Effect of addition of gelatin on the rheological and microstructural properties of acid milk protein gels

To gain an understanding of the gelation mechanism of mixtures of milk proteins and gelatin, rheological and microstructural properties of the mixtures were characterized following four stages. During the acidification stage (at 45 °C), the presence of gelatin at sufficient concentration (higher than 1%) led to a lower storage modulus (G′) than that of the pure milk protein gels and a more heterogenous microstructure with larger milk protein clusters was formed. During the cooling (from 45 to 10 °C) and annealing stage s (at 10 °C), the G′ of the gels increased because of both milk gel enhancement and gelatin gelation. Higher concentrations of gelatin led to earlier formation of strand-like structures, seen in the micrographs. The gelation of gelatin changed the microstructure of whey protein isolate (WPI) gel dramatically, while gels of milk protein concentrate (MPC) and skim milk powder (SMP) maintained the typical milk gel network and gelatin formed strands and films without destroying the existing gels. During the heating stage (from 10 to 45 °C), gelatin strands were melted and the G′ of the mixed gels tended to revert to the value at the end of the acidification stage, indicating that the changes caused by gelatin in the microstructure of milk protein gels after acidification are reversible. Additionally, gelatin enhanced the water holding capacity (WHC) of the gels (no serum expulsion was observed for gels containing ≥1% gelatin), without increasing gel firmness significantly.