Abstract
<jats:p>Chitosan is a renewable, biodegradable, and intrinsically antimicrobial alternative to petroleumbased packaging, but most chitosan films contain plasticizers or cross-linkers that complicate foodcontact safety and end-of-life disposal. Here, additive-free films were prepared from chitosan, water, and near-stoichiometric acetic acid. A 128 cP grade at 2.0% (w/v) provided the best castability. Single-step drying (55 ℃/6 h) and two-step drying (55 ℃/6 h followed by 120 ℃/0.5 h) were compared. The second step reduced residual chitosan–acetate-associated mass loss and produced a stiffer film (elongation at break, 11.9 ± 1.4%; elastic modulus, 351.7 ± 56.4 MPa) than the more extensible single-step film (43.2 ± 2.8%; 251.1 ± 19.1 MPa). Both films were water-insoluble, enzymatically degradable, and lost 17.7–21.9% of their dry mass after 28 days in lysozymecontaining buffer. A non-destructive transparent-pot soil assay allowed one specimen to span airexposed, enclosed-air, and soil-contact zones. Both films disintegrated completely within 60 days only in direct soil contact, whereas the air-exposed and enclosed-air regions remained intact for 180 days. This within-specimen design directly visualized soil-contact-driven degradation without specimen retrieval. In a preliminary apple-wrapping trial, the film reduced browning and dehydration relative to unwrapped controls. Drying protocol therefore provides an additive-free route to mechanically distinct chitosan films for flexible or semi-rigid packaging applications. Keywords: Chitosan film; biodegradable packaging; drying protocol; soil biodegradation 1. Introduction Petroleum-derived plastic packaging is one of the most persistent forms of environmental pollution, with several hundred million tons of plastic produced annually and only a small fraction recovered through recycling or composting [1,2]. Food packaging accounts for a large share of this waste stream, and its short service life relative to the multi-decade persistence of conventional polyolefins in the environment has intensified the search for biodegradable alternatives derived from renewable resources [3,32]. Among the biopolymers considered for this purpose, chitosan is one of the most extensively studied because it is abundant, non-toxic, film-forming, and intrinsically antimicrobial [4,5,7,25]. Cast chitosan films are transparent, provide a useful oxygen barrier, and have been explored as edible coatings and stand-alone packaging materials for fruit, meat, and bakery products [6,8,9,10].</jats:p>