Improving Cocoa Drying Efficiency with a Mixed Forced Convection Solar Dryer in an Equatorial Climate

Abstract

A crucial stage in the post-harvest processing of cocoa beans, drying, has a direct effect on the finished product’s quality and market value. This study investigates the efficiency, quality outcomes, and environmental implications of a mixed forced convection solar dryer designed for drying cocoa beans in Ntui, Cameroon, compared to traditional open-air drying methods. The solar dryer’s design, incorporating a solar collector, forced ventilation, and thermal storage, leverages local materials and renewable energy, offering an environmentally sustainable alternative by reducing fossil fuel reliance and post-harvest losses. Experimental trials were conducted to assess key drying parameters, including the temperature, relative humidity, water removal rate, pH, and free fatty acid (FFA) content, under the equatorial climate conditions of high solar irradiation and humidity. Results demonstrate that the solar dryer significantly reduces drying time from an average of 4.83 days in open-air drying to 2.5 days, a 50% improvement, while maintaining optimal conditions for bean quality preservation. The solar-dried beans exhibited a stable pH (5.7–5.9), a low FFA content (0.282% oleic acid equivalent, well below the EU standard of 1.75%), and superior uniformity in texture and color, meeting international quality standards. In contrast, open-air drying showed greater variability in quality due to weather dependencies and contamination risks. The study highlights the dryer’s adaptability to equatorial climates and its potential to enhance cocoa yields and quality for small-scale producers. These findings underscore the viability of solar drying as a high-performance, eco-friendly solution, paving the way for its optimization and broader adoption in cocoa-producing regions. This research contributes to the growing body of knowledge on sustainable drying technologies, addressing both economic and environmental challenges in tropical agriculture.

1. Introduction

Cocoa is an essential crop in tropical regions, with annual world production exceeding 5 million tones, over 70% of which comes from West Africa, notably Côte d’Ivoire, Ghana, and Cameroon [1]. A key step in post-harvest processing is drying, which reduces the water content of cocoa beans from 50–60% to around 7%, guaranteeing their stability, organoleptic quality, and compliance with international standards [2,3,4,5]. In Cameroon, air-drying is the most commonly used method, due to its low cost and simplicity. However, this technique is highly dependent on climatic conditions, with risks of contamination, mold development, and economic losses [6,7]. To overcome these limitations, solar drying technologies are emerging as a sustainable, high-performance alternative, offering better control of drying parameters and a significant reduction in the processing time. However, their adoption in the cocoa industry remains limited due to the lack of experimental data adapted to tropical contexts [7]. In this context, the present study explores the performance of a mixed forced convection solar dryer for drying cocoa beans in Ntui, Cameroon. The aim is to optimize the drying efficiency in comparison with the traditional method, by assessing key criteria such as the drying time, pH, free fat content, and residual bean moisture, while meeting the sustainability and quality requirements of the agri-food industry.

Although solar dryers have proven their effectiveness in various agri-food sectors, their application to cocoa drying in equatorial climates remains not well studied [8]. Questions remain as to their ability to maintain optimum temperature and humidity conditions for efficient drying, their impact on the physico-chemical properties of the beans, notably the pH and free fatty acid content, and their comparison with traditional drying in terms of the drying time, final quality, and energy efficiency. This study aims to answer these questions by evaluating the performance of a forced convection solar dryer for drying cocoa beans in Ntui, Cameroon.

Several studies have explored the impact of drying conditions on cocoa quality [9,10,11,12]. According to [13], the integration of thermal storage materials in solar dryers improves temperature regulation and optimizes heat transfer, contributing to the reduction of post-harvest losses and the improvement of agricultural product quality. In [14], the authors have shown that the fermentation process influences the biochemical composition of beans, requiring controlled drying to preserve flavor precursors. In addition, the authors in [15] have shown that drying significantly affects the chemical and physical properties of beans, including the free fatty acid content and aroma profile. Poor drying control can lead to the enzymatic degradation of lipids and alter the characteristics of cocoa butter.

Research into the performance of solar dryers adapted to local conditions has led to a number of advances in cocoa production [16,17]. The authors in [18] studied the use of Arduino controllers to maintain optimal drying conditions in tropical environments. For his part, the authors in [19] highlighted the importance of controlling parameters such as the temperature and humidity to maximize dried product quality. In [20], the authors also demonstrated that optimizing fermentation and drying processes helps preserve the essential aromatic properties of cocoa beans.

Despite these advances, studies on solar drying applied to cocoa in equatorial environments remain limited [21,22]. Research carried out in [23] highlights the potential of solar dryers to improve sustainability and reduce environmental impacts in agricultural production, but their work remains general. Thus, it remains crucial to adapt these technologies to cocoa beans in equatorial contexts. With this in mind, the authors in [24,25] modeled heat and mass transfer phenomena during the drying of agricultural products, highlighting the importance of precise control of the evaporation rate.

