during
Fig. 4. Changes in drying rate during hot air drying of Japanese pear (sample D).
Fig. 5. Changes in moisture content of sample D and sample W during hot air drying.
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Table 1 Hardness of sample D and sample W before and after drying.a 40 C Sample D Distilled water Citric acid Ascorbic acid
Before drying(N) 0.58 ± 0.01 0.57 ± 0.01 0.55 ± 0.01 0.54 ± 0.01
After drying(N) 0.65 ± 0.02 0.58 ± 0.01 0.56 ± 0.01 0.57 ± 0.03
50 C Sample D Distilled water Citric acid Ascorbic acid
Before drying(N) 0.56 ± 0.02 0.58 ± 0.02 0.57 ± 0.02 0.56 ± 0.02
After drying(N) 0.68 ± 0.01 0.62 ± 0.03 0.61 ± 0.01 0.61 ± 0.03
60 C Sample D Distilled water Citric acid Ascorbic acid
Before drying(N) 0.55 ± 0.01 0.59 ± 0.01 0.60 ± 0.01 0.60 ± 0.02
After drying(N) 0.68 ± 0.01 0.62 ± 0.01 0.64 ± 0.01 0.63 ± 0.01
a
Values are mean ± S.D.
2e9%, respectively. The result clearly shows that the hardness of sample W is less than that of sample D because the sample surface was kept moist by the periodical spraying during drying, even when the temperature was high. Thus, sample hardening could be prevented by spraying during drying. The drying rate of sample W was greater than that of sample D, as described above. The drying rates of sample W increased 58e63, 16e19 and 20e22% at 40, 50 and 60 C, respectively. This finding suggested that if the hardening of the sample surface can be prevented, the decrease in the drying rate of Japanese pears will be inhibited. Water migration from the sample surface to the air was relatively slow at 40 C. In other words, most of the sprayed solution remained on the sample surface for a long time. Therefore, the solution spraying was effective for the drying rate at 40 C more than at 50 and 60 C. These results indicated that spraying during hot air drying inhibits hardening of the sample surface. 3.5. Color
Fig. 6. Changes in drying rate R of sample D and sample W during hot air drying.
where A0 is the initial surface area (m2), Wd is the dry matter mass (g) and RC is the drying rate for a constant-rate drying period. The moisture contents calculated using Eq. (9) are represented as curves in Fig. 5. The figures indicated that the measured results agreed well with the calculated results (RMSE ¼ 0.0001e0.009 kgwater/kg-solid). Thus, the hot air drying process of sample W was shown to be a constant-rate drying period, and the moisture content change over time could be predicted by Eq. (9). Superheated Steam was available for drying of some agricultural products (Somjai et al., 2009; Rumruaytum et al., 2014). Under the process of superheated steam drying, the surface of sample is covered by the water of condensation, which condition was similar to the condition in this study. So, the condition of the drying with solution spraying needs to be advanced much more simple and low cost process to keep same effect of superheated steam. 3.4. Surface hardening Table 1 shows the hardness at each sample before and after drying. The hardness of samples D and W increased 12e24% and
Table 2 shows the color difference before and after each drying. The color difference of sample W was less than that of sample D. Although the enzymatic reaction is the main factor in the browning of fruits, Japanese pears have a small amount of polyphenol, which becomes the substrate of the browning reaction (Shimohashi and Terada, 1996). Thus, it is unlikely that the color change was caused by the enzymatic reaction. Another major factor contributing to browning may be the Maillard reaction. This reaction causes non-enzymatic browning and is influenced by many factors, Table 2 Color difference of sample D and sample W after hot air drying.a 40 C Sample D Distilled water Citric acid Ascorbic acid
DE 10.6 7.7 6.4 5.9
± ± ± ±
2.1 0.6 0.5 0.6
50 C Sample D Distilled water Citric acid Ascorbic acid
DE 10.5 8.4 7.4 7.0
± ± ± ±
1.5 0.6 1.0 1.5
60 C Sample D Distilled water Citric acid Ascorbic acid
DE 10.1 10.9 8.7 8.7
± ± ± ±
2.4 0.2 1.3 2.1
a
Values are mean ± S.D.
