DRYING AND COLOR CHARACTERISTICS OF CORIANDER FOLIAGE USING CONVECTIVE THIN- LAYER AND MICROWAVE DRYING

Transcription

1 DRYING AND COLOR CHARACTERISTICS OF CORIANDER FOLIAGE USING CONVECTIVE THIN- LAYER AND MICROWAVE DRYING M. Shaw, V. Meda, L. Tabil Jr. and A. Opoku Department of Agricultural and Bioresource Engineering, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5A9 Heat sensitive properties (aromatic, medicinal, color) provide herbs and spices with their high market value. In order to prevent extreme loss of heat sensitive properties when drying herbs, they are normally dried at low temperatures for longer periods of time to preserve these sensory properties. High energy consumption often results from drying herbs over a long period. Coriander (Coriandrum sativum L., Umbelliferae) was dehydrated in two different drying units (thin layer convection and microwave dryers) in order to compare the drying and fi nal product quality (color) characteristics. Microwave drying of the coriander foliage was faster than convective drying. The entire drying process took place in the falling rate period for both microwave and convective dried samples. The drying rate for the microwave dried samples ranged from 42.3 to 48.2% db/min and that of the convective dried samples ranged from 7.1 to 12.5% db/min. The fresh sample color had the lowest L value at with higher L values for all dried samples. The results show that convective thin layer dried coriander samples exhibited a signifi cantly greater color change than microwave dried coriander samples. The color change index values for the microwave dried samples ranged from 2.67 to 3.27 and that of the convective dried samples varied from 4.59 to Submission Date: 30 November 2006 Acceptance Date: 19 June 2007 Publication Date: 30 August 2007 INTRODUCTION Keywords: Drying, dehydration, herbs, coriander, specialty crops, convection, microwave, color, quality. Coriander (Coriandrum sativum L., Umbelliferae) foliage is a culinary herb that is used to flavor dishes, soups and salads. It is one of the most consumed culinary herbs. Coriander leaves possess antioxidants and exhibit stronger antioxidant activity than the seeds (Wangensteen et al., 2004). Coriander has been used to treat worms, rheumatism, joint pains and indigestion [Wichtl, 1994]. Dehydration is the most common preservation method for prolonging the shelf-life of herbs and spices. Drying also facilitates easier handling and transportation characteristics of herbs and spices. The dried product can easily be milled or ground and can be used in the nutraceutical, baking and culinary industries. Coriander/cilantro is grown in the Canadian Prairies with yields ranging from 800 to 1000 kg/ha. Several dehydration methods exist which producers can use to produce a shelf stable dried product. Currently, it appears there is no existing efficient standard method for drying herbs and spices in Western Canada. Each producer employs his/her own techniques based on natural or forced-air convective drying. Small scale Journal of Microwave Power & Electromagnetic Energy Vol. 41, No. 2, 2007

