Understanding the impact of drying on the optical properties of cured muscle tissue: a case study of dried salt-cured cod

Optical techniques, such as near-infrared spectroscopy (NIRS), have previously been proposed for non-destructive monitoring of the industrial dry-curing of fish and meat, but existing prediction models often lack robustness due to the complexity of these products. This study investigates the interaction of light with cured muscle tissue during drying, focusing on dried salt-cured cod within a commercially relevant range (\(\qtyrange[range-units=single,range-phrase=-]{45}{56}{\percent}\) water content). We quantified the wavelength-dependent bulk optical properties (absorption and scattering coefficients) using double integrating sphere (DIS) measurements across the \(\qtyrange[range-units=single,range-phrase=-]{500}{1700}{\nano\meter}\) wavelength range on \(36\) samples of dried salt-cured cod muscle (excluding the salt layer). Complementary histological and low-field NMR analyses helped interpret the observed trends. As expected, light absorption by water decreased during drying. This was accompanied by an increase in the anisotropy coefficient, \(g\), and a corresponding decrease in the reduced scattering coefficient, \(\mu_s'\), both stabilizing below \(50%\) water content. A corresponding degradation of muscle microstructure, caused by increasing salt concentration, is proposed as a key mechanism behind these changes. We also observed that scattering variation was weaker than initially expected and influenced by factors such as salting methods, fibre orientation, and fish size. While this study focuses on salt-cured cod, the insights gained are also applicable to other dry-cured products, such as ham. The findings provide a fundamental understanding of the optical properties of dry-cured foods, offering valuable insights for optimising NIRS-based technologies, improving prediction models, and enabling non-invasive quality monitoring in the dry-curing industry.