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such as temperature and pH (Ames, 1990). The browning of sample W was inhibited because the temperature of sample W was decreased by spraying. Morifusa et al. (2012) reported the changes in sample temperatures of apple during hot air drying using the same solution spraying. From the report, the temperatures of sample W remain lower level than that of sample D during drying (5e10 C). We evaluated that the same effect was expressed in this study. The color difference of sample W was 27e44 and 20e33% less than that of sample D at 40 and 50 C, respectively, while there was little effect at 60 C. This observation suggested that the inhibition of discoloration is greater at lower temperatures because the sprayed solution remained on the sample surface for a long time. Spraying the citric acid and ascorbic acid solutions decreased the pH of the sample surface and controlled the Maillard reaction. These results indicate that the color change caused by drying can be inhibited by solution spraying. Further research into a method to prevent discoloration is expected in the future. 4. Conclusion Japanese pear slices were dried by hot air at 40, 50 and 60 C. The three types of solutions (distilled water, aqueous ascorbic acid solution and aqueous citric acid solution) were sprayed periodically on the sample surface, and the effect of solution spraying on the drying characteristics and the quality of the samples was examined. The results were summarized as follows: (1) Changes in the moisture content of non-sprayed Japanese pears during hot air drying were explained by the exponential model of first falling rate drying. (2) The drying rate of the samples sprayed with citric acid solution (0.2%, w/v), ascorbic acid solution (0.2%, w/v) and distilled water were greater than those of sample D. (3) The hardening of the sampling surface was controlled by applying the spraying process. (4) The discoloration was inhibited by the solution spraying because the Maillard reaction was occurred at lower temperatures and pH values. The spraying process during drying can be expected to become the new technology used for the drying processing of Japanese pears and other agricultural products. References AOAC. Official methods of analysis of AOAC International. 16th ed 1995. ASTM (American Society for Testing and Materials). Standard test method for rubber property e Durometer hardeness. 2003. http://www.astm.org/ Standards/D2240.htm. Ames JM. Control of the Maillerd reaction in food systems. Trends Food Sci Technol 1990;1:150e4. Arai Y, Muto H, Sano Y, Ito K. Food and food additives hypersensitivity in adult asthmatics. : III. Adverse reaction to sulfites in adult asthmatics. Jpn Soc Allergol 1998;47:1163e7.
Fujimaki M. Food chemistry. Tokyo: Asakura Publishing; 1976. p. 102e3. Hayashi S. Drying characteristics of Japanese-persimmon. J Jpn Soc Agric Mach 1989;51(6):71e6. Heaton JW, Alejandro GM. Chlorophyll degradation in processed foods and senescent plant tissues. Trends Food Sci Technol 1996;71:8e15. Japan Food Research Laboratories. Report of Japan food research laboratories. Tokyo, Japan: Printing Bureau, Ministry of Finance of Japan; 1973. p. 37e40. Karathanos V, Anglea S, Karel M. Collapse of structure during drying of celery. Dry Technol 1993;11(5):1005e23. MAFF (Japanese Ministry of Agriculture, Forestry and Fisheries). Pocket statistics of agriculture, forestry and fisheries in 2008. Tokyo, Japan: Association of Agriculture and Forestry Statics; 2008. p. 272e3. MAFF (Japanese Ministry of Agriculture, Forestry and Fisheries). Current states of fruits for processing or business use, and measures in the future. 2009. http:// www.maff.go.jp/j/seisan/kakou/yasai_kazitu/pdf/kazitu_meguzi4.pdf. Maharaj V, Sankat CK. Quality changes in dehydrated dasheen leaves: effects of blanching pre-treatments and drying conditions. Food Res Int 1996;29:563e8. Maskan M. Drying, shrinkage and rehydration characteristics of kiwifruits during hot air and microwave drying. J Food Eng 2001;48:177e82. McMinn WAM, Magee TRA. Physical characteristics of dehydrated potatoes- Part 1. J Food Eng 1997a;33:37e48. McMinn WAM, Magee TRA. Physical characteristics of dehydrated potatoes- Part 2. J Food Eng 1997b;33:49e55. Morifusa S, Orikasa T, Muramatsu Y, Tagawa A. Effect of solution spraying during hot air drying on surface hardening and browning of freshly cut apple pulp. J Jpn Soc Food Sci Technol 2012;59(11):583e90. Mrad ND, Boudhioua N, Kechaou