2 producers normally bundle herbs together and hang them to dry in warm, dark places such as sheds. On a larger scale, the labor requirements for this would be very high. Some common drying trends include the use of gas-fired dryers and commercially available dehumidifiers. The development of high-quality dried foods can be accomplished by utilizing novel drying technologies such as microwave drying, refractance window and vacuum drying methods in order to maximize and retain the quality attributes such as structure, color, flavor and nutrition [Sablani, 2006]. Studies of thin-layer (convection) drying of herbs and other food materials are numerous in literature [Li and Morey, 1987; Temple et al., 2001; Erenturk et al., 2004; Demir et al., 2004; Shaw et al., 2005; Akpinar et al., 2006; Doymaz, 2006]. The majority of research into herb dehydration examines the effect of convective drying on the ability of the dried material to retain its heat sensitive properties. Several studies have since concluded that temperature is the principal factor affecting drying rate and quality. Li and Morey [1987] reported that drying air temperature had the greatest effect on drying rates and quality during thin-layer drying of American ginseng, particularly below moisture contents of 28 to 30% wet basis (wb). Rocha et al. [1992] also found that air temperature was the main factor controlling the drying rate and color (quality) of mint and basil foliage when the materials were not blanched prior to drying. For a particular temperature, higher air humidity contributed to the degradation of the green color of the plants since drying times were prolonged. They indicated that low air temperatures (45 to 50 o C) should be used to dry mint and basil plants to prevent the deterioration of product quality. Chou and Chua [2001] noted several advantages of hybrid microwave drying units over conventional units. Along with enhanced diffusion of heat and mass, microwave energy is reported to increase drying rates without increasing surface temperature, thereby increasing product quality. However, higher capital investments and low energy efficiencies are significant disadvantages of microwave drying units. Microwave heating technology has been used to dry herbs and spices, and its effect on the quality of the dried product has been evaluated. The effect of microwave heating on the extraction of volatiles from herbs and spices has been studied [Balasundaram and Meda, 2003; Kaufmann and Christen, 2002]. Bertelli et al. [2004] investigated the effect of microwave drying on the antioxidant activity of black pepper, oregano, basil and sage. They indicated that there was no statistical difference between the antioxidant activity of the fresh and the microwave dried samples. Di Cesare et al. [2003] dried basil leaves using freeze-drying, air-drying at 50 o C and microwave drying. The fresh and dried samples were analyzed for chlorophyll and color changes during drying. It was found that microwave-dried samples had higher chlorophyll content and the green color was preserved better than for the air-dried and freeze-dried samples. Alibas [2006] indicated that chard leaves dried at 50 o C had a color quality close to the fresh sample. Microwave drying at 500 W produced a product color close to the fresh sample. A sample size of about 25 g was used in the tests. Air drying and its effects on the characteristics of coriander have been studied by researchers [Pande et al., 2000; Ahmed et al., 2001; Kaur et al., 2006]. Pande et al. [2000] reported that the entire coriander leaf drying process took place in the falling rate period, and there was no constant rate period. It was also noted that drying temperatures between o C are suitable for good quality finished products. Literature pertaining to the drying kinetics of coriander using microwave is scant. The effect of microwave drying on the quality (color) of coriander is also limited. Historically, herbs have been dried using temperatures ranging from o C. Drying temperature of 50 o C has been indicated to cause minimal loss of quality in drying herbs and spices. In this study, drying International Microwave Power Institute

3 Figure 1. Schematic diagram of the thin layer (convection) dryer [Kashaninejad and Tabil, 2004]. air temperature of 50 o C is selected to compare to the microwave drying characteristics of coriander. The objectives of this study were to compare the dehydration characteristics of high moisture (fresh) coriander using two dehydration units; namely, convective thin-layer and microwave dryers. The main drying characteristics that were investigated were the drying rate and product quality in terms of color. METHODS AND MATERIALS Sample Preparation Fresh coriander foliage (leaves and stems) was obtained from a local grocery store and stored in a refrigerator at 5 o C and relative humidity of 85%. Moisture content measurement followed that of ASAE Standard S358.2 [1997]. Approximately 30 minutes prior to dehydration experiments, coriander was removed from the refrigerator and allowed to equilibrate with ambient conditions. Stems and leaves were not separated and thus they were dried together. Thin Layer Convective Drying Figure 1 shows the schematic diagram of the thin layer drying unit. It consists of an air conditioning unit, fan, air straightener, sample tray, data acquisition system and instruments for measuring temperature, mass, relative humidity and airflow. The air conditioning unit (Model AH-213) was manufactured by Bryant Manufacturing Associates, Inc., Ayer, MA. The air was conditioned to the required temperature and humidity using the air conditioner and was forced to the drying chamber using the fan. Type T thermocouples were used to measure the temperatures to an accuracy of ±0.01 o C. The mass, relative humidity and airflow were measured to accuracies of 0.01 g, 1 o and 0.1 m/s, respectively. Approximately 60 g of coriander was placed on mesh trays within the thin layer drying unit. Average initial coriander moisture content was determined to be 93.37% wet basis (wb) for the first two tests, and 92.16% wb for the final test. Care was taken to ensure that the sample was distributed evenly across the trays, so that overlapping was minimized. Drying air temperature was set to 50 o C, while the relative humidity Journal of Microwave Power & Electromagnetic Energy Vol. 41, No. 2, 2007