The aim of this study was to analyze the performance of a forced convection solar dryer for drying cocoa beans in Ntui, Cameroon. This solar dryer is compared with open-air drying in order to assess its energy efficiency and the impact of environmental conditions on the process. It measures and analyzes drying parameters including the temperature, relative humidity, and air velocity, drying time, pH, water, and free fatty acid content, while studying the effects of drying on the physico-chemical characteristics of the cocoa. A rigorous experimental methodology was set up, incorporating precise measurements of climatic parameters and bean properties during drying, with a comparative evaluation of the performance of the two methods from an economic and environmental angle. The hypotheses put forward assume that the solar dryer significantly reduces the drying time, ensures better control of parameters, thus minimizing quality losses, and that optimizing its design could improve its efficiency and economic viability for small-scale producers. This work also aims to propose a sustainable technological solution that can be adapted on a large scale in similar regions, contributing to the sustainability of agricultural practices and improved cocoa quality, while opening up prospects for the wider adoption of this technology.

The experiment was carried out in Ntui, in the humid equatorial zone, where high temperatures and high humidity influence drying. A forced convection solar dryer was designed and tested, comprising a solar collector to capture energy, a drying chamber equipped with grids for even bean distribution, and a forced ventilation system to optimize hot air circulation. Two drying methods were compared: open-air drying, where the beans are spread out on tarpaulins under the sun, and drying in the solar dryer, with temperature and humidity parameters monitored. Analyses were carried out to measure the residual moisture content to assess the drying efficiency, pH evolution as an indicator of chemical stability, and free fatty acid content to judge the quality of the cocoa butter obtained.

The rest of this document is organized as follows: Section 2 presents the study site and environmental conditions and then describes the proposed solar dryer and its mode of operation. Section 3 details the experimental methodology, including sample selection and analysis protocols. Section 4 presents and discusses the results obtained by comparing solar drying with traditional drying, and finally, Section 5 concludes by addressing the prospects for improving and disseminating this technology.

2. Dryer Description and Technical Specification

2.1. Environmental Conditions at the Study Site

The solar dryer was designed, built, and tested in the climatic conditions of the Central Cameroon region, specifically in the town of Ntui. Figure 1 illustrates the map of the study site, generated using PVGIS, Version 5.2 (Photovoltaic Geographical Information System) software [26]. The geographical coordinates of the site are as follows: Latitude 4°466′00″ N, Longitude 11°628′00″ E, with an altitude of 546 m above sea level.

2.2. Schematic Diagram of the Proposed Dryer

The longitudinal section of an object presents a detailed view of its interior, facilitating an understanding of its internal structure and its various components. A schematic diagram of the dryer and its various parts is depicted in Figure 2. This graphic representation shows a longitudinal section of the proposed solar dryer, highlighting its essential components and their interconnections. The suggested drying system is designed in a structured way to optimize the drying process. It consists mainly of three units: a solar collector, a drying chamber, and a control box. The drying chamber consists of a sheet metal frame (3), reinforced by a fine mesh (4). It includes a fan (11) for air circulation, a drying rack (7) for the beans, and a wooden support (5) on which rests a thermal storage system (8) and a layer of polystyrene (9) to accelerate drying. The solar collector consists of three major components: a transparent cover (6), an absorber plate, and thermal insulation. It captures solar energy and transfers the heat to the drying chamber to ensure efficient drying of the cocoa beans. A fan (10) is integrated to promote the circulation of hot air, which is first absorbed by the plate and then stored as heat in a sand bed, optimizing the system’s thermal efficiency. The control box (2) includes a charge controller, battery, thermostat, and hygrostat, enabling efficient regulation of the drying process. The whole unit is powered by a solar panel (1), ensuring the device’s energy autonomy.

2.3. Operating Principle of the Proposed Dryer

In this study, three types of sensors are used: humidity, temperature, solar radiation and airflow sensors, the layout of which is illustrated in Figure 3. Four measurement positions have been defined in order to evaluate temperature and relative humidity at different strategic locations. The probes are distributed as follows: T_in and H_in at the inlet to the collector (1), T_out and H_out at its outlet (2), T_therm and H_therm inside the drying chamber (4), and T_ch and H_ch at the outlet from the chimney of the same chamber (3). These sensors enable temperature measurements between −40 °C and +125 °C with an accuracy of ±0.5 °C, as well as relative humidity measurements ranging from 0 to 100% with an accuracy of ±2% [27].

3. Cocoa Drying Process

3.1. Selection of Plant Material and Test Sample

Cocoa was chosen as the plant material for this study. Samples were carefully selected from varieties grown in the main cocoa-growing areas of Cameroon. A batch of over 300 beans, sourced from growers in the central region of Cameroon, was carefully selected for testing.