N, Countrois F, Bonazzi C. Influence of air drying temperature on kinetics, physicochemical properties, total phenolic content and ascorbic acid of pears. Food Bioprod Process 2012;90(3):433e41. Murata M. Recent progress in polyphenol oxidase and control of enzymatic browning. J Jpn Soc Food Sci Technol 1998;45(3):177e85. Murata S, Koide S, Miyauchi K. Drying characteristics of sliced radish. J Jpn Soc Agric Mach 1993;55(4):83e9. Orikasa T, Tagawa A, Soma S, Imoto M, Ogawa Y. Hot air drying characteristics of fruits and vegetables and surface hardening of samples during drying. J Jpn Soc Agric Mach 2005;67(6):62e70. Orikasa T, Wu L, Shiina T, Tagawa A. Drying characteristics of kiwifruit during hot air drying. J Food Eng 2008;85:303e8. Orikasa T, Koide S, Okamoto S, Imaizumi T, Muramatsu Y, Takeda J, et al. Impacts of hot air and vacuum drying on the quality attributes of kiwifruit slices. J Food Eng 2014;125:51e8. Rumruaytum P, Borompichaichartkul Chaleeda, Kongpensook V. Effect of drying involving fluidisation in superheated steam on physicochemical and antioxidant properties of Thai native rice cultivars. J Food Eng 2014;123:143e7. Raquel PFG. Influence of drying method on density and porosity. Food Bioprod Process 2006;84:179e85. Raquel PFG. Pear drying: experimental validation of a mathematical prediction model. Food Bioprod Process 2008;86:248e53. Science and Technology Agency in Japan. Standard tables of food composition in Japan. Tokyo, Japan: Department of publishing, Kagawa Nutrition University; 2002. p. 114. Shimohashi J, Terada K. Fractional quantification of polyphenol compound and browning in vegetables and fruits. Fac J Komazawa Women's Jr Coll 1996;29: 45e9. Sjoholm I, Gekas V. Apple shrinkage upon drying. J Food Eng 1995;25:123e30. Somjai T, Achariyaviriya S, Achariyaviriya A, Namsanguan K. Strategy for longan drying in two-stage superheated steam and hot air. J Food Eng 2009;95(2): 313e21. Sotome I, Sakamoto K, Takenaka M, Ogasawara Y, Nadachi Y, Isobe S. Heat transfer and drying properties of superheated steam containing Micro-droplets of water. Jpn J Food Eng 2005;6(4):229e36. Tsutsumi C. Moisture content. In: The Japanese Society for Food Science and Technology, editor. New food analysis methods. Tokyo, Japan: Printing Bureau, Korin; 1996. p. 5e30. Wu L, Orikasa T, Ogawa Y, Tagawa A. Vacuum drying characteristics of eggplants. J Food Eng 2007;83:422e9.
Please cite this article in press as: Imaizumi T, et al., Effects of solution spraying during hot air drying on drying rate, surface hardening and browning of fresh-cut Japanese pear, Engineering in Agriculture, Environment and Food (2015), http://dx.doi.org/10.1016/j.eaef.2015.01.001
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Research paper
Effects of solution spraying during hot air drying on drying rate, surface hardening and browning of fresh-cut Japanese pear Teppei Imaizumi a, Takahiro Orikasa b, *, Sonoko Morifusa c, Lam Van Man c, Yoshiki Muramatsu d, Shoji Koide b, Toshitaka Uchino e, Fumihiko Tanaka e, Daisuke Hamanaka e, Akio Tagawa c a
Graduate School of Bioresource and Bioenvironmental Sciences, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan Faculty of Agriculture, Iwate University, 3-18-8, Ueda, Morioka, Iwate, 020-8550, Japan Graduate School of Horticulture, Chiba University, 648, Matsudo, Matsudo, Chiba, 271-8510, Japan d Faculty of Regional Environment Science, 1-1-1, Sakuraoka, Setagaya-ku, Tokyo, 156-8502, Japan e Graduate School of Agriculture, Kyushu University, 6-10-1, Hakozaki, Higashi-ku, Fukuoka, 812-8581, Japan b c
a r t i c l e i n f o
a b s t r a c t
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Three types of solutions (distilled water, aqueous ascorbic acid solution and aqueous citric acid solution) were sprayed periodically on the surface of fresh-cut Japanese pear during hot air drying at 40, 50 and 60 C. The effects of solution spraying on the drying kinetics, the surface hardening, and the sample discoloration were investigated. The drying rates of the sprayed samples increased 58e63, 16e19 and 20 e22% in comparison with those of the non-sprayed samples at 40, 50 and 60 C, respectively. The hardness of the sprayed samples decreased 12e24% and 2e9% compared to that of the non-sprayed samples, and the total color difference (DE) was 27e44 and 20e33% less compared the non-sprayed samples at 40 and 50 C, respectively. © 2015, Asian Agricultural and Biological Engineering Association. Published by Elsevier B.V. All rights reserved.
