Label-Free Optical Investigation of Structural and Thermal Changes in Fat and Lean Beef Under Frozen Storage

We evaluated the microstructural and thermal stability of beef fat and lean meat tissue over 56 days of household frozen storage (-18°C) using Brillouin spectroscopy, Raman spectroscopy, Differential Scanning Calorimetry (DSC), and Fluorescence Lifetime Imaging Microscopy (FLIM). Mechanical stiffness represented by Brillouin shift values ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Δ</mml:mi> <mml:mi>ν</mml:mi></mml:math> ) and Brillouin line FWHM values ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Γ</mml:mi></mml:math> ), and thermal metrics ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mi>Δ</mml:mi> <mml:mi>H</mml:mi> <mml:mo>,</mml:mo> <mml:msub><mml:mrow><mml:mi>T</mml:mi></mml:mrow> <mml:mrow><mml:mi>m</mml:mi></mml:mrow> </mml:msub> </mml:math> ) of fat and lean meat samples remained within experimental uncertainty ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mo>±</mml:mo> <mml:mn>0.2</mml:mn> <mml:mspace></mml:mspace> <mml:mtext>GHz</mml:mtext> <mml:mo>;</mml:mo> <mml:mi>p</mml:mi> <mml:mo>></mml:mo> <mml:mn>0.05</mml:mn></mml:math> ) throughout storage. While Raman spectra showed minimal chemical contrast between tissue types, autofluorescence lifetime constants ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:msub><mml:mrow><mml:mi>τ</mml:mi></mml:mrow> <mml:mrow><mml:mn>1</mml:mn></mml:mrow> </mml:msub> <mml:mo>,</mml:mo> <mml:msub><mml:mrow><mml:mi>τ</mml:mi></mml:mrow> <mml:mrow><mml:mn>2</mml:mn></mml:mrow> </mml:msub> </mml:math> ) and Brillouin shifts enabled robust, label-free mechanical discrimination of fat versus lean meat domains. Compression and acetone treatments confirmed the techniques' sensitivity. These results demonstrate that common home-freezing preserves beef tissue microstructure and fat thermal behavior over two months, and that combined optical and calorimetric methods offer a non-destructive framework for meat quality assessment.