Effect of Temperature and Velocity of Drying Air on Kinetics, Quality and Energy Consumption in Drying Process of Rice Noodles

Transcription

1 Kasetsart J. (Nat. Sci.) 46 : (2012) Effect of Temperature and Velocity of Drying Air on Kinetics, Quality and Energy Consumption in Drying Process of Rice Noodles Pisut Kongkiattisak and Sirichai Songsermpong* ABSTRACT The drying characteristics of rice noodles were observed at three levels of air drying temperature and two levels of average air drying velocity on the rice noodle surface. The results indicated that the temperature significantly affected the drying process while the velocity was not significant in the terminal stage of the drying process. The drying process for rice noodles occurred during the falling rate period. The effective moisture diffusivity increased at higher air drying temperatures and velocities. The Two- Term model was the most appropriate of all the thin-layer drying equations to explain and predict the drying process of rice noodles. Drying at 85 C and 0.30 m.s -1 can be applied in the production of dried rice noodles to reduce both the primary specific energy in the drying process and the cooking loss of rice noodles. Moreover, this condition resulted in a structure, textural quality and water absorption index which did not differ from other drying conditions. Keywords: rice noodles, drying kinetics, energy consumption, quality NOMECLATURE A, B, C=Constants in thin-layer drying equations D eff =Effective moisture diffusivity (m 2.s -1 ) D 0 =Diffusivity constant (m 2.s -1 ) E a =Activation energy (kj.mol -1 ) k, k 1, k 2 =Rate constants in thin layer drying equations (min -1 ) L=Slab thickness (m) M 0 =Initial moisture content (dry basis) M t =Moisture content at time t (dry basis) M t+dt =Moisture content at time t + dt (dry basis), where dt = Time difference M eq =Equilibrium moisture content (dry basis) MR=Moisture ratio (dimensionless) MR exp,i =Experimental moisture ratio at observation (dimensionless) MR pre,i =Predicted moisture ratio at observation (dimensionless) n=positive integer number N=Number of experimental data points R=Gas constant (8.314 kj.kmolk -1 ) Food Engineering Program, Department of Food Science and Technology, Faculty of Agro-Industry, Kasetsart University, Bangkok 10900, Thailand. * Corresponding author, sirichai.so@ku.ac.th Received date : 04/10/11 Accepted date : 16/03/12

2 604 Kasetsart J. (Nat. Sci.) 46(4) R 2 =Coefficient of determination RMSE=Root mean square error T=Air drying temperature ( C) t=drying time or time (min) V=Average air drying velocity at rice noodles surface (m.s -1 ) %P=Relative percent error INTRODUCTION Noodles of various contents, formulations and shapes have been the staple foods in many Asian countries since ancient times. They can be made from wheat, rice, buckwheat and starches derived from potatoes, sweet potatoes and pulses (Fu, 2008). Rice noodles produced from rice flour are one of the most popular varieties of Asian noodles in Southeast Asia, particularly in Thailand (Juliano and Sakurai, 1985; Hormdok and Noomhorm, 2007). Traditionally, rice noodles are made from long grain rice with a high content of amylose namely, more than 25 g per100 g (Juliano and Sakurai, 1985). Rice noodle processing starts with cleaning and soaking the rice. The soaked rice is milled and the concentration of the milled rice in the slurry is adjusted. It is then steeped, spread, steamed and formed into sheets. Then, the sheets are aged to harden and finally cut into strips. Normally, rice noodles have a high moisture content; therefore, drying can prolong the shelf life. Improper drying could damage the noodle structure, causing overelongation, cracking, warping and splitting of the noodle strands. Moreover, the cooking properties and texture could be severely affected (Fu, 2008). Drying is one of the most ancient methods of food preservation. Drying or dehydration is defined as the removal of water by evaporation, from a solid or liquid food, with the purpose of obtaining a solid product sufficiently low in water content (Berk, 2009). Diffusivity is a physical property of the drying system (Saravacos, 2005; Srikiatden and Roberts, 2007). For practical engineering purposes, the overall transportation may be considered as molecular diffusion, and the overall (effective) diffusivity is usually estimated for the characterization of the mass transfer phenomena in very different processes and this aspect must be taken into account for the analysis and design of processes controlled by the diffusion mechanism (Saravacos, 2005; Welti-Chanes et al., 2005). Thin-layer drying equations are important tools in the mathematical modeling of drying; they are practical and give adequate results (Erbay and Icier, 2010). Thin-layer drying equations may incorporate theoretical, semi-theoretical and empirical models. Semi-theoretical models are generally derived from Fick s second law and modifications of its simplified forms such as the Newton, Page, modified Page, Henderson and Pabis, Logarithmic, Two-Term and Midilli models, while other semi theoretical models are derived by analogues with Newton s law of cooling (Erbay and Icier, 2010). The most important engineering and technological issues in food dehydration are the kinetics of drying, product quality and energy consumption (Berk, 2009). As there are no published reports on the kinetics, energy consumption and qualities of the drying process of rice noodles, this work was conducted with the following objectives: 1) to investigate the effects of air drying temperatures and velocities on the drying characteristics, energy consumption and quality of rice noodles; and 2) to compute the effective moisture diffusivity and fit the experimental data with a suitable thin-layer drying equation for the application of drying process of rice noodles.

