Development of solar tunnel dryer for drying of seasonal fruits in rural areas

Introduction

Why do rural growers continue to lose almost a third of their seasonal fruit harvest year after year? Part of the answer lies in the mismatch between glut-season production and the absence of drying infrastructure accessible to small holdings [1] . Mangoes, bananas, guavas and papayas mature in waves during a short window, and without rapid moisture removal they enter rapid spoilage within 72-96 hours of harvest [2,3] . Open-sun drying, the default village practice, exposes produce to dust, insects and unpredictable showers, often ending with browning and microbial contamination [4] , while mechanical cabinet dryers remain beyond the reach of most smallholders on capital cost [5] . Solar tunnel dryers sit between these two extremes. By enclosing the product inside a transparent plastic tunnel, they raise chamber temperatures to 50-65 °C during peak radiation hours, while a photovoltaic-driven fan prevents the chamber from becoming humid and stagnant [6] . Reports from Kenya, Germany and Thailand have shown drying times cut by 30-55% and better colour, aroma and vitamin retention compared with open drying [7,8] . Yet most published designs have been tested on large commercial farms; the rural Kenyan context is only thinly represented [9,10] . A locally fabricated, low-cost tunnel dryer that uses materials available in county-level markets could bridge this gap. The present research was therefore undertaken to design, fabricate and evaluate such a unit for three widely grown seasonal fruits. Specific objectives were: 1) To determine the moisture reduction pattern under three drying methods, 2) To compare key quality indicators in the dried product, and 3) To work out the economic feasibility for a representative rural user.

Theoretical background

Drying is a coupled heat-and-mass-transfer process in which moisture migrates from the interior of a hygroscopic material to its surface and evaporates into the surrounding air. In a solar tunnel dryer, the useful thermal energy is given by Q = η × Ir × A, where η is the International Journal of Agriculture and Food Science https://www.agriculturaljournals.com overall collector efficiency, Ir is incident radiation, and A is the effective aperture area [11] . Page's thin-layer model, MR = exp(-k•tⁿ), was adopted here to describe moisture ratio changes during the falling-rate period [12] .

Material and Methods

Material

The research was carried out at the post-harvest yard of Rift Valley College of Food Technology, Nakuru (0.28° S, 36.07° E, altitude 1850 m) in collaboration with Lake Victoria College of Food Science and Technology, Kisumu, between March 2023 and June 2024. Three fresh fruits were sourced from the local market: mango (Apple), banana (Grand Naine) and guava (White). Fruits were hand-selected for uniform maturity and processed within 6 hours of purchase. The tunnel dryer frame was fabricated from 25 mm galvanised iron pipes covered with 200 µm UVstabilised polyethylene sheet procured from a greenhouse supplier in Nairobi. A 12 V DC axial fan (92 mm) was coupled to a 20 W polycrystalline solar panel and a 7 Ah battery. Measurements were taken using a calibrated digital moisture analyser (MA-150, ±0.01%), Type-K thermocouples (±0.5 °C), a pyranometer, and a chroma meter (CR-400).

Methods

Fruits were washed in chlorinated water (50 ppm), peeled where relevant, and sliced to uniform thickness (mango 5 mm, banana 4 mm, guava 6 mm). Pre-treatment consisted of a 2% potassium metabisulphite dip for 10 minutes followed by hot-water blanching at 85 °C for 2 minutes. Samples of 4 kg per batch were placed on trays at a load density of 3.2 kg/m². Three drying methods were compared in parallel: Solar Tunnel Dryer (STD), Cabinet Dryer (CD) at 55 °C, and Open-Sun Drying (OSD). Moisture loss was recorded every 2 hours. Ascorbic acid was estimated by the 2,6dichlorophenol-indophenol method; rehydration ratio by the standard AOAC protocol; and microbial counts by pourplate on PCA and PDA [13] . Each treatment was replicated three times and data analysed by two-way ANOVA using SPSS v26, with mean separation by Duncan's multiple range test (p<0.05).

