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Effects of pulsed infra-red radiation followed by hot-press drying on the properties of mashed sweet potato chips
LWT - Food Science and Technology 82 (2017) 66e71
Contents lists available at ScienceDirect
LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Effects of pulsed infra-red radiation followed by hot-press drying on the properties of mashed sweet potato chips Suji Oh, Karna Ramachandraiah, Geun-Pyo Hong* Faculty of Food Science and Biotechnology, Sejong University, Seoul 05006, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2017 Received in revised form 10 April 2017 Accepted 10 April 2017
This study attempted to develop an additive-free dried sweet potato snack. To provide a crispy texture, sweet potatoes were dried by a two-stage process. At the first drying stage, steamed sweet potatoes were semi-dried for 6 h using hot-air convection or pulsed infra-red (IR) radiation, and drying rate was compared under varying sample thicknesses and drying temperatures. The IR exhibited enhanced drying speed, particularly the IR radiation at 60 C was favorable for application to the drying of sweet potatoes with large thickness. For the secondary drying, the IR-dried sweet potatoes with varying moisture content were applied to hot-press (HP) drying at 180 C for 2 s. The quality of final products indicated that the crispy texture of the products was generated when the semi-dried sample had a moisture content lower than 0.5 kg/kg dry base (d.b.). When the moisture content of samples prior to HP process was lower than 0.3 kg/kg d.b., the final product was easily broken with discoloration to dark-brown. Considering the entire processing procedure, the present study demonstrated that IR radiation at 60 C for 5 h followed by HP was an effective combination for the mass production of a crispy sweet potato snack. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Sweet potato Hot-air drying Infra-red radiation Hot-press Crispy texture
1. Introduction The sweet potato (Ipomoea batatas L.), an important root vegetable consumed world-wide, is rich in carbohydrates (~20 g/100 g) and, particularly, starch accounts for ~65% of the carbohydrates. Sweet potato remains as an important staple food in many developing countries (Scott, Best, Rosegrant, & Bokanga, 2000). Recently, the sweet potato has been regarded as a healthy food due to its various constituents such as dietary fiber, vitamins, minerals, and antioxidants (Yang, Chen, Zhao, & Mas, 2010). A common form of processed sweet potato is the French fries. However, more recently consumers are increasingly demanding products that are additivesfree (Oke & Workneh, 2013). The drying of foods provides various advantages such as long shelf-life, convenient application and increased concentration of nutrients (Choi et al., 2015; Kim & Chin, 2016). However, producing dried sweet potato products with acceptable texture is not simple by any means. Traditionally, sun light has been used to produce dried sweet potato, while commercial dried products are made by
* Corresponding author. Faculty of Food Science and Biotechnology, College of Life Science, Sejong University, Seoul 05006, South Korea. E-mail address: [email protected] (G.-P. Hong). http://dx.doi.org/10.1016/j.lwt.2017.04.023 0023-6438/© 2017 Elsevier Ltd. All rights reserved.
hot-air (HA) drying. Prior to drying, sweet potatoes are steamed to gelatinize starch which results in increased sweetness. However, the high amount of sugar in the steamed sweet potatoes lowers the drying rate and empirically manifests a very hard texture after complete drying due to crystallization of sugar. For the purpose of accelerating the overall drying rate, alternative techniques have been introduced, which include vacuum-ohmic heating and vacuum-microwave (Oke & Workneh, 2013; Zhong & Lima, 2003). Infra-red (IR) drying is recognized as the effective alternative to HA drying technique (Bondaruk, Markowski, & Blaszczak, 2007). It is well known that IR penetrates drying matter and efficiently generates heat inside the matter (Nowak & Lewicki, 2004). Various fruits and vegetables are dried by IR due to its effective reduction of drying time and improvement in product quality (Nowak & Lewicki, 2004; Sharma, Verma, & Pathare, 2005). However, the IR radiation technique can also cause negative effects such as surface overheating, lipid oxidation, and impingement damage due to generation of intensive heat (Doymaz, 2012; Sheridan & Shilton, 1999). It is important to note that the quality of dried sweet potato is dependent on the IR dosage. More precisely, drying temperature greatly influences the quality of the final product (Hong, Shim, Choi, & Min, 2009). Hot-press (HP) is a processing technique applied industrially to
S. Oh et al. / LWT - Food Science and Technology 82 (2017) 66e71
dry plywood, and has the potential for complete drying of materials. Target materials are dried under high temperature usually ranging from 120 C to 160 C and a pressure at about 0.4 MPa (Han, Zhan, Xu, Jiang, & Lu, 2015). Under such conditions, materials can be dried instantly (within 1e2 s), but the dryness of HP-treated final products depends on the moisture content of raw materials. During compression, samples are formed to various shapes depending on the mold, hence the HP can be adopted as a secondary or final drying process. In the present study, HP was used to make the dried sweet potato in the form of a chip (<1 mm thickness) with acceptable crispy texture. However, direct application of HP for sweet potato drying is limited because of its high sugar and moisture contents, which result in sticky final products (Bhandari, Datta, & Howes, 1997). Consequently, the steamed sweet potato has to be pretreated to produce an adequately semi-dried form prior to HP process. It is recommended to apply drying the sweet potato in the order of IR as a pretreatment and HP as a final drying process. Therefore, the present study was conducted to optimize IR and HP processing conditions to produce dried sweet potato chips with improved textural and sensorial properties. 