Drying Performance and Aflatoxin Content of Paddy Rice Applying an Inflatable Solar Dryer in Burkina Faso

Abstract

The drying performance of paddy rice using an inflatable solar dryer (ISD), or also known as GrainPro^® Solar Bubble Dryer™, was evaluated and compared to conventional sun drying in Burkina Faso. Drying time was around eight hours. Thermal imaging was conducted to observe temperature distribution in the ISD during drying and mixing. Shadow casting was observed in the ISD due to the round shape of the black plastic film, which reduced the temperature of the paddy rice to about 10 °C. The temperature inside the ISD was up to 13 °C higher than the ambient temperature, whereas the temperature of paddy rice on the top layer was about 5 °C higher than on the bottom. The final moisture content of paddy rice dried in the ISD and under the sun was not considerably different. Under certain circumstances, impurities in paddy rice dried in the ISD could be substantially lower than for sun drying. The aflatoxin level of paddy rice was under the maximum limit of the EU regulation. Drying paddy rice seemed to be effective to remove aflatoxin type AFG2 content. Further adaptation of the ISD design for drying operations on rough surfaces and sandy soils is suggested.

1. Introduction

Paddy rice (Oryza sativa L.) is one of the most consumed staple foods in the Sahel area of Africa, which includes Burkina Faso, Ghana, Benin and Niger [1,2]. Around 80 kg of rice is consumed per capita per year in Sub-Saharan African countries [3]. In the postharvest processing of paddy rice, drying is the most crucial step to preserve product quality and reduce losses [4]. The final moisture content of paddy rice should not exceed 14% w.b. for a maximum storage period of eight months [4,5]. In Burkina Faso, conventional sun drying on tarpaulin is the most common method due to its cheap investment costs and easy handling. However, this conventional method is known to be prone to contamination from dust, animals and rot, especially on sandy soils [4].

Mycotoxin contamination in foodstuffs is one of the key factors that cause severe food losses and health issues in developing countries [6]. As one of the mycotoxins, aflatoxins (AF) have become a major concern in the food value chain because of their negative impacts on human health [6,7,8]. As the proven carcinogen to humans, several aflatoxins have been classified as Group 1 mutagens, such as AFB1, AFB2, AFG1 and AFG2 [9]. Among them, AFB1 is one of the most toxic compounds, which is found in food mostly before and after being harvested [6,10]. Therefore, EU regulation applies a maximum limit of 5 μg/kg for AFB1 and 10 μg/kg for total aflatoxins [11]. Aflatoxin concentration could be reduced by employing a drying practice [12].

Various alternative low-cost drying practices, like solar, infrared, convective, desiccant, fluidized and spouted bed dryers, have been designed for developing countries [13]. Among all those practices, the use of solar dryers is one of the most suitable methods, due to their low external energy requirement and the considerable reduction of contamination compared to conventional sun drying [13,14,15,16]. At a village or farmer’s cooperative scale, high demand for solar dryers that are cheap, efficient, easy-to-operate and flexible has been acknowledged [13,14,16,17].

Drying performance of a solar dryer is affected by different factors, such as air movement, operating conditions, geometrical properties and thermal energy storage system [18,19]. Thermal energy for a solar dryer can be transferred by natural convection (passive) or forced convection (active) [19,20,21].

Over the last three decades, inflatable solar dryer (ISD), also known as GrainPro^® Solar Bubble Dryer™, which is an active solar dryer, has been constantly developed by the University of Hohenheim, the International Research Institute (IRRI) and GrainPro Inc. In the interest of improving drying operation and preserving product quality, the dryer has been tested to dry various crops such as paddy rice, maize, cassava, red beans and amaranth leaves, in Africa and Asia [22,23,24].

The objective of the present study is to evaluate an inflatable solar dryer (GrainPro SBD 50) to dry paddy rice in the Southwest region of Burkina Faso, with regard to drying performance and product quality, such as impurities and aflatoxins contamination.

2. Materials and Methods

2.1. Drying Experiment

The experiments were conducted in Bama, Burkina Faso (1.374437° N, 4.389404° E). The fresh paddy rice was obtained from the nearby plantation areas, and was provided by a farmer cooperative. After harvest, the paddy rice was kept in a storage house for approximately 24 h before drying. The drying experiments were conducted in three batches on 20, 22 and 23 November, 2017. For each batch, the experiments started at around 10:00 and were terminated at around 16:00.

In this study, an ISD (SBD 50, GrainPro Inc., Zambales, Philippines) was used for the field tests, as shown in Figure 1. The overall length and width of the ISD are 25 and 2 m, respectively. The ISD has a flexible body, which consists of two waterproof plastic films, namely, lower and upper cover, which are joined by a heavy-duty zipper. The lower cover, or also known as the drying floor where paddy rice is spread, is made of a black reinforced polyvinyl chloride (PVC), while the upper is made of a transparent UV-resistant low-density polyethylene (LDPE). An area with a length of 1 m from the ISD inlets was used as a solar air heater, later called a collector area, by not spreading the paddy rice on this part of the black plastic film.

