Processes of Thermal Treatment on Hazelnuts Investigated by NMR and MRI

ABSTRACT 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.