Citric acid-catalyzed cassava pyrodextrin: Molecular structural evolution and resistant starch formation mechanism

Pyrodextrins from cassava starch, especially when catalyzed by organic acids like citric acid, hold promise as low-digestibility food ingredients. However, the precise relationship between the time-dependent structural evolution during citric acid-catalyzed pyrodextrinization and the resulting digestibility remains unclear. This study aimed to elucidate how citric acid directs the structural transformation of cassava starch under pyrolysis (170 °C, up to 3 h) and to establish the mechanistic links between molecular features and in vitro digestibility. Compared to pyrolysis without acid, citric acid profoundly altered the reaction pathway: it initially promoted extensive hydrolysis, dramatically increasing solubility and short-chain content, and subsequently drove transglycosylation and repolymerization. These late-stage reactions generated novel digestion-resistant features, including diverse non-starch glycosidic bonds (α-1,2, β-1,2, β-1,4, β-1,6), high degrees of branching, and ester carbonyl groups. Consequently, the resistant starch (RS) content in citric acid-catalyzed pyrodextrins surged to 42.04%. Correlation analysis confirmed that RS content was positively associated with branching degree in acid-catalyzed samples. This work reveals that citric acid acts not merely as a catalyst but as a multifunctional modifier that steers pyroconversion toward highly branched, cross-linked structures rich in enzyme-resistant linkages. The findings provide a fundamental understanding for tailoring starch digestibility and advance the development of "clean-label" functional ingredients with health benefits.