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Pretreatment of Palm Fruit by Using a Conveyor Belt Microwave Prototype

This paper presents a microwave drying prototype, which is a conveyor belt design for oil palm pretreatment. The prototype consists of four 800 W magnetrons launching electromagnetic energy to the rectangular waveguide cavity. Palm fruit was fed in the cavity by the conveyor belt and pretreated in the cavity. The cavity size was designed optimally to ensure that temperature distribution in the palm fruit is uniform. Another conveyor belt is applied to the cavity output to feed out the pretreated fruit. Two corrugated waveguide filters were installed at the conveyor belt ends to suppress the microwave leakage. Three hundred sixty palm fruit were pretreated to prove the prototype concept. From the experiment results, the prototype heated palm fruit to the temperature required for inhibiting lipase enzyme within 120 seconds. It is found that free fatty acids in the treated palm fruit was well below 2% even 1 week storage.


Introduction
Oil palms, Elasis quineensis Jacq., are the crop with highest oil yield among the known oil plants [1,2]. Products from these palms include crude palm oil (CPO), and crude palm kernel oil (CPKO) obtained from the mesocarp and kernel layers of palm fruit. Both CPO and CPKO are used by, for example, food industries, consumers, and energy sector.
There is a need for pretreating oil palm before extracting CPO from palm bunch and fruit. Currently, there are two popular processes, hot steam heating and frying. This pretreatment process is required to inhibit the hydrolysis of free fatty acids (FFA) inherent in the palm fruit [3,4]. The FFA content is the key indicator of the CPO grade for producing vegetable oil. According to the commercial standard oil quality specifications, the FFA content of CPO should be well below 5% [4][5][6]. The formation of FFA in CPO can be considerably reduced by deactivating the lipase enzyme, and this is accomplished by heating the palm fruit to the specific temperature.
Many prior studies support microwave heating as rapid, uniformly, and environmentally friendly in comparison to convection heating, which is less energy efficient. Microwave energy generating from a high power microwave source penetrates heating objects and subsequently vibrates polar molecules in the objects. Friction around the vibrated molecules creates loss. Consequently, this dissipates microwave energy and heat objects [7]. There are several proposed works using microwave technique [3][4][5][8][9][10][11]. Several designs were based on a household microwave oven which was not a suitable solution for palm pretreatment. When applying it to the problem, poor uniformity of electromagnetic (EM) field and temperature distribution in the palm fruit were obtained [4,7,12]. Moreover, pretreatment process using a household microwave oven is not continuous since it lacks a mechanism to automatically feed palm fruit through the cavity [13][14][15].
Part of the solution is to have continuous-belt feeding of the palm fruit through the cavity. A design concept for this was proposed, and the EM field and temperature distribution were simulated using the finiteelement method [16]. Based on this design, the prototype implementation and its experiment results are first reported herein.