3 Kasetsart J. (Nat. Sci.) 46(4) 605 MATERIALS AND METHODS Rice noodle preparation The rice noodles used for this study were made from Thai rice grains (Luang 11 variety) with a high amylose content. Rice kernels were soaked in water for 3 hr and then ground with water in a super mass colloider. The slurry was centrifuged and dried at 40 C for 12 hr. The flour was ground in a hammer mill, passed through a 100-mesh sieve and stored at room temperature in sealed plastic bags. The moisture content of the dried flour was 12.87% (wet basis). Rice noodle sheets were prepared from rice flour slurry (40% rice flour, 60% water) which was left at room temperature for 3 hr. The rice flour slurry (30 g) was poured on a stainless tray ( cm) and steamed for five minutes to obtain sheets with a thickness of 0.56 ± 0.09 mm. The cooked noodle sheets were then cooled for 3 min at an ambient temperature prior to being kept in a refrigerator (4 C) for 12 hr and finally cut into strips of approximately 6.50 mm width, 0.56 mm thickness and cm length. The initial moisture content of the rice noodles was ± 0.92% (wet basis) or ± 3.95% (dry basis). Drying experiment Approximately 73 g of rice noodle strips were dried in a hot air oven (FED53; Binder GmbH; Tuttlingen, Germany) with an interior volume of 53 L. The drying operations were performed at air temperatures of 55, 70, and 85 C and an average air velocity at the rice noodle surface of 0.30 and 1.04 m.s -1 determined by an anemometer (Testo 400; Testo Ag; Lenzkirch, Germany). For the purposes of this experiment, rice noodles were chosen randomly to determine the moisture content at intervals of 5, 10, 20, 35, 55, 90, 135 min during the drying process. The moisture content of samples was determined with the air oven method at 135 C (AACC, 2000). All tests were performed in three replications. Drying rate The drying rate (Doymaz, 2007) and moisture ratio (MR; Parry, 1985; Parti, 1993; Srikiatden and Roberts, 2007) were calculated using Equations 1 and 2, respectively: Drying rate = M t+ dt - M t (1) dt ( Mt - Meq ) Moisture ratio (MR) = (2) M - M ( 0 eq ) where M 0, M t, M t+dt and M eq denote the initial moisture content, the moisture content at time t, the moisture content at time t + dt and the equilibrium moisture content (dry basis), respectively. To attain the equilibrium moisture content, the rice noodles had to be dried at a steady temperature for 36 hr until they had attained a constant weight. Effective moisture diffusivity of rice noodles Rice noodles were assumed to be in the form of an infinite slab. The drying data were analyzed using Fick s second law of diffusion for infinite slab objects. Crank (1975) provided the solutions for various geometries. The solution for infinite slab objects with constant effective moisture diffusivity is given by Equation 3: ( 2n + 1) Deff π t MR = 2 exp (3) 2 n n + 2 π L = 0 ( 2 1) 4 where D eff is the effective moisture diffusivity, (m 2.s -1 ), n is a positive integer number and L is the slab thickness (m), being half the thickness of the rice noodles as drying occurs from both sides. The form of Equation 3 is applicable under the assumption of uniform initial moisture distribution, constant diffusivity, negligible shrinkage, negligible external resistance and negligible temperature gradients, an isothermal condition of drying and macroscopically homogeneous and continuous porosity (Inazu and Iwasaki, 1999; Srikiatden and Roberts, 2007; Doymaz, 2007). The effective moisture diffusivity was estimated with six terms (n = 0, 1, 2, 3, 4, and 5) using Equation 3 by nonlinear estimation used for this purpose.

