1 Introduction

Recent evidence indicates a rising trend of world hunger since 2015. Approximately one out of every nine people in the world is undernourished [1]. In Sub-Saharan Africa (SSA), the prevalence of hunger increased from 20.7% in 2014 to 23.2% in 2017 [2]. In fact, most of the world’s acute hunger occurs during the pre-harvest months, coinciding with the period when household food stocks from the previous harvest have run out, food prices are generally high, and off-farm income opportunities are scarce [3]. The high levels of hunger are, in part, driven by a lack of adequate techniques for food processing, which causes substantial food losses [4, 5]. The Food and Agriculture Organization (FAO) defines food loss as a “decrease in mass or nutritional value of food that was originally intended for human consumption,” differentiating it from food waste, which is “food appropriate for human consumption being discarded, whether or not after it is kept beyond its expiry date or left to spoil” [6].

Worldwide, approximately 1.3 billion tons of food for human consumption is lost every year [7], which equals roughly one-third of total production (32.5%) [8]. The annual value lost is estimated at US$4 billion, which exceeds the total US$6.1 billion SSA received between 1998 and 2008 in the form of food aid. This US$4 billon lost is equivalent to the annual caloric requirements of at least 48 million people (at 2,500 kcal per person per day) [9]. In SSA, per capita food losses are 170 kg/year [7], representing approximately 14% of global food losses.

To address this challenge, diverse technologies and methods have been developed. For small-scale farmers in the developing world, the use of solar energy through solar food dryers—here defined as devices that use solar energy to dry substances, especially food products [10]—is highlighted as a particularly promising solution [11, 12].

Reasons for this claim are manifold, including benefits with regard to preservation of nutrients, improved time efficiency of different solar drying systems compared to traditional drying, and applicability for rural areas [13,14,15,16,17,18,19,20,21]. In addition, food drying works for many types of food, with dried food being lightweight, easily stored, and easily transported [22]. Consequently, various types and designs of solar drying systems have been developed. A review article by Sharma, Chen [23] outlines existing solar drying systems and presents solar dryers that can be suitably used in rural farming areas.

Depending up on how the heat is transferred to the food, solar dryers are classified as direct, indirect, or mixed-mode dryers [21, 24,25,26]. Direct solar dryers (DSD) consist of a transparent cover that allows direct exposure of food to solar radiation [14, 27]. In addition to reducing heat losses, the cover also protects products against external contaminants such as rain, dust, and insects [28]. Nevertheless, direct exposure to solar radiation can negatively affect the sensory and nutritional attributes of food [16, 29]. The use of indirect-type solar dryers (ISD) minimizes the negative effect of solar radiation on the dried product [21, 30], as the agricultural products are placed inside a drying chamber [31] and are dried through heated air coming from a separate solar air heater where solar energy is collected. Mixed solar driers (MSD) are an intermediate solution for PSD and DSD in which the product is heated by both a transparent drying chamber and an air heater [32]. Solar dryers are also classified as forced (or active) and natural (or passive) convective, based on the air movement mode [33]. Active solar dryers (ASD) combine solar energy with electricity or fossil fuels to provide heat and air circulation; therefore these are not optimal for rural and isolated areas, especially in SSA where electrification is uncommon and financial resources are scarce [34]. In fact, the economic vulnerability and the limited access to energy restricts the application of measures to reduce food loss and waste in developing countries [35]. Previous review studies of solar dryers indicate that passive solar dryers (PSD) are cheaper than ASD [16, 36], as such PSD are suitable for resource-constrained households like those in rural areas of SSA.

PSD is widely demonstrated as achieving high efficiency, shortening drying time [37,38,39,40,41,42,43], and protecting products against fungi, pests, and rodents compared to traditional open-sun drying (OSD) [33]. In addition, PSD requires low capital and maintenance costs. However, the level of solar dryers acceptance among smallholder farmers is still low [24], thus, its application as a strategy to sustainably preserve agricultural products and safeguard food storage is limited [14]. As such, the use of systematic approaches to learn about progress with respect to the development and performance assessment of food solar drying technologies can help to avoid underutilization of the existing knowledge base, thus allowing a better understanding and identification of the major constraints that discourage the effective utilization of solar energy for drying agricultural products.

