Factors Determining Large-Strain (Fracture) Rheological Properties of Model Processed Cheese

¹ Department of Food Science, North Carolina State Univ., Raleigh 27695

Torsional fracture was used to map changes in large-strain rheological properties of a model processed cheese that contained 20% protein (rennet casein), 27% anhydrous milk fat, 1.5% NaCl, and 1 to 3% Na₂HPO₄. Processing time (10, 20, or 30 min), Na₂HPO₄ (1, 2 or 3%), and pH of the Na₂HPO₄-NaCl solution (5.4, 5.6, or 5.8) were adjusted to alter the extent of casein micelle solubilization and heat-induced protein-protein interactions. A Box-Behnken experimental design was employed, and results were analyzed using response surface regression and ridge analysis techniques. Further investigation employed a single factorial design in which Na₂HPO₄ increased from 1 to 4%

Model processed cheeses had fracture stress, fracture strain, and fracture modulus (fracture stress/fracture strain) values ranging from 25.1 to 79.7 kPa, 0.66 to 1.88 kPa, and 15.2 to 96.0 kPa, respectively. The properties of fracture modulus and fracture strain formed a master curve, independent of the processing variable. Conditions favoring increased protein solubilization and heat-induced protein-protein interactions increase fracture modulus and decreased fracture strain. This finding coincided with an increase in slope ratio (fracture modulus/modulus at 30% of the fracture-strain), indicating a change in fracture mechanism from strain weakening to elastic fracture. These results suggested that large-strain (fracture) rheological properties could be used to characterize the effects of processing variables important to processed cheese quality.

Key Words: processed cheese • rheological analysis

Submitted on August 24, 1998
Accepted on May 10, 1999