2. Materials and methods 2.1. Materials and sample preparation Sweet potatoes of a domestic cultivar were purchased from a local farm (Jeongeup, Korea). The sweet potatoes were kept in a chilled dark room (~18 C) for 3 months which increased sweetness of the sweet potatoes. Prior to sample preparation, raw sweet potatoes were cleaned using running water, and steamed at 100 C for 30 min for saccharification. The steamed sweet potatoes were mashed using a food processor (KMX51, Kenwood Ltd., Havant, UK) for 5 min, and put in a container (210 140 40 mm) to form a block. After removing from the container, the blocks of mashed sweet potato were manually sliced to 6 mm and 8 mm thickness and used as sample without further storage. 2.2. Drying process In this study, the drying rate of the two drying methods (HA versus IR radiation) was compared under two sample thicknesses (6 mm versus 8 mm) and three drying temperatures (50, 55 or 60 C). For HA drying, samples were treated in a commercial HA dryer (LD-918H5, Lequip, Seoul, Korea). Vapor generated during drying was continuously removed by a ventilator located bottom of the device. To monitor drying temperature, a k-type thermocouple was inserted inside the dryer during entire drying, and drying was conducted for 6 h. The IR drying was applied using a lab-assembled IR dryer
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(Fig. 1A). In brief, the device consisted of an IR radiator (250 W, Philips, Brussel, Belgium), an air ventilator and a sample holder in a polystyrene housing coated by aluminum foil. The distance from IR ramp to sample was 25 cm, and the IR power was regulated by an electric transformer (500 V A, Hanil Transformer, Seoul, Korea). The temperature of sample surface was monitored by a k-type thermocouple. Since the IR dosage at constant voltage continuously increased the temperature in the dryer, pulsed IR radiation was applied at 220 V. When the sample reached the target temperature, the IR power was turned off until the sample cooled down to 3 C below the target temperature and then radiated again. This cycle was repeated for 6 h. This entire process of treatments was repeated three times on different days using a new batch of steamed sweet potatoes (n ¼ 3). 2.3. Hot-press (HP) treatment The dried sweet potatoes with varying moisture contents (0.2e0.6 kg/kg dry base (d.b.)) were applied to HP drying for the purpose of final complete drying and forming as chips. Sample with 6 mm thickness were semi-dried at 60 C in the pulsed IR radiator for various times. After tempering at ambient for 15 min, the semidried samples were subjected to the HP process. The HP consisted of a hot-plate with round shaped mold and pressor (Fig. 1B). The pressing cycle (5 s with intact pressing for 2 s) was regulated by rotor RPM and the plate was ohmically heated to 180 C. Dried samples were mounted on the hot plate and HP treated. Immediately after HP treatment, the final sample was removed from the plate and analyzed without storage. 2.4. Moisture content and sugar content The moisture content of samples was measured by the oven method at 105 C (AOAC, 1997), and expressed as dry weight basis. As an indicator of the sugar content (or sweetness) of the sample, brix of sample was determined. A 5 g sample was homogenized with 45 mL distilled water and kept at ambient temperature for 2 h while stirring gently. The homogenate was centrifuged at 15,000g for 5 min, and the brix of the supernatant was measured using a refractometer (RHB-55, Lumen Optical, Seoul, Korea). The total brix of the sample was numerically calculated by multiplying the obtained brix with a diluting factor of 10. 2.5. Texture The texture of both dried and HP treated sweet potatoes was measured using a texture analyzer (CT-3, Brookfield Engineering Laboratories, Inc., Middleboro, USA) but with different fixture. For the analysis of semi-dried sweet potatoes, multiple chip fixture
Fig. 1. Schematic diagram of (A) infra-red radiator and (B) hot-press dryer.
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S. Oh et al. / LWT - Food Science and Technology 82 (2017) 66e71
(TA-MCF, Brookfield Engineering Laboratories, Inc., Middleboro, USA) was selected. For one measurement, a total of 10 pieces of dried sweet potato strips were loaded together, and the sample hardness represents the force required the probe to penetrate 2 mm into the samples. A total of five measurements per treatment were conducted. To evaluate the texture of HP treated sweet potato chips, a three point bend fixture (TA-TPB, Brookfield Engineering Laboratories) was adopted. The sample was loaded on the fixture table and broken by a blade (TA7, Brookfield Engineering Laboratories). The fracture of the sample was estimated by a force at failure and the distance of blade at failure was recorded. All measurements were conducted by the conditions of 0.5 g trigger load and 60 mm/min head speed. 2.6. Sensory test Because the additive-free domestic sweet potato product is only produced by steamed followed by semi-dried form, the consumer perception of the developed sweet potato (treatment) was compared with a commercial product (control). The control was purchased from the local market, the treatment sample was prepared by IR (6 mm thickness, 60 C for 5 h) followed by HP. A total of 120 panelists of different nationality, age and gender was recruited for the consumer panel test. The sensorial evaluation was conducted by means of intensity and preference perception of aroma, sweetness, taste, stickiness (or mouthfeel) and crispiness (or texture) with a 6-point hedonic scale. All panelists received 3 pieces of each sample with water for rinsing their mouth between evaluations. 2.7. Statistical analysis A factorial design (2 2 3) was adopted to analyze the effects of two drying methods, two thicknesses and three temperatures on the quality of pretreated sweet potato samples. For HP-treated products, a completely randomized block design was adopted to analyze the relationship between moisture content and quality of final products. The means from three entirely repeated experiments were analyzed by one-way ANOVA (analysis of variance) using the SAS statistical program (ver. 9.1). When the main effect was significant (p < 0.05), the means were separated by Duncan's multiple range test as a post-hoc procedure. 3. Results and discussion 3.1. Primary drying process 3.1.1. Temperature profiles Temperature profiles during drying process are illustrated in Fig. 2. During HA drying, the temperature was maintained
