Zwitterionic engineering of chitosan enables highly soluble, antibacterial films for sustainable fruit preservation

Chitosan-based materials show considerable potential for fruit preservation but are limited by poor solubility and relatively weak antibacterial activity. In this study, we developed a zwitterionic betaine-modified chitosan (BM-CS) with tunable degrees of substitution using a low-temperature homogeneous alkali/urea system. The resulting BM-CS exhibited marked enhanced water solubility, film-forming ability, and broad-spectrum antibacterial activity, while maintaining good biocompatibility with both HMEC-1 and HIEC-6 cells (>90% viability). Compared with native chitosan, BM-CS films displayed reduced crystallinity, improved thermal stability, and increased tensile strength (up to 77.78 MPa vs. 57.79 MPa for unmodified chitosan). The films also demonstrated rapid biodegradability, reaching 93.4% degradation within 7 days in soil, as well as excellent storage stability, retaining >98% of their antibacterial activity after 50 days. When applied as edible coatings, BM-CS effectively reduced water loss, ethylene production, and nutrient consumption in grapes, cherry tomatoes, and strawberries, while maintaining total phenolic content and VC retention. Notably, the high-substitution BM-CS achieved weight losses of only 18.3% and 16.1% after 18 days, significantly lower than the control (>45%), and 2.41% after 7 days for strawberries (control >18%). These findings indicate that zwitterionic modification is an effective strategy to enhance the functional performance of chitosan, highlighting BM-CS as a promising sustainable material for postharvest fruit preservation.