Keywords: Solution spraying Japanese pear Hot air drying Drying rate Surface color
1. Introduction In Japan, the domestic consumption of Japanese pears is the fourth highest consumption after banana, orange and apple (Ministry of Agriculture, Forestry and Fisheries of Japan (MAFF), 2008). This fruit contains a substantial amount of dietary fiber and many minerals, such as potassium (140 mg/100 g) and phosphorus (11 mg/100 g) (Science and Technology Agency in Japan, 2002). Because the Japanese pear is mostly consumed fresh (MAFF, 2009), it is also necessary to establish the processing techniques for Japanese pears to expand the consumption of this fruit. Dehydration is one of the most important methods for longterm food preservation. Dehydration can be accomplished by several methods, including hot air drying (Maskan, 2001), vacuum drying (Wu et al., 2007) and superheated steam drying (Sotome et al., 2005); the hot air drying method has been mainly used in the drying of fruits and vegetables. Raquel (2008) reported the
* Corresponding author. E-mail address: [email protected] (T. Orikasa).
drying characteristics and the change in sugar content of pears during the drying process. Mrad et al. (2012) and Raquel (2006) also studied the influence of the drying method on the physical properties and chemical compositions of pears, such as density, porosity and ascorbic acid. This information can improve the drying equipment, the process design and the quality and nutritive value of the dried product. Little research has been performed on the drying of Japanese pears; in particular, the relationships between the drying conditions and the changes in the physical and chemical properties of Japanese pears, such as surface hardening and shrinkage, have not been elucidated. It was reported that surface hardening causes a decrease in the drying rate of kiwifruits (Maskan, 2001; Orikasa et al., 2008), sweet potato (Orikasa et al., 2005), apple (Morifusa et al., 2012) and persimmon (Hayashi, 1989). Orikasa et al. (2008) and Morifusa et al. (2012) reported that the surface hardening and the drying rate could be controlled by periodically wetting the surface of kiwifruits and apples during hot air drying. The discoloration of fruits and vegetables during drying also causes their quality to deteriorate (Heaton and Alejandro, 1996; Maharaj and Sankat, 1996). Although sulfur dioxide and sulfite
http://dx.doi.org/10.1016/j.eaef.2015.01.001 1881-8366/© 2015, Asian Agricultural and Biological Engineering Association. Published by Elsevier B.V. All rights reserved.
Please cite this article in press as: Imaizumi T, et al., Effects of solution spraying during hot air drying on drying rate, surface hardening and browning of fresh-cut Japanese pear, Engineering in Agriculture, Environment and Food (2015), http://dx.doi.org/10.1016/j.eaef.2015.01.001
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have been used to prevent discoloration (Fujimaki, 1976), some reports have indicated that using sulfur dioxide and sulfite causes some allergic disorders, such as adult bronchial asthma (Murata, 1998). Thus, alternative methods to prevent discoloration have been tried; for example, citric acid and ascorbic acid solutions are used as a substitute (Arai et al., 1998). Morifusa et al. (2012) examined the difference of the discoloration rate of cut apples by periodically spraying ascorbic acid, citric acid and saline solutions and distilled water on the material surface during hot air drying. The drying rate during hot air with spraying might increase because the sample surface is covered with the water of condensation same as superheated drying process. In this study, the effects of periodically spraying citric and ascorbic solutions were examined, and these operations were applied to Japanese pear drying to establish the optimal conditions for the drying of this fruit. In addition, a spraying method that maximized the drying rate and concurrently controlled the discoloration was evaluated. The objectives of this study were as follows: - Investigate the effects of spraying on the relationship between the surface hardening and the drying rate; - Describe the hot air drying kinetics of sliced Japanese pear, including volume change; - Examine the effects of spraying solutions on the surface hardening, drying rate and discoloration of sliced Japanese pear during hot air drying.