This paper systematically reviews the academic literature in which passive solar dryers systems are tested to assess their performance and impact on food quality, with the latter defined as the sum of all properties and assessable attributes of a food item (sensory value, suitability value, and health value) [44]. Subsequently, we identify the main limitations and challenges for application of PSD as sustainable alternatives for drying agricultural products in SSA. To capture relevant documents, we apply a structured approach for paper identification based on Liberati, Altman [45]. Possible solutions and directions for future research are proposed and discussed based on existing research results.

2 Methods

In this review, we searched for peer-reviewed papers and conference proceedings in the following academic search engines and bibliographic databases based on overall scientific impact and multidisciplinarity: Google Scholar, Web of Science, Agris, and Scopus. Reference lists of retrieved articles were checked for additional and possibly suitable literature (cf. Fig. 1). The applied search terms and (combined) keywords were “solar drying systems,” “solar dryer,” “solar dry + direct,” “solar dry + open sun,” “solar dry + mixed-mode,” and “solar dry + indirect.” After article identification, duplicates were removed, and then the following three steps implemented to remove articles that were defined as irrelevant: (1) title screening; (2) abstract screening; and (3) full text screening (cf. Figure 1). The documents included in our review had to meet the criteria presented in Table 1. The number of studies screened, assessed for eligibility, and included in the review are presented in a PRISMA flow diagram [45]. From a total of 27,745 studies, 125 studies were assessed for eligibility, as presented in Fig. 1.

Fig. 1
figure 1

Flow of information through the different phases

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Table 1 Inclusion and exclusion criteria
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The type of solar dryer, the study location, and the type of dried products were extracted from the reviewed articles and reported in summary tables. The parameters used to assess the solar dryer performances and the key findings were also extracted from each study; thereafter the main limitations, challenges, and possible solutions for more sustainable application of PSD in the future were derived.

3 Results and discussion

3.1 General findings

The results from our literature search reveal that very limited research on solar drying for food processing in SSA dedicated to smallholder farmers has been conducted since 2000 (n = 18, cf. Figure 1). Starting with 27,745 studies, only 688 abstracts were screened (2.5%), as the titles of the remaining articles reported information that did not meet the inclusion criteria (cf. Table 1). Of these, 670 studies were eliminated from the review, 563 for not meeting the criteria (cf. Table 1) and 107 due to lack of methodological clarity (28), type of solar dryer not mentioned (5), inclusion of non-passive solar dryers in the analysis (36), or a lack of included food quality parameters (38). Hence, only 18 studies are included in our review (Fig. 1).

3.2 Solar dryers’ performance

The performance of the different types of PSD reported in the studies that were included in our review is summarized in Table 2. Agarwal, Seretse [46] develop a direct passive solar dryer in Botswana for drying various products with a drying rate of 0.01488 kg/hr. It took 4 h to dry the groundnuts at maximum ambient and drying temperatures of 43.6 °C and 79.2 °C, respectively. Dissa et al. develop and test both ISD and DSD for drying mango in Burkina Faso. In the DSD experiment, the observed drying rate was 0.15 gkg−1 s−1, and the maximum drying efficiency was 34%. The ambient air humidity ranged from 30 to 66%, while the ambient temperature ranged from 30 to 36 °C [47]. For the ISD, the ambient air humidity ranged from 30.4 to 41%, while the ambient temperature ranged from 25.8 to 37.5 °C. The maximum drying rate observed was 0.18 gkg−1 s−1. The drying temperatures reached a maximum of 37.5 °C, and the drying relative humidity ranged from 16 to 51%. In this experiment, the drying efficiency was 35.04% [48].

Table 2 Summary of the solar dryers’ performance
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Another DSD was tested by Muhammadu and Abraham [49] in Nigeria to dry cassava. It took 76 h to reach a final water content of 12%. The maximum drying temperature was 43.8 °C, and the maximum ambient temperature was 30.3 °C. The observed drying efficiency was 52%, and the drying rate was 0.25 kg/hr.

