Analysis of Heterogeneous Biological Preparations in Turbid Media: Limitations of Standard Methods and a Proof-of-Concept Evaluation of Diffuse Reflectance Kinetics

(1) Background: Turbid biological formulations present a challenge for conventional optical protein analysis because scattering and particulate heterogeneity can distort analytical signals. (2) Methods: This study evaluated the limitations of ultraviolet–visible spectroscopy, intrinsic fluorescence, and dynamic light scattering for heterogeneous protein-containing systems and explored diffuse reflectance kinetics (DRK) as a non-destructive complementary approach. Bovine serum albumin model systems with and without particulate components were used to assess matrix-related effects, while a squalene-adjuvanted virus-like particle (VLP) formulation was investigated as a representative turbid biopharmaceutical system. DRK analysis was based on time-resolved diffuse-reflectance image sequences and numerical descriptors of spatiotemporal scattering dynamics. (3) Results: Conventional optical methods were strongly affected by turbidity, whereas selected DRK descriptors showed a concentration-dependent response in a squalene-containing emulsion matrix and acceptable repeatability under the investigated conditions. The results support the potential of DRK for comparative monitoring of apparent protein-related changes in turbid biological preparations. (4) Conclusions: Further validation using independent final vaccine batches and orthogonal reference methods is required before DRK can be considered for quantitative protein determination.