The ISD was equipped with two DC axial fans (RDF2589B12N18S, Runda Electronics CO., LTD, Shenzhen, China), which were attached at the inlets. The fans were driven by a 12 V storage battery with a capacity of 100 Ah (DIN100SMF, Guangzhou Tongli Storage Battery Co., Ltd, Guangzhou, China). The battery was charged by two solar panels (P100, Resun Solar Energy Co., Ltd, Jiangsu, China). The system was controlled by a solar charge controller (SR-SL 10 A, Shenzhen Shuori New Energy Technology Co., Ltd, Shenzhen, China). The amount of paddy rice dried in the ISD in batch 1, 2 and 3 was 587, 975 and 975 kg, respectively.

When the fans were switched on, the ISD body was inflated by air pressure, which built up inside. As solar radiation passed through the transparent upper cover, the inflated ISD body trapped the solar radiation by the greenhouse effect. Consequently, the air and product temperature in the ISD increased. As shown in Figure 2, the fans at the ISD inlets could easily suck unwanted material, like dust, into the ISD, due to sandy soils and a dusty environment. As a control measure, in batch 2 and 3, the ISD inlets were covered with two cardboard sheets at the left and right side, while the solar panel was placed right in front of the inlets.

The bulk height of the paddy rice in the ISD was 22 mm in batch 1, and 40 mm in batch 2 and 3. In all batches, the bulk width was around 1.83 m. As a control, for conventional sun drying, around 80 kg of paddy rice was spread on a 2.5 × 2.2 m tarpaulin. The bulk width and height were the same as the paddy rice in the ISD, while the length was adjusted proportionally. When short rains occurred in batch 1, paddy rice dried under the sun was covered with a tarpaulin, while the ISD was kept running. The rainfall duration was generally less than seven minutes.

The paddy rice drying in the ISD and under the sun was mixed every hour. For conventional sun drying, a rake was used, while for the ISD, a roller bar was used to mix the paddy rice without disturbing the drying process. The roller bar was delivered together with the ISD. Each end of the roller bar was attached to a castor wheel. In order to tow the roller bar underneath the drying floor along the ISD, two operators pulled a rope tied to each wheel. The purpose was to transmit an oscillating movement to the paddy rice during mixing [22].

2.2. Measurement of the Operating Conditions

Temperature (°C) and relative humidity (%) in the ISD were measured using USB data loggers (Voltcraft DL-220THP, Voltcraft^®, Hirschau, Germany) laid out on the surface of the paddy along the centre line of the dryer at 0.5, 1.1, 7.4, 12.9 and 22.1 m distance from the inlets of the ISD. For conventional sun drying, a similar USB data logger (Voltcraft DL-181THP, Voltcraft^®, Hirschau, Germany) was placed on the top layer of paddy rice, and for ambient meteorological conditions, a data logger of the same type was placed on a post 1.8 m above the ground. Solar radiation (W/m²) and photovoltaic (PV) voltage (V) were measured using USB data loggers (Voltcraft DL-131, DL-191V, Voltcraft^®, Hirschau, Germany). All measurements by USB data loggers were taken at a 1-min time interval, and the data was stored at the end of each drying batch. As additional information, the spatial temperature pattern in the ISD was visualized by taking thermal images using a hand-held thermal imaging camera (Testo 870-2, Testo SE & Co. KGaA, Lenzkirch, Germany). The thermal camera simultaneously records the true-colour (real) image to facilitate the spatial allocation. During drying, occasionally the thermal camera was held in one of the ISD outlets and directed to the entire length of the dryer. Air velocity (m/s) at the ISD outlet was measured using a digital anemometer (Voltcraft BL-30 AN, Voltcraft^®, Hirschau, Germany). In each batch, the total mass of paddy rice before and after drying was measured using a heavy-duty spring scale (NT-100kg, Camry Electronic Ltd, Guangdong, China).

2.3. Quality Determination

The samples of paddy rice were taken and stored in aluminium airtight bags. About 90 g of paddy rice was collected in each bag. An oxygen absorber ATCO FT 100 (Dry & Safe GmbH, Oensingen, Germany) was placed in each bag. Samples were delivered to the University of Hohenheim for moisture content (% w.b.) analysis using a convection oven (Memmert UM 700, Memmert GmbH & Co. KG, Schwabach, Germany) at 103 °C for 24 h according to DIN CEN/TS 14774-3 [25].

