Development of methodology for assessing flowability of milk protein powders using shear failure testing device

Abstract

Protein-rich milk powders can be susceptible to caking and clumping during manufacture and storage. A method was developed for objective and reliable assessment of their flowability, i.e., tendency of powders not to stick to the equipment surfaces. Milk protein powder (MPC 80) was subjected to three-point shear failure testing on a powder shear cell attached to a MCR302e rheometer. Flow function coefficients (ffc) were obtained after the Mohr circle analysis of pre-shear and shear-to-failure points. Due to their globular shape and significantly larger particle size (50–70 μm), milk protein powders exhibited more stick-slip phenomenon and lack of shear to failure point particularly at higher pre-shear (>6 kPa) and shearing normal stresses than the flat-shaped, smaller size (<20 μm) cohesive calcium carbonate powder (ffc value 2.1 at 3.0 kPa). Absence of shear-to-failure points in milk protein powders was attributed to instant failure of the powder at higher normal stresses due to larger particle size and globular shape, which was avoided by lowering pre-shear (3, 6, and 9 kPa to 1, 3 and 6 kPa) and shearing normal stresses, increasing data capturing interval and optimizing shear speed. Reliable ffc values at 3.0 kPa pre-shear normal stress, characterizing MPC 80 powder (4.6 ± 0.4) as easy flowing and MPI 85 as cohesive (3.7 ± 0.5) were obtained successfully using developed protocol.