Effects of cold-renneted and pre-heated milk protein concentrates (MPCs) addition on the properties of alginate composite gels

This study investigated the effects of milk protein concentrate (MPC85) (untreated, cold-renneted, and pre-heated (80 °C, 30 min)) addition on the physical/mechanical properties of sodium alginate (2% w/w) composite gels in millimeter-size (bead) and centrimeter-size gel forms. The gels were characterized for the degree of syneresis, swelling behavior, hardness, stiffness, viscoelastic behavior, and surface morphology of freeze-dried gel. The results showed that the addition of untreated and treated MPCs reduced the hardness, the stiffness and the solid-like behavior of the alginate gels. Untreated MPC and pre-heated MPC caused no effect on syneresis of alginate gels. The addition of cold-renneted MPC reduced the degree of syneresis in composite gels by 13.7%. Similarly, the addition of cold-renneted MPC reduced (by 16.4%) the degree of syneresis of the alginate composite gels after incubation in simulated gastric fluid (SGF). After incubation in simulated intestinal fluid (SIF), the gel containing alginate only swelled while gels containing untreated MPC and pre-heated MPC experienced degradation and a severe mass loss. Meanwhile, the gels containing cold-renneted MPC swelled and at the same time eroded. Moreover, only cold-renneted MPC composite gels showed lower shrinkage and wrinkles on the surface of the beads during lyophilization. Therefore, based on these results, it is indicated that cold-rennet induced gelation method could increase the effectiveness of MPC as a composite material for alginate gels.