STRUCTURE-COMPRESSIVE STRESS RELATIONSHIPS IN MIXED DAIRY GELS

Abstract

Mixed dairy gels (including a control without fat) of skim milk powder (SMP) and whey protein isolate (WPI) containing fat globules were formed by heating protein emulsions to 90-degrees-C and by acid release from glucono-delta-lactone to provide a pH of 4.3-4.4. Fat globules with artificial protein membranes (FGAPM) were prepared by homogenization of a butter oil/water mixture in the presence of WPI while fat globules without membranes were stabilized with polyoxyethylene sorbitan monolaurate (Tween 20). Both emulsions were added at a 4% (w/w) level to solutions having 3% SMP and 8.3% WPI. The gel containing FGAPM had significantly higher compressive strength than the control without fat (2.4 versus 1.8 kPa, respectively) and microstructurally it was a mixed gel in which the FGAPM, casein and whey protein aggregates formed a copolymer network. Addition of fat globules without membranes led to a filled gel weaker than the control without fat (1.4 versus 1.8 kPa, respectively). Bonding of the protein membrane in FGAPM to the gel network and presence of individually dispersed fat globules without membranes was demonstrated by transmission electron microscopy. The difference in microstructure is proposed to be responsible for the mechanical properties of each gel.