throughout the entire period of processing and the deviation of temperature was within ± 0.3 C. For the pulsed IR treatment, however, temperature fluctuation was observed during drying. A sharp decrease in temperature was noticed when the sample was removed from the dryer. The maximum temperature of the IR treatment did not exceed the target temperature, which provided lower thermal dosage of IR when compared to HA drying. 3.1.2. Drying rate Initial moisture content of raw sweet potatoes was 1.97 kg/kg d.b (Fig. 3). which was lower than that reported by Doymaz et al. (2012). The difference would be due to 3 months of storage of raw sweet potatoes and the steam cooking treatment. For HA drying, thickness was an important factor affecting the drying rate. In the case of 6 mm thickness, the drying rate was accelerated by increasing the drying temperature, and a moisture content of 0.61 kg/kg d.b. was reached after 6 h of drying at 60 C. Samples of 8 mm thickness showed 1.22e1.33 kg/kg d.b. after 6 h drying, and the drying rate of the sweet potatoes was not affected by the drying temperature. Considering industrial mass production, sample slicing with 6 mm thickness was not easy unless manually handled because of the soft texture of steamed sweet potato. If the thickness increased, drying temperature has to be higher than 60 C for effective drying, which could negatively affect the quality of dried product (Sablani & Mujumdar, 2007). Alternately, IR drying exhibited higher drying rates for samples with both thicknesses. For 50 C IR radiation, the sample with 6 mm thickness reached 0.66 kg/kg d.b. within 4 h. The IR treatment at the same temperature exhibited 0.56 kg/kg d.b. of moisture content after 6 h drying, which was significantly lower than 0.61 kg/kg d.b. of the sample obtained by 60 C HA drying (p < 0.05). When the temperature of the IR drying increased to 60 C, 0.36 kg/kg d.b. of moisture was obtained after 6 h treatment. The acceleration of IR drying rate was also shown when the sample thickness was increased to 8 mm. In particular, effects of thickness diminished when IR radiation was conducted at 60 C where the moisture content of sample with 8 mm thickness reached to 0.40 kg/kg d.b. after 6 h. In the present study, the maximum temperature of the pulsed IR treatment did not exceed the target drying temperature, hence it was obvious that IR contributed to the acceleration of the drying rate of sweet potatoes. Based on the literature, IR caused continuous increments in temperature during radiation, hence the main contributor of IR to accelerate drying rate was not distinct between convective and radiated heat (Hong et al., 2009). It should be noted that the pulsed IR process of this study did not exceed the target temperature, and the average drying temperature of IR treatment was 1.5 K lower than target temperature. According to Kirchhoff's law, moisture in sweet potato can act as an IR energy absorber, and effective drying was possible within short time (Lewicki, 2004). Consequently, the
Fig. 2. Temperature profiles of (A) hot-air and (B) pulsed infra-red dryer as a function of drying period.
S. Oh et al. / LWT - Food Science and Technology 82 (2017) 66e71
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Fig. 3. Changes in moisture content (dry weight basis) of sweet potato dried by (A, B) hot-air and (C, D) infra-red with thickness of (A, C) 6 mm and (B, D) 8 mm, respectively. Round, triangle and square indicate drying temperature of 50 C, 55 C and 60 C, respectively. Vertical bars indicate standard deviations (n ¼ 3).
result demonstrated that IR radiation showed an ability to improve drying rate of sweet potatoes. Due to its high heat efficiency, IR treatment had the problem of causing surface overheating, which would cause quality deterioration of the final product (Hong et al., 2009). However, applying pulses of IR processing enabled to prevent the overheating of the sweet potato samples in this study.
3.1.3. Brix and hardness Estimated brix was plotted as a function of moisture content of the samples, because the brix was reversely proportional to moisture content of sweet potatoes with irrespective of sample thickness (Fig. 4A). For each drying method, the relationship between brix and moisture content was estimated as linear equation
BrixHA ¼ 36:1WM þ 105:6 R2 ¼ 0:97 BrixIR ¼ 32:2WM
þ 97:1 R2 ¼ 0:98
well known that the sugar content in sweet potatoes increased with the storage period and heating time, which was evidenced by starch degradation to maltose (Van Den, Biermann, & Marlett, 1986). It was likely that the starch degradation was closely related to drying time. The IR drying was faster than HA drying, and the sweet potatoes dried by IR would show slightly lower starch degradation than that of HA treatment which was supported by the report of Purcell and Walter (1988). Nevertheless, a minimal difference in brix between the two drying methods showed that IR drying was favorable for commercial application in sweet potato drying techniques. Hardness of samples dried for 6 h in HA or IR radiator was plotted as a function of final moisture content (Fig. 4B). The hardness showed an exponential increase with decreasing moisture content of samples as expressed in the following equations:
(1)
HardnessHA ¼ 241:7e4:6WM (2)
where WM is moisture content (dry basis). Although the brix of the sample was rarely affected by the drying methods, HA dried samples showed slightly higher brix than those after IR treatment. It is
HardnessIR ¼ 608:7e5:0WM
R2 ¼ 0:99
R2 ¼ 0:93
(3)
(4)
The overall hardness of dried sweet potatoes was greater in the IR treatment than that in HA drying treatments. The result was not
Fig. 4. Plot of (A) brix and (B) hardness as a function of moisture content (dry weight basis) of dried sweet potatoes. Round and triangle indicate hot-air and infra-red drying, respectively. Opened and closed marks represent sample thickness of 6 mm and 8 mm, respectively.
