The Drying Kinetics of Chilies using a Rotating Fluidized Bed Technique

Problem statements: The present study investigates experimentally the drying of chilies (Capsicum annuum L.) with rotating fluidized bed technique and the biolo gical properties: capsaicin content, moisture diffusivity and activation energy . Approach: The drying experiment was conducted at the drying air temperatures ranged 70-120°C and the drying air velocity was fixed at 1.8 m sec , while the layer’s height was fixed at 4 ± 0.5 cm. T he chilies were dried starting 350 ± 5%db down to 10 ± 1%db. Results: The drying time requested to dry chilies ranged 69 -257 min. The average capsaicin content of dried chilies was decreased st arting 2725.4 down to 1617.4 ppm. The effective diffusivity and activation energy were also describ ed. Conclusion: Drying air temperature was significant factor on the decreasing of moisture co ntent, capsaicin content and redness of dried chili es. All samples can be marketable.


INTRODUCTION
The chilies are commonly used as condiment and they are the ripe fruits of the species of genus capsicum (Amaroek et al., 2010). After harvest, chilies contained high moisture estimated 400%db (Amaroek et al., 2010); nevertheless, moisture content of 16%db were acceptable in the export market (Amaroek et al., 2010). Then, chilies must be dried for making chili powder and for keeping in the short or long term storages.
A Rotating Fluidized Bed (RFB) which one of among of fluidized bed techniques is an air-distributor that can be rotated about its axis of symmetry with airflow introduced in the inward direction of the radius to fluidize the bed. Instead of having a fixed gravity field as in a conventional fluidized bed, the body force in a centrifugal bed becomes an adjustable parameter that is determined by the rotating speed and airdistributor radius. By using the strong centrifugal field much greater than gravity, the particle bed is able to withstand a large amount of airflow without serious formation of large bubbles and the gas-solid contract at high airflow is improved (Abdullah et al., 2010). The drying kinetics food from RFB were found only a few samples in published literature, e.g., soybean, green bean and rice (Chen et al., 1999 andElmi et al., 2009) that almost sphere.
On the other hand, the cylindrical food, such as, chilies are scanty; in addition, the capsaicin of dried chilies based on drying temperature are scanty, too.
For the objectives of this study are (i) to investigate experimentally the drying kinetics of chilies in RFB technique, (ii) and to investigate the capsaicin content in dried chilies, moisture diffusivities and activation energies.
are introduced inside an air-distributor and are force to the wall by a centrifugal force due to the rotation of the air-distributor. The air flows radially inward through the air-distributor and the forces on the grain (Fig. 1) are balanced by the airflow (drag force and buoyancy force) and the centrifugal force (Watanoa et al., 2003;Elmi et al., 2009;Opafunso et al., 2009). The variable defined r is radius of an air-distributor (m) and ω is an angular velocity (rad sec −1 ). The air-distributor always rotates in the clockwise direction in this study.
For RFB apparatus can be seen at Fig. 2, it is a schematic diagram of the experimental apparatus for this study. The air-distributor with 400 mm diameter and 200 mm width can be rotated around a horizontal axis. The rotating speed can be adjustable for the several drying conditions. Additional, the dimension of side surface of air-distributor are 2.5 mm in hole and 38.5% in open area. The rotating speed can be adjusted by a frequency inverter and measured by DTO6234N and Germany) with an accuracy of ±0.1%. For visualizing, the transparent glass was installed at the end wall. The high-speed video camera was observed the fluidization behaviors. For exhaust air, a filter of 100 mm diameter was located at the center of the air-distributor. The drying air was blown by a blower of 5 HP which was adjusted the rotating speeds by frequency inverter. The drying airflow was measured by an orifice meter with U-shaped manometer. The pressure drop through the bed was measured by a Ushaped manometer. An electric heater, the drying air was heated by an electric heater of 5 kW and adjusted by PID control (Suntree, SG6, USA) with an accuracy of ±0.1%. For temperature, J-type iron-constantan thermocouple was used: thermometer indicators (NS, YB05C-A1 and China) with an accuracy of ±0.05%. An apparatus was constructed for the experiment in this study it was installed at the Khon Kaen University, Thailand.
Drying method: For drying with RFB, the procedures were defined as following. First of all, an electric heater was started upon drying conditions with the six intervals of the drying air temperatures; namely, 70, 80, 90, 100, 110 and 120°C at the drying air velocity of 1.8 m sec −1 .
An experimental apparatus was operated with the operation time of 60 min for stabilizing the drying condition. The second, the 2 kg samples (estimated 4±0.5cm of the bed layer height) were filled into the air-distributor (drying chamber) they are an initial weight of the sample. The third, the air-distributor was started with the rotating speed of 106 ± 0.1 rpm. Finally, the moisture loss was recorded every 10 min during drying processes by a digital balance (OHAUS, PA512, USA) with an accuracy of ±0.01g. The samples are dried from the initial moisture content of 350 ± 5%db down to moisture of 10% ± 1d.b. The experiment was repeated three times and the average data was evaluated for analysis.
For drying with sunlight, a kilogram of chilies were exposed under the sunlight. The samples were kept in a container at night. This process was conducted continuously in a span of 12-15 days.

