Effects of Different Drying Methods on the Quality of Broccoli Stems and Leaves

In order to investigate and evaluate the processibility of broccoli by-products (stems and leaves) and hence to provide a reference for their rational utilization, this study investigated the physicochemical properties and nutritional quality of broccoli stem and leaf powders prepared by different drying methods, namely vacuum freeze drying (FD), microwave freeze drying (MFD), heat pump drying (HPD) and hot air drying (HAD). The results showed that the color difference ΔE between MFD-treated and fresh broccoli stems was 8.52 ± 0.02, which was smaller than that between HAD-treated and fresh samples (30.27 ± 0.28), indicating that MFD had better color preservation effect. The median particle size ranges of broccoli stem and leaf powders were 31.19–52.09 and 32.30–40.47 μm, respectively. Broccoli leaf powder had lower water and oil retention capacities but higher swelling force than that broccoli stem powder. The water retention capacity of FD-dried broccoli stem powder was (11.40 ± 0.46) g/g, and the oil retention capacity of MFD-dried broccoli stem powder was (1.40 ± 0.04) g/g. FD and MFD could effectively maintain the microstructure of broccoli stems and leaves, with obvious pores. The specific energy consumption of FD and MFD for treating an equal amount of broccoli stems was 7.35 and 3.26 kW·h/kg, respectively, so MFD reduced energy consumption by 55.65% when compared with FD. MFD maintained heat-sensitive and readily oxidized nutrients well, resulting in good antioxidant activity. The 1,1-diphenyl-2-picryl hydrazine (DPPH) scavenging capacity, 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) cationic radical scavenging capacity and ferric ion reducing antioxidant power (FRAP) of MFD broccoli leaf extract at a concentration of 0.125 mg/100 mL were 70.21%, 71.11% and (0.31 ± 0.01) mmol/g, respectively. In summary, broccoli stem and leaf powders had good physicochemical properties and nutritional quality and therefore could be used as raw materials for developing functional foods. The application of drying technology enables the rational utilization of broccoli stems and leaves.