Optimizing crispness and nutrient retention in carrot snacks: Multi-stage drying via microwave-infrared and negative pressure puffing with moisture-dependent transition

This study aims to optimize the crispness and nutrient retention of ready-to-eat carrot snacks by investigating a multi-stage combined drying process of microwave-infrared vibrating bed drying (MIVBD) and pulse-spouted microwave vacuum drying (PSMVD). Four groups of combined drying experiments were designed with moisture content as the process transition point (30 %, 40 %, 50 %, and 60 % w.b.)—denoted as MI → PS-X, where MI refers to MIVBD, PS refers to PSMVD, and X represents the threshold (e.g., 0.5 for 50 % w.b.)—and compared with single drying processes to systematically analyze drying characteristics, physical properties (hardness, brittleness, color, microstructure, rehydration rate), nutritional components (total phenolics, carotenoids), and antioxidant activity. The results showed that MIVBD had the highest drying efficiency (80 ± 2.13 min), while PSMVD formed a porous structure through negative pressure puffing, resulting in the best brittleness (31.18 ± 1.11 peak numbers). When the critical moisture content was 50–60 %, the drying time of combined drying was 21.5–25.4 % shorter than that of PSMVD, and the brittleness (24.98–27.73 peaks) was close to that of single PSMVD. The MI → PS-0.5 group had the highest rehydration rate and moderate hardness (1602.52 g) due to its honeycomb-like porous structure with moderate wall thickness. PSMVD retained carotenoids best (26 % loss), while the MI → PS-0.6 group exhibited the highest antioxidant activity (DPPH scavenging rate 0.53 mg TE/g) due to the controlled balance between phenolic retention and Maillard reaction product. Scanning electron microscopy and low-field nuclear magnetic resonance analysis showed that combined drying at higher critical moisture content (≥50 %) could form well-connected “sponge-like” pore structures, improving crispness and rehydration speed, but moisture-proof packaging was required. Considering drying efficiency, quality, and nutrient retention, the MI → PS-0.5 process provides a new path for the industrial production of ready-to-eat vegetable snacks through a segmented mechanism of “rapid dehydration-structural expansion”.