Impurities and aflatoxins of the samples were analysed at the University of Ouagadougou. Determination of impurities in the samples was carried out using an electrical sieving machine (ERIMAKI Snc, Milan, Italy) based on ISO 658 [26]. About 50 g of rice was sampled to conduct a sieve analysis in three replications. The weighed samples were sieved by vibration for 20 min using a screen opening of 3 mm. The mass passing the screen was calculated as % impurities.

Aflatoxins were analysed by ultraviolet light (365 nm) using a UVP Chromato-Vue cabinet (C-70G, Analytik Jena US LLC, California, USA). The stock standard solutions of AFB1, AFB2, AFG1 and AFG2 (10 µg/mL) were prepared according to AOAC [27]. The samples were ground, weighed and extracted according to Kamimura et al. [28]. The residues were conditioned based on Wilson and Romer [29]. The level of aflatoxin in the sample (µg/kg) was calculated by comparing the area of chromatographic peak of the samples with those of the standard calibration curve, as explained by Shantha [30].

2.4. Statistical Analysis

Least significant difference test (LSD) using SAS 9.0 (SAS Inc, North Carolina, USA) was used to analyse the differences of mean values, whereas the differences were significant at p ≤ 0.05. The data were plotted using OriginPro 2018 (OriginLab Corp, Massachusetts, USA).

3. Results and Discussion

3.1. Solar Radiation, Electricity Generation, Temperature and Relative Humidity

Solar radiation and electricity generation, along with temperature and relative humidity of ambient, inside the ISD and during sun drying, are shown in Figure 3. The solar panel generated a stable voltage between 19 and 21 V, even when solar radiation fluctuated. The highest solar radiation recorded in batch 1, 2 and 3 was 580.6, 788.2 and 673.5 W/m² at 13:53, 11:42 and 11:13, respectively. In batch 1, it was a cloudy and slightly rainy day. It can be seen that the stability of energy generation seemed not to be strongly affected by the fluctuation of solar radiation.

The temperatures in the ISD were generally higher than for sun drying. Moreover, the highest temperatures recorded in the ISD were 46.9, 60.0 and 51.5 °C at 13:55, 14:11 and 10:52 in batch 1, 2 and 3, respectively. Similar results were also reported from paddy rice drying using an ISD in the Philippines [22]. Hence, the temperature in the ISD was around 10 to 13 °C higher than the ambient temperature.

The surface of the solar panels could also easily become dusty. Therefore, the panels should be cleaned regularly, in order to prevent a greater loss of energy generation from the panels. Moreover, a high risk of a short circuit on the solar charge controller was observed due to faulty insulation and wiring. Any loose connection between the cables and wires should be avoided. The wire insulation at the plugs of the charge controller should be improved.

The shadow effect on the surface temperature of paddy rice is shown in Figure 4. High cover and round shape of the black plastic film blocked the sunlight and created a shady area on the right or left side of the ISD drying area. Temperature of the paddy rice in the shade was around 10 °C less than the paddy rice exposed to the sunlight. This could be one of the explanations for the low drying performance of the dryer [31,32]. The air velocity at each outlet was 5.05 ± 0.25 m/s. It could be estimated that the air velocity in the ISD was between 0.3 and 0.5 m/s. Since the fans were connected to the solar battery and therefore supplied with a nearly constant current, no substantial difference in the air velocity was observed among all batches. In contrast to the study of Salvatierra-Rojas et al. [22], in this study, condensation of water in the lower layers of the paddy rice did not occur in the ISD.

Temperature and relative humidity in the ISD at different distances from the ISD inlets at a particular time in each batch, where the ISD inlets were represented as 0 m, are presented in Figure 5. A rapid increase of temperature from the collector to half the length of the ISD was detected in all batches. For instance, in batch 2, the temperature increased from 40.8 °C at the collector to 55.3 °C in the middle at a distance of 12.4 m. Subsequently, the temperature was slightly lower near the outlet (about 1–2 °C) than the temperature in the middle of the ISD. On the other hand, the relative humidity was relatively constant along the ISD.

Figure 6 presents the temperature change of the paddy rice during a mixing process with the roller bar. It can be seen that temperature of the paddy rice in the upper layers was higher than in the lower layers. The bulk height of the paddy rice prevented a heat transfer from the top to the bottom layer, and therefore the temperature on the top was about 5 °C higher than on the bottom layer.

The roller bar seemed to pose some challenges during field application in Burkina Faso (See Figure 7). It was observed that the texture of the soil affected the efficiency of the roller bar. The sandy soil at the research site made it difficult to pull it smoothly over the ground. It was also pointed out that the heavy-duty zipper frequently got stuck, and therefore could not be closed or opened. This might be caused by sand grains that slipped into the chain and jammed the slider of the zipper. As a fast solution, the zipper could be cleaned using a brush periodically.

