Effects of isochoric assisted drying and nanocomposite packaging on the physicochemical, mechanical, and microbial quality of Hibiscus sabdariffa

Fresh Hibiscus sabdariffa calyces are highly susceptible to quality deterioration because of their high moisture content, while conventional drying can accelerate losses of nutritionally important constituents. This study developed an integrated preservation strategy combining isochoric-assisted pretreatment with yam starch–potato peel nanoparticle nanocomposite packaging and evaluated film functionality and dried H. sabdariffa quality. A 4.5-L Grade 316 stainless-steel isochoric system was operated at − 2.5, − 5.0, and − 7.5 °C for 5–15 min. Nanocomposite films of 8 and 10 μm were solution-cast and characterized for microstructure, mechanical properties, and thermal behaviour, while pretreated calyces were packaged, freeze-dried, and evaluated for microbial quality and vitamin C retention. Results showed that the 10 μm film exhibited higher tensile strength (0.322 ± 0.018 kN) but lower elongation (0.897 ± 0.095 mm) than the 8 μm film ( p < 0.05), indicating greater mechanical resistance. Microstructural analysis revealed a heterogeneous but relatively compact matrix with embedded and partially protruding fibrous structures, localized microvoids, and interfacial discontinuities, indicating heterogeneous nanoparticle distribution. The nanocomposite exhibited a lower glass-transition onset (38.39 versus 53.51 °C) but higher melting peak (110.53 versus 103.24 °C) than the control, indicating altered thermal behaviour. Isochoric pretreatment significantly reduced total viable counts, coliforms, and fungal populations compared with fresh calyces ( p < 0.05), demonstrating improved microbial stability. Vitamin C retention ranged from 9.65 to 144.03 mg/100 g, revealing treatment-dependent differences in nutrient preservation and emphasizing the need for process optimization. Overall, the integrated approach provides a bio-based strategy for improving microbial stability, packaging functionality, and nutritional quality of dried H. sabdariffa .