Ultrasound-Assisted Alkaline Extraction of Proteins from Fresh and Hot Air-Dried Duckweed (Wolffia arrhiza) and Their Functional Properties

Thai duckweed (Wolffia arrhiza) is currently considered as one of the highly interesting plants for a source of nutritious proteins and novel functional ingredient. However, duckweed contains high moisture of up to 95%, a practical processing for production of the duckweed proteins (DP) is therefore challenging. In this study, ultrasonication was applied to assist alkaline extraction of proteins from fresh and hot-air dried duckweed. Effects of duckweed/water extraction ratio (1/6-1/20) and ultrasound-assisted extraction (UAE) on extraction yield, protein content and protein recovery of the DP were investigated. Besides, water holding capacity (WHC), oil holding capacity (OHC) and emulsifying properties of the DP were examined. Fresh duckweed was hot air-dried at 52±1 ∘C to obtain dried duckweed with a moisture content of 5.5±0.5%. The dried duckweed was ground to fine powder (∼250 µm). Alkaline extraction of DP was carried out at pH 8.5 and precipitated at pH 4.2, then freeze-dried to obtain dried DP. For sample prepared with UAE applied, the duckweed was ultrasonicated before the alkaline extraction. The results showed that UAE significantly improved the extraction yield, protein content and protein recovery. Increasing duckweed/water ratio did not affect extraction yield; but it helped facilitate protein extraction in alkaline, thereby increasing protein content and protein recovery of the DP. DP’s protein recovery produced from fresh duckweed was higher than that prepared from dried duckweed. UAE affected WHC, OHC and emulsifying properties of the DP. Fresh/hot air-dried duckweed had the effects on the WHC and OHC, but not on the emulsifying properties.