Water molecular dynamics during bread staling by Nuclear Magnetic Resonance

Bread staling is a complex phenomenon that originates from multiple physico-chemical events (amylopectin retrogradation, water loss and redistribution) that are not yet completely elucidated. Molecular properties of white bread loaves were characterized by multiple proton Nuclear Magnetic Resonance (NMR) techniques (proton FID, T2 and T1 relaxation time) over 14 days of storage. Changes at a molecular level (faster decay of proton FIDs and shifting of proton T2 relaxation times distributions towards shorter times), indicating a proton mobility reduction of the bread matrix, were observed during storage. Multiple 1H T2 populations were observed and tentatively associated to water-gluten and water-starch domains. Proton T1 of bread was for the first time measured at variable frequencies (Fast Field Cycling NMR) and found to be strongly dependent upon frequency and to decrease in bread during storage, especially at frequencies ≤ 0.2 MHz. An additional proton T1 population, relaxing at 2 ms, was detected at 0.52 MHz only at early storage times and tentatively attributed to a water-gluten domain that lost mobility during storage.