Microstructure changes of processed cream cheeses observed using confocal Raman spectroscopy

Ingredients are often added to cream cheese to improve its technological functionality and shelf life. The objective of this research was to evaluate the applicability of confocal Raman microscopy to observe microstructural changes, caused by freezing and thawing in two different cream cheeses. The cream cheeses having a different composition in terms of macro components and stabilizers were frozen at -20°C. After 7 days of frozen storage at -20°C, cheeses were thawed at +4°C. Analyses were conducted on non-frozen cheese before freezing (control sample), and on the frozen cheese after an overnight period of subsequent thawing for both cheese formulations. Microstructural analyses of the cheeses were carried out using a confocal fluorescence microscope and a confocal Raman microscope equipped with a 532 nm excitation laser beam. To measure the degree of freezing damage caused by the process, the self-diffusion water coefficient was measured at 20°C using a low-resolution Nuclear Magnetic Field (NMR) spectroscope. The results showed an increase in the water self-diffusion coefficient, which can be related to a to a lower dynamic viscosity and a higher water mobility in both cheeses. Raman images were in good accordance with confocal images, furthermore, Raman spectra permitted to obtain an image of the water distribution at the sample’s surface plane. In accordance with NMR data, both microscopical techniques were able to show freeze-thaw damaged structures in the cheese. The main advantage of confocal Raman microscopy is the ability to observe samples without the use of staining reagents, and more importantly to observe all micro- or macromolecule with a distinguishable Raman shifting band signal.