Humidity-Responsive Starch Foam Pads for Humidity-triggered Release of 1-Methylcyclopropene in Litchi Fruit Preservation

Humidity-responsive delivery systems provide a promising strategy for improving the efficiency of volatile postharvest regulators, yet the role of porous biopolymer structure in governing moisture activation and gas release remains insufficiently clarified. In this study, extrusion-structured starch foam pads were developed as moisture-responsive carriers for 1-methylcyclopropene (1-MCP) delivery, and their structure–sorption–release relationships were investigated in association with litchi fruit preservation. A sandwich-type pad was prepared by embedding a commercial 1-MCP–α-cyclodextrin inclusion complex and calcium chloride between two starch foam sheets. The areal loading was expressed as the mass of 1-MCP–α-CD complex per unit pad area, corresponding to equivalent 1-MCP loadings calculated from the declared active content of the commercial complex. Increasing relative humidity promoted water uptake, moisture-induced plasticization, and structural rearrangement of the starch foam matrix, thereby enhancing the accessibility of the inclusion complex to moisture and facilitating 1-MCP diffusion. Release behavior was therefore governed not only by nominal active loading, but also by the balance between active availability and structure-mediated transport efficiency. Under 93% RH, the 18.5 g/m² 1-MCP–α-CD complex loading exhibited a more regular and sustained release profile than the lower and higher loading levels, whereas the 27.8 g/m² formulation showed incomplete and less regular release. In the subsequent litchi storage trial, among the formulations selected for fruit validation, the 18.5 g/m² treatment more effectively delayed pericarp browning, reduced ethylene production and respiration rate, maintained membrane stability, and alleviated quality deterioration during storage at 25 °C. These findings indicate that extrusion-structured starch foams can function as humidity-coupled transport media for controlled 1-MCP release and provide a biodegradable active packaging platform for postharvest litchi preservation. Further studies using other fruit species are needed before extending this system to broader fruit preservation applications.