Notably, there are very mixed results in terms of drying time and final water content among studies assessing the performance of solar dryers for the same crop. For example, Dissa, Bathiebo [47] needed 96 h to reach a water content of 24.8% for a variety of mangos, while Adepoju and Osunde [50] needed 8 h to reduce the water content from initially 85% to 6% while using the same crop; to reach 13% of the water content for mango, Gbaha, Yobouet Andoh [51] needed 27 h. Eke [52] reached 4% of the water content in tomatoes after 96 h of drying, reaching a maximum drying temperature of 60 °C, whereas Aliyu, Kabri [53] reached the same water content after 72 h with a maximum drying temperature of 47 °C. There are also differences in reported drying efficiencies and drying rates (cf. Table 2).

3.3 Challenges and limitations for the effective utilization of PSD in SSA

3.3.1 Limited research and difficult comparability between studies

Among the studies obtained during our literature search, only 18 met our inclusion criteria, which reveals a limited availability of academic literature on PSD in SSA. Most studies about solar dryers are on DSD, which are already proven inadequate for rural and low-income communities as they require additional energy sources and are not affordable. Moreover, based on Leon and Kumar [63] and Masud et al., [35], 27 parameters are important for different passive solar dryers’ performance analyses. These parameters are grouped into four categories, as shown in Fig. 2. However, the majority of studies in SSA generally report only a few selected parameters, mostly related to the thermal performance and physical features of the solar dryer. Nevertheless, when evaluating solar dryers, it is important to account for the requirements of both consumers and users.

Fig. 2
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adapted from Leon and Kumar [63] and Masud et al., [35]

Solar dryer evaluation parameters

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Most studies included in our review report thermal performance-related parameters, especially ambient temperature, drying time/drying rate, and drying air temperature. Moreover, physical features of dryers are also frequently reported, mainly the size and shape of solar dryers, collector areas, and solar apertures. However, parameters related to cost are rarely reported (Table 3). Hence, the evaluation procedures of solar dryers generally neglect financial feasibility. Out of a total of six parameters related to the quality of dried food, the moisture content is the only parameter generally measured. Parameters such as sensory quality and nutritional attributes are generally neglected (cf. Table 3).

Table 3 Number of parameters used to assess solar dryer performance in each study
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Although it is often generally assumed that solar drying is a good strategy to improve food preservation and nutrient retention, the data supporting this assertion is weak. A large heterogeneity in methods and parameters for assessing the performance of solar dryers is found among the included studies, which may explain the diversity and sometimes contradictory assessments of the technology. Mulokozi and Svanberg [60] report a higher retention of all-transβ-carotene with the use of a solar dryer compared to open-sun drying, whereas Bechoff, Tomlins [57] find no differences.

Variations in size, shape, color, and texture are usually observed in dried products [63,64,65], but the included studies rarely report these attributes. From our review, it can be derived that research on PSD in SSA rarely takes into account consumer needs, as it focuses on the thermal performance and physical features of the dryer, ignoring the final characteristics of the dried product. Leon, Kumar [63] report inconsistencies in the quality evaluation of dried products and further claim that there is “no quantification of food quality parameters in most literature, which makes evaluation and comparison between dryers difficult and often misleading.” However, the quality of dried products is associated with how a product satisfies the requirements of the user [66]. It includes attributes such as aroma, taste, color, microbial load, retention of nutrients, texture, and porosity, etc. [35]. Assessing the quality of dried products is necessary, as different solar driers may result in different sensory and nutritional parameters [21, 67], which, in turn, affects the price of the dried products and, therefore, the economic viability and subsequent adoption rate of the technology.

Parameters related to economics are the least reported in studies on the performance of PSD in SSA. Tiwari [68] indicates that financial viability is a key component, as solar dryers are generally capital intensive compared to other investments by farming and processing households. In fact, costs are the main determining factor for scaling up these technologies [20].

3.3.2 Time consumption and small quantities of dried products

A principle limitation of solar dryers in general is the quantity of product that can be dried per drying process. For example, Agarwal and Seretse [46] obtain less than 600 g of dried ground nuts in one day of the experiment. Abubakar and Umaru [61] dry only 2 kg of yam, which had 77.5% of moisture content lost, in a two-day experiment. This constitutes an additional burden for farming households, as more dryers or much longer periods would be required to dry, for example, the total seasonal production of maize, which averages 1800 kg/ha in SSA [69]. In fact, long drying times were also reported as one of the major drawbacks of sun drying. It can take up to 30 days to finalize the drying process, depending on the crop and the weather conditions of the site [11]. This would be aggravated by the fact that additional attention is necessary to ensure safe storage of dried products and avoid contamination after the drying process. Some crops may require adequate pretreatment to produce safer dried products [70]. Thus, it is also necessary to establish a daily routine to perform and monitor the drying process, which can overload daily activities and, therefore, constitute a limitation to the continuous use of PSD.