Theoretical considerations:
The moisture movement inside the particle was developed based on Fick's second law of diffusion (Quasem et al., 2009;Yahya et al., 2010). The chilies are assumed to the infinite length cylinder which is the equation of conservation of energy is: The variable, M is the moisture content at anytime in unit of decimal of dry basis (db), D eff is effective moisture diffusivity of grains (m 2 sec −1 ), t is drying time (s), r is radial distance from center of grains (m) and c is constant. Assuming uniform initial moisture distribution and negligible external resistance, the solution proposed by following (Amaroek et al., 2010): The variable, r s is radius of grain (m), M is average moisture content at anytime (dry basis) and b is root of Bessel's function of order zero. For long drying times (setting n = 1), an Eq. 2 can be further simplified to a straight line equation as (Sayyar et al., 2009) To determine the effective diffusivity coefficient, D eff , the slope of the relationships between ln (MR) and time (Eq. 3) is computed and then D eff is calculated by the following equation: Moistures ratio of sample during the thin-layer drying experiment is calculated with the following equation (Brooks et al., 2008;Charmongkolpradit et al., 2010): The equilibrium moisture content, M e , can be negligible because it is very small (Quasem et al., 2009;Sghaier et al., 2009) The moisture diffusivity (D eff ) increases with the increase in drying temperature; (Hii et al., 2009;Sayyar et al., 2009). The temperature effect is modeled using well know Arrhenius type relationship (Sayyar et al., 2009).
By taking the natural logarithm of both sides, the above exponential form of Arrhenius can be transfigured into a linear Logarithmic form: The variable, E a is the activation energy (kJ kgmol −1 ) and D o is the Arrhenius's factor (Sayyar et al., 2009;Sghaier et al., 2009). (m² sec −1 ) that were determined by plotting ln (D eff ) versus (1/T). The variable, R is universal gas constant (8.314 kJ kgmol.K −1 ) and T is absolute temperature (K). Analysis of variance was carried out to find the effects (p<0.05) of drying air temperature.

Drying kinetics:
The comparison to the drying kinetics of six intervals of drying air temperatures have shown the rate of moisture reduction that was greatest at a highest temperature (Fig. 3). It responded the influence of drying temperature on the ability to diffuse moisture (p<0.05). It can be described after that have ventilated the gas velocity in RFB technique, the turbulence and mixing of the fluidized particles were intensified. Then, the gas film on the particles becomes thin, so the gassolid heat transfer was improved (Kowalski et al., 2010). In addition, after increased the temperature of drying air from 70-120°C, the latent heat was intensified, so the dehydration from the moist produce was improved (Bovornsethanan and Wongwises, 2007;Brooks et al., 2008;Theansuwan et al., 2008).
The researchers have been recently reported the same phenomenon for other grain, such as red chilies (Anwarul Huq and Arshad, 2010), Bird's chilies (Amaroek et al., 2010), chopped coconut (Madhiyanon et al., 2009) and apple (Marjan et al., 2010). The drying time to dry chilies from initial moisture content of approximately 350 ± 5%db to final moisture content of approximately 10 ± 1%db were 257, 163, 131, 98, 78 and 69 min at 70, 80, 90, 100, 110 and 120°C of drying air temperature, respectively. It was reported that the using of another dryer took 13 h to dry chilies from 325-10.5%db at 65°C of drying air temperature (Amaroek et al., 2010;Mazloomi et al., 2010) and 106.7 min to dry the sliced bird's eye chilies to 16%db at 70°C of drying air temperature (Omar et al., 2008). Initially, the abundance of free water on the produce surface contributed the effortless moisture liberation; however, much more difficult it might be to expel water after that, when the produce surface becomes harder due to shrinkage (Kowalski et al., 2010).