On average, 100 g contain around 96 beans, with a margin of error of ±9 beans, corresponding to a standard size. As the beans get larger, their number per 100 g decreases. A total sample of 20 kg of fermented, moist beans is pre-sorted into a clean basket before being fed into the dryer. The average number of beans per 100 g complies with current standards [2,3].

3.2. Plant Material Preparation Stages

In this project, a structured protocol was developed for the preparation and processing of plant material (cocoa) through several key stages, as illustrated in Figure 4. Stage 1 corresponds to the initial preparation of the cocoa, including sorting and cleaning of the beans. Stage 2, prior to fermentation, involves conditioning the beans to optimize the upcoming process. Stage 3, during fermentation, is divided into two distinct phases: phase 1, marked by the start of alcoholic fermentation, and phase 2, where acetic fermentation begins. Finally, stage 4, after fermentation, includes the final operations of drying and analyzing the cocoa beans to guarantee their quality and compliance with the study objectives.

3.2.1. Stage 1: Preparation of Plant Material

The tests were carried out on cocoa beans freshly extracted from the pods. Harvesting took place during the second harvest season 2023–2024. The appearance of cocoa pods after bean sorting and cleaning is illustrated in Figure 5. The uniformity of the pods testifies to the rigorous choice of plant material, guaranteeing greater homogeneity in the fermentation process. Careful observation can detect any immature or contaminated pods, which could alter the quality of the final beans. A 20 kg quantity of cocoa was placed in a crate for drying, with regular stirring to ensure adequate fermentation. The first of the three necessary brews was completed on the third day, followed by the two subsequent brews, carried out every other day for a total period of eight days.

3.2.2. Stage 2: Before Fermentation

Figure 6 depicts a detailed view of the cocoa beans immediately after shelling, prior to fermentation. Pre-sorting reduces the amount of residue that may be present in the sample. This step is crucial to eliminate undesirable residues and ensure an even distribution of the beans in the fermentation crate. Bean color and integrity are key indicators of the initial product quality, directly influencing the aromatic profile of the final cocoa.

3.2.3. Stage 3: During Fermentation

In this step, the cocoa beans are covered with banana leaves. According to the authors in Reference [28], banana leaves have a dual function when enveloping cocoa beans. On the one hand, they provide yeast owing to their micro-organism-rich surface. On the other, they protect the cocoa beans from air and night-time cooling [29]. This stage consists of two phases: alcoholic fermentation and acetic fermentation.

• Phase 1: Start of Alcoholic Fermentation

For the start of the alcoholic fermentation of cocoa beans, Figure 7 reveals the appearance of the cocoa beans just at the start of the first fermentation and before the temperature is taken. Temperature recording is essential at this stage to monitor the microbial activity. The right temperature (generally between 30 °C and 35 °C) encourages the proliferation of yeasts, which convert the sugars in the pulp into alcohol. An excessive temperature difference could compromise the development of the microbial flora necessary for fermentation.

After cleaning, the beans are placed, for fermentation, in a wooden crate lined with banana leaves in the absence of oxygen for 3 days, as indicated in Figure 8. This initial fermentation liquefies the pulp. An abundant flow of water will be noticed during this stage. Figure 8a shows the first stirring of the beans after 24 h. This operation is crucial to ensure even aeration and promote the diffusion of micro-organisms throughout the bean mass. Figure 8b illustrates the temperature measurement used to monitor the thermal rise and assess the efficiency of the fermentation process. Too low a temperature would indicate insufficient microbial activity, while too high a temperature (>45 °C) could lead to premature bean degradation.

• Phase 2: Beginning of acetic fermentation

In the second phase, which lasts 3 to 5 days, the alcoholic pulp is transformed into vinegar. This process takes place aerobically. Figure 9a,b give the appearance of the cocoa beans during fermentation, as well as their longitudinal section. Two stirring operations are required before the process is complete, and temperatures must remain almost constant, between 40 °C and 43 °C. The external appearance of cocoa beans during acetic fermentation can be observed in Figure 9a. The transformation of alcohol into acetic acid, an essential step in the development of flavor precursors, is clearly visible. Controlled oxidation of the beans is manifested by a progressive change in hue. As for Figure 9b, the longitudinal section of the beans illustrates their internal evolution. A uniform brown color indicates a well-conducted fermentation, while the presence of whitish areas could suggest incomplete fermentation.

3.2.4. Stage 4: After Fermentation

At the end of the process, a longitudinal section of the fermented beans is taken to check their quality. This longitudinal section of the cocoa beans is illustrated in Figure 10. A homogeneous internal structure, with no whitish stains or pulp residues, attests to successful fermentation. The low presence of slate-colored beans is a positive indicator, confirming that the fermentation and brewing conditions were optimal. The bean texture and internal coloring are decisive factors in cocoa quality, influencing its aromatic potential and market value.

