Potential enhancement of thermal stability in soy protein nanofibrils by metal ions

Fibrillation, a universal property of proteins, serves as an effective strategy to enhance the functional properties of food-derived proteins. However, certain protein fibrils are susceptible to degradation at elevated temperatures, which limits their application. This study investigates the thermal destabilization behavior of soy protein nanofibril (SPNF) during prolonged heating (85 °C, 0–72 h) and explores the role of Na⁺ and Ca²⁺ in enhancing fibril thermal stability. The aggregation, morphology, and structural properties of the fibril systems were analyzed, revealing that all fibril systems developed weak gels after 24 h of incubation. SPNF and Na⁺-induced SPNF experienced gel network disruption and aggregation during prolonged heating, leading to increased particle size, turbidity, and reduced storage modulus, transitioning to fluid state at 36 h. Meanwhile, both exhibited a rapid decrease in ThT fluorescence and a transition from linear fibrils to particulate aggregates within 60 h. CD and FTIR indicated that as fibrils degraded, the β-sheet structure converted to β-turns, with a significant reduction in hydrogen bonding. Conversely, SPNF induced by 100 and 33 mM Ca²⁺ effectively resisted aggregation, maintaining gelation at 60 and 48 h, respectively. The ThT fluorescence, β-sheet structure, and hydrogen bonding decreased relatively slowly with prolonged heating, and the fibril morphology remained intact at 60 h. Thermogravimetric analysis confirmed that 100 mM Ca²⁺-induced fibrils had a higher thermal degradation temperature (341 °C). These findings suggest that Ca²⁺ enhances SPNF thermal stability, providing a theoretical basis for its application in high-temperature environments in food processing.