Label free noninvasive spatially resolved NaCl concentration measurements using Coherent Anti-Stokes Raman Scattering microscopy applied to butter

•A method to measure spatially resolved concentrations of NaCl is presented.•CARS provides quantitative, label free, spatially resolved measurements in butter.•CARS can discriminate butters based on the size of water droplets and NaCl conc.•CARS can determine salt conc. for the full range of solubility in aqueous solution. Imaging the microstructure of opaque composite foodstuffs and extracting quantitative chemical information about specific localized components is challenging. Herein, a method has been developed to determine spatially resolved concentrations of aqueous salt and applied to measure salt concentrations of water droplets in butter samples. This was done using Coherent Anti-Stokes Raman Scattering (CARS) microscopy which achieves non-invasive label free imaging based on visualization of specific chemical-bond vibrations. The concentration of salt in the dispersed water droplets in butter was determined based on the relative change in intensity of the CARS-signal at two distinct wavenumbers, which have been shown to be dependent on the inter-molecular coupling of water molecules and salt. The results provide the size and salt concentration distribution of the droplets in the samples. It is further shown that the average salt concentration in the whole sample can correctly be inferred from the concentration measured within the water droplets.