3.3. Drying Processes

Two types of drying processes were used in this study. These were an open-air drying system and a mixed solar drying system with forced convection. Figure 11 displays the cocoa beans on the air-drying device. This device consisted of a display (a tarpaulin) on which the cocoa beans were placed in the sun for the duration of the drying trials. Both the open-air tarpaulin and the forced-convection solar dryer were closely positioned in a sunny spot. The devices were positioned to prevent the dryer from shading the display. During the six days of drying, the beans were placed in the open air in the morning (07:30–07:50) and removed in the early evening (17:25–17:40). Around 20 kg of fermented beans were handled during this operation. The beans were turned over regularly, twice a day, to ensure even drying. At night, the beans were stacked in plastic packaging and kept in a dry environment to prevent rewetting. Bean samples were taken and weighed at two-hour intervals throughout the day. When successive weighings indicated little or no change in mass, the beans were considered effectively dry. During this experiment, the condition of the dried beans was recorded to better appreciate the changes.

The second drying mode is a mixed solar dryer with forced convection, illustrated in Figure 1. The prototype is detailed in the results section. Inside the dryer, the beans were arranged on a rack to ensure optimum exposure. The orientation of the dryer was carefully chosen: it is aligned along a north–south axis, with its long axis parallel to the east–west direction and the collector positioned to the south, thus maximizing the absorption of solar energy.

3.4. Analysis of Characteristic Parameters

In this study, to determine the characteristic parameters of cocoa, a data analysis protocol was drawn up, structured in several essential stages. The analysis began by determining the mass of water to be extracted, accurately assessing the quantity of water to be removed to guarantee reliable characterization. Next, the rate of water removal was measured, ensuring rigorous control adapted to the specific characteristics of the product. These steps enabled us to establish the moisture content, a key indicator of quality. For specific products such as cocoa and its derivatives, specialized analytical methods, including pH measurements, are used to complete the overall assessment. This protocol guarantees a rigorous and precise analysis of final product quality.

3.4.1. Product Water Content

According to the authors in References [30,31], products of animal, vegetable, or mineral origin contain a certain amount of water m_w. If mi is the initial total mass of the product and m_ew is the mass remaining after extracting the water, the relationship given by Equation (1) is obtained. The latter is called the anhydrous mass, also known as the dry mass (m_dry).

$$m_{dry}=m_w-m_{ew}$$

(1)

Compared with moist air, the moisture or water content of a product (cocoa in this case) is referred to as X, or the dry basis. Its expression is given by Equation (2).

$$X={\displaystyle \frac{m_w}{m_{ew}}}$$

(2)

Similarly, if the moisture content of a product is denoted by W_h, then the moisture content is defined as the ratio of the mass of water in kg to the mass of the product with its so-called “wet basis” water. Its expression is defined by Equation (3).

$$W_h={\displaystyle \frac{m_w}{m_{dry}}}$$

(3)

The notions of moisture or water content on a dry basis (noted X) and the quantity of moisture in a product (noted W_h) are therefore correlated based on the relationships in Equations (4) and (5), respectively [30,31].

$$W_h={\displaystyle \frac{100\cdot X}{100+X}}$$

(4)

$$X={\displaystyle \frac{W_h\cdot 100}{100-W_h}}$$

(5)

3.4.2. Water Mass to Be Extracted from Product

According to the authors in [32,33], it symbolizes what needs to be removed from the product in order for it to reach a certain predetermined water content. This characteristic is generally mentioned in “product data sheets” and is regulated to guarantee product protection by specifying a given final water content. The expression of its formula is defined by Equation (6).

$$m_e={\displaystyle \frac{\left(W_{hi}-W_{hf}\right)m_i}{100-W_{hf}}}$$

(6)

where the initial water content of the fresh product, W_hi, is expressed as a percentage, as is the final water content after drying, W_hf. The quantity of water to be extracted from the product, denoted m_e, is calculated in kilograms. Finally, m_i represents the mass of the fresh product to be dried after cleaning, also expressed in kilograms.

3.4.3. Product Water Removal Rate

The expression in Equation (7) allows us to accurately estimate the rate at which water is extracted from the product. The associated parameters are as follows: V_em, the water removal rate (kg/h); m_e, the total quantity of water to be extracted (kg); and t_s, the optimum drying time for the product concerned (h).

$$V_{em}={\displaystyle \frac{m_e}{t_s}}$$

(7)

3.4.4. Moisture Content Estimation

The moisture content refers to the proportion of water vapor and several other volatile elements present in a given sample [33,34]. The percentage moisture content is determined from the formula in Equation (8), where W_h represents the moisture content (%), m_i is the initial mass of the sample before drying (kg), and m_f is the final mass of the sample after drying (kg).

