Effect of pentasodium triphosphate concentration on physicochemical properties, microstructure, and formation of casein fibrils in model processed cheese

The effects of varying the concentration of pentasodium triphosphate (PP) emulsifying salt [0, 0.6, 1.2, 1.5, and 1.8%, plus 0.9% of a mixture of citric acid (CA) and disodium phosphate (DSP) to adjust cheese pH to 5.85] on rheological, textural, physicochemical, and microstructural properties were studied in a processed cheese model system containing ~20% micellar casein concentrate, ~20% sunflower oil, and ~59% water. Special emphasis was placed on the unique casein fibrils recently described in a comparable processed cheese model system. Our results show that during processing (90°C, 17.37 rpm over 270 min) the apparent viscosity increased more and faster for formulations containing higher concentrations of PP, in analogy to the so-called creaming reaction, a general thickening of the molten cheese mass with prolonged processing. We found that 1.2% PP (plus 0.9% CA-DSP) appeared to be the threshold for the creaming reaction to take place. With increasing PP concentrations, cheese hardness increased in a sigmoidal fashion, and insoluble (protein-bound) calcium concentration decreased exponentially. Light micrographs of samples taken at the end of processing indicated initially large and dense casein aggregates within the matrix that disappeared with higher levels of PP, in parallel with the development of a finer emulsion. With transmission electron microscopy analysis on the same samples, the highly complex restructuring of the casein matrix was evident; casein fibrils had formed de novo at the periphery of the loosening casein aggregates. With higher levels of PP, amorphous areas were observed in place of the dense casein aggregates that appeared progressively void of protein, whereas fibril concentration increased throughout the rest of the matrix. Fibrils progressively attached to the surface of fat globules, thereby emulsifying them. Reverse-phase HPLC analysis of insoluble and soluble fractions indicated κ-casein to be the most likely constituent of the newly formed fibrils. The results of this study suggest that PP induced a concentration-dependent dissociation of caseins (through increased calcium chelation) and further led to their spatial separation. In essence, their chaperone activity was hindered, which resulted in amorphous aggregation on the one hand and fibril formation on the other.