Introduction

Cocoa is a crop with a long tradition in our country. It is considered that its primary center of origin according to the experts was Central America, from where it is estimated was dispersed to different regions of America, most likely through the various indigenous groups that populated this continent (^{ASERCA, 1995}). Cocoa is a culturally and economically important crop in Tabasco, as it´s a traditional agroecosystem and is the first producer nationwide with 68% of the surface and 61% of production (^{SIAP, 2018}). The drying of cocoa in the region of Chontalpa, Tabasco, Mexico carried out by the farmers is carried out mainly by solar drying in the open air, in concrete yards; in some cases dryers constructed with wood are used.

With this type of drying there can be contamination with dust, fungi and animals that affect the safety of the dry grain.). The beneficiaries of the region use gas as an input for artificial drying, which increases the cost and the ecological footprint of the product obtained. During the dry season (March-May) the drying process requires up to 3.5 days and in rainy or northerly season (November-February), few producers carry it out. The NMX-F-352-S-1980 standard establishes that the maximum moisture content should be 7.5% wb, which allows to conserve the safety of the grain during storage and decreases the risk of the appearance of fungi (Normas, 1980). The drying process consists of reducing the moisture content of the product to reduce its deterioration in a period of time called "safe period of storage" (^{Ekechukwu, 1997}).

The decrease of humidity in agricultural products is carried out through the evaporation that entails the transfer of mass and heat (^{Ekechukwu and Norton, 1999}). The decrease in moisture in a product can be achieved naturally or by forced convection. The natural way is when the hot air circulates naturally through the product due to the temperature difference and in the forced convection, the hot air is circulated through a blower or fan.

The use of solar energy in countries near the equator has great potential, however the development of technologies based on this energy is still limited (^{Eggers-Lura, 1979}). Obtaining electricity through the use of solar cells has become popular as a sustainable alternative (^{Labouret and Villoz, 2010}). There are different types of dryers that take advantage of solar radiation, which have been designed or adapted to the conditions of the tropics and that take advantage of the materials of the region to reduce costs (^{Ekechukwu and Norton, 1999}). Solar radiation can be used more effectively for the drying of products by controlling the parameters of air temperature, humidity, rate of drying, moisture content and air flow rate (^{Garg, 1987}).

In this regard, ^{Hii et al. (2006)} designed and evaluated a solar dryer type greenhouse, with capacity of 20 kg and polyethylene cover with ultraviolet protection, which allows to isolate the grain from the contaminants with a minimum drying time of 4 days. ^{Dina et al. (2015)} used an indirect solar dryer prototype with a capacity of 1 kg and recorded a drying time of 2 days when using molecular sieve 13x (Na86((AlO₂)86*(SiO₂)106)*264H₂O) and CaCl₂, as desiccants ^{Barnwal and Tiwari (2008)} tested a greenhouse dryer, with plastic cover and extractors activated by solar cells, in order to obtain evaporated moisture predictions, with multilinear expressions in forced convection mode. ^{Arjoo et al. (2017)} performed the evaluation of a tunnel-type solar dryer, with a capacity of 400 kg and covered with polyethylene with UV stabilizer, for the drying of garlic whose initial humidity was 66% (wb) and final of 9% (wb) in a period of 9 days.

The objective of this work was to test the efficiency in the drying time of a polycarbonate tunnel-type solar dryer, by forced convection, in cocoa bean during the canicular period in the Chontalpa, Tabasco region and the satisfaction of the participating producer.

Materials and methods

Polycarbonate tunnel type solar dryer

The polycarbonate tunnel type solar dryer used is based on a Thai design (Figure 1) proposed by ^{Janjai (2012)}, developed for the use of food drying by small scale industries. This dryer consists of a parabolic roof covered with polycarbonate sheets, galvanized steel structure and concrete floor. The dimensions of this dryer are 8 m wide, 20 m long and 3.5 m high, with a loading capacity of 1 000 kg of fruits or vegetables. The main modifications made to the original design were the integration of fans in the ceiling to homogenize the internal temperature and accelerate the drying process, activated by a solar cell, and the use of aluminum in its structure to make it lighter, more durable and portable.

Figure 1 Greenhouse solar dryer designed by ^{Janjai (2012)} for small-scale industries. 

The dimensions of the solar dryer used are: 1.20 m wide by 2.40 m long and 0.6 m high, with maximum drying capacity of 21 kg of cocoa in a single layer. The Makrolon Bayer brand unicellular polycarbonate cover used has a thickness of 6 mm, has UV protection and is mounted on an aluminum structure and galvanized steel base. Inside the cover, installed on the roof, there are four 4-inch 12 V fans to homogenize the internal heat and in the back there are three 4-inch 12 V fans for air extraction. The fans work with energy generated by a solar panel type SE-156 * 26-25P-36 of 25 W and 21.7 V with dimension of 55 x 36 x 2.5 cm, manufactured by Solarever. On the front part there are two air intakes of 15 x 5 cm each, with movable windows to regulate the air intake.