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S. Oh et al. / LWT - Food Science and Technology 82 (2017) 66e71
in agreement with the finding of Antal (2015) wherein IR-assisted freeze dried apple exhibited lower hardness than that assisted by hot-air. The lack in results consistency could be due to differences in the applied drying process. The texture of dried products was closely related toh the porous structure, and the pore size was affected by drying conditions such as temperature and time (Sinh, Pan, McHuge, Wood, & Hirschberg, 2008). With regards to IR treatment, rapid drying would manifest a collapse of surface cells in sweet potato, which probably produced a dense surface, resulting in higher mechanical force than that of the HA treatment (Ayungulu & Pan, 2011). 3.2. Hot-press (HP) processed final product 3.2.1. Quality characteristics The exponential relationship between hardness and moisture content reflected that the dried sweet potato was too hard to chew, hence textural modification was necessary for the better consumers’ preference. Based on the preliminary study, HP was regarded as an alternative drying method of steamed sweet potatoes. However, it was difficult to produce dried sweet potato chip using HP alone, due to instant processing time (~2 s), hence the raw materials had to be semi-dried to an adequate moisture range. From the data obtained by Fig. 3C, the rate constant of IR drying at 60 C was estimated to be 8.64 105/s based on the 1st order reaction kinetic model. Inversely, total IR drying times to obtain final moisture contents (0.2e0.6 kg/kg d.b.) were numerically calculated and actual final moisture content of sweet potato as a function of IR drying time is depicted in Fig. 5A. After HP treatment, the moisture contents were dependent on those before HP and showed around 50% of moisture in final product compared to the sample prior to HP treatment. Therefore, HP was recognized as the best final drying technique for preparing sweet potato snacks. The texture of the final products indicated that initial drying of the raw
material to 0.4 kg/kg d.b. was necessary for a successful preparation of the sweet potato chip. When the initial moisture content of the sample was higher than 0.4 kg/kg d.b., the samples did not break but stretched during test which was characterized by long deformation (10 mm). Compared to the sample with 0.66 kg/kg d.b., the texture of the sample with 0.53 kg/kg d.b. showed a significantly higher hardness without any breakage (p < 0.05). Therefore, HPtreated samples with high moisture content exhibited a flexible texture that was not suitable for sweet potato snack. In this case, the sample was found to be sticky and hence was difficult to remove from the HP plate, resulting in burning. Meanwhile, the sample initially dried to lower than 4.5 kg/kg d.b. showed a brittle texture, which was characterized by high fracture with short deformation (Blahovec, 2007). To produce a crispy texture, therefore sweet potato had to be semi-dried properly then treated HP. In addition, texture of the samples with initial moisture contents of 0.2e0.4 kg/kg d.b. was not affected by initial moisture, and there was no significant difference in fracture and deformation. However, the sample with a moisture content of 0.22 kg/kg d.b. was too brittle upon HP treatment, which caused a defective final product. Furthermore, the lower the initial moisture content of the sweet potatoes, the darker was the final product. From the result, the best condition of sweet potato to apply for HP process was pre-drying the sample to 0.3e0.4 kg/kg d.b., which led to a crispy texture with good appearance. 3.2.2. Consumer panel test In this study, treatment (IR followed by HP treatment) exhibited different intensity perception with that of control (commercial semi-dried product) (Fig. 6A). Aroma of treatment tended to be higher than that of control, however the difference was not significant. Meanwhile, treatment showed enhanced sweetness, taste and crispiness than those of control (p < 0.05). In addition, the treatment showed less stickiness in mouth comparing to control
Fig. 5. Moisture content (A) and textural properties (B) of infra-red (IR) followed by hot-press (HP) treated sweet potato chip as a function of moisture contents. Dotted line with rhombus indicates predicted moisture content of sweet potatoes after IR drying. Solid line with open round and dashed line with closed round reflect measured moisture content of sweet potatoes after IR drying and HP process, respectively. Lower case (aee) with the same letter indicated the same treatment.
Fig. 6. Consumer panel test by means of (A) intensity and (B) preference evaluation of commercial semi-dried (control, solid line) and infra-red followed by hot-press treated sweet potato snacks (treatment, dashed line).
S. Oh et al. / LWT - Food Science and Technology 82 (2017) 66e71
(p < 0.05), which indicated that overall properties of the treatment was distinctive from the control. Based on the preference conception, consumers were highly favorable to treatment compared to the control, which were evidenced by higher grading of preference scores at all tested properties (Fig. 6B). Besides taste and textural properties, increasing the dryness of final product satisfied the mouthfeel. Because the treatment was a crispy chip, the product was lower in the stickiness during mastication and probably resulted in proper mouthfeel. Consequently, the result indicated that application of IR followed by HP had a potential application to produce a textural modified dried sweet potato product. 4. Conclusion The HP had a potential application in the production of crispy sweet potato snack with reduced processing time. To apply the HP process, it was essential to regulate the moisture content of the raw sweet potatoes, and 0.3e0.4 kg/kg d.b. was best condition for the HP process. For the effective removal of moisture from raw sweet potatoes, IR exhibited a greater advantage comparing to HA from the point of view of drying time. Finally, the present study demonstrated that IR radiation followed by HP was an effective combination for the mass production of dried sweet potato chips. For the improved industrial application of IR followed by HP process, optimization of HP temperature and time was still required, which warranted further exploration. Acknowledgement This work was supported by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry and Fisheries (iPET) through the Export Promotion Technology Development Program, funded by Ministry of Agriculture, Food and Rural Affairs (MAFRA) (grant number: 115071-2). References Antal, T. (2015). Comparative study of three drying methods: Freeze, hot air-assisted freeze and infrared-assisted freeze modes. Agronomy Research, 13, 863e878. AOAC. (1997). Official methods of analysis (15th ed.). Washington, DC: Association of Official Analytical Chemists. Ayungulu, G. G., & Pan, Z. (2011). Combined infrared radiation and freeze-drying. In
