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The influence of various drying methods, namely hot-air drying (HAD), heat pump drying (HPD), freeze-drying (FD), and microwave freeze drying (MFD), on the structural characterization and browning substrates of daylily was investigated. The chemical structure as determined by Fourier Transform infrared spectroscopy of the products from different drying methods were similar and the microphysical structure (microstructures and texture) were significantly different. High-quality daylily products with a bright appearance and a lower degree of browning were produced by FD and MFD. Higher contents of browning substrates (phenolic acid, total flavonoids, and total phenols) were found in products with lower browning (color, browning degree and Maillard fluorescent compounds). FD products kept the highest contents of gallic acid (27.39 mg/100 g), chlorogenic acid (35.92 mg/100 g), and quercetin (15.03 mg/100 g), and had a high nitrite scavenging activity. MFD products kept the highest contents of catechin (144.90 mg/100 g), rutin (71.5 mg/100 g), and kaempferol (33.87 mg/100 g). The results showed that the final products produced by FD and MFD had superior quality than those produced with HAD and HPD. FD and MFD are suitable for daylily powder with high nutritional value and HAD and HPD are suitable for large-scale production and affordable dried daylily products.
Chu et al. (Thu,) studied this question.