Multi-Reflectance-Spectroscopy,Part III: In-Line Monitoring of Milk Fat Globule Size via Sauter Diameter Using an Optical Sensor

Abstract

In milk-based formulations, fat globule size is a fundamental quality parameter. Accurate measurement of globule size is critical for ensuring consistent product performance, preferably through in-line monitoring. However, most conventional techniques are off-line and unsuitable for integration into manufacturing processes. The objective of this study was to demonstrate the feasibility of a recently developed optical sensor based on Multi-Reflectance Spectroscopy (MRS) for in-line estimation of the fat globule Sauter diameter. The MRS sensor acquires multidimensional reflectance data across multiple wavelengths and defined illumination–detection geometries. Industrial validation was performed in a milk standardization plant using low-fat (1.5 wt%) and whole-fat (3.5 wt%) milk. Fat globule sizes were systematically varied by stepwise adjustment of homogenization pressure. Reference globule size distributions were obtained via analytical centrifugation and expressed as the Sauter diameter. Analytical reflectances derived from the reference size distributions were compared with experimental spectra to assess globule size dependency of the optical responses. Principal component analysis revealed a clear separation between fat concentration levels, motivating the development of concentration-specific partial least squares regression models. Two PLS models were trained and validated using independent datasets. Across the combined concentration range and full Sauter diameter span (0.7–3.5  $\mu$ m), resultet in a test RMSE of 271 nm. For the separate 1.5 and 3.5 wt% fat lines, RMSE values of approximately 80 and 40 nm were obtained, respectively. These results demonstrate that the MRS-based optical sensor enables accurate, real-time in-line monitoring of fat globule size in complex dairy emulsions and represents a promising process-analytical tool for industrial process control.