Summary
In this study, drying of red‐fleshed dragon fruit at different temperatures (40, 50, 60, 70 and 80 °C) and air velocities (1.0 and 1.5 m s−1) was conducted under the specific humidity of 25 g H2O kg−1 dry air. The results showed that drying at higher temperatures resulted in shorter drying times. However, temperature and air velocity did not significantly affect the total betacyanin contents and antioxidant capacities in dried products. Seven betacyanins identified by LC‐MS were betanin, isobetanin, phyllocactin, isophyllocactin, betanidin 5 –O‐(6′O‐3‐hydroxybutyryl)‐β‐glucoside, isobetanidin 5 –O‐(6′O‐3‐hydroxybutyryl)‐β‐glucoside and decarboxylated phyllocactin. In addition, the temperature increased the isomerisation of betacyanins. The most preferable condition for preserving the colour and betacyanin contents of red‐fleshed dragon fruit was the drying at a temperature of 80 °C and air velocity of 1.5 m s−1 since it could shorten the drying time and give a bright red colour to the dried product as well as it had no significant impact on the betacyanins.
Dried turmeric is used as a spice and traditional medicine. The common drying methods for turmeric (Curcuma longa L.) are sun drying and solar drying. In this study, turmeric slices with a thickness of 2 mm were dried at 40, 50, 60, and 70 °C in a laboratory hot-air dryer with a simulated solar radiation applied through transparent polycarbonate cover (UV impermeable) and PMMA cover (UV permeable). Air velocity and relative humidity of drying air were fixed at 1.0 M·s−1 and 25 g H2O kg−1 dry air, respectively. Light significantly increased the sample temperature under both covers. Page was the best model to predict the drying characteristics of turmeric slices. Drying rate correlated with the effective moisture diffusivity, which increased at higher temperature. The hue angle (h°) of turmeric was distinctly lower at 70 °C under both covers. The dried products were of intensive orange color. Curcumin, demethoxycurcumin, and total curcuminoids were affected by the cumulated thermal load (CTL). The lowest curcumin content was found at 40 °C under PMMA (highest CTL). The optimum drying condition was 70 °C under polycarbonate cover due to shorter drying time and better preservation of color and curcuminoids in the dried product.
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