Hydroxypropyl Methylcellulose (HPMC) Edible Films Incorporated with Melatonin: Physicochemical Properties and Quality Preservation of Coated Green Asparagus (Asparagus officinalis L.) During Cold Storage

Green asparagus (Asparagus officinalis L.) has a short shelf life due to its high respiration rate, leading to rapid senescence, weight loss, and toughening after harvest. While melatonin (ML) is known to delay postharvest senescence, its application is limited by environmental instability. This study developed hydroxypropyl methylcellulose (HPMC)-based edible films incorporating ML (0, 25, 50, and 100 µM) and evaluated their physicochemical properties. The corresponding formulations were applied as edible coatings to the basal region of asparagus spears to evaluate their preservation performance during 25 days at 4 °C and 85% relative humidity (RH). The incorporation of ML reduced the water vapor transmission rate (WVTR) without compromising the mechanical properties of the films. Compared with uncoated green asparagus, HPMC/ML coatings suppressed the activities of lignification-related enzymes, phenylalanine ammonia-lyase (PAL) and peroxidase (POD), thereby attenuating postharvest toughening. Furthermore, the HPMC/ML treatment enhanced the activities of antioxidant-related enzymes, catalase (CAT) and superoxide dismutase (SOD), which were associated with reduced hydrogen peroxide (H2O2) accumulation. The HPMC/ML coating also reduced weight loss, retained chlorophyll content, and attenuated the change in firmness of the storage green asparagus. Considering the overall properties, HPMC/ML50 was identified as the recommended formulation for delaying postharvest senescence and maintaining green asparagus quality during cold storage.