Rheology of animal and plant milks featuring dripping-onto-substrate protocols

Animal milk is a multicomponent colloidal dispersion that contains fat globules and proteins dispersed in an aqueous continuous phase that contains dissolved sugars and minerals. The rapid increase in demand and production of plant milks, i.e., the plant-based alternatives to animal milks, is driven by lactose intolerance, allergens, and religious, cultural, environmental, and health choices or concerns. Many challenges for emulating animal milk's physicochemical, functional, and sensory attributes can be translated as problems related to the characterization and analysis of flow behavior, motivating this study. Only a few studies characterize shear rheology and almost none show characterization of extensional rheology of animal milks (cow and goat) and plant milks (soy, almond, rice, coconut, pea, and oat). We find that the two animal milks have rate-independent shear viscosity responses similar to Newtonian fluids. In contrast, the plant milks, except rice milk, display shear thinning. We characterize the pinching dynamics and extensional rheology response by employing the dripping-onto-substrate (DoS) protocols that we have shown are suitable for formulations such as milk that display a constant or shear thinning response but no measurable elasticity or viscoelasticity. In the last stage of pinching, plant milks appeared to show inertiocapillary, power law, or elastocapillary pinching, whereas animal milk displayed a viscocapillary response. The distinct rheology response of animal and plant milks, the illustration of suitable measurement protocols, and the discussion of the role of dispersed colloidal particles, drops, and macromolecules are presented as three key contributions of this study, aimed at facilitating the formulation and design of more nutritious and sustainable future milks.