$$W_h={\displaystyle \frac{m_i-m_f}{m_i}}\cdot 100$$

(8)

3.4.5. Assessment of Water Content

The water content refers to the proportion of water in a sample. After tarring an empty m₀ capsule, a quantity of fresh m_fr product equivalent to approximately 10 g of ground cocoa beans is weighed. The capsule containing the ground cocoa beans is then placed in the dryer. After drying, the capsule is removed and placed on a weighing scale to obtain the total mass m_p. The water content is determined using the formula in Equation (9), where W (%) is the water content, mi is the fresh mass of the cocoa, m_p is the total mass of the product, and m₀ is the mass of the empty capsule after drying.

$$W={\displaystyle \frac{m_i-\left(m_p-m_0\right)}{m_i}}\cdot 100$$

(9)

3.4.6. The Amount of Water to Be Removed from the Product

The product to be dried contains a certain amount of water, which the drying process will need to extract to ensure good preservation. Using the initial and final water content, this mass can be calculated using the relationship in Equation (10), where W_i is the initial water content in the product (in %), W_f is the final water content of the dried product (in %), W_e is the mass of water extracted from the product (in kg), and m_p is the mass of fresh product to be dried (in kg).

$$W_e=m_p\cdot {\displaystyle \frac{W_i-W_f}{100-W_f}}$$

(10)

3.4.7. Experiment Size

According to References [35,36], the size of an experiment can be influenced by increasing or decreasing the number of repetitions. The calculation of this number depends on several factors, including the variance estimate (σ²) obtained from previous experiments, the magnitude of the difference (δ) to be detected, the statistical power level (p = 1 − β) guaranteeing the probability of detecting a real difference, and the significance level (α) determining the risk of Type I error. In addition, the choice of statistical test, whether one-sided or two-sided, also influences this estimate. Thus, to determine the optimum number of repetitions, it is best to apply the formula in Equation (11).

$$R_{rep}\ge 2{\left(t_1-t_2\right)}^2{\left({\displaystyle \frac{\sigma }{\delta }}\right)}^2$$

(11)

where t₁ = z_α/2 is the critical value of the standard normal distribution for a significance level of α/2 (typically 0.025) and t₂ = z_β is the critical value of the standard normal distribution for statistical power 1 − β (typically 0.80).

3.4.8. Analysis of Cocoa Quality

The terms and grading levels for cocoa beans are precisely defined by ISO 2451 [37]. This standard establishes classifications based on cutting tests, which are essential for detecting and limiting imperfections that can alter cocoa flavors. In line with these standards, beans must meet strict requirements: they must be free from visible adulteration, practically free from live insects and any other infestation, and free from foreign matter. They must also contain a minimum of broken beans, fragments, and shell fragments. Fermentation and drying of the beans must guarantee a moisture content not exceeding 7.5% by mass, while eliminating any olfactory contamination. The bean size must be uniformly adapted to food production, and they must be reasonably free from flat, sprouted beans, residues, sifting debris, and crab. Finally, the maximum permissible values for the internal criteria of fermented beans, in accordance with this standard, are specified in

Table 1. Maximum limits for the internal classification of fermented beans.

Moldy (%)	Slate (%)	Attacked by Insects, Sprouted, or Flat (%)
3	3	3
4	8	6

[38].

3.4.9. Analytical Methods for Cocoa Analysis

In this study, 30 g of cocoa beans was finely ground in a kitchen coffee grinder (Moulinex Deluxe Stainless Steel Mill 843, France), and cocoa butter was extracted from the powder on a Soxhlet apparatus for 8 h in hexane [39]. For acid titration measurements, 5 g of the extracted cocoa butter sample was weighed at each stage. Figure 12 depicts the 250 mL Erlenmeyer flask containing the cocoa butter, for which the mass was known to be empty.

In the rest of the experiment, 50 mL of a hexane/ethanol mixture was added to dissolve each sample; 2 mL of phenopthaline was also added as a color indicator, as shown in Figure 13a. Figure 13b shows titration on a magnetic stirrer (Searchtech Instruments, Germany), carried out with 0.1 N alcoholic KOH solution until a pink solution was formed (the color persists for 30 s), and the final volume V is noted.

