Nutritional evaluation of beetroot dried using novel thermal storage-based passive solar dryers

Nutritional Evaluation of Solar-Dried Beetroot

Abstract

This study investigates the thermal performance and drying efficiency of an indirect passive solar dryer (IPSD) integrated with three thermal energy storage (TES) materials-river pebbles, grit, and recycled metal bottle caps-for dehydration of beetroot (Beta vulgaris) slices of 2, 4, and 6 mm thickness under semi-arid climatic conditions in Gujarat, India. The IPSD consisted of a 1.5 m² blackened aluminium absorber plate covered with 4 mm toughened glass glazing and inclined at 25°. TES materials were arranged in a 3 cm thick bed, exhibiting thermal conductivities of 0.22 W m⁻¹ K⁻¹ (pebbles), 1.5 W m⁻¹ K⁻¹ (grit), and 160 W m⁻¹ K⁻¹ (metal bottle caps). Thermal analysis revealed that the metal bottle cap configuration achieved the best overall performance, attaining a peak heat recovery ratio of 161.24% during the discharging phase and the highest average charging-phase thermal efficiency of 59.70%, outperforming grit (36.32%) and pebbles (42.26%). Although grit recorded the highest peak collector temperature (112 °C), metal bottle caps sustained the greatest rise in outlet air temperature (up to 45 °C above ambient), ensuring stable drying conditions. Drying experiments showed that metal bottle caps reduced moisture content to 5.59% for 2 mm slices in the shortest drying duration while minimising nutrient losses. Proximate analysis indicated enhanced retention of protein (10.17%), ascorbic acid (10.23 mg/100 g), and total phenolics (128 mg/100 g) compared to open sun drying. A 4 mm slice thickness provided an optimal balance between drying rate and quality retention. The study establishes recycled metal bottle caps as an efficient, low-cost, and sustainable TES material for passive solar drying applications.