4 Figure 2. Microwave unit used in the dehydration of coriander foliage (a), and the placement of fi ber-optic probes on the material tray (b). was maintained below 5%, and the air velocity was kept at 1.1 m/s. Three replicates were performed for this treatment. Temperature, relative humidity, air-flow and product mass data were logged throughout the drying process using Lab- VIEW TM 6.0 (National Instruments, Austin, TX). Following dehydration, the dried samples were placed in polyethylene bags and returned to the refrigerator set at 5 o C to be used for subsequent color measurements. Microwave Drying Figure 2 shows a combined microwave-convective dryer, Panasonic Model NNC980W (Panasonic Canada Ltd, Mississauga, ON, Canada) used in drying the coriander samples. The microwave unit employs an inverter technology for continuous control of the microwave power output. The internal chamber dimensions were 242 mm (H) x 412 mm (W) x 426 mm (D). The microwave dryer has 10 different power levels with rated power ranging from 220 to 1000 W. It was operated at a frequency of 2450 MHz. The desired drying temperature was unable to be preset; instead, an appropriate power setting was chosen (Power level 3 corresponding to measured power of 295 W) which maintained the average product temperature as close as possible to 50 o C. The microwave power output was determined according to the standard procedure prescribed by Schiffmann [1987]. Temperature was logged using 4 fiber-optic probes connected to a 4-channel UMI Signal Controller (FISO Technologies Inc., Quebec, QB). The fiber-optic probe had a temperature measurement range of -50 to 250 o C with an accuracy of ±0.01 o C. Three of the fiber optic probes were placed on the product surface with each probe s spatial distance equally spaced. The fourth probe was laid on the floor of the microwave chamber to monitor ambient air temperature. Sixty grams of coriander at 91.20% wb was placed on a ceramic plate within the microwave chamber. The plate was not rotated during drying, but remained stationary throughout due to temperature measurement restrictions. The sample tray was removed from the chamber and weighed periodically at regular intervals. The mass of the samples was measured to an accuracy of 0.01 g. Color Measurement One of the indicators of final dehydrated product quality is color [Soysal, 2004; Kaur et al., 2006; Di Cesare et al., 2003; Alibas, 2006]. Exces- International Microwave Power Institute

5 sive color deterioration may be an indicator of the loss of heat sensitive properties, as color is heat sensitive. Therefore, fresh coriander was selected as the standard for color measurement. Shears were used to cut fresh and dehydrated samples into approximately 10 mm pieces. Approximately 5 g of the chopped sample was placed in a 250 ml glass beaker from which the color of the coriander was measured using a LabScan 6000 spectro-colorimeter (Hunter Associates Laboratory Inc., Reston, VA). Color calculations followed the same methodology as in Kashaninejad and Tabil et al. [2004], and thus color measurements were reported based on Hunter L, a, and b values. Three replicates of color measurement were made for each sample. Following each replicate, the sample was mixed thoroughly in the beaker before a new set of readings were taken. The relative color of the dried samples was compared using color difference index value, E (Equation 1). E = ( L) + ( a) + ( b) (1) The base (standard) values of Hunter L, a, and b represent that of the fresh (un-dried) sample. The Hunter L value represents the lightness or darkness of a sample on a scale of 0 to 100 (100 being white and 0 being black). Hunter a value represents the greenness or redness of the sample (-50 being green and +50 being red). Hunter b value is also rated on a scale of -50 to +50, with -50 representing blue and +50 representing yellow. The Hunter values prefixed by a greek delta (Δ) represent the difference of the base value from the dried sample measured value. Drying Data Analysis Ahmed et al. [2001] reported that Page s Equation (Eq. 2) adequately described the convective drying of coriander leaves over a wide range of drying temperatures in the falling rate period. Page s equation has been used to model the microwave drying characteristics of herbs and other products [Soysal, 2004; Alibas, 2006; Panigrahi et al., 2006]. Equation 2 was fitted to model coriander foliage drying for microwave and convective air drying at 50 o C. m me MR = m m o e n = exp( kt ) (2) where k is the drying rate constant (min -1 ), n is a dimensionless coefficient, MR is the moisture ratio, m is the instantaneous moisture content (% db), m e is the equilibrium moisture content (% db), m o is the initial moisture content (% db), and t is time (min). The equilibrium mois- ture content of the thin layer dried samples of coriander was determined by using the model provided by Soysal and Oztekin [2001]. For the microwave dried coriander samples, the equilibrium moisture content was assumed to be zero [Maskan, 2000; Soysal, 2004]. The k and n parameters in Page s equation were determined for the dried materials by using a curve-fitting software (TableCurve 2D, Jandel Scientific, San Rafael, CA, USA). The drying rates of the dried coriander samples were determined by finding the derivative of Page s equation. RESULTS AND DISCUSSION M i c r o w a v e a n d c o n v e c t i v e d r y i n g characteristics Typical drying curves for convective thin layer and microwave drying are shown in Figure 3. The drying curves show that microwave drying is faster than convective drying. Microwave drying was rapid and convective drying was slow and gradual to reach the desired final moisture content. For the convective drying, it could be seen that replicates 1 and 2 required 267 min utes to dry approximately 60 g of coriander from 93.37% wb to approximately 12% wb, while Journal of Microwave Power & Electromagnetic Energy Vol. 41, No. 2, 2007