FFA (free fatty acid) is a term used to designate fatty acids that are not bound to glycerol molecules in fats or oils. In the context of vegetable and animal oils, FFA is often an indicator of quality and freshness. In this study, the FFA (oleic acid) was calculated according to Equation (12), where FFA is the free fatty acids (%), m_b is the mass of the weighted cocoa butter sample (kg), N is the normality of the alcoholic solution of KOH (potassium hydroxide), V_KOH is the volume of KOH base at the end point, and 28.2 is the molecular weight of oleic acid.

$$FFA={\displaystyle \frac{28.2\cdot V_{KOH}\cdot N}{m_b}}$$

(12)

3.4.10. pH Assessment

A pH meter is a scientific instrument for measuring the acidity or basicity of a solution, expressed by its pH (hydrogen potential). In this study, the digital pH shown in Figure 14 was used for the measurements. These measurements are made on a logarithmic scale generally ranging from 0 to 14, where a pH below 7 indicates an acidic solution, a pH of 7 corresponds to a neutral solution, and a pH above 7 reveals a basic solution. To determine the pH, 10 g of cocoa paste was dissolved in 90 mL of boiling distilled water, homogenized, and then cooled to 25 °C. The analysis was carried out using a digital pH meter with automatic two-point calibration, operating over a range from 0 to 80 °C, with a resolution of 0.01 pH. The solution was thoroughly mixed before the measurement, and the absolute error based on the value obtained was ±0.01.

4. Experimental Results and Discussion

After completing the construction of the solar dryer, an in-depth experimental evaluation was carried out to analyze the drying performance, both inside and outside the drying chamber. The results obtained are presented in several stages. Firstly, the prototype dryer is described, followed by its behavior in the absence of cocoa beans. Next, the performance of the dryer under load, when cocoa beans are placed in it, will be analyzed. Experiments were carried out to examine the evolution of various internal drying parameters, taking into account the climatic conditions of the site. Finally, the results of laboratory analyses to assess the quality of dried cocoa will be presented.

4.1. Prototype and Dimensions of the Solar Dryer

The dryer was our main experimental tool, and the prototype of the mixed solar dryer with forced convection is shown in Figure 15. Its main components are a transparent cover (1) over the drying chamber, a solar panel (2) for energy self-sufficiency, and a chimney (3) for evacuating air flows. The control box (4) groups together the device’s control elements. A drying rack (5) is provided for the beans, while a solar collector (6) and a fan (7) ensure air circulation. Finally, an opening in the upper section (8) provides ventilation. The dimensions of the proposed solar dryer are 0.80 m wide × 2.60 m long × 0.70 m high, with a 0.60 m rise from the ground. The drying chamber is equipped with three 0.75 m wide × 1.30 m long racks, each topped by a hole to facilitate the evacuation of air that has passed through the products. The whole unit is covered with black-painted 3 mm sheet metal, optimizing the absorption of solar radiation. Designed to be functional and modular, the dryer features an upper compartment that can be opened to allow ventilation of the unit, as well as access for cleaning and maintenance, guaranteeing optimal operation.

The dimensions of the solar dryer are 0.80 m × 2.6 m × 0.70 m and are detailed in

Table 2. Typical material values for dryer parts.

Product Name	Materials	Specifications (Width × Length × Thickness)	Units
Dryer	Wood	0.80 × 2.60 × 0.70	m
Collector		0.70 × 1.40 × 0.09	m
Solar drying room		0.75 × 1.30 × 0.55	m
Drying rack	Plastic mesh	0.65 × 1 × 1.20	m
Absorber material	Stainless steel sheet	0.004	m
Transparent cover	Flexible polycarbonate	0.003	m
Insulating materials	Polystyrene and plywood	0.015	m
Electrical characteristics of fans	DC Voltage	12	V
	Power	7 and 5	W
Battery	Li-ion	10	Ah
Solar module	Monocrystalline semiconductor	30	W

; it is built from materials readily available in Cameroon. The frame consists of plywood sheets and sheet metal on the inside bottom. A black-painted steel sheet separates the drying chamber from the acceleration chamber. The maximum thickness of the polyethylene cover and wood insulation is 20 cm. The bottom of the dryer is also insulated with a wooden panel and polystyrene, on top of which a 6 cm-thick layer of sand is poured for thermal storage. The entire frame of the solar dryer rests on four supports, each 50 cm high, allowing the dryer to stand firmly on the ground.

4.2. Environmental Parameters

Figure 16 depicts the environmental data collected during the test days, with a particular emphasis on the first day. It consists of three graphs illustrating the key parameters. Figure 16a displays the hourly irradiance measured on each test day, revealing a significant variation in solar radiation over the course of the day, with a peak reaching 800 W/m² around midday on the second test day, indicating maximum energy availability for drying in the middle of the day. Figure 16b illustrates the temperatures recorded, showing a rising curve in the morning, a plateau at maximum around midday, followed by a gradual decrease in the late afternoon. Finally, Figure 16c represents variations in the relative humidity of the ambient air, which evolves inversely with the temperature, with maximum values at the beginning and end of the day. These parameters are essential for assessing the efficiency of the solar dryer and understanding the atmospheric variations influencing the drying process.