2. Materials and methods 2.1. Materials The fully ripe Japanese pear fruits were purchased from a local market and were immediately used for the experiments. The skin of the fruit was peeled off manually using a knife. The flesh then was cut into crescent-shaped pieces and its thickness was 10.5 ± 0.1 mm. The initial moisture content of the pear was measured by the method of using film with diatomite (Japan Food Research Laboratories, 1973) and was determined to be 6.4 ± 0.1 kgwater/kg-solid (86.5%(w.b.)). This method can dry completely high moisture content sample by using a diatomite as dehydrate additive, which prevent hardening of the sample surface and accelerate moving moisture in the dried sample (Tsutsumi, 1996; Orikasa et al., 2014). Tsutsumi (1996) reported that the value by the method using film with diatomite was almost the same as that by AOAC method (AOAC, 1995).
Fig. 1. Schematic diagram of the experimental hot air drying system.
study, to examine the ameliorating on surface hardening and discoloration during hot air drying, distilled water, 0.2% citric acid and 0.2% ascorbic acid solutions were sprayed on the sample surface. The sprayings were performed from the top to the bottom of the sample at intervals of 30 min using an atomizer (material: PET, capacity: 50 ml). The sprayings did not substantially increase the moisture content of the sample because the amount of liquid delivered in each spraying was less than 0.01 g. The solutionsprayed sample (sample “W”) was dried until the moisture content reached 2.0 kg-water/kg-solid (66.7%(w.b.)). 2.3. Volume The volume of sample D was measured by the water substitution method (Sjoholm and Gekas, 1995). The volume change (SV) was calculated using the following equation:
SV ¼
V V0
(1)
where V0 is the initial volume of the sample before drying (m3), and V is the volume of the dried sample (m3).
2.2. Hot air drying
2.4. Hardness
A schematic diagram of the experimental hot air drying system is shown in Fig. 1. The apparatus consisted of a forced convection drying oven (WFO-400, Tokyo Science Machine, Japan) for maintaining the desired drying temperatures, an acrylic pipe (diameter 15 cm, length 31 cm), a fan (San Ace 120, Sanyo Electric Machine, Japan), a DC power source (AD-8724D, A & D, Japan) for controlling air velocity (1.0 m/s), a timer (Lab. clock ELT-2, As One, Japan) and a digital balance connected to a personal computer. The sample mass was measured at 10 min intervals using a digital balance and automatically recorded on the personal computer. The fan was stopped 1 min before measuring the sample weight and was turned back on after the measurement. The changes in the moisture content of the sample were measured until the final moisture content reached 1.0 kg-water/kg-solid (50%(w.b.)) by using the non-sprayed sample (sample “D”) at temperatures of 40, 50 and 60 C. In this
The hardness of all the samples was measured using a hardness tester (GS-754G, TECLOCK, Japan) before and after the drying test. The hardness was measured at three points on the sample surface and then converted into the unit of force (N), according to the ASTM-D-2240 (ASTM, 2003) standard. Measurements of each sample were taken in triplicate. 2.5. Color The L* (lightness: L* ¼ 0 for black, L* ¼ 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) indexes of the CIEclairage, L*, a*, b*) LAB (Commission Internationale de l'e colorimetric system were used to evaluate the color change of the samples. The L*, a* and b* values of all the samples were measured
Please cite this article in press as: Imaizumi T, et al., Effects of solution spraying during hot air drying on drying rate, surface hardening and browning of fresh-cut Japanese pear, Engineering in Agriculture, Environment and Food (2015), http://dx.doi.org/10.1016/j.eaef.2015.01.001
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using a chromameter (CR-200b, MINOLTA, Japan) before and after the drying test. Measurements of each sample were taken in triplicate. The color difference DE after drying was calculated using the following equation:
DE ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðL0 L*Þ2 þ ða0 a*Þ2 þ ðb0 b*Þ2
(2)
