Extensional rheology of condensed milk treated with glucosyl transferases

This study proposes a constant velocity scheme for filament stretching rheometry as a valuable and differentiating tool in food science. The methodology is developed to overcome challenges with constant rate filament stretching rheology regarding strains and strain rates accessible in lower viscosity fluids relevant to foods. It also maintains the accuracy and versatility of filament stretching rheometry, and range of material properties that can be investigated. We demonstrate this technique on a condensed milk system thickened by a mixture of two specific glucosyl transferases (GTFs), which allow tuning of the stringiness of food systems. The qualitative concept of stringiness often assessed by a spoon drip method is replaced by a stringiness parameter directly obtained from the extensional kinematics and mapped against the capillary number. The latter allowed to demonstrate that the filament stabilization is not a shear viscosity but rather an elastic effect. This allows further discussion of the colloidal structures and interactions that may cause the specific dependence of stringiness of enzyme treatment. Accurate stress data is obtained up to higher Hencky strains by data graduation, enabling the observation of highly nonlinear effects that provide valuable insights into the structural rearrangements of the colloidal and polymeric components of the system. Enabled by these improvements, the study highlights the strength of extensional rheology in its ability to detect subtle differences in the colloidal structure of GTF thickened foods that are not measurable in shear.