4.3. Results of Evaluation of the Load Dryer

The behavior of the dryer under load, i.e., when the various compartments have received cocoa, was studied from day two to day three. Three (03) experimental solar bean drying trials were carried out between 14 January and 16 January 2024 to evaluate the performance of this new solar dryer under load. Typical results of the experimental trials are shown in the figures below. During the drying experiment, values for solar radiation, temperature, and relative humidity of the ambient air ranged from 68 W/m² to 800 W/m², 16–39 °C, and 22–78% respectively, as illustrated in Figure 17 and Figure 18. Drying temperatures and relative humidity values were significantly different from those of the ambient air throughout the drying period. During the drying period, solar radiation rose sharply from 8:00 a.m. to midday and then fell considerably in the afternoon. There were occasional untimely variations in solar radiation. However, in general, solar radiation was similar over the drying period.

Figure 18 demonstrates a comparison of air temperatures at the following three locations: in the collector (T_in), in the drying chamber (T_therm), at the outlet from the chimney (T_ch), and in the open air (ambient temperature). Temperatures in the first three locations differed considerably from the ambient air temperature. From the information provided in Figure 18, it is clear that the drying air temperature remains relatively high compared to the ambient temperature during the period from 2:00 p.m. to 6:00 p.m. At around 6:00 p.m., when solar radiation is almost negligible, the drying air temperature is still slightly higher than the collector outlet temperature. This shows that there is a heat input that can only originate from thermal storage.

4.4. Drying Time and Number of Experiment Replicates

The samples were dried at a constant air flow rate of 1 m/s. In all drying tests, the initial moisture content of the wet-base product in the samples was around 0.51. The drying time required to reach a final moisture content of 0.056 was not the same in the dryer and the full-scale system. The drying time was 18 h in the drying chamber and 30 h in the open air on a tarpaulin, for an average drying time of 8 h per day, corresponding to the average duration of sunshine. The rates of water removal by the drying process were 0.53 kg/h and 0.27 kg/h in the drying chamber and in the open air on a tarpaulin, respectively. The total quantity of water to be removed by the drying process was 9.62 kg of water for a final product dry mass of 9.83 kg.

For a statistical power of 80% (β = 0.20) and a significance level of 5% (α = 0.05), the cocoa drying time standard deviation (σ) is estimated at 1 day and the minimum detectable effect difference (δ), i.e., the threshold at which a difference in results is considered statistically significant at 2 days. Applying the formula in Equation (11), these results are as follows:

-

t₁ is the z-score for a significance level of 5%, corresponding to 1.96;

-

t₂ is the z-score for a statistical power of 80%, corresponding to 0.84.

Thus, the calculated value of R_rep is 3.92, corresponding to four repetitions.

4.5. Results of pH Analysis

The pH was measured by dissolving 10 g of cocoa paste in 90 mL of distilled water brought to a boil and then homogenized and cooled to 25 °C. The pH of cocoa beans plays a crucial role during fermentation, directly influencing the biochemical reactions taking place. In the following sections, the results of pH evolution during fermentation and drying are presented.

4.5.1. Control of pH During Fermentation

During fermentation, the pH of the cocoa varied between 5 and 5.9. After the first two days, it dropped to 4.2 before gradually rising to 5.9. It is essential to note that cocoa fermentation is characterized by a high temperature, up to 45 °C, and a duration of between 48 and 72 h [40,41]. Figure 19 illustrates the evolution of the pH throughout the fermentation process of our fresh cocoa bean sample.

4.5.2. Control pH During Drying

• For air-drying

During air-drying, there is a slight decrease in pH, from around 5.9 to almost 5.7, followed by a steady increase to reach a value of 6.3 at the end of drying, as indicated in Figure 20. This suggests that beans that have been air-dried tend to release acids.

• For the proposed dryer

In the proposed mixed forced convection dryer, a slight decrease in pH from 5.9 to 5.7 was observed in comparison with open-air drying, as illustrated in Figure 21. This result indicates that forced convection drying tends to intensify the acidic flavor of cocoa beans.

Table 3. Comparison of drying performance.

Test	Solar Dryer (Days)	Free Air (Days)
Test 1	2	5
Test 2	3	4.5
Test 3	2.5	5

compares the time needed to reach a moisture content of 8% in cocoa beans using two drying methods: solar drying and open-air drying. The results show that solar drying enables faster drying, varying between 2 and 3 days, while open-air drying requires between 4.5 and 5 days. On average, the solar dryer reduces drying time to 2.5 days, compared with 4.83 days for open air, a savings of around 50%. This difference can be explained by the thermal efficiency of the solar dryer, which generates more intense and stable heat, favoring rapid evaporation of the water, whereas drying in the open air depends on climatic conditions, notably the amount of sunshine and ambient humidity, which lengthens the process. The use of solar dryers offers a number of advantages, including significant time savings for growers, reduced weather-related risks, and better control of the drying process, guaranteeing more consistent bean quality.

