Scattering properties of meat and its relation to microstructure

Light scattering properties are generally related to the sample’s physical properties and provide information on its microstructure and texture. Standard spectroscopic measurements in the visual (VIS) to near infrared (NIR) wavelength range contain information of both absorption and scattering properties, making it difficult to interpret. In this study, total reflectance (MR), total transmittance (MT) and unscattered transmittance (MU) were measured and combined with inverse adding doubling (IAD) calculations to acquire bulk absorption and scattering properties separately.
Two types of muscle, the semitendinosus (ST) and semimembranosus (SM) of beef were considered in this study. Since the samples needed to be thin (mm-range) to achieve enough transmittance signal, the samples were sliced to 0.5 mm with a cryostat after being embedded in optical cutting temperature (OCT) compound and rapidly frozen in liquid nitrogen. Twenty eight muscle slices (12 slices of ST and 16 of SM) were acquired from the same animal. MR, MT, and MU of these samples were measured with the double integrating sphere and unscattered transmittance system, in the wavelength range of 500-1850 nm. Using IAD calculations, the bulk scattering coefficient, anisotropy factor and bulk absorption coefficient were calculated from this data.
As a reference measurement for texture, the toughness of the meat was measured using the Warner Bratzler shear force measurement method. In this method, the meat is cooked for 50 min at 75 ℃, cored into cylinder shapes with a diameter of 1.25 cm and sheared perpendicular to the muscle fibers. The maximum shear force is used as an index of toughness. Additionally, the microstructure of the samples was also observed with light microscopy. For microscopic observations, the samples were sliced to 10 micrometer thickness and observed in phase difference mode.
Scattering coefficients were higher for the ST muscle, with an average bulk scattering coefficient for all wavelengths being 13.9 cm-1 for the ST and 11.2 cm-1 for the SM muscle. Warner Bratzler shear force measurements gave lower values (more tender) for the ST, being 23.4 [N] compared to 35.4 [N] for the SM. Microscopic images showed that the fibers in the ST were much thinner than that of SM. This suggested that the thinner fibers in the ST scattered more light and lead to a more tender texture. Although measurements in this study are limited to two types of muscle, the relation between light scattering properties and muscle fibers suggests that light scattering measurements could be used to predict the toughness of meat.