The optimal activity of most proteins is observed at physiological temperature
however, thermophilic proteins including papain show reduced activity at physiological temperature and their maximum activity is observed at elevated temperature. The question of what structural alteration in papain is materialised via such temperature decrease from higher-to-physiological condition becomes important, and it is unclear that, apart from the loss of functional conformation, what other defects are introduced via temperature drop. Here, we revealed that 65 °C➔37 °C temperature switching causes a minimalistic conformational alteration in papain's active site geometry that not only makes it non-functional but also it entrenches an aggregation propensity and drives a phase separation into cross-β-rich cytotoxic amyloid fibrils. Papain-fibrils were found to catalyse cross-seeding of proteins, and enhanced autooxidation of neurotransmitters was observed in the presence of papain-fibrils. Since papain is extensively used in protein-based food processors and therapeutic-formulations, the results provide fundamental insights into structural conversion of papain into amyloid-prone-conformers via a high-low temperature switch, revealing possible food amyloid risks.