4 606 Kasetsart J. (Nat. Sci.) 46(4) The dependence of effective moisture diffusivity can be described with the Arrhenius equation by Equation 4: D eff = D 0 Ea exp RT ( ) which can be reformulated as Equation 5: Ea ln ( Deff )= ln( D0 ) RT ( ) (4) (5) where D 0 is a diffusivity constant, R is the gas constant (8.314 kj.kmol.k -1 ), E a is the activation energy (kj.mol -1 ) and T is the air drying temperature ( C). Mathematical model of rice noodle drying The drying data were fitted with the seven thin-layer drying equations given in Table 1. Nonlinear estimation was used to evaluate the parameters of the selected models. The goodness of fit was determined with three statistical parameters namely, the coefficient of determination (R 2 ), root mean square error (RMSE) and the relative percent error (%P). These parameters are formulated in Equations 6 8: R 2 = N N N N MRpre, imrexp, i MRpre, i MRexp, i i= 1 i= 1 i= 1 N N N N ( N MRpre, i ( MRpre, i) ) ( N MRexp, i ( MRexp, i) ) i= 1 i= 1 i= 1 i= 1 12 / 2 (6) RMSE = 1 N 2 ( MRpre, i MRexp, i ) (7) N i= 1 %P = 100 N MRexp, i MRpre, i (8) N = 1 MR i exp, i where MR exp,i is the experimental moisture ratio at observation i, MR pre,i is the predicted moisture ratio at the given observation and N is the number of experimental data points. The higher values of R 2 and the lower values of RMSE and %P were chosen to determine goodness of fit (Madamba et al., 1996; Doymaz, 2007; Hacihafizoğlu et al., 2008). Validation with the production of dried rice noodles The drying process of rice noodles was repeated to confirm that the model could predict the moisture content of rice noodles during drying. The production of dried rice noodles with a moisture content of 10% (dry basis) was performed in three replications. The moisture contents of the observed and predicted values with the thin-layer drying equations at the various drying conditions were compared with a t-test (95% confidence level). Energy consumption The energy consumption in the drying process of rice noodles was analyzed in terms of electrical energy. The electrical power was analyzed according to the Thai Yazaki Electric Wire Company Limited (1991). The secondary specific energy (kilojoules per gram of water evaporated) in the production of dried rice noodles Table 1 Selected thin-layer drying equations of rice noodle for the study. Model name Model equation Newton MR = exp(-kt) Page MR = exp(-kt n ) Modified page MR = exp(-(kt) n ) Henderson and Pabis MR = Aexp(-kt) Logarithmic MR = A(exp(-kt)) + C Two-term MR = Aexp(-k 1 t)+ Bexp(-k 2 t) Midilli MR = A(exp(-kt n )) + Bt