4.6. Cutting Test Results

Figure 22 presents the results of the cutting tests conducted on fermented and dried cocoa beans. These tests allow for the observation of the internal appearance of the beans after drying, using two distinct methods: solar chamber drying (Figure 22a) and air drying (Figure 22b). The cocoa beans dried in the solar chamber exhibit a homogeneous texture and a uniform color, characteristic of effective drying. These beans are also clean and emit a pleasant chocolate aroma, indicating the good quality of the drying process, as illustrated in Figure 22a. Controlled temperature and dust protection promote this quality. As shown in Figure 22b, the air-dried beans show color variations, with slightly muted areas, which could be attributed to partial exposure to dust and climatic fluctuations, particularly during the dry season. The beans are less uniform and may exhibit visual defects, which could affect the final quality of the produced cocoa. The analysis of Figure 22a,b highlights the superiority of the beans dried in the solar chamber compared to those dried in the open air. In terms of quality, the homogeneous texture and color, along with the absence of impurities, promote an increased commercial value for the beans dried in a controlled manner. On the other hand, air drying, although more traditional and economical, leads to variations that can reduce the perceived quality of the finished product. The use of a solar chamber for drying cocoa beans appears to be a more efficient and qualitative method compared to air drying, particularly in terms of preserving the homogeneity and cleanliness of the beans. These observations suggest an increased interest in adopting this technique in cocoa production areas to improve the overall quality of the product.

4.7. Results of Laboratory Analysis of Product Quality

The release of free fatty acids (FFAs) is triggered by the action of an enzyme called lipase (E.C. 3.1.1.3). FFAs are derived from triglycerides or from an oxidation reaction [42,43,44]. Lipids are also broken down with water in the presence of oxygen and possibly micro-organisms. Heat and light accelerate degradation. In addition, the strength of cocoa butter is determined by the amount of saturated and unsaturated fatty acids present in the triglycerides, as well as the amount of free fatty acids (FFAs). Consequently, a fatty acid hydrolysis reaction can be illustrated in Figure 23. The cocoa butter hardness depends on the saturated and unsaturated fatty acid contents bound in triglycerides and on the free fatty acid (FFA) content. For reasons of quality, the directive 73/241/EEC [45] limits the maximum FFA content to a 1.75% oleic acid equivalent in cocoa butter.

Acid–base titration or an alkalimetric method was used in this experiment for the free oleic acid analysis. The cocoa butter samples were reacted with a base solution, 0.1 N potassium hydroxide solutions, to produce soap and water according to the chemical expression detailed in Figure 24.

As a base, the soap product is detected based on the solution color changing from colorless to pink with the addition of a phenolphthalein indicator, as illustrated in Figure 25a–c. The changing color of sample solutions was used for the free fatty acid qualitative analysis. After four titrations, the free fatty acid (oleic acid) content in the cocoa butter was found to be equal to 0.282%. This very low FFA content, in accordance with UE standards (1.75% oleic acid equivalent), confirms the good quality and health of the cocoa bean sample used for the analysis. Therefore, post-harvest techniques, such as fermentation and a mixed forced convection solar dryer for cocoa bean drying, were efficient and led to cocoa beans with good standards.

5. Conclusions

In this paper, the potential of a mixed forced convection solar dryer for drying cocoa beans in an equatorial context, particularly in the Ntui region of Cameroon, is highlighted. This solution constitutes a sustainable and efficient alternative to traditional open-air drying. The design of the dryer, using local materials, a solar collector, forced ventilation, and thermal storage, ensures energy autonomy and adaptability to high solar irradiance and humidity in the equatorial climate, making it a practical solution for small farmers. The experimental results reveal a substantial reduction in the drying time, decreasing it from 30 h under open-air drying conditions to 18 h in the solar dryer, while preserving critical quality attributes, such as pH stability (5.7–5.9), low free fatty acid content (0.282% oleic acid equivalent), and a uniform texture and color, all of which comply with international standards. These results highlight the solar dryer’s ability to mitigate the limitations of sun drying, including the dependence on weather conditions, contamination risks, and prolonged processing times, which often compromise the quality of cocoa and the economic returns for producers. Beyond the efficiency and quality, the solar dryer offers significant environmental benefits by reducing dependence on fossil fuels and minimizing post-harvest losses, thus aligning with global sustainability goals. Its successful implementation in Ntui suggests scalability to other cocoa-producing regions, provided that further optimization improves the thermal efficiency and profitability. Future research should focus on refining the dryer design, integrating advanced control systems, and evaluating its performance in various climatic contexts to confirm its robustness. Moreover, promoting adoption through awareness campaigns and financial incentives could strengthen local producers, thereby improving livelihoods and the resilience of the cocoa supply chain. This work highlights the role of innovative drying technologies in addressing agricultural challenges, offering a sustainable cocoa production model that balances quality, economic viability, and environmental management, with broader implications for tropical agro-industries.