A review on the application of biodegradable alginate-based films in post-harvest quality preservation of fruits and vegetables.

Postharvest fruits and vegetables (FVs) are highly susceptible to quality deterioration caused by microbial contamination and endogenous metabolic processes, thereby increasing the demand for green and efficient preservation materials. Sodium alginate (SA), a naturally derived polysaccharide renowned for its outstanding film-forming capacity, biocompatibility, and biodegradability, represents a promising bio-based candidate for fruit and vegetable preservation. However, pure SA films have inherent drawbacks, including poor mechanical strength, excessive hydrophilicity and insufficient biological activity. This review provides a systematic synthesis of the structural characteristics, fabrication methodologies, and preservation mechanisms of SA-based films. Furthermore, the advantages and limitations of four principal fabrication techniques-direct coating, solution casting, electrospinning, and layer-by-layer self-assembly-are critically evaluated. Five core mechanisms are clarified: physical barrier; antibacterial activity; antioxidant capacity; nano-reinforcement and intelligent responsiveness. To overcome the limitations of pure SA, this paper systematically analyses the effects of adding bioactive substances-such as plant extracts, antioxidants and nanofillers-on the functional properties of the films. The performance of the functional composite films produced through these modifications was significantly enhanced: for example, cross-linking with tannic acid increased the tensile strength of the film by 22.54%, reduced water vapour permeability by 25.36%, and simultaneously achieved near-total UV blocking. These performance improvements can lead to significant freshness-preserving effects in practical applications. Research has shown that a SA/sodium carboxymethyl cellulose/ε-polylysine composite coating can extend the shelf life of mangoes from 4 days to 10 days, reduce the incidence of black spot disease by 46.04%, and achieve an inhibition rate of 58.10% against Colletotrichum glabrum. Furthermore, after 12 days of storage, the total phenolic content of the coated mangoes was 27.44% higher than that of the control group, whilst the total flavonoid content was 52.17% higher, confirming that this system can delay fruit decay and maintain nutritional quality. In summary, this review confirms that functionalised SA composite films can achieve targeted preservation and real-time freshness monitoring of fruit and vegetables. Future research directions are proposed, encompassing scalable manufacturing processes, rational design of customized formulations, integrated safety and techno-economic assessments, and in-depth exploration of underlying molecular mechanisms-collectively offering a robust scientific foundation for the innovation of SA-based green preservation materials.