6 Moisture Ratio Drying time (min) Figure 3. Typical coriander dehydration curves for thin-layer and microwave drying. Table 1. Thin layer and microwave drying time and average drying rate. Thin layer drying Microwave drying Test 1 Test 2 Test 3 Test 1 Test 2 Test 3 Initial moisture content (%, wb*) Final moisture content (%,wb) Drying time (min) Average drying rate (% db**/min) *wb is the wet basis and db is the drying basis. replicate 3 took 236 minutes to dry the coriander from 92.16% wb to approximately 12% wb (Table 1). The difference in the drying time may be due to the initial moisture content of the sample. For higher initial moisture content more time may be required for the moisture to diffuse through the material. Similar phenomena were reported by Tang and Sokhansanj [1993] and Wadsworth et al. [1990] for convective drying of lentil seeds and microwave-vacuum drying of parboiled rice, respectively. Microwave power was able to reduce the coriander moisture content to the 12%, wb target in 21 minutes in replicate 1, and 22 minutes for replicates 2 and 3. Kaur et al. [2006] indicated that 300 min utes was required to dry coriander leaves to a final moisture content of 4.17% db using a convective dryer. Coriander leaves were dried at 55 o C but International Microwave Power Institute

7 Table 2. Parameters for the Page s equation fitted to the convective thin layer and microwave drying curves. Thin layer k (min -1 ) n R 2 Standard error Test Test Test Microwave k (min -1 ) n R 2 Standard error Test Test Test the relative humidity was not reported. Ahmed et al. [2001] reported the drying characteristics of blanched coriander leaves. Less time was required to dry the blanched coriander samples at 45 o C than the time needed by Kaur et al. [2006] and the present study. Figure 4 shows the typical drying rate curves for convective thin layer and microwave drying. The drying rate for the microwave-dried sample increased rapidly at the beginning of the drying process, peaked and then started to decrease quickly. The rapid increase in the drying rate of the coriander was probably due to its high moisture content which resulted in the higher absorption of the microwave energy at the start of the drying process. The higher absorption of microwave energy at the onset of the drying process resulted in higher temperatures and higher moisture diffusion from inside the microwave-dried coriander samples compared to convective-dried samples. The drying rate started to decline after most of the moisture had been removed by microwave heating. The drying rates for the microwave-dried samples peaked at 5 to 6 minutes. The drying rate for the convective dried coriander samples decreased gradually with time. The average drying rates for the microwave dried samples were much higher than the convective dried samples as presented in Table 1, ranging from 42.3 to 48.2% db/min. Alibas [2006] reported the average drying rate of microwave dried chard leaves as 56.9, 72.4 and 92.2% db/min for microwave output power of 350, 500 and 650 W, respectively. For the convective dried chard leaves at 50 o C, the average drying rate was 2.5% db/min. There was no constant drying rate period for both drying methods. The entire drying process took place in the falling rate period for both drying methods. Similar observation was reported by Ahmed et al. [2001] and Alibas [2006]. The parameters (k k and n) for the microwave and convective drying data fitted to Page s equation are presented in Table 2. Page s equation adequately described the drying process for the microwave and the convective drying of coriander foliage. The coefficient of determination (R 2 ) for microwave dried samples ranged from to with standard error range of to The R 2 values for the convective dried samples were mainly with a standard error range of to The k and n parameters for the microwave dried samples were mostly higher than the convective dried samples. Color quality of the dried coriander Results from color analysis of the fresh and dried material can be found in Table 3. Oneway analysis of variance was used to compare the means using SAS 9.1 (Statistical Analysis System, Cary, North Carolina, USA) at 5% significance level. The fresh sample had the Journal of Microwave Power & Electromagnetic Energy Vol. 41, No. 2, 2007

