Processing Routes Influence on the Structure–Property Relationships of Citric Acid/Glycerol-Modified Thermoplastic Starch

Citric acid can modify thermoplastic starch (TPS) through competing mechanisms involving intermolecular interactions, acid-catalyzed hydrolysis, and esterification. However, the products formed and their effects on TPS properties depend on the conditions under which citric acid is incorporated. This work aimed to investigate how the sequence of citric acid and glycerol incorporation affects the molecular transformations and structure–property relationships of cassava starch-based materials. Two pretreatment routes with different citric acid concentrations were compared. In Route 1, citric acid was incorporated directly into cassava starch before adding glycerol, whereas in Route 2, citric acid was thermally pretreated with glycerol before adding it to the starch. The resulting materials were subsequently processed by extrusion and compression molding. FTIR spectra showed carbonyl bands at approximately 1722–1728 cm−1 in citric-acid-containing materials, supporting the formation of ester-containing species. Route 1 showed more pronounced changes in the apparent molecular-weight distribution in the presence of citric acid, including low-molecular-weight fractions consistent with acid-catalyzed starch chain cleavage. These changes were accompanied by distinct intermediate thermal degradation events and increasing susceptibility to water-induced structural damage at higher citric acid concentrations. In contrast, Route 2 preserved the apparent molecular-weight distribution more effectively and produced materials with substantially greater elongation at break. Citric-acid-containing materials prepared by this route also exhibited greater dimensional stability during water immersion, despite their lower stiffness. Overall, the results demonstrate that the sequence of citric acid and glycerol incorporation directs distinct chemical and molecular transformations during processing, providing a practical strategy for tailoring the mechanical and water-response properties of TPS.