Tomatoes drying in Santorini, Greece. Photo by Klearchos Kapoutsis via Flickr, licensed under CC by 2.0
Written by: Custodio Matavel and Harry Hoffmann. Custodio Matavel is a second year PhD student at the Humboldt University of Berlin and affiliated with the Leibniz Centre for Agricultural Landscape Research (ZALF). Dr Hoffmann has a PhD in Agriculture and holds a research post at ZALF. His work focuses on food security and nutrition projects in East Africa.
When you think about sun-dried food, you might think of pantry staples and beloved ingredients, from the raisins in your morning müsli to the sun-dried tomatoes in a restaurant risotto. But did you know that solar drying is also an essential component of sustainable food systems? In this article we explore how solar drying works and how it fits into the bigger sustainability picture.
Solar Drying and Sustainability: How It All Fits Together
Sustainable access to sufficient, safe and nutritious food is one of today’s biggest global challenges. The United Nations’ Sustainable Development Goal (SDG) 2 acknowledges the need to reduce the number of people who suffer from hunger to zero, while simultaneously addressing climate change and protecting the environment. Similarly, SDG 12 focuses on sustainable consumption and production patterns.
Currently, around 690 million people worldwide – or roughly 9% of the global population – are undernourished. This number may exceed 840 million by 2030, even without the increase in global hunger that is likely to follow the COVID-19 pandemic (FAO et al., 2020). These challenges are likely to become even more difficult to address as the population continues to grow and food needs increase (Gerland et al., 2014).
Three overarching strategies could address this challenge: reducing food demand, which is difficult to do due to the growing global population and evolving dietary habits (Hasegawa et al., 2019); increasing food production – a challenge due to the environmental side effects of intensive or industrial agriculture (Wu et al., 2018); and reducing current levels of food loss and waste (Lemaire and Limbourg, 2019).
This third strategy has received particular attention, especially as current food production is more than sufficient to feed everyone in the world. Yet in 2019 the UN’s Food and Agriculture Organisation (FAO) reported that 14% of all global food is lost (FAO, 2019) due to the failure to adequately process agricultural products after harvesting, causing them to spoil. Conversely, food waste refers to food that is suitable for human consumption being discarded or spoilt at the retail, food service provider and consumer level. In 2011, the FAO estimated that one third of all food is lost or wasted, but has yet to publish a more detailed estimate of the scale of food waste specifically.
Reducing food waste and loss is therefore an essential step towards reaching the world’s SDG 2 (ending hunger) and SDG 12 (ensuring sustainable consumption and production patterns) goals. It can also potentially decrease the pressure that agricultural activities exert on the environment. Estimates by Springmann et al. (2018) reveal that reducing food loss and waste is an important factor in improving the environmental sustainability of food systems as it would limit the use of cropland, blue water, nitrogen and phosphorus.
This in turn requires sustainable and affordable technological solutions to increase food shelf-life while optimising nutrient availability and food quality.
Reducing Food Waste Through Preservation Technologies
Refrigeration is one effective way to reduce the loss of a variety of food products (van Holsteijn and Kemna, 2018). However, keeping products at low temperatures throughout the food distribution chain requires a reliable electricity supply and financial output, and can contribute to environmental pollution (Helo and Ala-Harja, 2018).
Solar drying is a suitable alternative to refrigeration, especially in places where cold chains are inadequate (Prakash and Kumar, 2017). It uses solar energy to reduce the water content in foods to the point at which spoilage and the growth of bacteria, yeast and mould are inhibited. In many cases, the loss of water content also results in a smaller volume, making products easier to handle and transport. Finally, it makes it possible to store food at ambient temperatures (Abrol et al., 2014), which increases shelf life and therefore likely contributes to food security.
In the context of resource-constrained communities and particularly in tropical and subtropical regions, solar drying is the preferred method of food processing due to its low cost, as sunlight is plentiful and freely available (Kumar et al., 2016). Moreover, some solar drying methods can preserve the colour, flavour, texture and nutritional value of the food. However, drying food items can also result in the loss of important nutrients (e.g. vitamins) as well as changes in sensory attributes (Mohammed et al., 2020) if drying is carried out incorrectly.
The most common method of solar drying is open-air sun drying (El-Sebaii and Shalaby, 2012). This is the traditional drying method for reducing the moisture content of foodstuffs. It involves spreading the commodity on the ground or on another surface until dry. However, this method can only incorporate very limited quality control, so open-air sun drying is often inadequate and likely to cause losses in both the quality and quantity of agricultural products due to contamination by dust, development of toxins or inadequate drying rates. Substantial losses associated with open-air sun drying have been reported in several studies (Poblete et al., 2018, Lingayat et al., 2017, Essalhi et al., 2018, Patil and Gawande, 2018).
Drying chillies in a direct dryer in Uganda. Photo courtesy of Farm Africa (copyright Farm Africa / Jjumba Martin) from a joint solar drying project by the North East Chilli Producers Association (NECPA) and Farm Africa.
Reducing Losses in Drying: Solar Drying Technologies
This loss in quantity and quality during sun drying mainly results from the fact that crops have to be left out in the fields until they reach a suitably low level of moisture content for traditional methods to be effective. In the process, they are exposed to weather and pests. Alternative drying methods make it possible to harvest crops earlier, when the moisture content is still higher.
In order to be effective in conserving nutrients, drying should be conducted at temperatures lower than 70 °C (Prakash et al., 2016). Solar drying systems can provide this heat under controlled conditions to minimise nutrient loss.