Redness:
The redness of dried chilies at different conditions was observed. The results shown the redness deterioration after drying both RFB technique and sunlit drying were investigated. They are differences in redness from two different methods; namely, the dried chilies from sunlit drying appear the light red; on the other hand, from a RFB appear dark redness (Fig. 4). When the drying air temperature increases starting 70-120°C, the redness is also affected adversely; on the other hand, it can be marketable. It may be due to the oxidation of carotenoid pigments at the elevated temperatures. This behavior was similarly reported by Omar et al. (2008). The chilies dried under the sunlight for about 12-15 days can be registered the moisture content of 10 ± 1%db. This drying kinetics was similar reported by Amaroek et al. (2010). Nevertheless, the drying time for sunlit drying is far too long compared to drying with a RFB with only requires a few hours.
Capsaicin content: Capsaicin is the main capsaicinoid in chilies, followed by Dihydrocapsaicin. These two compounds are also about twice as potent to the taste and nerves while the minor capsaicinoids are Nordihydrocapsaicin, Homodihydro-capsaicin, Homocapsaicin and Nonivamide (Cisneros-Pineda et al., 2007;Ornela-Paz et al., 2010). The analysis of capsaicinoids content in this study represented to the capsaicin (8-Methyl-N-vanillyl-trans-6-nonenamide) in dried chilies (Capsicum annuum L.). The method to investigate the capsaicinoid content was made by HPLC based on AOAC (2005) 995.03 in house method and tested at Central Laboratory (Thailand) Co., Ltd., Khon Kaen Branch, Thailand-that certificated from the laboratory accreditation with ISO/IEC 17025. Each dried chilies was analyzed in four replications while the level of capsaicin content with the use of ppm (mg kg −1 ) unit was also determined. The volume of the lowest significant difference was determined with p≤0.05. They were determined in among dried chilies in difference of drying air temperature and represented more than 70% of the total capsaicinoids in the dried chilies (Omar et al., 2008;Pompimon et al., 2009). For the first repeated sample at drying air temperature of 70°C allowed us to infer that peak at RetTime of 3.403 min and 4.341 min for capsaicin content of 2709.1 ppm and dihydrocapsaicin content of 1488.6 ppm, respectively while other detail can be seen in Fig. 5 and Fig. 6; in addition, the results of other repeated drying can be seen in Table 1. The results shown the average capsaicin contents of dried chilies decreased from 2725.4 to 1617.4 ppm as the drying air temperature increased from 70-120°C while the average capsaicin of dried chilies after sunlit drying method are 2943.8 ppm. It clearly shows that the capsaicin of chilies was affected adversely when higher temperatures were used for drying (p<0.05). Similar results were quoted in the case of red chilies drying (Amaroek et al., 2010). The polynomial equation was fitted capsaicin "Cap" (ppm) versus drying air temperature "T" that can be seen at Fig. 5 as follows: Cap = 0.3495T 2 -86. 924T + 7050.132 (9) where, R² = 0.982 and temperature(T) ranged 70-120°C.  The 1st, 2nd and 3rd are the repeated drying sample, respectively. "STD" and "Sunlight" are the standard deviation and chilies drying by sunlight, respectively   (Wang et al., 2007) Fig. 5: Relationship between capsaicin Vs temperature Moisture diffusivity: The Fick's law can be adopted into the simulation (Eq. 2). The values of D eff associated with temperatures of 70-120°C (Table 2) are 0.42×10-9 to 1.67×10-9 m² sec −1 . As expected, drying air temperature had an appreciable effect on D eff , which increased vigorously with increase in temperature that can be fitted in linear equation as following.
Characteristic length is inversely related to internal mass transfer resistance (Karim and Hawlader, 2005;Madhiyanon et al., 2009), so that the longer the length of chilies to be dried, the greater moisture immigration to the chilies surface.

Activation energy:
The ln-values of activation energy, analyzed by Arrhenius expression (Eq. 7) and plotted versus the reciprocal of absolute temperature (Fig. 7) has shown a linear relationship. The activation energy, E a , gained from the line slop yielded a value of 30.39 kJ mol −1 and the pre-exponential, D o , derived from the axis intercept point is 1.99×10-5 m² sec −1 . As summarized in Table 3, the comparisons of E a with other researcher for a diversity of agricultural product are suggested. The present values shown the black tea it was rather high.

CONCLUSION
The chilies drying on the Rotating Fluidized Bed (RFB) technique can be concluded as following. Drying kinetic, the influence of drying temperature on the ability to diffuse moisture that can be described after that have ventilated the gas velocity in RFB technique, the turbulence and mixing of the fluidized particles were intensified; in addition, after increased the temperature of drying air from 70-120°C, the latent heat was intensified, so the dehydration from the moist produce was improved (Sghaier et al., 2009;Brian Boswell et al., 2009;Singh and Pandey, 2010). Redness, they are differences in redness from two different methods; namely, the dried chilies from sunlit drying appear the light red; on the other hand, from a RFB appear dark redness. It may be due to the oxidation of carotenoid pigments at the elevated temperatures (Amaroek et al., 2010). Capsaicin, the average capsaicin contents of dried chilies decreased from 2725.4-1617.4 ppm as the drying air temperature increased while the average capsaicin of dried chilies after sunlit drying method are 2943.8 ppm. It clearly shows that the capsaicin of chilies was affected adversely when higher temperatures were used for drying. Similar results were quoted in the case of red chilies drying (Amaroek et al., 2010). Moisture diffusivities, D eff , drying air temperature had an appreciable effect on its and increased vigorously with increase in temperature. Activation energy, E a , gained from the line slop yielded a value of 30.39 kJ mol −1 .

ACKNOWLEDGEMENT
The researchers wish to thank the Center for Alternative Energy Research and Development (AERD), KhonKaen University, Thailand. That supported the financial research under project of R10/2552.