STRUCTURE-COMPRESSIVE STRESS RELATIONSHIPS IN MIXED DAIRY GELS

Mixed dairy gels (including a control without fat) of skim milk powder (SMP) and whey protein isolate (WPI) containing fat globules were for med by heating protein emulsions to 90-degrees-C and by acid release f rom glucono-delta-lactone to provide a pH of 4.3-4.4. Fat globules wit h artificial protein membranes (FGAPM) were prepared by homogenization of a butter oil/water mixture in the presence of WPI while fat globul es without membranes were stabilized with polyoxyethylene sorbitan mon olaurate (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 sig nificantly 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 co polymer network. Addition of fat globules without membranes led to a f illed gel weaker than the control without fat (1.4 versus 1.8 kPa, res pectively). Bonding of the protein membrane in FGAPM to the gel networ k and presence of individually dispersed fat globules without membrane s was demonstrated by transmission electron microscopy. The difference in microstructure is proposed to be responsible for the mechanical pr operties of each gel.