Effect of microwave freeze drying on quality and energy supply in drying of barley grass

BACKGROUNDYoung barley grass leaves are well-known for containing the antioxidant substances flavonoid and chlorophyll. However, low product quality and energy efficiency exist with respect to the dehydration of barley grass leaves. To improve energy supply and the quality of barley grass, microwave heating instead of contact heat was applied for the freeze drying of barley grass at a pilot scale at 1, 1.5 and 2W g(-1), respectively; After drying, energy supply and quality parameters of color, moisture content, chlorophyll, flavonoids, odors of dried barley grass were determined to evaluate the feasibility of the study. RESULTSMicrowave freeze drying (MFD) allowed a low energy supply and high contents of chlorophyll and flavonoids. A lightness value of 60.0, a green value of -11.5 and an energy supply of 0.61kW h(-1)g(-1) were observed in 1.5W g(-1) MFD; whereas drying time (7h) decreased by 42% compared to contact heating. Maximum content of flavonoid and chlorophyll was 11.7 and 12.8g kg(-1) barley grass. Microwave heating leads to an odor change larger than that for contact heating observed for the freeze drying of barley grass. CONCLUSIONMFD retains chlorophyll and flavonoids, as well as colors and odors of samples, and also decreases energy consumption in the freeze drying of barley grass. (c) 2017 Society of Chemical Industry