Free Radical Generation during Extrusion: A Critical Contributor to Texturization

EPR has detected free radicals in extruded cornmeal, wheat flour, potato granules, and plantain flour. The g-values in all cases are in the range g=2.0055-2.0060, indicating nitrogen-centered radicals, most likely on proteins. Extensive EPR and chemical analyses have shown that the radicals are integrally involved in the chemical changes leading to texturization in these materials. High heat causes peptide scission, which yields N radicals that are the major component of EPR signals. Under conditions of high shear, S-S bonds are also broken, and sulfur radicals then are evident in the EPR spectra, oxidizing lipids transfer radicals to protein side chains, and this is reflected in spectral lineshapes. Addition of free radical scavengers during extrusion dramatically diminishes and alters both EPR spectra and extrudate textures. Chemical and microscopic evidence supports EPR data and shows that free radical-mediated protein-protein and protein-starch crosslinking are key determinants of extrudate physical structure.