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Z. Pan, & G. G. Atungulu (Eds.), Infrared heating for food and agricultural processing (pp. 117e140). Boca Raton: CRC press. Bhandari, B. R., Datta, N., & Howes, T. (1997). Problems associated with spray drying of sugar-rich foods. Drying Technology, 15, 671e684. Blahovec, J. (2007). Role of water content in food and product texture. International Agrophysics, 21, 209e215. Bondaruk, J., Markowski, M., & Blaszczak, W. (2007). Effect of drying conditions on the quality of vacuum-microwave dried potato cubes. Journal of Food Engineering, 81, 306e312. Choi, Y. S., Ku, S. K., Park, J. D., Kim, H. J., Jang, A., & Kim, Y. B. (2015). Effects of drying condition and binding agent on the quality characteristics of ground dried-pork meat products. Korean Journal for Food Science of Animal Resources, 35, 597e603. Doymaz, I. (2012). Infrared drying of sweet potato (Ipomoea batatas L.) slices. Journal of Food Science and Technology, 49, 760e766. Han, C., Zhan, T., Xu, J., Jiang, J., & Lu, J. (2015). Process optimization for multi-veneer hot-press drying. Drying Technology, 33, 735e741. Hong, G. P., Shim, K. B., Choi, M. J., & Min, S. G. (2009). Effects of air blast thawing combined with infrared radiation on physical properties of pork. Korean Journal for Food Science of Animal Resources, 29, 302e309. Kim, H. S., & Chin, K. B. (2016). Effects of drying temperature on antioxidant activities of tomato powder and storage stability of pork patties. Korean Journal for Food Science of Animal Resources, 36, 51e60. Lewicki, P. P. (2004). Water as the determinant of food engineering properties. A review. Journal of Food Engineering, 61, 483e495. Nowak, D., & Lewicki, P. P. (2004). Infrared drying of apple slices. Innovative Food Science and Emerging Technologies, 5, 353e360. Oke, M. O., & Workneh, T. S. (2013). A review on sweet potato postharvest processing and preservation technology. International Journal of Agricultural Research and Reviews, 1, 1e14. Purcell, A. E., & Walter, W. M., Jr. (1988). Comparison of carbohydrate components in sweet potatoes baked by convection heating and microwave heating. Journal of Agricultural and Food Chemistry, 36, 360e362. Sablani, S. S., & Mujumdar, A. S. (2007). Drying of potato, sweet potato, and other roots. In A. S. Mujumdar (Ed.), Handbook of industrial drying (pp. 647e662). Boca Raton: CRC press. Scott, G. J., Best, R., Rosegrant, M., & Bokanga, M. (2000). Roots and tubers in the global food system. A vision statement to the year 2020. Lima: International Potato Center. Sharma, G. P., Verma, R. C., & Pathare, P. B. (2005). Thin-layer infrared radiation drying of onion slices. Journal of Food Engineering, 67, 361e366. Sheridan, P., & Shilton, N. (1999). Application of infra-red radiation to cooking of meat products. Journal of Food Engineering, 41, 203e208. Sinh, C., Pan, Z., McHuge, T. H., Wood, D., & Hirschberg, E. (2008). Sequential infrared radiation and freeze-drying method for producing crispy strawberries. Transactions of the ASABE, 51, 205e216. Van Den, T., Biermann, C. J., & Marlett, J. A. (1986). Simple sugars, oligosaccharides, and starch concentrations in raw and cooked sweet potato. Journal of Agricultural and Food Chemistry, 34, 421e425. Yang, J., Chen, J. F., Zhao, Y. Y., & Mas, L. C. (2010). Effects of drying processes on the antioxidant properties in sweet potatoes. Agricultural Sciences in China, 9, 1522e1529. Zhong, T., & Lima, M. (2003). The effect of ohmic heating on vacuum drying rate of sweet potato tissue. Bioresource Technology, 87, 215e220.
Contents lists available at ScienceDirect
LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt
Effects of pulsed infra-red radiation followed by hot-press drying on the properties of mashed sweet potato chips Suji Oh, Karna Ramachandraiah, Geun-Pyo Hong* Faculty of Food Science and Biotechnology, Sejong University, Seoul 05006, South Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2017 Received in revised form 10 April 2017 Accepted 10 April 2017
This study attempted to develop an additive-free dried sweet potato snack. To provide a crispy texture, sweet potatoes were dried by a two-stage process. At the first drying stage, steamed sweet potatoes were semi-dried for 6 h using hot-air convection or pulsed infra-red (IR) radiation, and drying rate was compared under varying sample thicknesses and drying temperatures. The IR exhibited enhanced drying speed, particularly the IR radiation at 60 C was favorable for application to the drying of sweet potatoes with large thickness. For the secondary drying, the IR-dried sweet potatoes with varying moisture content were applied to hot-press (HP) drying at 180 C for 2 s. The quality of final products indicated that the crispy texture of the products was generated when the semi-dried sample had a moisture content lower than 0.5 kg/kg dry base (d.b.). When the moisture content of samples prior to HP process was lower than 0.3 kg/kg d.b., the final product was easily broken with discoloration to dark-brown. Considering the entire processing procedure, the present study demonstrated that IR radiation at 60 C for 5 h followed by HP was an effective combination for the mass production of a crispy sweet potato snack. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Sweet potato Hot-air drying Infra-red radiation Hot-press Crispy texture
1. Introduction The sweet potato (Ipomoea batatas L.), an important root vegetable consumed world-wide, is rich in carbohydrates (~20 g/100 g) and, particularly, starch accounts for ~65% of the carbohydrates. Sweet potato remains as an important staple food in many developing countries (Scott, Best, Rosegrant, & Bokanga, 2000). Recently, the sweet potato has been regarded as a healthy food due to its various constituents such as dietary fiber, vitamins, minerals, and antioxidants (Yang, Chen, Zhao, & Mas, 2010). A common form of processed sweet potato is the French fries. However, more recently consumers are increasingly demanding products that are additivesfree (Oke & Workneh, 2013). The drying of foods provides various advantages such as long shelf-life, convenient application and increased concentration of nutrients (Choi et al., 2015; Kim & Chin, 2016). However, producing dried sweet potato products with acceptable texture is not simple by any means. Traditionally, sun light has been used to produce dried sweet potato, while commercial dried products are made by
* Corresponding author. Faculty of Food Science and Biotechnology, College of Life Science, Sejong University, Seoul 05006, South Korea. E-mail address: [email protected] (G.-P. Hong). http://dx.doi.org/10.1016/j.lwt.2017.04.023 0023-6438/© 2017 Elsevier Ltd. All rights reserved.
