Changes occurring in potatoes during cooking and reheating as affected by salting and cool or frozen storage – a LF-NMR study

Low field nuclear magnetic resonance (LF-NMR) was measured continuously during cooking of salted and unsalted potatoes to investigate the effect of salt on the cooking process. The cooked potatoes were subjected to cool storage (1, 2, 24 and 48 h) or frozen storage (1, 3, 5 or 14 days). Upon storage they were reheated in the LF-NMR instrument to investigate the effect of different storage regimes and salt on the state of water in potatoes during a continuous reheating process. In salted potatoes, a broader distribution of T2 times leaning towards longer relaxation times were observed for extra-cellular/cytoplasmic water at low temperatures during cooking. This could indicate that the extra-cellular space is increased due to salting. In samples subjected to cool and frozen storage an additional free water component was found, which reflects that syneresis occurs as a result of retrogradation. In the frozen samples, the T2,1 and T2,2 water populations merge, illustrating an increased cell wall permeability or freeze damage of the cell walls. This is more evident in the unsalted samples showing that the cell walls are stabilized by salt during freezing. In conclusion, the results showed that LF-NMR could monitor the changes during starch gelatinization, starch retrogradation and melting of retrograded starch in salted and non-salted potatoes.