Addressing food loss and waste is crucial in combating global food insecurity. Globally, one-third of food produced for human consumption is lost between harvest and the consumer stage (FAO, 2011). It is estimated that 30-50% of total food production is lost during postharvest processes. The issue is particularly severe in developing countries, where 30-40% of production is lost before reaching the market. These losses primarily affect staple crops like pulses, grains, and cereals, as well as horticultural crops (fruits and vegetables). Storage losses result from mold, bacteria, insects, and rodents, often due to inadequate postharvest management. Further, food spoilage is especially severe in humid climates where high moisture levels create an ideal environment for mycotoxins to thrive.

Food losses have a significant economic impact globally, affecting efforts made to meet the Sustainable Development Goals (SDGs) of reducing poverty and hunger by 2030. The dry chain offers a viable option for further drying of grains, fruits, and vegetables, enabling value addition and extending availability for off-season consumption.

Moisture Levels in dried Commodities

Throughout history, sun-drying has been a near-universal method for preserving commodities. Traditionally, farmers spread products on tarps, tables, or floors to harness the sun's heat. While this sounds an effective drying method, achieving safe moisture levels requires additional innovations.  For example, maize is typically harvested with a moisture content (MC) between 28-32%. Proper drying requires an equilibrium relative humidity (ERH) below 65% to prevent mold growth. The ERH is the humidity level at which moisture in the crop is balanced with the surrounding air.

If commodities are not dried to safe storage levels, mycotoxins—harmful secondary metabolites will proliferate. Aflatoxins, a type of mycotoxin, pose severe health risks when they contaminate food directly (through the soil) or through toxigenic fungi. Therefore, inadequate drying can result in microbial infections, causing acute and chronic diseases in humans and animals. In humid regions, where ambient relative humidity is insufficient for effective drying, further innovations are needed to prevent spoilage.

The Need for the dry chain

Using the cold chain model where fresh produce is maintained through continuous refrigeration after harvest, the dry chain ensures that dried commodities reach safe moisture levels before storage. Many grain quality issues stem from storing products with unsafe moisture and temperature levels. In humid regions and other regions of the world with unpredictable weather conditions, small-scale farmers relying solely on sun drying need affordable drying innovations to maintain product quality and prevent postharvest losses. It is essential to lower the commodity’s Equilibrium Relative Humidity (ERH) to the desired safe levels and proper assessment is critical to minimize degradation of the product during storage. 

Critical Role of Research in the Dry Chain

The Feed the Future Innovation Lab for Horticulture, in collaboration with researchers from the University of California, Davis has developed drying innovations applicable to both staple and horticultural crops. Small-scale farmers bear the greatest burden of postharvest losses due to inadequate drying methods. One such innovation, the DryCard™, developed by Michael Reid and Jim Thompson from UC Davis, helps farmers assess moisture levels in stored crops.

Collaborating with private sector innovators, the Horticulture Innovation Lab has introduced low-cost, accessible drying solutions like the chimney dryer and pallet dryer, creating an efficient dry chain that reduces moisture levels. These innovations are detailed below.

Solar Chimney Dryer: The chimney solar dryer is a great innovation in the dry chain, utilizing solar heat collection and rapid airflow for quick and efficient product drying. This safe, clean drying method minimizes postharvest losses. Made from locally available materials such as wood, tarp and greenhouse plastic, it is affordable and adaptable across various climates. Implemented in Ghana, Guinea, Tanzania, Bangladesh, Nepal, Guinea Bissau, Guatemala, Honduras, and the United States, the technology can easily be scaled through farmer cooperatives and research centers, fostering long-term sustainability.

Pallet Dryer:  Unlike the chimney solar dryer, the pallet dryer is an active, indirect solar dryer that uses a fan to pull out the excess heat out of  the product, allowing air circulation and reducing drying time. With a 90-kilogram capacity for products like maize, rice, coffee etc., it ensures efficient drying while preventing over-drying that may cause cracking. Drying time varies based on sunlight intensity and weather conditions. In 2024, the Horticulture Innovation Lab, led by Project Manager Kristen Becker and UC Davis Professor Dr. Erwin built solar chimney dryer and pallet dryers at Universidad del Valle de in Solola, Guatemala to test their adaptability and effectiveness. It serves as a learning center to both researchers and the community.

DryCard™: A Critical Dry Chain Tool

The DryCard™ : As a low-cost technology, it works with dried grains, cereals, pulses, fruits, and vegetables.  As all hygrometers work to determine the level of relative humidity (RH) in the product, the DryCard™ uses a cobalt chloride humidity indicator strip that changes color based on relative humidity. Users compare the strip's color against an RH scale to determine whether a commodity is dry enough for safe storage.

If the indicator strip turns pink, the product requires further drying or immediate consumption. If the color of the indicator is blue, the product is dry and ready for storage. By ensuring commodities are sufficiently dried before storage, the DryCard™ plays a crucial role in reducing food spoilage and postharvest losses.

Best Practices for the Dry Chain Process

While sun drying remains widely used, innovations such as pallet and chimney solar dryers enhance drying efficiency and protect commodities from contamination. Once a commodity is dried, small-scale farmers can use tools like the DryCard™ or a hygrometer to verify moisture levels before storage. Proper packaging in moisture-proof, hermetic storage materials, such as Purdue Improved Crop Storage (PICS) bags or sealed metal bins, further preserves quality. In humid climates, desiccant-based dryers provide additional drying support for seeds before storage.

Scaling DryCard™ and the Dry Chain

The Horticulture Innovation Lab actively promotes dry chain technologies worldwide. Over the past four years, it has conducted DryCard™ and dry chain workshops, collaborating with stakeholders from government, private sector, academia, NGOs, and research institutions.

The Lab offers training programs both onsite and online. Over the past six years, more than 800 people have received dry chain training, and over 25,000 DryCard™ pieces have been distributed globally. The Lab’s DryCard™ entrepreneurship model enables private business owners to produce and sell the technology affordably while earning a profit. Currently, ten DryCard™ entrepreneurs operate globally, providing training and promoting waterproof hermetic bags. Active regions include Nepal, Bangladesh, Kenya, Uganda, Nigeria, Tanzania, Niger, Guinea Bissau, and Ghana.

The Lab continues to provide DryCard™ resources, including brochures, construction manuals for chimney and pellet dryers, and materials relating to climate-smart agriculture.  

The Promising Future of DryCard™

In 2024, the Horticulture Innovation Lab conducted a study to assess scaling pathways for the DryCard™, with a focus on reducing food safety risks associated with improper drying. The findings indicate a strong market demand, emphasizing the need for sustained awareness campaigns to improve food safety. Widespread adoption of DryCard™ technology can enhance food security and improve livelihoods by reducing postharvest losses.

Demonstration Centers: Showcasing Innovations

To raise awareness, the Horticulture Innovation Lab has established a Demonstration Center at University of California, Davis that serves as a learning hub for students, local communities, and international visitors. This center offers hands-on experience with low-cost drying innovations and reinforces the importance of the dry chain. Currently, there are additional demonstration centers in other regions including Niger, and Maynmar.

Interested in learning more about dry chain innovations? Visit the Horticulture Innovation Lab for additional resources.