hot-air (HA) drying. Prior to drying, sweet potatoes are steamed to gelatinize starch which results in increased sweetness. However, the high amount of sugar in the steamed sweet potatoes lowers the drying rate and empirically manifests a very hard texture after complete drying due to crystallization of sugar. For the purpose of accelerating the overall drying rate, alternative techniques have been introduced, which include vacuum-ohmic heating and vacuum-microwave (Oke & Workneh, 2013; Zhong & Lima, 2003). Infra-red (IR) drying is recognized as the effective alternative to HA drying technique (Bondaruk, Markowski, & Blaszczak, 2007). It is well known that IR penetrates drying matter and efficiently generates heat inside the matter (Nowak & Lewicki, 2004). Various fruits and vegetables are dried by IR due to its effective reduction of drying time and improvement in product quality (Nowak & Lewicki, 2004; Sharma, Verma, & Pathare, 2005). However, the IR radiation technique can also cause negative effects such as surface overheating, lipid oxidation, and impingement damage due to generation of intensive heat (Doymaz, 2012; Sheridan & Shilton, 1999). It is important to note that the quality of dried sweet potato is dependent on the IR dosage. More precisely, drying temperature greatly influences the quality of the final product (Hong, Shim, Choi, & Min, 2009). Hot-press (HP) is a processing technique applied industrially to
S. Oh et al. / LWT - Food Science and Technology 82 (2017) 66e71
dry plywood, and has the potential for complete drying of materials. Target materials are dried under high temperature usually ranging from 120 C to 160 C and a pressure at about 0.4 MPa (Han, Zhan, Xu, Jiang, & Lu, 2015). Under such conditions, materials can be dried instantly (within 1e2 s), but the dryness of HP-treated final products depends on the moisture content of raw materials. During compression, samples are formed to various shapes depending on the mold, hence the HP can be adopted as a secondary or final drying process. In the present study, HP was used to make the dried sweet potato in the form of a chip (<1 mm thickness) with acceptable crispy texture. However, direct application of HP for sweet potato drying is limited because of its high sugar and moisture contents, which result in sticky final products (Bhandari, Datta, & Howes, 1997). Consequently, the steamed sweet potato has to be pretreated to produce an adequately semi-dried form prior to HP process. It is recommended to apply drying the sweet potato in the order of IR as a pretreatment and HP as a final drying process. Therefore, the present study was conducted to optimize IR and HP processing conditions to produce dried sweet potato chips with improved textural and sensorial properties. 2. Materials and methods 2.1. Materials and sample preparation Sweet potatoes of a domestic cultivar were purchased from a local farm (Jeongeup, Korea). The sweet potatoes were kept in a chilled dark room (~18 C) for 3 months which increased sweetness of the sweet potatoes. Prior to sample preparation, raw sweet potatoes were cleaned using running water, and steamed at 100 C for 30 min for saccharification. The steamed sweet potatoes were mashed using a food processor (KMX51, Kenwood Ltd., Havant, UK) for 5 min, and put in a container (210 140 40 mm) to form a block. After removing from the container, the blocks of mashed sweet potato were manually sliced to 6 mm and 8 mm thickness and used as sample without further storage. 2.2. Drying process In this study, the drying rate of the two drying methods (HA versus IR radiation) was compared under two sample thicknesses (6 mm versus 8 mm) and three drying temperatures (50, 55 or 60 C). For HA drying, samples were treated in a commercial HA dryer (LD-918H5, Lequip, Seoul, Korea). Vapor generated during drying was continuously removed by a ventilator located bottom of the device. To monitor drying temperature, a k-type thermocouple was inserted inside the dryer during entire drying, and drying was conducted for 6 h. The IR drying was applied using a lab-assembled IR dryer
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(Fig. 1A). In brief, the device consisted of an IR radiator (250 W, Philips, Brussel, Belgium), an air ventilator and a sample holder in a polystyrene housing coated by aluminum foil. The distance from IR ramp to sample was 25 cm, and the IR power was regulated by an electric transformer (500 V A, Hanil Transformer, Seoul, Korea). The temperature of sample surface was monitored by a k-type thermocouple. Since the IR dosage at constant voltage continuously increased the temperature in the dryer, pulsed IR radiation was applied at 220 V. When the sample reached the target temperature, the IR power was turned off until the sample cooled down to 3 C below the target temperature and then radiated again. This cycle was repeated for 6 h. This entire process of treatments was repeated three times on different days using a new batch of steamed sweet potatoes (n ¼ 3). 2.3. Hot-press (HP) treatment The dried sweet potatoes with varying moisture contents (0.2e0.6 kg/kg dry base (d.b.)) were applied to HP drying for the purpose of final complete drying and forming as chips. Sample with 6 mm thickness were semi-dried at 60 C in the pulsed IR radiator for various times. After tempering at ambient for 15 min, the semidried samples were subjected to the HP process. The HP consisted of a hot-plate with round shaped mold and pressor (Fig. 1B). The pressing cycle (5 s with intact pressing for 2 s) was regulated by rotor RPM and the plate was ohmically heated to 180 C. Dried samples were mounted on the hot plate and HP treated. Immediately after HP treatment, the final sample was removed from the plate and analyzed without storage. 2.4. Moisture content and sugar content The moisture content of samples was measured by the oven method at 105 C (AOAC, 1997), and expressed as dry weight basis. As an indicator of the sugar content (or sweetness) of the sample, brix of sample was determined. A 5 g sample was homogenized with 45 mL distilled water and kept at ambient temperature for 2 h while stirring gently. The homogenate was centrifuged at 15,000g for 5 min, and the brix of the supernatant was measured using a refractometer (RHB-55, Lumen Optical, Seoul, Korea). The total brix of the sample was numerically calculated by multiplying the obtained brix with a diluting factor of 10. 2.5. Texture The texture of both dried and HP treated sweet potatoes was measured using a texture analyzer (CT-3, Brookfield Engineering Laboratories, Inc., Middleboro, USA) but with different fixture. For the analysis of semi-dried sweet potatoes, multiple chip fixture
Fig. 1. Schematic diagram of (A) infra-red radiator and (B) hot-press dryer.
