Non-monotonic regulation of kelp quality by relative humidity during hot-air drying: Cascade effects linking moisture migration-microstructure-macroscopic quality

Kelp, a nutrient-dense marine vegetable, necessitating immediate postharvest dehydration due to its exceptionally high moisture content. While industrial hot-air drying prioritizes temperature optimization, the quality-modulating role of relative humidity (RH) remains insufficiently characterized. This study systematically investigated RH effects (20-60%) on drying kinetics and sensory-nutritional quality of kelp through multiscale analysis. Key findings reveal: (1) a transition of moisture migration pattern from inverted-V biphasic to S-shaped triphasic above RH40%; (2) the non-monotonic moisture migration pattern governs the graded structural evolution of cell walls, transitioning from lamellar delamination (RH >40%) to uniform contraction (RH 40%) and ultimately to shriveled fracture (RH <40%); (3) an inverted-V trend in physicochemical properties, with non-monotonic nutrient retention peaking at RH40%; (4) monotonic textural modification, where 10% RH increments increased viscoelasticity/chewiness by 24.7%/6.2% but reduced hardness/brittleness by 5.5%/5.6%. Surprisingly, low RH (20%) significantly enhanced umami constituents (163.6% and 127.7% increases in glutamate and aspartate, respectively) while concurrently inhibited undesirable flavor compounds (8.56% and 32.01% reductions in aldehydes and ketones, respectively). These findings establish RH as a critical parameter for simultaneous quality and efficiency optimization in kelp processing. • RH drives drying kinetics shift from biphasic to triphasic moisture migration. • RH40% yields gradient porous structures resembling fresh kelp. • Nutrient retention peaks at RH40% and texture hardens monotonically. • RH20% boosts umami (Glu and Asp) and reduces off-flavors (aldehydes and ketones). • Moisture-microstructure-quality cascade mechanism is revealed.