Effect of different pretreatments and air temperatures on drying and quality characteristics of Sauropus androgynus Merr

Abstract

Sauropus androgynus Merr. is one of the widely consumed indigenous plants in Thailand due to its high nutritional value and antioxidant compounds. Naturally, as green leafy vegetable, the Sauropus androgynus Merr. in fresh form be easily deteriorate Dried powder form is an alternative means to apply this plant as healthy supplement ingredient to increase nutritional value in bakery and snack products. In this study, the effects of different pretreatments and air temperatures on drying and quality characteristics of Sauropus androgynus Merr leaves were investigated. The pretreatment conditions before drying included 1) rinse with water, 2) blanch with hot water at 100°C for 15 second, and 3) blanch with 28 kHz of ultrasonic wave at 25°C for 15 minute. The drying process was carried out in a tray dryer with 0.25 m/s of air velocity and followed by drying with hot air at temperature of 45°C, 55°C, and 65°C. The drying kinetics was modeled based on Newton and Page equations. The results showed that the ultrasonic pretreatment could improve drying rate of the Sauropus androgynus Merr. leaves significantly faster than did the hot water blanching and the water rinse pretreatment respectively. The Newton's model gave a better fit to the experimental result than the Page's model. The kinetic parameters in the models were also presented as the function of initial leaves moisture and air temperature. Qualities of the dried product, including final moisture content, water activity, color and total phenolic content, were analyzed. The highest amount of total phenolic content of 52.70 mg GAE/ g was found in the dried product obtained from the water rinse pretreatment and dried at 45°C. However, the results also indicated that the high amount of the total phenolic content could be achieved by using the ultrasonic blanching and drying at high temperature with shorter time.