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(TA-MCF, Brookfield Engineering Laboratories, Inc., Middleboro, USA) was selected. For one measurement, a total of 10 pieces of dried sweet potato strips were loaded together, and the sample hardness represents the force required the probe to penetrate 2 mm into the samples. A total of five measurements per treatment were conducted. To evaluate the texture of HP treated sweet potato chips, a three point bend fixture (TA-TPB, Brookfield Engineering Laboratories) was adopted. The sample was loaded on the fixture table and broken by a blade (TA7, Brookfield Engineering Laboratories). The fracture of the sample was estimated by a force at failure and the distance of blade at failure was recorded. All measurements were conducted by the conditions of 0.5 g trigger load and 60 mm/min head speed. 2.6. Sensory test Because the additive-free domestic sweet potato product is only produced by steamed followed by semi-dried form, the consumer perception of the developed sweet potato (treatment) was compared with a commercial product (control). The control was purchased from the local market, the treatment sample was prepared by IR (6 mm thickness, 60 C for 5 h) followed by HP. A total of 120 panelists of different nationality, age and gender was recruited for the consumer panel test. The sensorial evaluation was conducted by means of intensity and preference perception of aroma, sweetness, taste, stickiness (or mouthfeel) and crispiness (or texture) with a 6-point hedonic scale. All panelists received 3 pieces of each sample with water for rinsing their mouth between evaluations. 2.7. Statistical analysis A factorial design (2 2 3) was adopted to analyze the effects of two drying methods, two thicknesses and three temperatures on the quality of pretreated sweet potato samples. For HP-treated products, a completely randomized block design was adopted to analyze the relationship between moisture content and quality of final products. The means from three entirely repeated experiments were analyzed by one-way ANOVA (analysis of variance) using the SAS statistical program (ver. 9.1). When the main effect was significant (p < 0.05), the means were separated by Duncan's multiple range test as a post-hoc procedure. 3. Results and discussion 3.1. Primary drying process 3.1.1. Temperature profiles Temperature profiles during drying process are illustrated in Fig. 2. During HA drying, the temperature was maintained
throughout the entire period of processing and the deviation of temperature was within ± 0.3 C. For the pulsed IR treatment, however, temperature fluctuation was observed during drying. A sharp decrease in temperature was noticed when the sample was removed from the dryer. The maximum temperature of the IR treatment did not exceed the target temperature, which provided lower thermal dosage of IR when compared to HA drying. 3.1.2. Drying rate Initial moisture content of raw sweet potatoes was 1.97 kg/kg d.b (Fig. 3). which was lower than that reported by Doymaz et al. (2012). The difference would be due to 3 months of storage of raw sweet potatoes and the steam cooking treatment. For HA drying, thickness was an important factor affecting the drying rate. In the case of 6 mm thickness, the drying rate was accelerated by increasing the drying temperature, and a moisture content of 0.61 kg/kg d.b. was reached after 6 h of drying at 60 C. Samples of 8 mm thickness showed 1.22e1.33 kg/kg d.b. after 6 h drying, and the drying rate of the sweet potatoes was not affected by the drying temperature. Considering industrial mass production, sample slicing with 6 mm thickness was not easy unless manually handled because of the soft texture of steamed sweet potato. If the thickness increased, drying temperature has to be higher than 60 C for effective drying, which could negatively affect the quality of dried product (Sablani & Mujumdar, 2007). Alternately, IR drying exhibited higher drying rates for samples with both thicknesses. For 50 C IR radiation, the sample with 6 mm thickness reached 0.66 kg/kg d.b. within 4 h. The IR treatment at the same temperature exhibited 0.56 kg/kg d.b. of moisture content after 6 h drying, which was significantly lower than 0.61 kg/kg d.b. of the sample obtained by 60 C HA drying (p < 0.05). When the temperature of the IR drying increased to 60 C, 0.36 kg/kg d.b. of moisture was obtained after 6 h treatment. The acceleration of IR drying rate was also shown when the sample thickness was increased to 8 mm. In particular, effects of thickness diminished when IR radiation was conducted at 60 C where the moisture content of sample with 8 mm thickness reached to 0.40 kg/kg d.b. after 6 h. In the present study, the maximum temperature of the pulsed IR treatment did not exceed the target drying temperature, hence it was obvious that IR contributed to the acceleration of the drying rate of sweet potatoes. Based on the literature, IR caused continuous increments in temperature during radiation, hence the main contributor of IR to accelerate drying rate was not distinct between convective and radiated heat (Hong et al., 2009). It should be noted that the pulsed IR process of this study did not exceed the target temperature, and the average drying temperature of IR treatment was 1.5 K lower than target temperature. According to Kirchhoff's law, moisture in sweet potato can act as an IR energy absorber, and effective drying was possible within short time (Lewicki, 2004). Consequently, the
Fig. 2. Temperature profiles of (A) hot-air and (B) pulsed infra-red dryer as a function of drying period.
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Fig. 3. Changes in moisture content (dry weight basis) of sweet potato dried by (A, B) hot-air and (C, D) infra-red with thickness of (A, C) 6 mm and (B, D) 8 mm, respectively. Round, triangle and square indicate drying temperature of 50 C, 55 C and 60 C, respectively. Vertical bars indicate standard deviations (n ¼ 3).
result demonstrated that IR radiation showed an ability to improve drying rate of sweet potatoes. Due to its high heat efficiency, IR treatment had the problem of causing surface overheating, which would cause quality deterioration of the final product (Hong et al., 2009). However, applying pulses of IR processing enabled to prevent the overheating of the sweet potato samples in this study.
3.1.3. Brix and hardness Estimated brix was plotted as a function of moisture content of the samples, because the brix was reversely proportional to moisture content of sweet potatoes with irrespective of sample thickness (Fig. 4A). For each drying method, the relationship between brix and moisture content was estimated as linear equation
BrixHA ¼ 36:1WM þ 105:6 R2 ¼ 0:97 BrixIR ¼ 32:2WM
þ 97:1 R2 ¼ 0:98
well known that the sugar content in sweet potatoes increased with the storage period and heating time, which was evidenced by starch degradation to maltose (Van Den, Biermann, & Marlett, 1986). It was likely that the starch degradation was closely related to drying time. The IR drying was faster than HA drying, and the sweet potatoes dried by IR would show slightly lower starch degradation than that of HA treatment which was supported by the report of Purcell and Walter (1988). Nevertheless, a minimal difference in brix between the two drying methods showed that IR drying was favorable for commercial application in sweet potato drying techniques. Hardness of samples dried for 6 h in HA or IR radiator was plotted as a function of final moisture content (Fig. 4B). The hardness showed an exponential increase with decreasing moisture content of samples as expressed in the following equations:
(1)
HardnessHA ¼ 241:7e4:6WM (2)
where WM is moisture content (dry basis). Although the brix of the sample was rarely affected by the drying methods, HA dried samples showed slightly higher brix than those after IR treatment. It is
HardnessIR ¼ 608:7e5:0WM
R2 ¼ 0:99
R2 ¼ 0:93
(3)
(4)
The overall hardness of dried sweet potatoes was greater in the IR treatment than that in HA drying treatments. The result was not
Fig. 4. Plot of (A) brix and (B) hardness as a function of moisture content (dry weight basis) of dried sweet potatoes. Round and triangle indicate hot-air and infra-red drying, respectively. Opened and closed marks represent sample thickness of 6 mm and 8 mm, respectively.