Therefore, various solar dryer models have been developed over decades in many countries (e.g. Alamu et al., 2010, Ananno et al., 2020, Mohammed et al., 2020) as a sustainable substitute for traditional, open-air sun drying, to reduce losses, improve the quality of dried products and potentially improve food security. This has resulted in a wide range of solar dryers, from simple, locally manufactured models to more sophisticated dryers that are manufactured industrially or semi-industrially.
Depending on how heat is transferred to the food, solar dryers can be classified as direct, indirect, mixed mode or hybrid. Direct models dry crops through direct heating from the sun. They usually take the form of a box with a transparent lid so that the foodstuff is directly exposed to solar radiation. Indirect dryers, on the other hand, do not directly expose the products to the sun but use sunlight to heat air, which then flows through the product to be dried. Mixed mode dryers, in turn, are indirect dryers in which the products are positioned in such a way that they receive direct heating from the sun as well as heated air. Hybrid dryers use solar energy and another energy source – such as wood, gas or electricity – as a heating source.
The drying efficiency of an indirect solar dryer is lower than that of direct solar dryer (Kumar et al., 2016), but the former performs better as it minimises changes in colour and loss of vitamins (Tomar et al., 2017). Hybrid models expedite the drying process whilst ensuring high product quality (Udomkun et al., 2020).
What Factors Influences the Choice of Drying Method?
The choice of dryer type also depends on local weather conditions, incoming radiation intensities, the type of agricultural product, users’ needs and resources, the amount of moisture and the size of food items, as well as the potential financial benefits of using a solar dryer.
Drying as a means of food preservation is practicable for a wide variety of fruits, vegetables, spices, grains, legumes, roots, tubers and meats. Drying can be an ideal preservation method for fruits in particular as their high acidity limits bacterial growth and their high sugar content binds moisture, preventing or slowing the growth of bacteria, moulds and yeast (Guiné and Castro, 2002, Mohammed et al., 2020). Drying dark-green leafy vegetables is particularly desirable as they are important sources of nutrients, but not available all year round (Nawiri et al., 2013). However, drying vegetables and meats is more challenging as vegetables have a low sugar and acid content and meat is sensitive to spoilage through contamination by pathogens. Therefore, food type determines ease of drying.
Local climate conditions such as temperature and relative humidity also determine the efficacy and efficiency of drying methods, as well as the amount of solar radiation needed for successful completion. Traditional open-air methods are most effective in areas where temperatures are high and relative humidity is low. In rainy seasons, open-air sun drying is more difficult and might result in development of toxic fungi (Bradford et al., 2018).
This is where solar dryers can help. The comparably higher temperatures in the drying chambers shorten the drying process and yield a higher quality product (Mohammed et al., 2020).
An industrial indirect dryer. Photo by Cachogaray, licensed under CC BY-SA 4.0
Solar Drying and Food Safety
The quality standards for dried foods vary quite a bit depending on products’ end uses and target consumers. Nevertheless, hygienic processing and packaging can improve the safety of dried food consumption.
As already noted, the point of drying is to lower the moisture content of the food in order to inhibit microbial growth. These moisture levels should be maintained as long as possible to limit microbial spoilage (Deng et al., 2020).
However, some pathogenic microorganisms can survive the drying process, compromising food safety. Trichinella spiralis (Hill et al., 2017), for example, is a foodborne parasite that survives drying. Such parasites remain dormant for longer periods and become active once the foods are rehydrated. Likewise, Salmonella, a foodborne pathogenic microorganism, can survive at very high temperatures when subjected to dry heat (Chitrakar et al, 2018).
This means that rehydrated products must still be refrigerated and thoroughly cooked. Furthermore, hygiene, cleaning and disinfection procedures must be adhered to in order to avoid contamination before, during and after the drying process.
Solar Dryers and Greenhouse Gas Emissions
Solar dryers not only help reduce food loss, but they are also clean-energy technologies. Thus, one potential benefit is a reduction of greenhouse gas (GHG) emissions compared to processing technologies that require fossil fuels. This means that a shift towards solar drying – or at least a combination of solar and conventional energy – could decrease conventional energy consumption and GHG emissions (Udomkun et al., 2020).
Importance to Food Security and Nutrition
One of the greatest challenges many countries currently face is how to ensure an adequate and dependable food supply. In many areas, especially in the Global South, households often face food shortages as locally produced foods cannot meet nutritional requirements all year round due to seasonal and insufficient crop production (Devereux and Tavener-Smith, 2019). Moreover, traditionally dried and stored food can be degraded by microorganisms, making these foods unsuitable for human consumption. It is therefore essential to ensure adequate food safety and to minimise the loss of its nutritional characteristics, as well as to ease and facilitate transport and handling. The use of solar dryers is increasing in many countries (Udomkun et al., 2020). They contribute to making basic food items more readily available for household consumption throughout the year. Small-scale, community-based solar drying systems also help bolster family incomes as processing increases the market value of agricultural products.
Increasing the shelf life of food can make it easier to transport it to areas where it is not produced, allowing it to reach urban communities for example. This could drive up the consumption of food crops produced in rural areas, thus strengthening rural-urban economic links and reducing urban dependence on foreign food imports.
In conclusion, the use of solar drying technologies, especially in agriculture-focused areas, is an economically and environmentally sustainable way of preserving many food items. It can help address the constraints of traditional open-air sun drying (food contamination, loss of food quantity and nutritional attributes, etc.) and reduce the use of fossil fuels, thus potentially improving not only the availability and quality of food, but also quality of life.
Peer review declaration: This is a peer-reviewed article written by an academic researcher. It has been put through double-blind peer review.