8 Drying rate (%db/min) Drying time (min) Figure 4. Typical drying rate curves for convective thin layer and microwave drying of coriander foliage. lowest brightness L value of and was significantly different from the dried samples except the microwave dried sample. The convective dried samples had higher L values than the microwave dried sample. The greenness a value of the fresh sample statistically differed from the dried coriander samples. The yellowness b values for the dried and the fresh samples differed from each other. The color change index ( E) values for the microwave dried samples were statistically different from the convective dried samples. The microwave dried samples had lower color change index values than the convective dried samples. This indicates that microwave drying did not affect the color quality of coriander samples as much as convection drying did. Kaur et al. [2006] indicated a color change index of 1.19 for a pretreated coriander dried at 55 o C by convection. This value appears to be lower than the values obtained in this present investigation. Thus, it shows that pretreatment has an effect on maintaining the color of the dried coriander. Soysal [2004] reported that L value for fresh parsley differed significantly from the microwave dried samples. However, a and b values of the fresh samples did not differ significantly from the microwave dried samples. CONCLUSIONS From the drying and quality analysis of coriander samples conducted, the following conclusions can be drawn: 1.Microwave drying of the coriander foliage was faster than for convective drying. Microwave drying was able to reduce the coriander moisture content to the 12%, wb target within 21 to 22 minutes. About 236 to 267 minutes were required to dry the coriander samples to the target moisture content using convection drying. International Microwave Power Institute

9 Table 3. Coriander color analysis from thin-layer and microwave dryers. Thin layer drying L* a* b* E Fresh 26.83a -5.90a 11.32bc c -4.92c 11.63cd 4.59b d -5.14c 11.93d 5.03b e -5.58b 11.91d 6.58c Microwave Drying L* a* b* b* a -3.75d 10.03a 2.67a b -5.19c 10.34a 3.21a bc -5.90a 11.49cd 3.27a * n = 3 Mean values with the same letter in the same column are not significantly different. 2. There was no constant rate period for the convective and microwave dried coriander samples, and the entire drying process took place in the falling rate period. The average drying rates for the microwave dried samples were higher compared to the convective dried samples. The drying rate for the microwave dried samples ranged from 42.3 to 48.2% db/min and that of the convective dried samples ranged from 7.1 to 12.5% db/min. 3. The fresh sample had the lowest brightness L value of and was significantly different from the dried samples. The convective dried samples produced higher L values than the microwave dried sample. The greenness a value of the fresh sample statistically differed from the dried coriander samples. The yellowness b values for the dried and the fresh samples differed from each other. The results show that convective thin layer dried coriander samples exhibited a significantly greater color change than microwave dried coriander samples. The color change index values for the microwave dried samples ranged from 2.67 to 3.27 and that of the convective dried samples varied from 4.59 to ACKNOWLEDGMENTS The authors would like to thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for financial assistance. Also acknowledged are Mr. Phil Leduc from the Prairie Agricultural Machinery Institute (PAMI), and the Saskatchewan Herb and Spice Association (SHSA) for their valuable information REFERENCES Ahmed, J., U.S. Shivhare and G. Singh (2001). Drying characteristics and product quality of coriander leaves. Food and Bioproducts Processing, 79(2), pp Akpinar, E.K., Y. Bicer and F. Cetinkaya (2006). Modeling of thin layer drying of parsley leaves in a convective dryer and under open sun. Journal of Food Engineering, 75(3), pp Alibas, I. (2006). Characteristics of chard leaves during microwave, convective, and combined microwaveconvective drying. Drying Technology, 24(11), pp ASAE (1997). ASAE Standard S358.2 DEC93 Moisture Measurement Forages. ASAE Standards 43rd ed., 499. American Society of Agricultural Engineers, St. Joseph, MI. Balasundaram, R. and V. Meda (2003). Utilization of microwaves in applied natural product chemistry. CSAE/ASAE 2003 Annual Intersectional Meeting, Journal of Microwave Power & Electromagnetic Energy Vol. 41, No. 2, 2007