where L0, a0, and b0 are the values of the fresh sample. 3. Results and discussion 3.1. Drying shrinkage It is evident that the loss of water during the drying process of fruits and vegetables causes a change in their volume. This volume change is expressed as shrinkage (Sjoholm and Gekas, 1995; Wu et al., 2007). Drying shrinkage affects the product quality, the drying process and the rehydration capability of the dried food materials (Karathanos et al., 1993; Maskan, 2001; McMinn and Magee, 1997a, 1997b). In this study, the volume of sample D was measured, and the volume change SV was calculated using Eq. (1). Fig. 2 shows the relationship between SV and the moisture content at three different drying temperatures. The SV was observed to decrease linearly as the moisture content decreased. Therefore, the SV of Japanese pears during hot air drying was expressed using the following equation:
SV ¼
V ¼ nM þ l V0
(3)
where M is the moisture content (kg-water/kg-solid), and n and l are constants. The solid lines in Fig. 2 represent the results that were calculated from Eq. (3). As shown in Fig. 2, the measured results agreed well with the calculated results. Thus, the volume of the sample at the specific moisture content could be predicted using Eq. (3). The values of n and l at each temperature are presented in Fig. 2. There was no significant difference in the temperature range of 40e60 C in the values of n and l. Therefore, the volume change was not significantly dependent on the drying temperature for the temperature and moisture content ranges tested in this study, and SV could be approximated by the linear equation of the moisture content using the mean values of n and l (n ¼ 0.13, l ¼ 0.13). The calculated value of SV using the mean values of n and l is shown with the short dashed line in Fig. 2. It is necessary to consider the shrinkage of the surface area to understand the drying characteristics. Murata et al., 1993 reported that the surface area shrinkage ratio SA of agricultural products during drying can be expressed according to the following equation:
SA ¼
A ¼ A0
V V0
2 3
2
¼ ðnM þ lÞ3
Fig. 2. Volume change SV as a function of moisture content of samples during hot air drying (sample D). Solid line: Calculated value of a non least square method. Short dashes line: Calculated value of the Eq (3)
(4)
where A (m2) is the surface area of the sample in the corresponding moisture content and A0 (m2) is the initial surface area. Thus, the surface area at a specific moisture content could be predicted using A0. 3.2. Drying kinetics of sample D The drying characteristics of sample D were investigated at temperatures of 40, 50 and 60 C with an air velocity of 1.0 m s1. Fig. 3 shows the changes in the moisture content during hot air
Fig. 3. Changes in moisture content during hot air drying of Japanese pear (sample D).
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drying of sample D. The moisture content decreases gradually as the drying time increases, and the drying time was shortened when the temperature was high. Because shrinkage of fruits and vegetables is caused by hot air drying (Maskan, 2001; Orikasa et al., 2005), the changes in the volume and the surface area must be determined to analyze the drying characteristics of Japanese pears. In the experiment, the volume change was measured to predict the surface area of the sample. As shown in Fig. 2, the volume of the sample decreased linearly during drying. The drying rate of the samples that shrank during drying is shown by the following equation (Murata et al., 1993):
R¼
Wd dM , A dt
(5)
where R is the drying rate (kg$m2$h1), Wd is the dry mass(kg) of the sample, and A is the surface area (m2) at the specific moisture content calculated from Eq. (4). Fig. 4 shows the relationship between the drying rate R calculated using Eq. (5) and the moisture content M at each drying temperature. The drying rates decreased linearly as the moisture content decreased. Thus, the drying of sample D assumed to be the first falling rate period. The changes in the moisture content during the first falling rate period can be defined by the following equation:
dM ¼ kf ðM Me Þ dt
drying were discussed. The sprayings were performed at intervals of 30 min. The sprayed sample (sample “W”) was dried until the moisture content reached 2.0 kg-water/kg-solid (66.7%(w.b.)). Fig. 5 shows the changes in the moisture content of each drying sample. The drying time of sample W was 0.5e1.0 h less than that of sample D at all temperatures. Fig. 6 shows the changes in the drying rate R of sample D and sample W during hot air drying. According to these results, the drying rate of sample W was greater than that of sample D at any temperature. Morifusa et al. (2012) and Orikasa et al. (2008) reported that drying rate of fruit and vegetables are decreased by the surface hardening. Thus, these results in this study indicated the possibility that the surface hardening was controlled by the solution sprayings. The drying rate is obtained using Eqs. (4) and (5).
