Processes of Thermal Treatment on Hazelnuts Investigated by NMR and MRI

NMR and MRI provide a variety of customizable methods for process monitoring. A selection was applied to monitor structural and compositional changes in hazelnuts during thermal treatment, with particular focus on the roasting and aging behavior of hazelnut oil. Hazelnuts contain a high oil fraction stored in subcellular oleosomes, whose stability is crucial for product quality and shelf life. Thermal stress can alter these microscopic oil-containing structures, affecting oil mobility and oxidative stability. In situ MRI measurements were combined with pulsed field gradient stimulated echo (PFG-STE) NMR diffusion experiments to investigate structural changes across multiple length scales. MRI detected mesostructural alterations in the hazelnut matrix from ~50 μm to several millimeters, corresponding to features above the cellular level. At roasting temperatures below 150°C, only minor structural changes occurred, whereas at 200°C, pronounced void formation and cellular collapse were observed. A dedicated experimental setup enabled in situ measurements during roasting under controlled temperature, allowing spatially resolved monitoring of oil redistribution in coarse nut structure. Complementary PFG-STE NMR diffusion measurements provided insight into the microstructure (100 nm-10 μm), revealing subcellular structural changes and oil mobility. These results showed that oleosomes were largely destroyed already at 100°C. Furthermore, NMR spectroscopy demonstrated temperature-dependent oxidation kinetics of unsaturated fatty acids in hazelnut oil on a molecular level, with clear formation of oxidation products upon heating, whereas ambient storage caused only minor chemical changes. The combined use of MRI and NMR enables quasi-nondestructive, in situ monitoring of molecular, microstructural, and mesostructural transformations in hazelnuts and their oil under controlled thermal processing conditions.