Design
EM field distribution in a microwave cavity, in which heating material contains, depends on the cavity dimensions and dielectric properties of the material. It is necessary to obtain a proper design for the cavity to provide a spatially uniform field distribution, which subsequently leads to spatially uniform heating in the heating material [7,9]. A multi-mode rectangle cavity was designed and built. Both cavity ends are connected with two conveyor belts. The cavity has 0.41 m width, 0.18 m height and 0.61 m length. The capacity of the cavity can pretreat palm fruit at the rate of 60 kg per hour. The target internal temperature of the fruit was around 70 °C, which would ensure inhibition of the lipase enzyme in the palm fruit and keep the FFA content of palm oil at very low level. This leads to determine the motor speed for controlling the conveyor belt at 30 cm/minute. Four 800 W magnetrons lie along the cavity length. Microwave power from these 2.45 GHz magnetrons is combined to obtain 3.20 kW radiated power. Four magnetrons are arranged in a parallel position to achieve the maximum combining and obtain spatially uniform heat distribution. The design was verified with simulation results performed using COMSOL TM Multiphysics software. To ensure the uniform heat distribution, the position nonorthogonal arrangement of magnetrons as shown in Fig.  1 was investigated. EM field along the x-axis in the cavity was analyzed by simulation. The electric field (E-field) distribution and mode generation were investigated.  A three-dimensional Finite Element Method (FEM) was performed using COMSOL TM Multiphysics software. High power microwave with TE 10 mode was launched from each magnetron via the waveguide. Since the cavity is symmetry around x = 0 plane, symmetric field distribution is guaranteed. Hence the simulation was performed in the cavity half only. This reduced the computation task considerably. Figure 1(a) show simulation results of the E-field ( E  ) in the cavity half. The field distribution exhibits TE 901 mode. This is observed from the pattern on the xz-plane which shows four full and a half wave along the x-axis and one full wave along the z-axis. The mode of propagation was generated by the reflections from the walls of cavity constructively reinforce each other to produce a standing wave [17]. The maximum in the cavity was 20.46 kV/m while the minimum was close to zero.
To provide good temperature distribution in palm fruit in the cavity, a continuous-belt design is proposed. To achieve this, a three-layer dielectric model of palm fruit was developed. The model includes both temperature-dependence dielectric ( ) ( T  ) and thermal properties of each fruit layer [7]. In practice, several palm fruit is fed continuously in the cavity by the moving belt. This can be visualized as a line of palm fruit in the cavity along the x-axis. In such scenario, we use a cylindrical dielectric model to represent several palm fruit moving in the cavity. The dielectric cylinder as shown in Fig. 1(b) has its length equal to the cavity length where its crosssection is equal to the cross section of palm fruit. The model consists of three dielectric layers of palm fruit [16]. The ) ( T  at 2.45 GHz of all three palm fruit are formed with polynomial functions shown in Table 1. The thermal properties of the microwave application for heating the palm fruit, the three following parameters, thermal conductivity (k), density (  ), and specific heat capacity ( p C ), were included in the model [18,7].  at the air and palm fruit exocarp interface is reduced as compared with no fruit scenario in Fig. 1(a). The magnitude of E  intensity ranges from 1 to 4 kV/m, depending on the dielectric property of the palm fruit material. The field variation varies with space which is related to envelope of the standing wave in the cavity. When fruit is exposed to E  , energy of the field will be absorbed and subsequently converted to heat. Hence the increase of temperature in the palm fruit results from the increase of absorbed power with elapsed time [10]. To investigate this, we observed the temperature change in the fruit with the heat-electromagnetic conversion rule at 60, 120, 180 and 240 seconds. The temperature results were calculated with the FEM using COMSOL TM Multiphysics software. The initial temperature of palm fruit was assumed at 30.0 °C. Heating time of 180 and 240 seconds may burn the palm fruit. At 60 seconds the temperature produced is not high enough. For a temperature around 70 °C, in our case 69 °C, the corresponding heating time was 120 seconds, and this is appropriate to deactivate the lipase enzyme that produces FFA. The information obtained can be use to configure a motor speed for controlling the continuous belt. Figure 2 is the photograph of the prototype. A conveyor belt feeds the palm fruit continuously through the cavity. The conveyor is driven with a 4.2V/2.9A stepping motor and a stress roller. The belt speed is accurately controlled with a control circuit. A high AC voltage at 2.2 kV obtained from a high-voltage transformer (1:10 ratio) drives the four magnetrons. The system includes a protection circuit for safety. Highpower electric devices and four electric fans were chosen. During the pretreatment, microwave leakage via the cavity port is minimized. According to the standard issued by International Commission on Non-Ionizing Radiation Protection (ICNIRP), the specific absorption rate (SAR) must be kept below 10 mW/cm 2 at 2.45 GHz [19][20][21]. Two corrugated waveguide filters installing at the input and output ports of the cavity were applied to reduce the leakage to meet the ICNIRP requirement

Moisture Ratio
The transport of moisture in the palm fruit is assumed to follow Fick's second law, as follows [22]. t is the heating time. Equation (2) can be simplified further by neglecting the higher order terms [25,26] and assuming e M to be zero [26][27][28]. Hence

