Light modulation through spectrally selective materials in solar dryers for the retention of bio-compounds

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In this work, we explore the requirements for preserving natural pigments such as anthocyanins, carotenoids, and chlorophylls after solar drying. We focus primarily on how to modulate lighting conditions inside the drying chamber of solar dryers using spectrally selective glazing materials: Green material (G), red material (R), and a nonmetallic plasmon (Cu2-xSe coated polycarbonate, C). These materials modulated the solar irradiance intensity in the spectral range: 450–600, 600–1050, and 450–1050 nm, respectively. This affected to different extents the stability and consequent retention of bio-compounds contained in the food. The changes in the optical absorption spectrum during the drying process of strawberry and cilantro organic extracts (OE) were analyzed. Glycosylation ratio (GR) and acylation ratio (AR) were the chosen variables in the experiments with strawberry OE. Chlorophylls and total carotenoid concentration were the variables chosen in the experiments with cilantro OE. The maximum temperature, 66 °C, was reached using C material, which allows for light transmission in the green to the near-infrared (NIR) spectral interval. The absorption spectrum of the natural pigments evolved over time, influenced by each treatment: G, R, C and open sun drying S. GR and AR increased with the drying time in all the treatments. Better stability of the pigments was observed in strawberry OE in solar drying under G, R, and C covering materials compared with S. A greater retention of chlorophylls and carotenes in cilantro OE was obtained using G and C covering materials. Thus, modulation of light is desirable to improve the retention of bioactive compounds during food drying.

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