Maturity-driven changes in radio frequency-vacuum drying of kiwifruit: Mechanistic insights from dielectric properties and microstructure evolution.

Postharvest maturity critically influences the physicochemical properties and processing adaptability of kiwifruit. However, its specific impact on the behavior of radio-frequency vacuum drying (RFVD) remains poorly understood. The evolution of water state distribution, dielectric and thermal properties, drying behavior, and dried product quality of 'Xuxiang' kiwifruit at five postharvest maturity stages (S1-S5) was systematically investigated. Results showed that increasing maturity (S1-S5) induced a gradual transformation of bound and immobilized water toward free water, accompanied by cell wall relaxation and tissue degradation. Compared with S1, S2-S4 samples exhibited relatively higher dielectric loss factors under intermediate moisture contents, together with favorable specific heat capacity, which may contribute to improved radio frequency (RF) heating performance. Meanwhile, S2-S4 samples exhibited relatively shorter drying times (6.83-7.67 h) and lower total energy consumption (22.18-22.94 kW·h). Microstructural observations revealed that S4 developed a continuous and porous structure, which facilitated moisture migration (Deff = 9.80 × 10-10 ± 0.00 m2/s) and improved rehydration capacity (2.48 ± 0.03). Additionally, samples at S2-S4 exhibited lower volume shrinkage (73.37-74.63%), while higher maturity stages (S4-S5) significantly reduced hardness and chewiness (P < 0.05) and raised the sugar-acid ratio (7.70-8.96). Regarding product quality, total phenolics, flavonoids, vitamin C content, and antioxidant activity followed a decreasing-then-increasing trend with maturity. Overall, these findings elucidated the maturity-driven mechanisms underlying RFVD behavior in kiwifruit and provided a theoretical basis for optimizing maturity selection to achieve both drying efficiency and quality retention.