Distinctive yielding of dilute colloidal gel
Résumé
The main protein fraction of milk is composed of four proteins, namely theαs1-,αs2-,β- andκ-casein (≈20 kDa). They naturally complex minerals, formingcolloidal structures called casein micelles (≈1.3×106kDa). Their colloidal sta-bility arises from theκ-casein that protrudes in the aqueous phase, providingsteric repulsions. Addition of a proteolytic enzyme that hydrolyses the pro-truding segment suppresses the colloidal stability and leads to aggregation andgelation. Here, we study the yielding of the enzymatic milk gel during stresssweep through the changes in viscoelastic moduli. We observe a 3-step yielding:both moduli show a softening followed by a hardening at intermediate stressesuntil the gel completely breaks. Such behavior has not been observed in sim-plified casein gel and does not fit the state of art simulations. Creep and fatigueexperiments are also performed at stresses in the softening or hardening stageto discuss the mechanisms of the yielding process.