RC ¼
Wd dM , 2 A0 ðnM þ lÞ3 dt
(8)
The equation is integrated with respect to t with the initial condition (M ¼ M0). Thus, the moisture content can be obtained using following equation:
M¼
1 n
1 n A R , 0 C ,t þ ðnM0 þ lÞ3 3 Wd
3
l
(9)
(6)
where kf is the drying rate constant (h1) and Me is the equilibrium moisture content (kg-water/kg-solid). Eq. (7) can be obtained by solving Eq. (6) under the initial conditions (M ¼ M0 and t ¼ 0).
M Me ¼ exp kf t M0 Me
(7)
The parameters kf and Me were determined using a nonlinear least squares method and the measured moisture content of the sample. The calculated results are represented as curves in Fig. 3. The measured results agreed well with the calculated results (RMSE ¼ 0.06e0.11 kg-water/kg-solid. According to these results, the drying process of sample D from the initial moisture content to 1.0 kg-water/kg-solid (50%(w.b.)) could be estimated using the exponential model (7).
3.3. Drying kinetics of sample W In this study, distilled water, 0.2% citric acid and 0.2% ascorbic acid solutions were sprayed on the sample surface, and the effect of the spraying process on the drying rate of the samples during
Fig. 4. Changes in drying rate during hot air drying of Japanese pear (sample D).
Fig. 5. Changes in moisture content of sample D and sample W during hot air drying.
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Table 1 Hardness of sample D and sample W before and after drying.a 40 C Sample D Distilled water Citric acid Ascorbic acid
Before drying(N) 0.58 ± 0.01 0.57 ± 0.01 0.55 ± 0.01 0.54 ± 0.01
After drying(N) 0.65 ± 0.02 0.58 ± 0.01 0.56 ± 0.01 0.57 ± 0.03
50 C Sample D Distilled water Citric acid Ascorbic acid
Before drying(N) 0.56 ± 0.02 0.58 ± 0.02 0.57 ± 0.02 0.56 ± 0.02
After drying(N) 0.68 ± 0.01 0.62 ± 0.03 0.61 ± 0.01 0.61 ± 0.03
60 C Sample D Distilled water Citric acid Ascorbic acid
Before drying(N) 0.55 ± 0.01 0.59 ± 0.01 0.60 ± 0.01 0.60 ± 0.02
After drying(N) 0.68 ± 0.01 0.62 ± 0.01 0.64 ± 0.01 0.63 ± 0.01
a
Values are mean ± S.D.
2e9%, respectively. The result clearly shows that the hardness of sample W is less than that of sample D because the sample surface was kept moist by the periodical spraying during drying, even when the temperature was high. Thus, sample hardening could be prevented by spraying during drying. The drying rate of sample W was greater than that of sample D, as described above. The drying rates of sample W increased 58e63, 16e19 and 20e22% at 40, 50 and 60 C, respectively. This finding suggested that if the hardening of the sample surface can be prevented, the decrease in the drying rate of Japanese pears will be inhibited. Water migration from the sample surface to the air was relatively slow at 40 C. In other words, most of the sprayed solution remained on the sample surface for a long time. Therefore, the solution spraying was effective for the drying rate at 40 C more than at 50 and 60 C. These results indicated that spraying during hot air drying inhibits hardening of the sample surface. 3.5. Color
Fig. 6. Changes in drying rate R of sample D and sample W during hot air drying.