Activation Energy
Diffusivity of moisture content in the solid phase, normally defined as eff D , depends on activation energy; a E . The temperature distribution in heating palm fruit influences the drying process more than the initial moisture content of the palm fruit [26][27][28][29]. From the Arrhenius equation,

Materials and Methods
All palm fruit samples were collected locally in Songkhla province, Thailand. Several fresh palm fruit bunches were collected from thirteen-year-old trees. Specifically, only ripe palm fruit bunches were selected. Three hundred sixty fruit samples were collected from several bunches and were pretreated with the prototype within 24 hours. Before the heating, the weight of each fruit was measured with a digital balance (0.1 gram uncertainty) and the diameters were measured with a digital caliper (Mitutoyo, 150 mm/6" Absolute Digital Digimatic Vernier Caliper, Japan) with an accuracy of 0.01 mm. The lengths, widths and weights of the samples were 40.32±3.60 mm, 30.16±2.24 mm, and 15.71±2.90 g, respectively. After the pretreatment with the prototype, the weights of the palm fruit were immediately measured. The recorded weights after and before pretreatment are applied to computed the MR . It is noted that the initial moisture content 0 M of the samples ranges from 29.40 to 29.80% wet basis, with 29.60 mean and 0.20 standard deviation.
We observed the temperature change in the palm fruit at 120, 180 and 240 seconds. The 60 second pretreatment time was omitted since, according to the simulation results, it gives a final temperature far below 70 °C which is insufficient to inhibit the lipase enzyme. The temperatures in the palm fruit at different pretreatment durations were recorded and compared with those obtained from the simulation results. A FLIR i3 thermal camera with an infrared sensor (accurate up to ±2% of its reading), was used to measure the average temperature in the mesocarp layer.
The FFA contents in CPO extracted from treated and untreated palm fruit were measured. Each treatment group was stored separately in a moisture-controlled cabinet for up to 7 days. On days 1, 3, 5, and 7, 192 palm fruits were sampled from each group to extract CPO, and the FFA contents of CPO samples were measured. All palm fruit samples were cut in small pieces and then pressed with a hydraulic presser to squezee the CPO. After that, the standard FFA measurement was performed by Specialized R&D Center for Alternative Energy from Palm Oil and Oil Crops, Prince of Songkla University. It was determined by using the titration method as written in the MPOB Test Method with some modification [30,31]. It used neutralized 2-propanol in amount of 50 ml to neutralize 0.5-2.0 g of palm oil sample. After regulating the temperature to 40°C, the sample was titrated against standard sodium hydroxide (0.1 mol) using phenolphthalein 1% as indicator to first permanent pink.   Figure 4 shows the thermal image of one of the test samples. The blue background represents the surrounding environment and the color change from green to pink shows that the inner part had a higher temperature than the surface. Figure 5 shows the mean temperature profile in the mesocarp layer of all palm fruit samples. The calculated and experimented temperature values are compared for 3.2 kW power (four 800 Watt magnetrons) at 30, 60, 120, 180, and 240 seconds. The temperature in the mesocarp layer from computations at 30, 60, 120, 180, and 240 sec are 43, 53, 69, 82, and 93 °C, respectively. While the experimented average temperature means in the mesocarp layer at 30, 60, 120, 180 and 240 sec are 47.29±0.32 °C, 52.75±2.25 °C, 66.73±2.21 °C, and 90.70±5.79 °C, respectively. The initial moisture content of the palm fruit samples was high, and the electromagnetic waves penetrated the palm fruit and made the water molecules rotate with the wave polarity. This created thermal energy and the temperature of palm fruit increased rapidly. The water molecules that absorbed energy moved towards the palm fruit surface so that moisture content was transferred from core to surface, following Fick's second law of diffusion [22][23][24].

