The Young's Modulus, Fracture Stress, and Fracture Strain of Gellan Hydrogels Filled with Whey Protein Microparticles.

Texture modifying abilities of whey protein microparticles are expected to be dependent on pH during heat-induced aggregation of whey protein in the microparticulation process. Therefore, whey protein microparticles were prepared at either pH 5.5 or 6.8 and their effects on small and large deformation properties of gellan gels containing whey protein microparticles as fillers were investigated. The majority of whey protein microparticles had diameters around 2 μm. Atomic force microscopy images showed that whey protein microparticles prepared at pH 6.8 partially collapsed and flatted by air-drying, while those prepared at pH 5.5 did not. The Young's modulus of filled gels adjusted to pH 5.5 decreased by the addition of whey protein microparticles, while those of filled gels adjusted to pH 6.8 increased with increasing volume fraction of filler particles. These results suggest that filler particles were weakly bonded to gel matrices at pH 5.5 but strongly at pH 6.8. Whey protein microparticles prepared at pH 5.5 showed more enhanced increases in the Young's modulus than those prepared at pH 6.8 at volume fractions between 0.2 and 0.4, indicating that microparticles prepared at pH 5.5 were mechanically stronger. The fracture stress of filled gels showed trends somewhat similar to those of the Young's modulus, while their fracture strains decreased by the addition of whey protein microparticles in all examined conditions, indicating that the primary effect of these filler particles was to enhance the brittleness of filled gels.