Although PSD are considered suitable for rural areas and are relatively cheaper than ASD, the quantities of dried products obtained might lead to unrealistic expectations. The current levels of productivity and efficiency are not ideal, since despite being relatively cheap, the initial costs that are necessary to obtain PSD may not be recovered, which is an unbearable burden in the context of resource-constrained communities.

3.3.3 Safety of food

In general, bacterial pathogens, fungi, and most waterborne viruses are highly sensitive to solar radiation and high temperatures. During solar drying, especially in direct solar drying, food products are exposed to relatively high temperatures and ultraviolet (UV) radiation, which can prevent microorganisms from replicating. Nevertheless, it is still crucial to note that certain microorganisms can be resistant to drying temperatures, compromising food safety. In addition, food handlers or drying platforms with low sanitary standards can contaminate food after sun drying [71, 72]. Exposure to rain, wind, dust, insects, and other contaminants may also lead to a final increase in microbial populations [70]. Moreover, the food itself can offer protective compounds and structures that may increase the survivability of microorganisms [70]. Many compounds found in fruits and vegetables, such as sugars, polypeptides, amino acids, glycerol, and carboxylic acids, may increase the chances of microorganism survival during the drying process [73]. Nevertheless, in most of the studies, the only parameter related to food quality generally reported is the level of moisture content, which creates an urgent need to also incorporate the microbiological load during performance evaluation to ensure food safety during and after dying.

3.3.4 Periodic availability of solar radiation

The use of PSD depends largely on the availability of solar radiation and seasonal changes; therefore, the heating of air and/or the drying of food occurs in real time when solar energy reaches the solar dyer. Thus, when solar radiation is not available, the rate of drying decreases to almost zero. This means that during rainy days or cloudy days and during nights, drying by using PSD may not occur. Although some solar radiation arrives during cloudy days, the system's capacity to heat the air decreases and, in regions that experience successive days of rain, it becomes impractical to use PSD during the rainy season. Solutions such as the use of thermal storage systems, auxiliary energy sources, or larger solar collectors are proposed in the literature [74, 75]. However, these solutions are not affordable for low-income households in the Global South.

Most solar dryers assessed in the reviewed studies report drying temperatures below or equal to 60 °C, which indicate that they are ineffective for crops that require high-temperature drying. As such, these solar dryers can only dry a limited number of agricultural products (especially those that require low energy input). In this context, the development of low cost and efficient solar collectors should be a main focus of research on PSD in order to find a trade-off between energy efficiency and cost-effectiveness [75].

3.3.5 Socio-cultural barriers

Adopting new technology may alter social and economic organization, thus redefining people or communities’ identity [76]. Therefore, during the implementation of a PSD, strategies must be undertaken to ensure that the number of people who decide to adopt and use the PSD are enough to outweigh what happens when individuals eventually abandon its use. Although PSD is potentially a beneficial technology, traditional uses of solar energy may persist due to people’s familiarity with traditional practices. Moreover, individual social beliefs can strongly limit the acceptance, adoption, and continued use of new technologies. Therefore, the population must believe that PSD can effectively add value to agricultural products compared to traditional OSD for more effective utilization. They must have knowledge on how to operate the PSD correctly and have an interest in the idea of alternative technology use. In addition, the construction materials and labor necessary for installation, operation, and maintenance must be locally available.

3.4 Future directions for a more sustainable and effective utilization of PSD in SSA

Based on the main limitations identified in this study, it can be derived that the design and development of a solar dryer in SSA should consider the following aspects:

  1. a.

    The solar dryers must be inexpensive and exclusively operated by solar energy;

  2. b.

    It must not require high-skilled labor for construction;

  3. c.

    It must be simple and easy to construct using locally available materials;

  4. d.

    The temperatures inside the drying chamber must be higher than the ambient temperature;

  5. e.

    It must be capable of drying crops that require both low energy and high energy drying;

  6. f.

    It must be able to ensure a relatively rapid achievement of a safe moisture level;

  7. g.

