By Jörg Schemminger, Founder, SchemTech; Mohamadú Saido Baldé, Sustainable Energy Expert, Basel Agency for Sustainable Energy; Rebecca Botello, Sustainable Finance Specialist, Basel Agency for Sustainable Energy; and Simran Singh, Sustainable Finance Specialist, Basel Agency for Sustainable Energy

Beneath aggregate figures charting declining global hunger and progress toward SDG 2 (zero hunger) lingers a significant caveat: food insecurity continues to be on the rise in Africa. Projections for 2025-2030 suggest that even as undernourishment recedes globally, 60% of those still caught in the hunger trap will be in this region.

While postharvest loss is a critical lever for food security, it has long been overlooked in favor of a historical policy focus on increasing production. This imbalance is increasingly untenable. Between 20 and 50% of produce grown in sub-Saharan Africa, depending on the crop, is lost before it ever reaches the consumer – demonstrating that production volume is no longer the sole metric that matters. GAIN finds that reducing these postharvest losses can make nutrient-dense foods, particularly fruits and vegetables, more accessible and affordable, marking a necessary shift away from business-as-usual supply patterns that overburden land, labor, and natural resources while leaving persistent nutrient gaps.

While active cooling technologies often dominate discussions on postharvest loss, this article draws on the Your Virtual Cold Chain Assistant (Your VCCA) initiative’s work in Guinea-Bissau to illustrate a complementary path. Using experimental data and community feedback, we show that passive cooling can significantly extend crop shelf life while increasing uptake of improved postharvest storage practices. By fostering familiarity with cooling methods, such social adaptation enables farmers to more readily adopt complex technologies over time.

Agriculture in Guinea-Bissau

Although vegetables can theoretically be grown year-round, labor shifts heavily toward the cashew harvest from March to June, leaving horticulture neglected as the dry season peaks. This competition for labor, combined with limited water availability during the dry season, results in a pronounced “lean season,” marked by severe shortages of fruits and vegetables between June and October. During these months, Guinea-Bissau is forced to import staples such as tomatoes and onions from neighboring Senegal and Guinea, often at prices that strain household budgets. These pressures can be partially eased by preserving the abundance of the preceding months.

Traditional storage practices in Guinea-Bissau are largely limited to shaded or lightly ventilated structures, offering little protection from extreme ambient heat, but high-tech cold chain alternatives remain out of reach for most farmers. In light of these constraints, Your VCCA’s post-harvest assessment identifies low-cost passive evaporative cooling, such as clay pot (Zeer pot) coolers, as a viable improvement pathway, given their affordability, local manufacturability, and lack of electricity requirements.

Although passive cooling cannot fully bridge the lean season, it provides a vital buffer that supports household nutrition and modest economic gains by enabling safer storage for self-consumption and short-term local sale. By operating at the household level, passive cooling also helps establish improved post-harvest routines, allowing farmers to build familiarity with cooling practices before more complex and capital-intensive technologies are introduced.

Performance analysis

Validation of the clay pot coolers began in St Gallen, Switzerland, where controlled trials were conducted to establish a performance baseline. The setup consisted of two concentric clay pots separated by a 2-3 centimeter water-wetted sand layer and sealed with a clay lid, with five 0.5 liter water bottles placed inside to simulate the thermal mass of a fully loaded unit. These laboratory trials were carried out in a temperature-controlled indoor environment with ambient temperatures of 21-23°C and relative humidity levels of 40-60%. Building on these results, field trials were then undertaken in Bafatá, Guinea-Bissau, during the 2025 dry season (January-February), using locally available materials and installing the clay pot coolers in a shaded, well-ventilated residential vestibule to reflect typical user conditions.

The performance of the clay pot cooler is driven by evaporative cooling, in which water evaporating from the moist sand layer through the porous vessel walls extracts heat from the interior. Field trials in Bafatá showed that the system consistently maintained internal temperatures 3-5°C below ambient levels, with peak performance during the dry season when low relative humidity accelerates evaporation; during the rainy season, higher humidity reduced the cooling effect. In addition to lowering temperature, the cooler maintained internal relative humidity at approximately 95%, substantially reducing moisture loss from stored produce and helping preserve weight and visual quality. The combined effect of high humidity and the thermal mass of the clay and water-wetted sand also dampened extreme mid-day temperature spikes, protecting produce from heat stress.

Under fluctuating field temperatures of 28-34°C, reference tomatoes stored in open baskets spoiled almost entirely within seven days, while those stored in the clay pot cooler remained largely saleable for 10-12 days (view graph here). Similar gains were observed across other crops: lettuce storage extended from one day to three, and okra storage time was nearly tripled compared to open-basket storage.

Community-led validation

To gauge community interest and willingness to adopt the solution, the Your VCCA team conducted hands-on demonstrations with Farmer Clubs across Bafatá, Gabú, Quinara, and Bolama, focusing on how the coolers could be constructed using locally available materials.

During these sessions, communities were not only shown how to assemble the structures but were also presented with a compelling live comparison. Tomatoes and eggplants transported across the country were displayed side by side, with half stored in a clay pot cooler and half in a traditional open basket. The contrast was immediately apparent: produce in the open basket showed wilting, bruising, and early spoilage, while produce stored in the clay pot remained visibly plump and fresh.

To foster local ownership, each Farmer Club was invited to identify an “Engineer of the Community,” a member with an interest in experimentation or technical tinkering, to take the lead on the technology. These individuals were equipped with basic sensors to track temperature differential inside and outside the pots and to share their observations with peers, reinforcing trust in the results through locally generated evidence.

Over the course of 2025, Your VCCA implemented capacity-bridging programs in which each participant constructed their own clay pot cooler, helping move passive cooling from a concept to a widespread practice. These efforts were supported by training materials developed by CoolVeg. In total, the training sessions reached 490 women and 174 men.

Planning cooling interventions

The experience in Guinea-Bissau shows that passive cooling is not merely an interim or “low-tech” measure, but a foundational element of resilient food systems where infrastructure, energy access, and climate stability cannot be assumed. Its impact is greatest during the dry season, when low relative humidity maximizes evaporative cooling efficiency, improving household nutrition, reducing post-harvest loss, and extending short windows of abundance into more stable food and income outcomes.

For policymakers, the significance of these findings lies in sequencing rather than substitution – introducing passive cooling as a first step that informs whether, where, and when more energy-intensive active cooling is required. Embedding passive cooling within agricultural extension services, the design of shaded and ventilated market facilities, and nutrition-sensitive public procurement programs offers an immediately actionable pathway to strengthen nutrition outcomes. It also enables ongoing assessment of postharvest needs, given current and targeted production levels, target markets, and other contextual factors that have been studied in works by Wageningen Food & Biobased Research and Empa. This approach can help reduce the risk of premature investment in active cooling and ensure that cold rooms are deployed where they are expected to add value while minimizing the risk of becoming stranded assets.