5 Kasetsart J. (Nat. Sci.) 46(4) 607 with a moisture content of 10% (dry basis) was calculated using the electrical power (kilowatt hours) multiplied by the drying time (seconds) divided by the weight of the water evaporated (grams).the primary specific energy was evaluated by the secondary specific energy multiplied by 2.6 (Soponronnarit, 1997; Jittanit et al., 2010). Structure of rice noodles The structure of dried rice noodles was analyzed with a scanning electron microscope (SU- 1500; Hitachi Co.; Tokyo, Japan). The samples were cut, mounted on a stub and photographed in cross-section at an accelerating voltage of 5 kv. Qualities of rice noodles Textural quality The dried rice noodles were rehydrated by soaking in water for 10 min, cooked in boiling water for 2 min, cooled in water for 1 min and drained for 5 min. The tensile strength of the cooked rice noodles was studied using a texture analyzer (TA-XT2; Stable Micro systems Ltd.; Godalming, UK) with spaghetti tensile grips and a 25 kg load cell. The analysis was conducted by winding the rice noodles two to three times around the parallel rollers of a probe with the upper arm set to travel apart from the lower arm at a speed of 3.0 mm.s -1 for a length of 50 mm. The tensile strength was measured as the maximum force at which the rice noodles broke and thus indicates the resistance to the breakdown of the sample. The breaking distance at which the rice noodles started to break indicates the extensibility (as applied by Hormdok and Noomhorm, 2007; Satmalee and Charoenrein, 2009). For the hardness, a rehydrated rice noodle was placed under a light knife blade probe with the axis of the product at right angles to the blade. The probe was compressed with 50% strain at a pre-test speed of 0.5 mm.s -1, a test speed of 0.2 mm.s -1 and a post test speed of 10 mm.s -1. The hardness of the sample was determined from the maximum force (using the method of Suksomboon, 2007). Cooking quality The method according to Hormdok and Noomhorm (2007) was applied. The dried rice noodles (1.0 g) were cut into small pieces (2.0 cm in length) and rehydrated in boiling distilled water (60 ml) until completely cooked (4 min). The cooked rice noodles were washed with distilled water (20 g), drained for 5 min and weighed immediately. The water absorption index was calculated as the percentage increase in the weight of the cooked rice noodles compared with the weight of the dried rice noodles. The cooked water was collected and dried at 105 C to a constant weight. The cooking loss was calculated as the percentage of dry matter in the cooked water compared with the weight of the dried rice noodles. The quality analysis was performed with three replicates. A factorial experimental design was used and the mean separations were determined using Duncan s method (at the 95% confidence level). RESULTS AND DISCUSSION Drying characteristics The effects of the temperature and velocity of air drying are shown in Figure 1. The results indicated that drying at high temperatures and velocities decreases the moisture content (dry basis) more effectively than at lower temperatures and velocities. In the initial period of drying, an air velocity of 1.04 m.s -1 decreased the moisture content more effectively than at an air velocity of 0.30 m.s -1. However, during the terminal stages of the drying period, the moisture content of the samples exposed to the aforementioned air drying velocities were not significantly affected. This result revealed that to attain a low moisture content in rice noodles, an average air velocity of 0.30 m.s -1 made only a slight difference in the moisture content than drying at 1.04 m.s -1 (with an identical duration for the drying process). Conversely, the air drying temperature significantly affected the

6 608 Kasetsart J. (Nat. Sci.) 46(4) drying performance of rice noodles. The results shown in Figure 2 reveal an absence of a constant rate period in the drying process of rice noodles, but rather the drying process occurred during the falling rate period because rice noodles have a low initial moisture content and the water is bound with starch. Furthermore, this indicates that molecular Moisture content (dry basis) C 1.04 m.s C 0.30 m.s C 1.04 m.s C 0.30 m.s C 1.04 m.s C 0.30 m.s Drying time (min) Figure 1 Effect of air drying temperatures and velocities on the moisture content (dry basis) of rice noodles. (Vertical bars represent ± SE.) 0.09 Drying rate (g water /(g dry solid min)) C 1.04 m.s C 0.30 m.s C 1.04 m.s C 0.30 m.s C 1.04 m.s C 0.30 m.s Moisture content (dry basis) Figure 2 Effect of air drying temperature and velocity on drying rate of rice noodles.

7 Kasetsart J. (Nat. Sci.) 46(4) 609 diffusion is the dominant physical mechanism which governs the moisture transfer in the sample (Sarsavadia et al., 1999; Saravacos, 2005; Doymaz, 2007). The air velocity has no direct effect on the internal water transport and therefore should not affect the drying rate during the falling rate period (Berk, 2009). Therefore, the air velocity has no effect on this process which was confirmed by the results shown in Figure 1. During drying, the noodle surface moisture becomes vaporized and is removed by the surrounding air. This creates a moisture content gradient within the noodle strands and the moisture diffuses as liquid from the centre moves to the surface of the noodle strands along the moisture gradient (Fu, 2008). Effective moisture diffusivity of rice noodles As described in the previous sections, the drying process of rice noodles occurred during the falling rate period. The data can be analyzed with Fick s second law of diffusion. The results are shown in Table 2. The value of D eff was in the range to m 2.s -1. At an identical velocity, the effective moisture diffusivity increased with higher drying temperatures. At the same temperature, this value increased with higher velocities. The activation energy (E a ) and the diffusivity constant (D 0 ) were calculated using Equation 5 by plotting ln(d eff ) versus 1/(T ) in Figure 3. This result shows that the activation energy was and kj.mol -1 for velocities of 0.30 and 1.04 m.s -1, respectively. The diffusivity constant was and m 2.s -1 for velocities of 0.30 and 1.04 m.s -1, respectively. An air flow velocity of 0.30 m.s -1 had a higher activation energy and lower diffusivity constant than an air flow velocity of 1.04 m.s -1. D 0 is generally defined as the reference diffusion coefficient at infinitely high temperatures. The value of E a shows the sensibility of the diffusivity to the temperature namely, the greater the value of E a, the greater the sensibility of D eff to the temperature (Erbay and Icier, 2010). The effective moisture diffusivity can be described by the air drying temperature and velocity using multiple regressions as demonstrated in Equation 9: D eff = ( T 0.011V TV) 10 11, R 2 = (9) where T is the air drying temperature ( C) and V is the average air drying velocity at the surface (m.s -1 ). The effective moisture diffusivities in noodle products are shown in Table 3, which indicates that the effective moisture diffusivities have various values ( to m 2.s -1 ). Despite being for the same product, the effective moisture diffusivity had various values which depended on the temperature, velocity and structure. Mathematical model of rice noodle drying The model constants and statistical parameters are shown in Table 4. For all Table 2 Effective moisture diffusivity, activation energy and diffusivity constant of rice noodles. Velocity Temperature D eff E a D 0 (m.s -1 ) ( C) ( m 2.s -1 ) (kj.mol -1 ) ( 10-8 m 2.s -1 )

