Rheology and Microstructure of Chemically Superphosphorylated Whole Casein

¹ United States Department of Agriculture, Agricultural Research Service, Eastern Regional Research Center, Dairy Products Research Unit, Wyndmoor, PA 19038

Whole bovine casein was chemically modified using POCl₃ to contain 9.1, 10.6, and 12.5 mmol of bound P/mmol of casein (160, 190, and 220% P, respectively) relative to the 5.6 mmol of P/mmol of unmodified casein (100% P). Superphosphorylation produced two types of modified caseins. Solutions made with 220% P casein had low viscosities, which remained constant between pH 5 and 9 and between protein concentrations of 0.2 and 0.7%; these solutions remained fluid when exposed to up to 30 mM Ca²⁺. Solutions made with 160 or 190% P caseins increased in viscosity as protein concentration and pH increased; the solutions formed gels at 1% protein, which increased in elastic modulus, viscous modulus, and complex viscosity as the protein and Ca²⁺. concentrations increased. When exposed to Ca²⁺, gels became more curd-like as protein aggregated and then underwent syneresis. Electron microscopy showed that the gel microstructure consisted of an open matrix of folded strands and sheets of casein in irregular sizes that condensed upon exposure to Ca²⁺. These unique interactions among proteins and unique rheological properties suggest that superphosphorylation could be useful in creating novel dairy foods with added value and enhanced functionality.

Key Words: casein • gelation • phosphorylation • rheology

Submitted on October 8, 1996
Accepted on April 17, 1997