3.2. Drying Performance of Inflatable Solar Dryer Compared to Conventional Sun Drying

The drying curve of paddy rice inside the ISD and for conventional sun drying in each batch is presented in Figure 8. The final mass of the material dried in the ISD in batch 1, 2 and 3 was 581.5, 937.0 and 896.0 kg, respectively. In batch 1, the moisture content of fresh paddy rice (15.6% w.b.) was barely reduced during drying in the ISD and under the sun. The reason might have been clouds in the sky, which occurred predominantly on that day. An increase of the moisture content of the paddy rice dried under the sun was due to short rains, which happened suddenly, and therefore the paddy rice was not immediately covered by a tarpaulin. On the other hand, the ISD proofed to be effective in protecting the paddy rice from rainwater.

After about eight hours of drying, it can be observed in batch 2 and 3 that the moisture content decreased considerably in both paddy rice dried in the ISD and under the sun. However, the final moisture content of paddy rice dried in the ISD was significantly higher than the one under the sun in batch 2 and 3 at p ≤ 0.05.

In the case of solar drying, where a simultaneous mass and heat transfer occurs on a defined amount of products, the thermal energy supply is not flexible because it is strongly dependent on solar radiation [19,33]. Therefore, drying efficiency of the ISD was often hard to predict, since it was strongly related to the climatic conditions and the loading capacity [18,34].

3.3. Impurities and Aflatoxins of Paddy Rice

Figure 9 shows the impurities in fresh and dried paddy rice collected from each drying batch. Among the batches, the amount of impurities in fresh paddy rice was different because it was dependent on the field conditions and the harvesting activities. A significant difference was observed in all batches between the impurities in the fresh paddy rice and those in ISD or sun-dried paddy rice (p ≤ 0.05). In batch 1, the impurities were significantly higher in the paddy rice dried in the ISD than for the paddy rice dried under the sun at p ≤ 0.05. The reason could be because the fans sucked a lot of dusty air into the dryer due to the low position of the ISD inlets near to the ground. Since the experiments were conducted on sandy soils, the dust was easily generated by people walking nearby. However, when the ISD inlets were covered in batch 2, the amount of impurities was significantly lower than for paddy rice dried under the sun at p ≤ 0.05. This suggests that the ISD inlets should be placed at a higher position from the ground or covered, especially for drying operations on sandy soils. Besides dust, other impurities found included leaves, straw, stones and other unwanted materials [35]. Remarkably, most of these materials were found amidst the paddy rice in batch 3.

The content of aflatoxins in fresh paddy rice, along with paddy rice dried in the ISD and under the sun in batch 1, is presented in Figure 10. Fresh paddy rice had a much higher AFB1 than dried paddy rice. The AFB1 of fresh paddy rice was 1.06 μg/kg, which was already under 5 μg/kg as the maximum limit regulated by European Commission-EC [11]. The paddy rice dried in the ISD showed higher AFB1, AFB2 and AFG1 contents than under the sun. Moreover, no content of AFG1 was detected from the paddy rice dried under the sun. The content of AFG2 of fresh paddy rice was 0.22 μg/kg, but no trace of AFG2 was detected in dried paddy rice. It could be hypothesized that drying seemed to affect the content of AFG2. To the best of our knowledge, no studies have been reported the thermal stability of AFG2 [6,36]. In general, the total aflatoxins content for all samples revealed that the levels of aflatoxins were still under the threshold of the EU regulation, namely, 10 μg/kg [11].

Aflatoxins can be found in tropical and subtropical countries due to the higher temperatures and humidity that induce the growth of the moulds. However, this does not seem to be the case for Burkina Faso. This might be related to the high solar radiation in this country, which leads to substantial photodecomposition of aflatoxins [36]. Although the level of aflatoxins was at a safe limit, it should be monitored during long-term storage [6].

4. Conclusions

The performance of the ISD for drying paddy rice in Burkina Faso was analysed and compared to conventional sun drying. Several improvement strategies for the ISD were pointed out in this study. Temperature in the ISD was higher than ambient as well as conventional sun drying. Under specific weather conditions, the drying performance of the ISD was not substantially faster than for sun drying. A considerable temperature change of paddy rice during mixing was observed. In the ISD, temperature promptly raised from the collector to the middle of the ISD. Nevertheless, the shadow casted in the ISD by the high and round black plastic film caused a slower drying time and required more intense mixing. Under some circumstances on sandy soils and in dusty environments, the ISD could reduce impurities in paddy rice. Solar drying proofed to significantly reduce the aflatoxin level in fresh paddy rice (p ≤ 0.05). The aflatoxin level generally fulfilled the EU regulation.

Reducing the ratio of the black plastic film of the ISD is necessary to reduce the shady area. Investigation on the proper capacity of paddy rice filled into the ISD with regard to weather conditions should be further conducted. Performance evaluation of the ISD in drying other bulk materials such as maize and sesame grains should be conducted. Further adaptation of the ISD design should be investigated, in order to ensure a successful dissemination in sub-Saharan Africa.