8 610 Kasetsart J. (Nat. Sci.) 46(4) experiments, the fitted thin-layer drying equations each had a high R 2 (R 2 > 0.97) and a value for %P of less than 5, which indicates an excellent fit, while values in excess of 10 are indicators of a poor fit (Singh and Gupta, 2007). The results revealed that the Two-Term model generally produced the highest R 2 and the lowest RMSE and %P. The coefficients in the Two-Term model were correlated and expressed as shown in Equations 10 13: A = T 0.280V TV T 2, R 2 = (10) k 1 = V T TV T 2, R 2 = (11) B = ln(V) ln(T) (ln(T)) ln(T) ln(v), R 2 = (12) k2 = , R 2 = 2 T V TV T (13) Validation with the production of dried rice noodles The results in Table 5 prove that the Two-Term and Logarithmic models can predict the moisture content in the production of dried rice noodles (the observed and predicted values were not significantly different). However, the moisture content which was predicted by the Logarithmic model seemed to differ more from the observed values than the moisture content predicted by the Two-Term model. For the other models, the results indicated that they inaccurately predicted the moisture content when compared with the observed values. Furthermore, the results in Table 5 confirm that the drying times to produce dried rice noodles at identical temperatures at air velocities of 0.30 and 1.04 m.s -1 were only slightly different. Energy consumption The electrical power and specific energy are shown in Table 6. Drying at high temperatures and velocities consumed additional electrical power compared with drying at low temperatures and velocities. For the production of dried rice 1/(T ) m.s m.s ln(d eff ) y = x R² = y = x R² = Figure 3 Relationships between effective moisture diffusivity and air drying temperature by Arrhenius equation. (D eff = effective moisture diffusivity (m 2.s -1 ); T = air drying temperature ( C).)

9 Kasetsart J. (Nat. Sci.) 46(4) 611 noodles (10% dry basis), the results indicated that drying at lower velocities (at the same temperature) required less primary specific energy, because the drying times at both velocities did not diverge significantly as the drying process ends in the terminal stage of the drying period and the electrical power required for drying at low velocity was less than required for drying at high velocity. The application of higher drying temperatures required augmented electrical power but resulted in shorter drying times compared to the drying procedures at lower temperatures. Therefore, the primary specific energy required for drying at higher temperatures was less than that of drying at lower temperatures. Thus, high temperatures and low velocities for the reduction of the energy consumption can be applied for the production of dried rice noodles. Moreover, when control of the drying process is required, the drying time can be predicted with the Two-Term model and this can be used to evaluate the energy consumption of the drying process. Structure of rice noodles The structure of dried rice noodles is shown in Figures 4 and 5. The images indicate that the dried rice noodles have a dense structure. The structure of rice noodles dried under various drying conditions was not different. This result was similar to that reported for banana foam mats (Thuwapanichayanan et al., 2008), where the porous structure was not significantly different for banana foam mats which were dried at high (80 C) or low (60 C) temperatures. In addition, Luangmalawat et al. (2008) reported that the morphology of dried cooked rice after rehydration could not be differentiated among samples obtained from low (50 C) or high (120 C) drying temperatures. Table 3 Effective moisture diffusivity of various noodles with drying temperature and velocity. Noodle Temperature ( C) Velocity (m.s -1 ) D eff (m 2.s -1 ) Reference Pasta Xiong et al. (1991) Pasta Litchfield and Okos (1992) Pasta (6%porosity) Waananen and Okos (1996) Pasta (26% porosity) Waananen and Okos (1996) Spaghetti Küçük and Özilgen (1997) Udon Inazu and Iwasaki (1999) Rice noodle Current study

