Process optimization of microwave dried purple sweet potato extract and its stability in aqueous system

Purple sweet potato anthocyanin is a potential natural food colorant with health beneficial bioactive compound. Encapsulation of bioactive compound by microwave drying technology could assist in limiting the loss in food. In this study, the process of microwave drying of purple sweet potato extract (PSPE) was studied. Response surface methodology (RSM) was used to optimise the microwave drying power (330 to 770 W), drying time (215 to 375 s) and maltodextrin concentration (5 to 35 %) for the process of microwave drying of PSPE. The optimised condition obtained with desirability 0.915 was at microwave power 550 W, drying time 298 s and maltodextrin concentration 19.42 %. Validation of optimum process parameter microwave purple sweet potato (MD-PSPE) resulted in an actual value of antioxidant capacity (DPPH), monomeric anthocyanin content (MAC), and moisture content of 79.92 ± 0.8 %, 498.09 ± 5.18 mg/L and 4.56 ± 0.06 % respectively. The errors obtained were between 6.12 – 8.50 % lower than expected value. The stability of MD-PSPE during 10 weeks of storage was evaluated and the stability followed first-order kinetic reaction. After 10 weeks of storage at temperature 26 ± 2 °C, the anthocyanin content of MD-PSPE successfully retain 95% of MAC compared to 65 % MAC retention in the nonencapsulated PSPE. Additionally, the stability of MD-PSPE applied in aqueous system towards thermal process (70 to 90 °C) and storage at temperature (26 ± 2 °C) was evaluated. Anthocyanin degradation towards thermal processing and storage was following first-order kinetic reaction. Throughout heating, MAC of MD-PSPE in aqueous system showed a higher stability at 70 °C with half-life ??1/2 14.4 weeks. MDPSPE in aqueous system (pH 2.0 to 6.0) during storage (26 ± 2 °C) had been successfully retained more than 50 % of MAC. MD-PSPE in aqueous was most stable at pH 3.0-4.0 with half-life ??1/2 20.1 to 21.1 weeks. Hence, MD-PSPE produced is suitable for the application into low pH beverages system