Thermal Transitions, Mechanical Properties, and Molecular Mobility in Cornflakes as Affected by Water Content

Common cornfl akes (CCF) and sugar - frosted cornfl akes (SCF) were ground and stored at relative humidities (RHs) of 11% - 80% at 25 ° C to achieve a water content (WC) between 5% and 20% dry basis (db). Time - resolved proton nuclear magnetic resonance (TR - 1 H NMR) measuring the spin - spin transverse relaxation time ( T 2 ) was used to determine the mobility of water and solids. T 2 analysis following a single 90 ° pulse (free - induction decay [FID]) was used to evaluate molecular mobility coupled to the solid matrix in the temperature range of 4 ° - 90 ° C. Water mobility was evaluated using a spin - echo (90 ° - o - 180 ° ) sequence. The FID analysis in both types of cornfl akes (CF) showed a single T 2 . At a WC close to the Guggenheim - Anderson - de Boer (GAB) monolayer value (m 0 = 5% db), the spin - echo analysis showed a short - relaxing T 2 component ( T 2S ) at 25 - 30 i s that was attributed to the mobility of protons from solids and the most closely associated water molecules. T 2S increased at temperatures above the glass transition temperature ( T g ), indicating higher proton mobility due to plasticization of solids by water. A long - relaxing T 2 component ( T 2L ) was observed at RHs above m 0 and increased as WC increased in the range of 0.4 - 2.5 ms, manifesting the higher mobility of water protons. At very low WC, water acts as an antiplasticizer, but beyond a critical level, the plasticizing effect became evident in mechanical properties. The temperature dependence of water and solid molecular mobility can be analyzed by TR - 1 H NMR. © 2010 Blackwell Publishing.