where A0 is the initial surface area (m2), Wd is the dry matter mass (g) and RC is the drying rate for a constant-rate drying period. The moisture contents calculated using Eq. (9) are represented as curves in Fig. 5. The figures indicated that the measured results agreed well with the calculated results (RMSE ¼ 0.0001e0.009 kgwater/kg-solid). Thus, the hot air drying process of sample W was shown to be a constant-rate drying period, and the moisture content change over time could be predicted by Eq. (9). Superheated Steam was available for drying of some agricultural products (Somjai et al., 2009; Rumruaytum et al., 2014). Under the process of superheated steam drying, the surface of sample is covered by the water of condensation, which condition was similar to the condition in this study. So, the condition of the drying with solution spraying needs to be advanced much more simple and low cost process to keep same effect of superheated steam. 3.4. Surface hardening Table 1 shows the hardness at each sample before and after drying. The hardness of samples D and W increased 12e24% and
Table 2 shows the color difference before and after each drying. The color difference of sample W was less than that of sample D. Although the enzymatic reaction is the main factor in the browning of fruits, Japanese pears have a small amount of polyphenol, which becomes the substrate of the browning reaction (Shimohashi and Terada, 1996). Thus, it is unlikely that the color change was caused by the enzymatic reaction. Another major factor contributing to browning may be the Maillard reaction. This reaction causes non-enzymatic browning and is influenced by many factors, Table 2 Color difference of sample D and sample W after hot air drying.a 40 C Sample D Distilled water Citric acid Ascorbic acid
DE 10.6 7.7 6.4 5.9
± ± ± ±
2.1 0.6 0.5 0.6
50 C Sample D Distilled water Citric acid Ascorbic acid
DE 10.5 8.4 7.4 7.0
± ± ± ±
1.5 0.6 1.0 1.5
60 C Sample D Distilled water Citric acid Ascorbic acid
DE 10.1 10.9 8.7 8.7
± ± ± ±
2.4 0.2 1.3 2.1
a
Values are mean ± S.D.
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such as temperature and pH (Ames, 1990). The browning of sample W was inhibited because the temperature of sample W was decreased by spraying. Morifusa et al. (2012) reported the changes in sample temperatures of apple during hot air drying using the same solution spraying. From the report, the temperatures of sample W remain lower level than that of sample D during drying (5e10 C). We evaluated that the same effect was expressed in this study. The color difference of sample W was 27e44 and 20e33% less than that of sample D at 40 and 50 C, respectively, while there was little effect at 60 C. This observation suggested that the inhibition of discoloration is greater at lower temperatures because the sprayed solution remained on the sample surface for a long time. Spraying the citric acid and ascorbic acid solutions decreased the pH of the sample surface and controlled the Maillard reaction. These results indicate that the color change caused by drying can be inhibited by solution spraying. Further research into a method to prevent discoloration is expected in the future. 4. Conclusion Japanese pear slices were dried by hot air at 40, 50 and 60 C. The three types of solutions (distilled water, aqueous ascorbic acid solution and aqueous citric acid solution) were sprayed periodically on the sample surface, and the effect of solution spraying on the drying characteristics and the quality of the samples was examined. The results were summarized as follows: (1) Changes in the moisture content of non-sprayed Japanese pears during hot air drying were explained by the exponential model of first falling rate drying. (2) The drying rate of the samples sprayed with citric acid solution (0.2%, w/v), ascorbic acid solution (0.2%, w/v) and distilled water were greater than those of sample D. (3) The hardening of the sampling surface was controlled by applying the spraying process. (4) The discoloration was inhibited by the solution spraying because the Maillard reaction was occurred at lower temperatures and pH values. The spraying process during drying can be expected to become the new technology used for the drying processing of Japanese pears and other agricultural products. References AOAC. Official methods of analysis of AOAC International. 16th ed 1995. ASTM (American Society for Testing and Materials). Standard test method for rubber property e Durometer hardeness. 2003. http://www.astm.org/ Standards/D2240.htm. Ames JM. Control of the Maillerd reaction in food systems. Trends Food Sci Technol 1990;1:150e4. Arai Y, Muto H, Sano Y, Ito K. Food and food additives hypersensitivity in adult asthmatics. : III. Adverse reaction to sulfites in adult asthmatics. Jpn Soc Allergol 1998;47:1163e7.
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Please cite this article in press as: Imaizumi T, et al., Effects of solution spraying during hot air drying on drying rate, surface hardening and browning of fresh-cut Japanese pear, Engineering in Agriculture, Environment and Food (2015), http://dx.doi.org/10.1016/j.eaef.2015.01.001