Casein micelle reassembly affects the interfacial properties of fluid-fluid interfaces.

Hypothesis: Precision fermentation is a novel approach to produce microbial-based caseins, which promises to be important in the production of animal-free dairy. Caseins are important stabilizers in foam- and emulsion-based dairy products. A solid understanding of how caseins behave at the air-water and oil-water interfaces is essential for the development of new products based on microbial-based caseins. Here, we compared the interfacial behavior of air-water and oil-water interfaces stabilized by bovine casein micelles (CM), sodium caseinate, and reassembled casein micelles (RCM) made from sodium caseinate. The effects of reassembly are important to assess, since precision fermentation produces individual casein fractions (i.e., caseinate) instead of micelles.

Experiments: The micellar structure of RCM and CM increased the interfacial stiffness of oil-water interfaces compared to caseinate and -free caseinate. A combination of the general stress decomposition and Fourier-transform rheology revealed that the micellar structure is able to resist the network weakening by polar oils better than -free caseinate through the presence of interfacial aggregates. The behavior of RCM and CM at the air-water interfaces was different from oil-water interfaces. RCM did not have higher interfacial stiffness compared to a -free caseinate sample, while CM did. Surprisingly, imaging the microstructure with atomic force microscopy (AFM) on Langmuir-Blodgett films did not reveal many differences in microstructure between RCM and CM. However, AFM showed an increased interfacial connectivity in all samples containing compared to -free samples.

Findings: This study showed that oil-water interfacial viscoelasticity is dominated by aggregated casein due to a high subphase polarity. Hence, the highly aggregated RCM successfully mimicked the oil-water functionality of CM. Aggregation was not as important for air-water which resulted in a different functionality of RCM and CM.