Structural changes of myoglobin in pressure-treated pork meat probed by resonance Raman spectroscopy

Pork meat was pressurised at 600–700 MPa under conditions applicable for non-thermal food preservation and studied by resonance Raman spectroscopy with 413-nm excitation to probe selectively myoglobin, which is the origin of the red colour of meat. The spectra of intact, non-pressurised meat tissue exclusively display the resonance Raman bands of the ferrous deoxy-form of myoglobin whereas upon pressure treatment a new six-coordinated low spin ferrous species is formed (>60%), that is assigned to a bis–histidine complex including the distal histidine 64. This structural change is associated with a shift of the electronic transitions of the haeme and thus affects the colour of the meat. In contrast, solutions containing myoglobin extracted from pressurised and non-pressurised pork meat give rise to resonance Raman spectra characteristic of the ferrous oxy-form of myoglobin, evidently due to the accessibility of the proteins for oxygen in solution. Upon pressure treatment of the extracted myoglobin solution, the oxy-form is partially converted to the met-(like) ferric form implying a pressure-induced oxidation of the haeme. Thus, this structural transition does not only cause a colour change but also may initiate unwanted oxidative side reactions involving further components of meat. Evidently, such effects can be largely avoided when the oxy- to deoxy-myoglobin ratio is kept small prior to pressure treatment.