Hierarchically Porous Cellulose-Based Radiative Cooler for Zero-Energy Food Preservation

Low temperature preservation is an essential route to extend the food shelf life but remains challenging due to its increasing energy consumption and carbon emission, as well as the need for decentralized food preservation. Passive radiative cooling, which can dissipate an object's thermal energy through an atmospheric transparency window to ultracold outer space without consuming any energy, provides a sustainable route for food preservation. Here, a hierarchically porous (0.006–10 μm) cellulose acetate/zinc oxide (CA/ZnO) nanocomposite film is constructebased on water-assisted induced phase separation to achieve passive cooling without any energy input. The film with 30 wt % ZnO concentration achieves subambient temperature cooling through high solar reflectivity (97.0%) and high mid-infrared emissivity (94.0%), achieving a daytime temperature drop of 13.8 °C under strong solar illumination. When used in food packaging, the CA/ZnO film can decrease the temperature of enoki mushrooms by up to 18 °C under direct sunlight. The preservation temperature reduction outperforms other commercially available food packaging materials, which can extend strawberries storage time to 9 days. Meanwhile, the film also demonstrates excellent antimicrobial, self-cleaning, and reusable properties. This energy-free cooling film has great potential in food preservation as well as other applications where a controlled environmental temperature is required.