Characterization of the anisotropy in proteinaceous semi-solid food matrices through polarized fluorescence spectroscopy

This study investigates the possibilities and limitations of fluorescence anisotropy (FA) as a technique to assess the intricate structures of (semi) solid food samples. Sensory and consumer preferences strongly depend on food texture and structural anisotropy which presents a formidable challenge for the food industry e.g. when developing plant-based alternatives. Plant-based foods are formulated to mimic the appearance, flavor, and fibrous texture of well-known animal-based products. In particular, anisotropy is a property of solid foods which is very difficult to control and quantify. Anisotropy is direction-dependent. Processed food matrices typically exhibit intricate micro- to macro-structural properties that differ in their anisotropic characteristics. FA represents a putative analytical tool to characterize and perhaps even quantify anisotropy, but so far research has provided ambiguous results. This study introduces a systematic protocol for measuring FA using fluorescein, a classic fluorescent dye, followed by a contextual quantitative comparison of anisotropy using solutions of tryptophan and BSA. Furthermore, high moisture extrusion products made from pea protein isolate and whey protein concentrate have been analyzed. In addition, the anisotropy of an industrially relevant food, namely, the Mozzarella cheese is quantified and compared to the Cagliata cheese. The results show - as expected - that Cagliata has a lower FA compared to Mozzarella produced via the pasta-filata process, quantified from the tryptophan fluorescence signal. This paper aims to clarify the complexities of quantifying fluorescence-based anisotropy in semi-solid food matrices and pinpoint challenges that hinder its broader application.