Effect of heat treatment on casein micelle structure and acid and rennet gel properties in goat and cow milk – a SESANS approach

Acid and rennet gelation are essential processing steps for dairy products such as cheese and yoghurt. The coagulation and gel properties of these gels differ depending on whether unheated or heated milk was used. However, these heat-induced effects occur to a different extent in goat and cow milk gels, and have yet to be determined on a micro- and nanoscale structural level. To obtain a better understanding of heat-induced structural changes, this study investigates non-heated and heated casein micelle solutions from goat and cow milk and their derived rennet and acid-induced gels by a multi-technique approach, combining rheology, confocal scanning laser microscopy, and spin-echo small-angle neutron scattering (SESANS). This work is thereby the first to describe heat-induced changes of caseins and casein gels using SESANS. Modeling casein micelles as sticky hard spheres shows that goat caseins are larger and less hydrated than their cow counterparts. SESANS data reveal that structural changes observed in rennet and acid gels follow opposite trends when milk was pre-heated. Modeling gels as fractal gels suggests a connection between heat treatment and changes in the arrangement of caseins to form more loose or compact aggregates. Cow milk gels showed larger heat-induced changes in SESANS and corresponding larger rheological changes than goat milk counterparts. This work shows that rennet and acid gels respond differently to pre-heat treatment, and that they differ in the compactness of their casein network. The findings from this study can contribute to the design and characterization of dairy products of different animal species. • Differences in micelle and gel structure of cow and goat samples were observed • Cow milk gels show greater heat-induced structural changes in SESANS data • Heating mainly affects the compactness of casein aggregates in milk gels • Correlation length is less affected than the fractal dimensions upon heating • Compactness of aggregates influences gel strength