Impact of citric acid-mediated starch crosslinking on the properties of films derived from stale bread particles: A comparative study of solvent casting and melt-mixing/hot pressing processing

The influence of citric acid (CA) on the properties of films fabricated from stale bread materials via solvent casting (SC) and extrusion/hot-pressing (HP) was investigated. Wheat starch, wheat flour (WF), stale bread particles (BFc), decrusted stale bread particles (BF), and a blend of BF and deep eutectic solvent-acetylated bread flour (AcBF) were comparatively analyzed. The presence of CA was confirmed in HP and SC films via FTIR microscopy, evidenced by an esterification band at 1714 cm^-1. CA increased the water contact angle (WCA) of the bottom surface of SC WF, BF, BFc, and BF + AcBF films, while no effect was observed for SC starch and HP films. Of prime interest, the tensile analysis revealed a substantial increase in elongation at break for SC BF, BFc and BF + AcBF samples, with values rising by 856 %, 606 %, and 516 %, respectively. In contrast, no significant effects were observed in SC starch and WF, suggesting that crosslinking was more effective in the bread matrix. In HP films, the incorporation of CA drastically reduced the elongation at break for BF, BFc, and BF + AcBF films, while having negligible effect on the maximum tensile strength, suggesting lower CA-mediated crosslinking in HP films. The presence of AcBF generally worsened WCA and tensile properties of all SC samples, though these effects were partially mitigated with CA. The crust generally reduced WCA in the top surface of SC films, though no such effect was observed in HP films. The crust did not impact the tensile properties of either SC or HP films.