10 612 Kasetsart J. (Nat. Sci.) 46(4) Table 4 Model constants and statistical parameters for the drying process of rice noodles. Drying conditions Model name Model constants R 2 RMSE ( 10 2 ) %P 55 C, 0.30 m.s -1 Newton k = Page k = , n = Modified page k = , n = Henderson and Pabis A = , k = Logarithmic A = , k = , C = Two-term A = , k 1 = , B = k 2 = Midilli A = , k 1 = , n = B = C, 1.04 m.s -1 Newton k = Page k = , n = Modified page k = , n = Henderson and Pabis A = , k = Logarithmic A = , k = , C = Two-term A = , k 1 = B = , k 2 = Midilli A = , k 1 = n = , B = C, 0.30 m.s -1 Newton k = Page k = , n = Modified page k = , n = Henderson and Pabis A = , k = Logarithmic A = , k = , C = Two-term A = , k 1 = , B = k 2 = Midilli A = , k 1 = , n = B =

11 Kasetsart J. (Nat. Sci.) 46(4) 613 Table 4 (cont.) Drying conditions Model name Model constants R 2 RMSE ( 10 2 ) %P 70 C, 1.04 m.s -1 Newton k = Page k = , n = Modified page k = , n = Henderson and Pabis A = , k = Logarithmic A = , k = , C = Two-term A = , k 1 = , B = k 2 = Midilli A = , k 1 = , n = B = C, 0.30 m.s -1 Newton k = Page k = , n = Modified page k = , n = Henderson and Pabis A = , k = Logarithmic A = , k = , C = Two-term A = , k 1 = , B = k 2 = Midilli A = , k 1 = , n = B = C, 1.04 m.s -1 Newton k = Page k = , n = Modified page k = , n = Henderson and Pabis A = , k = Logarithmic A = , k = , C = Two-term A = , k 1 = , B = k 2 = Midilli A = , k 1 = , n = B =

12 614 Kasetsart J. (Nat. Sci.) 46(4) Table 5 Moisture content of observed and predicted values by mathematical model. Moisture content Moisture content of predicted value by mathematical model (% dry basis) Drying conditions of observed value Newton Page Modified page Henderson Logarithmic Two-term Midilli (% dry basis) and Pabis 55 C, 0.30 m.s a ± b 6.82 b 6.82 b 6.32 b 9.65 a 9.66 a 9.11 a 55 C, 1.04 m.s a ± b 7.09 b 7.09 b 5.84 b a a 9.01 b 70 C, 0.30 m.s a ± b 7.70 b 7.70 b 6.79 b 9.51 a 9.75 a 8.37 b 70 C, 1.04 m.s a ± b 6.65 b 6.65 b 5.22 b 8.84 a 9.37 a 7.53 b 85 C, 0.30 m.s a ± b 9.79 a 9.79 a 8.33 b a a 9.63 a 85 C, 1.04 m.s a ± b 8.79 b 8.79 b 6.57 b 8.98 a 9.69 a 8.65 b Different lower case superscript letters in the same row indicate a significant difference between the observed value and predicted value with t test (95% confidence level). Qualities of dried rice noodles Tables 7 and 8 show that the textural quality of rice noodles was not affected by various drying conditions. The drying temperature (25 65 C) exerted no significant effects on the textural properties of the cooked noodles (Lee et al., 2005). The velocity was not a significant factor on the breaking strength of noodles (Pronyk et al., 2008). For the cooking quality, the results indicated that the water absorption index was not affected by various drying conditions. However, the rice noodles which were dried at 55 C had significantly higher cooking loss than the rice noodles which were dried at 70 and 85 C because it was possible that drying at 70 and 85 C caused strengthening of the surface and prevented the loss of solids during cooking. Alternatively, it was possible that the uncompleted gelatinization of starch in the rice noodles continued during the drying process. Udomrati (2005) found that rice flour (Luang 11 variety) was completely gelatinized when the moisture content was more than 70% (by weight) and the onset gelatinization temperature was C at 30% moisture content and C at 50% moisture content, while the moisture content of the rice flour slurry and fresh rice noodles was 60% and 51.49%, respectively. Therefore, it was possible that fresh rice noodles had uncompleted gelatinization starch which was gelatinized during drying at 70 and 85 C. CONCLUSION The investigation of the drying characteristics of rice noodles revealed that the temperature significantly affected the drying rate, whereas the air flow velocity only slightly influenced the effectiveness of the drying process performance. The drying process of rice noodles revealed a falling rate period. The effective moisture diffusivity increased with augmented air drying temperature and air flow velocities and could be correlated with the regression model.