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in agreement with the finding of Antal (2015) wherein IR-assisted freeze dried apple exhibited lower hardness than that assisted by hot-air. The lack in results consistency could be due to differences in the applied drying process. The texture of dried products was closely related toh the porous structure, and the pore size was affected by drying conditions such as temperature and time (Sinh, Pan, McHuge, Wood, & Hirschberg, 2008). With regards to IR treatment, rapid drying would manifest a collapse of surface cells in sweet potato, which probably produced a dense surface, resulting in higher mechanical force than that of the HA treatment (Ayungulu & Pan, 2011). 3.2. Hot-press (HP) processed final product 3.2.1. Quality characteristics The exponential relationship between hardness and moisture content reflected that the dried sweet potato was too hard to chew, hence textural modification was necessary for the better consumers’ preference. Based on the preliminary study, HP was regarded as an alternative drying method of steamed sweet potatoes. However, it was difficult to produce dried sweet potato chip using HP alone, due to instant processing time (~2 s), hence the raw materials had to be semi-dried to an adequate moisture range. From the data obtained by Fig. 3C, the rate constant of IR drying at 60 C was estimated to be 8.64 105/s based on the 1st order reaction kinetic model. Inversely, total IR drying times to obtain final moisture contents (0.2e0.6 kg/kg d.b.) were numerically calculated and actual final moisture content of sweet potato as a function of IR drying time is depicted in Fig. 5A. After HP treatment, the moisture contents were dependent on those before HP and showed around 50% of moisture in final product compared to the sample prior to HP treatment. Therefore, HP was recognized as the best final drying technique for preparing sweet potato snacks. The texture of the final products indicated that initial drying of the raw
material to 0.4 kg/kg d.b. was necessary for a successful preparation of the sweet potato chip. When the initial moisture content of the sample was higher than 0.4 kg/kg d.b., the samples did not break but stretched during test which was characterized by long deformation (10 mm). Compared to the sample with 0.66 kg/kg d.b., the texture of the sample with 0.53 kg/kg d.b. showed a significantly higher hardness without any breakage (p < 0.05). Therefore, HPtreated samples with high moisture content exhibited a flexible texture that was not suitable for sweet potato snack. In this case, the sample was found to be sticky and hence was difficult to remove from the HP plate, resulting in burning. Meanwhile, the sample initially dried to lower than 4.5 kg/kg d.b. showed a brittle texture, which was characterized by high fracture with short deformation (Blahovec, 2007). To produce a crispy texture, therefore sweet potato had to be semi-dried properly then treated HP. In addition, texture of the samples with initial moisture contents of 0.2e0.4 kg/kg d.b. was not affected by initial moisture, and there was no significant difference in fracture and deformation. However, the sample with a moisture content of 0.22 kg/kg d.b. was too brittle upon HP treatment, which caused a defective final product. Furthermore, the lower the initial moisture content of the sweet potatoes, the darker was the final product. From the result, the best condition of sweet potato to apply for HP process was pre-drying the sample to 0.3e0.4 kg/kg d.b., which led to a crispy texture with good appearance. 3.2.2. Consumer panel test In this study, treatment (IR followed by HP treatment) exhibited different intensity perception with that of control (commercial semi-dried product) (Fig. 6A). Aroma of treatment tended to be higher than that of control, however the difference was not significant. Meanwhile, treatment showed enhanced sweetness, taste and crispiness than those of control (p < 0.05). In addition, the treatment showed less stickiness in mouth comparing to control
Fig. 5. Moisture content (A) and textural properties (B) of infra-red (IR) followed by hot-press (HP) treated sweet potato chip as a function of moisture contents. Dotted line with rhombus indicates predicted moisture content of sweet potatoes after IR drying. Solid line with open round and dashed line with closed round reflect measured moisture content of sweet potatoes after IR drying and HP process, respectively. Lower case (aee) with the same letter indicated the same treatment.
Fig. 6. Consumer panel test by means of (A) intensity and (B) preference evaluation of commercial semi-dried (control, solid line) and infra-red followed by hot-press treated sweet potato snacks (treatment, dashed line).
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(p < 0.05), which indicated that overall properties of the treatment was distinctive from the control. Based on the preference conception, consumers were highly favorable to treatment compared to the control, which were evidenced by higher grading of preference scores at all tested properties (Fig. 6B). Besides taste and textural properties, increasing the dryness of final product satisfied the mouthfeel. Because the treatment was a crispy chip, the product was lower in the stickiness during mastication and probably resulted in proper mouthfeel. Consequently, the result indicated that application of IR followed by HP had a potential application to produce a textural modified dried sweet potato product. 4. Conclusion The HP had a potential application in the production of crispy sweet potato snack with reduced processing time. To apply the HP process, it was essential to regulate the moisture content of the raw sweet potatoes, and 0.3e0.4 kg/kg d.b. was best condition for the HP process. For the effective removal of moisture from raw sweet potatoes, IR exhibited a greater advantage comparing to HA from the point of view of drying time. Finally, the present study demonstrated that IR radiation followed by HP was an effective combination for the mass production of dried sweet potato chips. For the improved industrial application of IR followed by HP process, optimization of HP temperature and time was still required, which warranted further exploration. Acknowledgement This work was supported by the Korea Institute of Planning and Evaluation for Technology in Food, Agriculture, Forestry and Fisheries (iPET) through the Export Promotion Technology Development Program, funded by Ministry of Agriculture, Food and Rural Affairs (MAFRA) (grant number: 115071-2). References Antal, T. (2015). Comparative study of three drying methods: Freeze, hot air-assisted freeze and infrared-assisted freeze modes. Agronomy Research, 13, 863e878. AOAC. (1997). Official methods of analysis (15th ed.). Washington, DC: Association of Official Analytical Chemists. Ayungulu, G. G., & Pan, Z. (2011). Combined infrared radiation and freeze-drying. In
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