Microwave drying of garlic ( Allium sativum L.) leaves: kinetics modelling and changes in phenolic compounds profile

Garlic ( Allium sativum L.) leaves, or green garlic characterized by a mild flavour, were a rich source of biologically active compounds as was found in garlic cloves. Influence of microwave drying (100–450 W) on the color, phenolics profile and radical scavenging activity of garlic leaves was studied. Effect of microwave power density level (W/g DM) on drying kinetics modelling was also investigated. Page’s model was found to be the best for fitting the experimental drying kinetics. Specific energy consumption increased with increasing microwave power from 80 to 137 MJ/kg of water at 100 and 450 W, respectively, whereas the energy efficiency decreased. Drying at 450 W produced a very distinct product because the sample’s ΔE value (24.87) was higher than those dried at lower microwave powers (ΔE = 14.66–16.86). Analysis by liquid chromatography coupled with mass spectrometry of garlic extracts resulted in identification of 5 phenolic acids and 11 flavonoids. Total identified phenolics in the fresh leaves was 1.28 mg/g extract, which increased for dried leaves at 100 W (5.6 mg/g extract), then gradually decreased to 2.7 mg/g extract at 450 W. Quercetin 3-O-rhamnoside was the main flavonoid extracted with 4 mg/g extract for the sample dried at 100 W. Drying garlic leaves at 100 W seems to be better in terms of lowest specific energy consumption and maximum phenolics extraction yield (12.45 g extract/100 g DM) and thereby enhanced antioxidant activity as evaluated by DPPH• radical-scavenging activity. Graphical Abstract