    It must dry relatively large quantities of products in the shortest time possible; and

  8. h.

    It must maintain food quality attributes that appeal to general consumers (e.g., color, nutrients, texture, aroma, and taste).

The use of passive greenhouse dryers is a particular solution that can meet the requirements mentioned above. It can increase the volume of dried food and, consequently, shorten the time necessary for each drying process. A greenhouse dryer is a direct type of solar dryer that is easy to fabricate, presents a simple design; and does not have any operating costs. It can achieve large quantities of dried product, higher drying rates, and can be used throughout the year for different purposes (e.g. cultivation and drying); therefore, it is more economically feasible [77,78,79]. It also ensures that some doses of UV are received by the food, which is important for microorganism deactivation. Ideally, a solar collector should be incorporated, thus allowing a mixed mode operation to optimize the use of solar radiation and temperature.

Taking into account that agriculture in SSA is typically small scale and for subsistence purposes, producers are likely to have the same characteristics as consumers, so testing for many subjective quality aspects would be easy. However, a standard evaluation procedure is still necessary; for example, more rigorous statistical methods would have been used in this review, but the studies included use a variety of methodological approaches. One key factor of a successful study is replicability. Thus, a test protocol is necessary for studies evaluating the performance of solar dryers in order to reduce methodological differences between studies. The protocol should clearly state which parameters are crucial for solar dryers’ evaluation, the procedures to conduct the experiments, and how the data should be analyzed and presented. Thus, future uncertainties are more likely to be avoided.

To ensure food safety, heated air and radiation must be enough to ensure complete deactivation of microorganisms. Thus, the use of a mixed mode greenhouse dryer may be able to increase temperatures above the optimum temperature for microbial growth, including those resistant to the drying process. In general, these temperatures must be above 40 °C [80]. Nevertheless, the application of effective procedures, including hygiene, cleaning, and disinfection, is still crucial for effectively avoiding the growth of harmful microorganisms during the drying process, especially foodborne pathogens. The latter are critically important and dangerous microorganisms, including bacteria, viruses, and fungi (mycotoxins), that are found in foods [81].

The use of PSD with higher capacity in terms of the daily quantity of dried food and better capacity to increase drying temperature can make it possible to reduce the large dependence on sunlight availability due to associated daily productivity and short exposure time necessary.

Efforts directed at overcoming social barriers should focus on the establishment of centers for community learning. Experience with solar dryer implementation shows that involving local communities, as well as local partners, such as local governments and universities or training institutes, is crucial for successful implementation, especially in rural contexts in developing countries.

4 Conclusion

In this paper, we systematically review the academic literature in which passive solar dryer systems are tested to assess their performance and impact on food quality in SSA. The results reveal a general shortage of research and a lack of study comparability regarding PSD performance in SSA. Furthermore, the few existing papers focus their analyses mainly on parameters related to the thermal performance and physical features of solar dryers. Parameters related to the quality of dried products and economics are rarely reported. The only parameter related to the quality of dried products generally reported is the water content after drying. Therefore, it is necessary to include not just the load density; size variation; shape, color, and texture of dried product; microbial load; costs of the dryer; operating costs; but also all 10 thermal performance parameters and 8 physical features parameters presented in Fig. 2. Moreover, more standardized and robust methodologies should be followed to achieve more accurate conclusions and improved comparability of results across studies. This would provide greater certainty on the potential benefits of PSD and avoid costly technology development and distribution, which may lead to economic losses for both potential producers of the PSD and clients (farmers). Recommendations for non-adapted and non-beneficial models can destroy the confidence of stakeholders not only with respect to the specific models but also the (potentially beneficial) technology in general, especially in SSA where the use solar drying technologies in very limited, despite the need to significantly reduce food loss and waste.

The small quantities of dried products obtained per drying process, inadequate handling of drying equipment, and the intermittent nature of solar radiation availability are some of the main limitations for the effective implementation, adoption, and continued use of PSD in SSA. Thus, given its potential to overcome most of these limitations, the use of mixed mode passive greenhouse dryers is proposed in this study. It can significantly improve the shelf-life of food and does not require external sources of energy. Future research should focus on developing and testing solar dryers that reflect the needs of both processors and consumers to ensure market penetration and effective utilization of PSD, thus improving the food loss and waste situation in SSA along with environmental sustainability.