Development and mechanical optimization of edible protein-based films from fresh duck egg white using alkali–thermal processing

Surplus fresh duck egg white (DEW), generated during intensive duck egg processing, is primarily utilized in gel-based products but remains underexplored for thin film applications due to its limited shelf life and pronounced protein reactivity under thermal or alkaline conditions. This study aimed to develop edible protein-based films directly from fresh DEW and to elucidate the effects of alkali-thermal treatment and formulation variables on film formation and mechanical performance. Fresh DEW was diluted to 45% (w/w) and adjusted to pH 11 to maintain fluidity during thermal processing. The resulting film-forming solution (FFS) remained sufficiently flowable after alkali-thermal treatment at 70-75°C, enabling subsequent casting and drying. Increasing heating temperature promoted protein unfolding, intermolecular association, and aggregate development, leading to significant increases in tensile strength and puncture resistance (P < 0.05), accompanied by progressive darkening. Glycerol was essential for film formation, and increasing glycerol content significantly improved the elongation at break and toughness of the films (P < 0.05). Heating temperature and glycerol concentration were therefore selected as key variables and optimized using response surface methodology with a central composite design. A well-fitted quadratic model was established to describe film toughness, and sequential quadratic programming identified the optimal condition. The highest toughness (4.42 MJ m⁻³) was obtained at a heating temperature of 75°C with 65.02% glycerol (w/w of protein). This study demonstrates the feasibility of producing mechanically robust edible films directly from fresh duck egg white and provides a processing-oriented strategy for converting surplus DEW into value-added protein-based materials with potential applications in edible food packaging systems.