Temperature Distribution
As shown in Fig. 5, the increase rate of temperature slowly decays with time because the water was evaporated from the palm fruit surface. The target temperature around 70 °C is reached when the palm fruit was pretreated for 120 seconds. The simulation results are in good agreement with experiments with 0.996 correlation coefficient, which confirms the accuracy of temperature prediction by the simulation under the microwave irradiation.  <8.285×10 -6 . As shown in Fig. 6, a very low MR is obtained in the A-B interval due to the initial stage of heating. In the B-C interval, a large amount of water has absorbed microwave energy and is evaporated, so that MR drops rapidly. At this stage, a large amount of water has diffused to the exocarp layer. At point C, the heating process is called the critical moisture content. Water around surface of sample is unstable distribution. When the heating process is continued, it reaches to the C-D interval. At this stage, the moisture evaporated rate is in the first falling rate state. The transfer rate of moisture from the endocarp layer to the exocarp layer becomes the rate-limiting step that slows down the evaporation at the exocarp layer, and the sample surface becomes drier. At point D, no water remains in the exocarp. The D-E interval is in the second falling rate stage, in which the rate of evaporation further drops. During this stage, water continuously evaporates until its quantity reaches equilibrium at point E in Fig. 6.

Moisture Ratio
With full-power delivering (3.2 kW) for 240 seconds, the moisture content in fruit samples was, on average, reduced from 29.86% (w.b.) to 5.23% (w.b.). The upscale of the microwave pretreatment system for the palm fruit has evaporated moisture from the palm fruit, resulting in lower cavity temperature. To upscale, system solves the problem of installing a hot-air heater to expel the moisture content from the cavity [32,33].[

FFA
The FFA content of CPO for the trading purpose must not exceed 5% [4][5][6]. When the palm bunch is cut during palm fruit harvesting, enzymes start to catalase and breaking down the triglycerides into FFA and partial glycerides. The increasing FFA deteriorates the CPO. Thus, it is important to keep the FFA content as low as possible. Fig. 7 Time profiles of FFA content (%) in CPO with several heat treatments in the microwave pretreatment system. Figure 7 shows the performance of a microwave pretreatment system to deactivate the lipase enzyme. The FFA content in CPO extracted from treated and untreated control palm fruit were measured. The actual treatments had three alternative heating times: 120, 180, and 240 sec. It is shown in Fig. 7 that the CPO extracts from the heated samples had much lower FFA contents than the controls. The FFA in control exceeded 15%, which is no longer acceptable. On the other hand, with actual treatments the FFA contents in CPO stayed well below 5%, remaining acceptable; they ranged from 0.683±0.001 to 2.409±0.008%. These cases would satisfy the CPO quality standards after 7 days of storage. The correlation coefficient between the 7-day storage period and the FFA content in CPO from treated at 120, 180, and 240 sec are 0.824, 0.760, and 0.712, respectively.
We achieved to extend the shelf life of the CPO extracts with the microwave-based pretreatment system. We found from the experiment that the heated samples could be kept for at least seven days with the FFA contents remaining below 5%. At a pretreatment time of 120 seconds, we obtained 67 °C for the average temperature in the palm fruit. This temperature is sufficient to deactivate the lipase enzyme that produces and slowly increases FFA. It is shown in Fig. 7 that the FFA is well below 2.00%, even one-week storage. Also, the physical colour of CPO remained reddish-brown, making the colour of oil product marketable. Meanwhile, the palm fruit texture was still soft so that the oil production at the subsequent stage can be readily done. On the other hands, the process caused burns of the palm fruits and changed the CPO into dark colour at a heating time of 180 and 240 seconds. Consequently, the optimum pretreatment duration is 120 seconds with the concerns of FFA level and physical properties of the CPO after 7-days storage as shown in Fig. 8.

Conclusions
A prototype continuous-belt microwave pretreatment system for palm fruit has been designed, developed and experimentally demonstrated. The system had four 800W magnetrons operating at 2.45 GHz for heating palm fruit to 67 degrees Celsius within 120 seconds. The pretreated palm fruit could be stored for at least seven days while the FFA contents stayed well below 5%, the standard limit observed by the oil industry. We believe that this prototype is a possible solution for using in small-scale and medium-scale plants for palm oil production.