10 ASAE Paper No. RR Bertelli, D., M. Plessi and F. Miglietta (2004). Effect of industrial microwave treatment on the antioxidant activity of herbs and spices. Italian Journal of Food Science, 16(1), pp Chou, S.K. and K.J. Chua (2001). New hybrid drying technologies for heat sensitive foodstuffs. Trends in Food Science & Technology, 12(10), pp Demir, V., T. Gunhan, A.K. Yagcioglu and A. Degirmencioglu (2004). Mathematical modeling and the determination of some quality parameters of air-dried bay leaves. Biosystems Engineering, 88(3), pp Di Cesare, L.F., E. Forni, D. Viscardi and R.C. Nani (2003). Changes in the chemical composition of basil caused by different drying procedures. Journal of Agricultural and Food Chemistry, 51(12), pp Doymaz, I. (2006). Thin-layer drying behaviour of mint leaves. Journal of Food Engineering, 74(3), pp Erenturk, S., M.S. Gulaboglu and S. Gultekin (2004). The thin-layer drying characteristics of rosehip. Biosystems Engineering, 89(2), pp Kashaninejad, M. and L.G. Tabil (2004). Drying characteristics of Purslane (Portulaca oleraceae L.). Drying Technology, 22(9), pp Kaufmann, B and P. Christen (2002). Recent extraction techniques for natural products: Microwave-assisted extraction and pressurized solvent extraction. Phytochemical Analysis, 13(2), pp Kaur, P., A. Kumar, S. Arora and B.S. Ghuman (2006). Quality of dried coriander leaves as affected by pretreatments and method of drying. European Food Research and Technology, 223(2), pp Li, Y. and R.V. Morey (1987). Thin-layer drying rates and quality of cultivated American ginseng. Transactions of the ASAE, 30(3), pp Maskan, M. (2000). Microwave/air and microwave finish drying of banana. Journal of Food Engineering, 44(2), pp Panigrahi, S., A. Ghazanfari and V. Meda (2006). Dehydrating of flax fiber with microwave heating for biocomposite production. Journal of Microwave Power and Electromagnetic Energy, 40(2), pp Pande, V.K., A.V. Sonune and S.K. Philip (2000). Solar drying of coriander and methi leaves. Journal of Food Science and Technology, 37(6), pp Rocha, T., A. Lebert and C.M. Audouin (1992). Effect of drying conditions and of blanching on drying kinetics of mint (Mentha spicata Huds.) and basil (Ocimum basilicum). Drying 92, pp Sablani, S.S. (2006). Drying of fruits and vegetables: retention of nutritional/functional quality. Drying Technology, 24(2), pp Shiffmann, R.F. (1987). Microwave and dielectric drying. Handbook of Industrial Drying, ed A.S. Mujumdar. Marcel Dekker, New York. Shaw, M., V. Meda, L. Tabil Jr. and P. Leduc (2005). Development and trends in drying of herbs and specialty crops in Western Canada. CSAE/SCGR 2005 Meeting, CSAE/SCGR Paper No Soysal, Y. (2004). Microwave drying characteristics of parsley. Biosystems Engineering, 89(2), pp Soysal, Y. and S. Oztekin (2001). Comparison of seven equilibrium moisture content equations for some medicinal and aromatic plants. Journal of Agricultural Engineering Research, 78(1), pp Tang, J. and S. Sokhansanj (1993). Drying parameter effects on lentil seed viability. Transactions of the ASAE, 36(3), pp Temple, S.J., C.M. Temple, A.J.B. van Boxtel and M.N. Clifford (2001). The effect of drying on black tea quality. Journal of the Science of Food and Agriculture, 81(8), pp Wadworth, J.I., L. Velupillai and L.R. Verma (1990). Microwave-vacuum drying of parboiled rice. Transactions of the ASAE, 33(1), pp Wangensteen, H., A.B. Samuelsen and K.E. Malterud (2004). Antioxidant activity in extracts from coriander. Food Chemistry, 88(2), pp Wichtl, M.W. (1994). Herbal drugs and phytopharmaceuticals. Medpharm GmbH Scientific Publishers, Stuttgart. International Microwave Power Institute