Results

Peak tunnel temperatures ranged between 52.4 °C and 62.7 °C, while open-sun surface temperatures rarely crossed 41 °C. Solar radiation during the trial period averaged 673 W/m² between 10:00 and 16:00 hours. Drying and quality outcomes across the three methods are summarised in Table 1. Moisture removal was most rapid in the first 8 hours for all three methods; beyond that, open-sun drying entered a clear stagnation phase as afternoon humidity rose. Figure 1 presents the process flow of the tunnel dryer from fresh fruit input through to the final dried product. The moisture reduction profile over the 18-hour drying period (mango slices) is shown in Figure 2, plotted for all three methods together with the drying-rate curve of the tunnel dryer. Taken together, the tables and figures give a consistent picture. The tunnel dryer delivers faster moisture removal and holds the product in a narrow, controlled temperature band that protects colour pigments and heat-sensitive vitamins. Sensory panel scores on a 9-point hedonic scale averaged 7.8 for STD mango against 5.5 for OSD mango, with STD banana at 7.6 and STD guava at 7.2, confirming the instrumental quality data. Open-sun drying ends with a product that is darker, lower in ascorbic acid, has poor rehydration behaviour, and carries a microbial load close to the spoilage threshold.

Discussion

The 38-46% reduction in drying time achieved by the tunnel dryer reflects its ability to hold chamber air at 15-22 °C above ambient while forced convection continuously flushes out moisture-laden vapour. Similar gains (35-50%) were reported by Ssemwanga and co-workers for tomato slices in Uganda and by Sontakke and Salve for banana figs in western Kenya, pointing to a design principle that travels well across geographies [14,15] . The higher ascorbic acid retention (81.2% in the tunnel dryer against 46.8% in open-sun drying) is explained by shorter residence time, which limits oxidative losses, and the absence of direct ultraviolet exposure, which accelerates photo-degradation of vitamin C [16] . Colour retention follows the same logic. The L* values of mango slices in the tunnel dryer remained close to the fresh sample because enzymatic browning is largely inactivated once tissue temperature clears 52-55 °C, and non-enzymatic browning is held in check by the short drying window. At a practical level, the choice of locally available materials GI pipe, UV polyethylene, wire mesh keeps capital costs within reach of farmer groups and small rural entrepreneurs, opening a path to batch drying of off-season vegetables and direct access to urban niche markets.

Cost analysis

Total fabrication cost of the 4.2 m × 1.6 m tunnel dryer worked out to KSh 48,500, made up of GI pipe (KSh 15,600), polyethylene cover (KSh 8,100), fan and wiring (KSh 3,600), solar panel and battery (KSh 9,800), drying trays (KSh 7,100) and fabrication labour (KSh 4,300). At 20 drying batches in a mango season, yield is 14-16 kg of dried mango sold at KSh 450-520 per kg. Adding similar gains from banana and guava seasons, gross annual revenue potential is KSh 65,400. The benefit-cost ratio comes to 1.87 with payback in 18-21 months.

Conclusion

This research set out to design, fabricate and evaluate a lowcost solar tunnel dryer for three widely grown seasonal fruits under conditions reflecting a small rural holding. The fabricated unit met every one of those objectives: drying times were cut by 38-46% relative to the open-sun method; final moisture fell to safe storage levels of 11.4-13.7% against 28-31% in open drying; and quality indicators colour, aroma, taste and vitamin retention stayed appreciably closer to the fresh produce benchmark. A tunnel dryer of this scale, built from materials already available at district markets, can be assembled for roughly KSh 47,000-51,000, recovers its cost within the second season, and operates without drawing on grid electricity. Follow-up work should expand trials to leafy vegetables and medicinal plants, test variable-speed fan controllers to push efficiency beyond 31.8%, and scale the design to a community-level model serving a cluster of 20-30 households the most promising route for rural adoption.

Acknowledgement

Acknowledgements Funding sourcesThe authors acknowledge the financial support received from internal research development grants available to faculty members during the project period. No external funding body was involved. Institutional supportThe authors gratefully acknowledge the infrastructural support provided by the Department of Post-Harvest Management, Rift Valley College of Food Technology, Nakuru, and the Department of Food Science, Lake Victoria College of Food Science and Technology, Kisumu.

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