Retrogradation Regulates the Thermal Puffing Quality of Tigernut (Cyperus esculentus L.) Starch via Modulating Its Multi-Scale Structure and Water State

Starch retrogradation is a key process governing the final quality of indirectly thermally puffed starchy foods. This study investigated the effect of retrogradation duration (0–72 h) on the multi-scale structure, water distribution, thermal puffing performance and textural properties of tigernut starch-based products. XRD, FTIR and synchrotron SAXS analyses confirmed that retrogradation progressively promoted molecular ordering, as reflected by reduced lamellar long period (dBragg) and radius of gyration (Rg), along with enhanced short-range order (R1047/1022) and relative crystallinity. LF-NMR results showed that among all tested retrogradation durations, the 4 h treatment presented the highest proportion of immobilized water (A22), while longer retrogradation periods led to a gradual increase in the content of free water. Puffing performance and texture showed a non-linear dependence on retrogradation time: the 4 h retrograded sample presented the best comprehensive quality among the seven tested time points, with the highest expansion ratio (9.75), specific volume (8.42 mL/g) and crispness (36.67), and the lowest hardness (1165.53 g). Pearson correlation analysis identified water activity (aw) as the dominant factor governing puffing performance, which was in turn regulated by retrogradation-induced structural evolution. This study elucidates that a moderate retrogradation degree (approximately 4 h) facilitates the formation of a moderately ordered lamellar structure with the most favorable water retention performance within the current experimental range, creating ideal thermodynamic conditions for puffing, while excessive retrogradation causes structural rigidity and poor expansion.