13 Kasetsart J. (Nat. Sci.) 46(4) 615 Using a thin-layer drying equation to fit the experimental results, the Two-Term model was considered the most appropriate for the elucidation of the drying characteristics of rice noodles and thus can be applied to predict and control the drying process of rice noodles. Drying rice noodles at 85 C and 0.30 m.s -1 could reduce the energy consumption and cooking loss and did not affect the textural quality, water absorption index and structure when compared with alternative drying conditions. Table 6 Electrical power and primary specific energy for the production of dried rice noodles. For the production of dried rice noodles Velocity Temperature Electrical (10% dry basis) (m.s -1 ) ( C) power (kw) Drying time Primary specific energy (min) (kj/g water evaporated) Table 7 Textural quality of rice noodles under various drying conditions. Textural quality Velocity Temperature Tensile strength NS Breaking distance NS Hardness NS (m.s -1 ) ( C) (N) (mm) (N) ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± NS = Not significant by Duncan s method (95% confidence level). Table 8 Cooking quality of rice noodles under various drying conditions. Cooking quality Velocity Temperature Water absorption index NS Cooking loss (m.s -1 ) ( C) (%) (%) ± a ± ± b ± ± b ± ± a ± ± b ± ± b ± 0.16 NS = Not significant by Duncan s method (95% confidence level). Different lower case superscript letters in the same row indicate a significant difference between the observed value and predicted value with t test (95% confidence level).

14 616 Kasetsart J. (Nat. Sci.) 46(4) Figure 4 Scanning electron micrographs of dried rice noodles (magnification 170) with a cross-section at: (a) 55 C, 0.30 m.s -1 ; (b) 55 C, 1.04 m.s -1 ; (c) 70 C, 0.30 m.s -1 ; (d) 70 C, 1.04 m.s -1 ; (e) 85 C, 0.30 m.s -1 ; and (f) 85 C, 1.04 m.s -1.

15 Kasetsart J. (Nat. Sci.) 46(4) 617 Figure 5 Scanning electron micrographs of dried rice noodles (magnification 2,000) with a crosssection at: (a) 55 C, 0.30 m.s -1 ; (b) 55 C, 1.04 m.s -1 ; (c) 70 C, 0.30 m.s -1 ; (d) 70 C, 1.04 m.s -1 ; (e) 85 C, 0.30 m.s -1 ; and (f) 85 C, 1.04 m.s -1.

