Valorisation of Leek Processing Residues Through Microwave Pretreatment to Produce Functional Powdered Ingredients

The transformation of vegetable wastes into powders allows for their integral valorisation by applying three basic stages: grinding, dehydration and milling. Additionally, pretreatments can be applied to improve process efficiency and modify the properties of the powders obtained. In this study, the effect of microwave (MW) pretreatment on the physicochemical and antioxidant properties of leek processing residues was investigated. A total of 16 MW pretreatment conditions, combining four power levels (2, 4, 6, and 9 W/g) and four treatment times (2, 4, 6, and 8 min), were initially evaluated to determine their effects on the physicochemical (water activity, moisture content, and soluble solids) and antioxidant characteristics (total phenols, total flavonoids, reducing sugars, and antioxidant capacity assessed by DPPH and ABTS radical methods) of leek processing residues. Based on the results obtained, a treatment time of 4 min was selected for all MW power levels evaluated. The effects of these MW pretreatments, applied prior to hot-air drying (HAD), on drying kinetics at 70 °C and on the physicochemical, antioxidant, FTIR, and techno-functional properties of the resulting powders were subsequently evaluated. Results confirmed that MW pretreatment reduced drying time, improving process efficiency. MW also induced modifications in powder structure, particle size distribution, hydration-related properties, and antioxidant capacity. Moderate MW intensities (2–4 W/g for 4 min) produced powders with lower final moisture contents (0.5–0.6% vs. 1.7% in the untreated control) and improved hydration-related properties, increasing water-holding capacity by up to 17% and water-retention capacity by up to 13%. In contrast, higher MW intensities (6–9 W/g) reduced drying kinetic parameters and promoted smaller particle sizes and poorer wettability. Although untreated powders exhibited the highest final antioxidant values, the greatest increases in antioxidant-related properties during drying were generally observed in samples pretreated at 2 W/g. FTIR analysis suggested structural modifications of the plant matrix without major changes in the main functional groups. In conclusion, moderate MW intensities produced the most favourable combination between drying behaviour, antioxidant preservation and techno-functional performance, supporting the use of MW and HAD for the valorisation of leek processing residues.