Three double trays made of aluminum, with a height of 10 cm from the ground, are placed in line on the ground, with a separation of 10 cm between them. Six trays, constructed in aluminum and teflon mesh of 50 x 60 cm, with a capacity of 3.5 kg each, are placed on the tray holders, Figure 2.

Figure 2 Main parts of the polycarbonate tunnel type solar dryer. 1) polycarbonate cover with UV protection; 2) windows that allow air to enter; 3) fans to homogenize the internal heat; 4) exhaust fans; 5) carriage holder; and 6 trays. 

Results and discussion

In this section, the conditions under which the cocoa bean drying test was carried out using the tunnel type solar dryer with polycarbonate cover are presented. The parameters of temperature and relative humidity inside the dryer, as well as the solar radiation, moisture content and humidity ratio in the cocoa bean are discussed.

The drying process required two days and one night, a total of 35 h. Figure 3 shows the records made of the ambient and interior temperatures of the polycarbonate tunnel-type solar dryer. The difference between the ambient and internal temperature at 10:30 h was 8.7 °C. The maximum temperature difference is observed at 15:30 h with a difference of 25.7 °C.

Figure 3 Comparison of ambient temperature with the interior temperature of the polycarbonate tunnel-type solar dryer in the Ranchería Miahuatlán, Cunduacán, Tabasco, Mexico using data Logger HOBO UX100-003. 

The Figure 4 shows the solar radiation for the region of Chontalpa, Tabasco during the days in which the test was performed. It is observed that the maximum value is 900 W/m², coincides with the maximum temperature of 59 °C recorded inside the solar dryer type tunnel polycarbonate. According to the results of ^{Jinap et al. (1994)}, with 60 °C of temperature, dry cocoa beans are obtained with a high concentration of volatile fatty acids (VFA) that intensifies the flavor of the chocolate produced with said cocoa. It is considered that the polycarbonate tunnel type solar dryer achieves similar conditions in the quality of the dry cocoa bean recorded by ^{Jinap et al. (1994)}.

Figure 4 Solar radiation in the region near Cunduacán, Tabasco, Mexico. http://smn.cna.gob.mx/tools/GUI/EMAS.php. 

It should be noted that the variations in solar radiation observed (Figure 4) are due to the presence of cloudiness during the day, which seems to be directly related to the temperature inside the dryer (Figure 3).

The Figure 5 shows the records of environmental relative humidity and inside the polycarbonate tunnel- type solar dryer. It is observed that the relative humidity values inside the solar dryer are kept below the values of relative humidity in the two days of the test, which confirms the efficiency of the dryer used for the drying process of the grain of cocoa and the improvement of the original design with the integration of the fans in the inner part of the solar dryer that favors the homogeneous drying in spite of the double layer of grains (Figure 6). The latter eliminates the need for the constant turning of the cocoa beans that is carried out with the drying methods used by the region's producers.

Figure 5 Comparison of the relative humidity (%) with the interior of the polycarbonate tunnel- type solar dryer in Miahuatlán, Cunduacán. Measurements taken every hour. 

Figure 6 Aspect of the cocoa dried in two layers with the tunnel-type solar dryer of polycarbonate. 

The Figure 7 a shows the decrease in moisture content, from 56.40% (wb) to 6.2% (wb) in cocoa bean. The final value obtained was lower than the maximum value established by the Mexican standard NMX-F-129-S-1979. The Figure 7 b shows the time in which the drying process was carried out until reaching a period without loss or humidity gain that goes from 7:30 p.m. on the first day to 8:30 p.m. on the second day, despite the presence of rain at night and dew in the morning during the trial period. To obtain cocoa beans with a final moisture content of 6.26% (wb), two days of drying (24 h) were required. The drying time obtained is 6 hours less than the time obtained with the integrated solar dryer with desiccant thermal energy storage and the use of desiccant type adsorbent, and 5 hours more compared to the use of desiccant type absorbent ^{Dina et al. (2015)}.

Figure 7 a) Moisture content in percentage of wet bulb (% wb) with respect to time; and b) ratio of humidity loss measured in wet bulb (wb) to the drying time. 

Conclusions

The drying of 12.94 kg of fermented cocoa of the Trinitaria variety was carried out with the polycarbonate tunnel- type solar dryer. As a result, drying was obtained in less time than sun drying in the open air carried out by the producers of the region. The final weight was 6.19 kg (47.82% with respect to the initial weight) with a loss of 52.18% of water. A marked difference was registered between the ambient temperature with respect to the interior temperature of the greenhouse, as well as the relative humidity, which indicates the efficiency of the dryer used. The humidity of the grain remained with a gain almost null during the night despite the presence of rain at night and dew in the morning.

The final moisture content test on a wet basis indicates a percentage of 6.6% (wb) achieved in two days of experiment, which is less than that established by the Mexican norm, which is a maximum of 7.5% (wb). The satisfaction of the producer of the drying process of the cocoa bean with the polycarbonate tunnel-type solar dryer was greater than 4 to 6 points than with the table-type wood dryer.