16 618 Kasetsart J. (Nat. Sci.) 46(4) ACKNOWLEDGEMENT The authors gratefully acknowledge the financial support given by the Office of the Higher Education Commission, Bangkok, Thailand. LITERATURE CITED American Association of Cereal Chemists, (AACC) Approved Methods of the AACC. 10th ed. St. Paul, MN, USA. Berk, Z Food Process Engineering and Technology.1st ed. Academic Press. Oxford, UK. 605 pp. Crank, J Mathematics of Diffusion. 2nd ed. Clarendon Press. Oxford, UK. 414 pp. Doymaz, İ The kinetics of forced convective air-drying of pumpkin slices. J. Food Eng. 78: Erbay, Z. and F. Icier A review of thin layer drying of foods: Theory, modeling, and experimental results. Crit. Rev. Food Sci. Nutr. 50: Fu, B.X Asian noodle: History, classification, raw materials, and processing. Food Res. Int. 41: Hacihafizoğlu, O., A. Cihan and K. Kahveci Mathematical modelling of drying of thin layer rough rice. Food Bioprod. Process. 86: Hormdok, R. and A. Noomhorm Hydrothermal treatments of rice starch for improvement of rice noodle quality. LWT. 40: Inazu, T. and K.I. Iwasaki Effective moisture diffusivity of fresh Japanese noodle (udon) as a function of temperature. Biosci. Biotech. Biochem. 63: Jittanit, W., N. Saeteaw and A. Charoenchaisri Industrial paddy drying and energy saving options. J. Stored Products Res. 46: Juliano, B.O. and J. Sakurai Miscellaneous rice products, pp In B.O. Juliano, (ed.). Rice: Chemistry and Technology. 2nd ed. AACC. St. Paul, MN, USA. 714 pp. Küçük, R. and M. Özilgen Predicting drying behavior of spaghetti in a continuous industrial dryer with the models determined in a lab scale batch system. J. Food Process Pres. 21: Lee, S.Y., K.S. Woo, J.K. Lim, H.I. Kim and S.T. Lim Effect of processing variables on texture of sweet potato starch noodles prepared in a nonfreezing process. Cereal Chem. 82(4): Litchfield, J.B. and M.R. Okos Moisture diffusivity in pasta during drying. J. Food Eng. 17: Luangmalawat, P., S. Prachayawarakorn, A. Nathakaranakule and S. Soponronnarit Effect of temperature on drying characteristics and quality of cooked rice. LWT. 41: Madamba, P.S., R.H. Driscoll and K.A. Buckle The thin-layer drying characteristics of garlic slices. J. Food Eng. 26: Parry, J.L Mathematical modeling and computer simulation of heat and mass transfer in agricultural grain drying: A review. J. Agr. Eng. Res. 32: Parti, M Selection of mathematical models for drying grain in thin-layer. J. Agr. Eng. Res. 54: Pronyk, C., S. Cenkowski, W.E. Muir and O.M. Lukow Effect of superheated steam processing on the textural and physical properties of Asian noodles. Dry. Technol. 26: Saravacos, G.D Mass transfer properties of foods, pp In M.A. Rao, S.S.H. Rizvi and A.K. Dautta, (eds.). Engineering Properties of Foods. Taylor & Francis. Boca Raton, NY, USA. 738 pp. Sarsavadia, P.N., R.L. Sawhney, D.R. Pangavhane and S.P. Singh Drying behaviour of brined onion slices. J. Food Eng. 40:

17 Kasetsart J. (Nat. Sci.) 46(4) 619 Satmalee, P. and S. Charoenrein Acceleration of ageing in rice stick noodle sheets using low temperature. Int. J. Food Sci. Tech. 44: Singh, B. and A.K. Gupta Mass transfer kinetics and determination of effective diffusivity during convective dehydration of pre-osmosed carrot cubes. J. Food Eng. 79: Soponronnarit, S Drying of Grain and Some Kinds of Food. 7th ed. King Mongkut s University of Technology. Thonburi, Bangkok, Thailand. 338 pp. Srikiatden, J. and J.S. Roberts Moisture transfer in solid food materials: A review of mechanisms, models, and measurements. Int. J. Food Prop. 10: Suksomboon, A Effect of Dry-and Wetmilling Process on Rice Flour, Rice Starch and Rice Noodle Process. PhD. Thesis. Kasetsart University. Bangkok, Thailand. 162 pp. Thai Yazaki Electric Wire Company Limited Technical Information and Specification for Electric Wires and Cables. 1st ed. Thai Yazaki Electric Wire Company Limited. Bangkok. 102 pp. Thuwapanichayanan, R., S. Prachayawarakorn and S. Soponronnarit Drying characteristics and quality of banana foam mat. J.Food Eng. 86: Udomrati, S Gelatinization and Retrogradation of Starch from Four Thai Rice Varieties. MSc. Thesis. Kasetsart University. Bangkok, Thailand. 162 pp. Waananen, K.M. and M.R. Okos Effect of porosity on moisture diffusion during drying of pasta. J. Food Eng. 28: Welti-Channes, J., F. Vergara-Balderas and D. Bermúdez-Aguirre Transport phenomena in food engineering: Basic concept and advances. J. Food Eng. 67: Xiong, X., G. Narsimhan and M.R. Okos Effect of composition and pore structure on binding energy and effective diffusivity of moisture in porous food. J. Food Eng. 15: