**1. Introduction**

Sustainable food processing is becoming an increasingly important issue in developing countries. To improve the local living conditions and redress global inequalities, a system-oriented approach in food production considering the whole value chain, including the economic, environmental, and social impacts, is essential. Moreover, future economic development depends particularly on how processing capacities in the agricultural sector can successfully be improved instead of exporting agricultural products unprocessed, and with no value added, to foreign markets. In this way, new quality jobs can be created at the same time.

Pineapple, after banana and citrus, is the third most important tropical fruit in international trade [1]. The seasonal pineapple processing in Togo, where pineapple is a major crop, was selected for the present study. According to [2], in 2016, Togolese farmers grew pineapples on a cultivation area of 237 ha and the annual production was 1908 t. The main commercial pineapple products are currently canned pineapple slices and chunks, juice and nectar, and frozen and dried fruit. Of these, the most common are dried fruit and juice, usually in organic quality, which are exported primarily to

European countries. The processing itself takes place in various decentralized small- and medium-sized enterprises. As described in [3], a typical such enterprise processes 1–2 t/d of fresh fruit, while the resulting pineapple waste (peels, cores, stems, and crowns) is about 40% of this amount. Other sources give even higher figures up to 75% [4]. The waste, together with discarded fruits with high moisture content and various other farm production wastes (cow dung, etc.) can be e fficiently utilized for energy recovery via biogas production, and the resulting sludge can be used as fertilizer. While sophisticated biogas production technologies are employed in the developed countries, the technologies in developing countries must be tailored to the local conditions. Both the adaptation of European biogas production technologies to the specific requirements of pineapple waste processing in Togo and the corresponding experimental results were discussed by the authors of the present paper in [3].

### *1.1. Key Role of the Drying Process*

Drying—that is, a continuous or intermittent process associated with heat and mass transfer—significantly influences the shelf life, appearance, composition, taste, shape, structure, and other characteristics of the product. With respect to the fact that this process is probably the most energy-intensive one in the food processing industry [5], any small increase in energy e fficiency will contribute to sustainable development in the respective industrial sector. It is obvious that moisture content in fresh fruit (generally about 85 wt % [1]) is of crucial importance. Water activity, that is, the ratio of vapor pressure to saturation vapor pressure at a given temperature, must also be considered, because it too a ffects the shelf life of a food product [6]. This ratio should be around 20% to reduce physical, chemical, and biochemical reactions and to minimize microbiological growth in a food product [7].

Recent developments in drying technologies of agricultural produce in general were discussed in [8], while another study [9] focused solely on fruits and vegetables. Comprehensive reviews of various drying technologies used in Africa [10] or just the sub-Saharan zone [11], but without any special focus on the feasibility of (semi-)industrial-scale indirect solar drying of pineapples, are also available. Similarly, reviews focusing on the drying of single fruit (e.g., fig [12], mango [13], or mulberry [14]) or vegetable varieties (e.g., chili peppers [15]) have been published as well.

Hot air dryers have been addressed abundantly in the literature, mainly with respect to the properties of the dried produce. One can find studies discussing color change [16], shrinkage [17], remaining bioactive compounds [18], microstructure [19], e ffective moisture di ffusivity [20], surface polyphenol accumulation [21], or even shear strength [22]. Apart from those mentioned above, there are also papers focusing on the optimum hot air dryer setup [23], real-time monitoring of the properties of the product being dried [24], the e ffects of pre-treatments [25], low-temperature drying [26], or various combined drying processes such as, for example, freezing-hot air drying [27], ultrasound-assisted hot air drying [28], or pressure drop-assisted hot air drying [29].

Considering the solar drying process in particular, one can encounter review articles covering both solar drying in specific African countries [30] and regions outside of Africa [31]. Analogously, there are papers focusing on the solar drying of banana [32], date [33], apple [34], coconut [35], Moroccan sweet cherry [36], grapes [37], blackberry [38], cocoa beans [39], tomato [40], potato [41], black turmeric [42], lemon balm leaves [43], medicinal plants [44], ber fruit [45], or algae [46]. Studies discussing in detail the solar drying of pineapple, however, are relatively rare, and they involve prohibitively long drying times [47] (which are not really feasible if the process is to be implemented (semi-)industrially), focus more on product shelf life and sensorial and bromatological analysis [48] than on the actual drying process, discuss a specific dryer chamber design in terms of a Computational Fluid Dynamics (CFD) simulation of air flow therein [49], or address drying of thin pineapple slices pre-treated with a sucrose solution of various concentrations [50] or coated with di fferent edible coatings [51]. Other studies focus on drying in a ventilated tunnel [52] instead of in a drying chamber into which hot air is supplied from a solar thermal collector, or employ various mixed mode [53] or hybrid [54] dryers, where both direct and indirect solar radiation is in e ffect or di fferent hot media are used instead of air, respectively.

### *1.2. Circular Economy Strategy for Pineapple Processing in Togo*

Togolese pineapple processing companies currently use conventional dryers, where the heat required for drying is generated by the combustion of liquefied gases (e.g., propane or butane) [55]. The fuel is bought in gas cylinders and thus the process is very demanding in terms of operating cost. Moreover, the liquefied gas supply is not su fficiently decentralized, and, therefore, at least a partial shift to renewable energy sources would be desirable. Following the findings presented in [3], the current paper proposes a circular economy strategy for pineapple processing in Togo, which is shown in Figure 1. This strategy, being somewhat similar to the one presented in [56] which involves cattle market wastes in Nigeria, lies in the e fficient utilization of pineapple processing wastes. The starting point is fresh pineapple (as highlighted in Figure 1), from which two cycles are originating—the production cycle and the waste cycle. The production cycle involves the preparation of slices, which are then pre-dried in the solar dryer and, as an intermediate product, continue to the conventional dryer. There, the final product (dried pineapple fruit) is obtained. The waste cycle begins with the wastes, which are inoculated with cow dung and enter the digester, where biogas is produced. This gas, being the primary product of the cycle, is then used as a fuel in the conventional dryer. The secondary waste cycle product (sludge) is utilized as fertilizer in pineapple cultivation, and thus the circular economy cycle is closed.

**Figure 1.** The circular economy principle presented for the example of pineapple processing in Togo.

For the strategy in Figure 1 to fully work, the biogas production (as discussed in [3]) must cover the fuel consumption in the conventional dryer. Therefore, the amount of fuel needed is decreased by pre-drying the pineapple slices in the solar dryer and, consequently, operating parameters of the dryer are essential for the optimal setup of the entire system. This is why the remaining portion of the present paper discusses in detail the solar drying process and the results of the corresponding pineapple drying experiments. The study's aim was to establish whether the industrial-scale application of an indirect convective solar dryer, implemented as indicated in Figure 1, would be feasible in the West African region. Further objectives were to determine the main parameters influencing the drying process and their optimal values, and to quantify the resulting decrease in the consumption of fossil fuels. The discussed energy-saving drying process, which preserves the nutritional quality of dried pineapple fruit, would then provide farmers with limited access to fossil fuels an option to process their agricultural products locally in a simple ye<sup>t</sup> reliable manner.

### **2. Materials and Methods**

For this initial feasibility study, a laboratory-scale solar dryer for pineapple processing was built and tested. The regional conditions (i.e., the temperature and solar radiation availability in Togo) were reproduced using an indoor test facility for solar thermal collectors and photovoltaic panels. A two-level factorial design was used to evaluate the main factors a ffecting the drying process and their interactions. The experimental work was conducted at Augsburg University of Applied Sciences, Laboratory of Energy and Process Engineering, Germany.

### *2.1. Experimental Device and Measuring Devices*

The newly built experimental device used was a convective solar dryer consisting of a 1.5 × 0.8 m solar thermal collector and a drying chamber with the dimensions of 0.47 × 0.37 × 0.6 m. The non-concentrating solar thermal collector for low-thermal applications adopted a modified flat plate format with a zig-zag geometry of the aluminum absorber, resulting in a surface area of 1.49 m<sup>2</sup> (Figure 2a). The bottom of the collector was insulated using a 20 mm thick expanded polystyrene panel (EPS; thermal conductivity: 0.034 W/(m K)). The wooden drying chamber was equipped with three drying trays with wire meshing, each having a net surface area of 0.11 m<sup>2</sup> (Figure 2b). The collector and the drying chamber were connected using a flexible aluminum tube insulated using a 25 mm thick, foil-faced polyethylene foam layer (thermal conductivity: 0.400 W/(m K), see Figure 3). The air flow around the material to be dried was ensured by the presence of a 2 W axial fan (type EE92251S1-000U-A99, Sunonwealth Electric Machine Industry Co. Ltd., Kaohsiung, Taiwan; flow rate up to 87.4 m<sup>3</sup>/h) at the solar collector outlet. Power was supplied to the fan by a small photovoltaic module with a capacity of 5 Wp. The experimental device was designed using inexpensive materials which are readily available in Togo. The total cost amounted to 377 EUR [57].

**Figure 3.** Indoor test facility with the solar collector, drying chamber, and relevant measuring points: (**1**) solar collector inlet, (**2**) solar collector outlet, and (**3**) drying chamber outlet.

The working principle of the solar drying system is based on the conversion of solar radiation to thermal energy in the black-coated aluminum absorber shown in Figure 2a. The absorber itself features a high absorption factor of more than 90% and a high thermal conductivity of 215 W/(m K). A transparent acrylic glass cover with high transmittance of 90% and high mechanical resistance reduces thermal losses to the surroundings. Simultaneously, the acrylic glass cover lets through only very little long-wave radiation emitted by the absorber. The air that acts as the heat transfer medium flows through and is heated up in the space between the absorber and the cover. The heated air is then passed through the drying chamber, where it circulates around the dried material by means of forced convection. This results in a gentle drying process without direct contact of the product with solar radiation.

The solar thermal collector was placed in the indoor test facility (Figure 3) containing 28 mercury vapor lamps and 27 halogen lamps. The distance between the source of radiation and the collector was variable between 0.05 m and 0.99 m so that di fferent radiation intensities could be simulated. The respective drying experiments were carried out with two di fferent solar radiation intensities, 650 <sup>W</sup>/m<sup>2</sup> and 1100 <sup>W</sup>/m2. Eight laterally arranged fans were used for wind simulation. The test unit was equipped with Testo 635 sensors (Testo SE & Co. KGaA, Titisee-Neustadt, Germany) to measure temperature (range: −20 ◦C to +70 ◦C, resolution: 0.1 ◦C, accuracy: ±0.3 K) and relative humidity (0–100%, 0.1%, ±2%) of the air at the inlet and outlet of the solar collector and at the drying chamber outlet as shown in Figure 3. It was assumed that the temperature and relative humidity at the solar collector outlet were approximately equivalent to the conditions at the drying chamber inlet. A pyranometer type 8101/8102 (Philipp Schenk GmbH, Vienna, Austria) was used to measure solar radiation (measuring range: 0–1500 <sup>W</sup>/m2, spectral range: 0.3–3 μm, resolution: 1 <sup>W</sup>/m2, accuracy: ±3%). To obtain the air volumetric flow rate, a hot wire anemometer type FV A915 S120 (Ahlborn Mess- und Regelungstechnik GmbH, Holzkirchen, Germany) was used (measuring range: 0.1–25 m/s, resolution: 0.01 m/s, accuracy: ±5%). Weight of the samples was measured using a Mettler PM 4600 electronic scale (N.V. Mettler-Toledo S.A., Zaventem, Belgium; accuracy: ±0.02 g).

### *2.2. Preparation of Samples*

Pineapples were peeled, cored, trimmed, and cut into slices in a single step using a pineapple slicer (Figure 4). The diameter of the prepared slices was 90–100 mm. In order to identify the e ffects of slice thickness on drying, two varying thicknesses, 6–8 mm and 12–14 mm, were considered. In addition, the rings were cut into eighths in order to examine the drying kinetics. The initial moisture content (87.3 ± 1.2 wt %) was determined according to DIN EN 322 [58] and DIN EN 15414-3 [59], that is, the samples were weighed before being placed in an oven at 105 ± 2 ◦C for 24 h to be fully dried, and then they were weighed again.

**Figure 4.** Preparation of samples: (**1**) fresh pineapples, (**2**) cutting of leaf crown, (**3**) cutting by means of rotational motion, (**4**) removing of the flesh, and (**5**) final spiral of flesh without core.

Commercially produced dried pineapple fruit served as a reference product and its moisture content (13.7 ± 0.67 wt %) was obtained in the same manner. This was then used to evaluate the performance of the solar dryer.
