Analysis of modified whole casein with different phosphorus contents using phosphorus-31 nuclear magnetic resonance and fourier transform infrared spectroscopy

The different types of P in modified caseins and the role of P in casein interactions were studied using 31P nuclear magnetic resonance spectroscopy of whole casein with reduced or elevated P concentrations and using Fourier transform infrared spectroscopy of whole casein with elevated P concentrations. Bovine whole casein (5.7 mmol of P/mmol of casein, 100% P) was either enzymatically dephosphorylated or chemically superphosphorylated to obtain casein containing 0.4, 3.3, 9.1, 10.6, or 12.5 mmol of bound P/mmol of casein (7, 60, 160, 190, or 220% P, respectively). The nuclear magnetic resonance spectra showed that all casein samples contained serine monophosphates; the caseins containing 160, 190, and 220% P also contained additional diphosphates, and the 220% P casein contained inorganic phosphate and other di- and polyphosphates. When the concentration of the 160 and 190% P caseins increased, they gelled, yet the nuclear magnetic resonance spectra did not show any differences that were typical of changes in conformation. Analysis by Fourier transform infrared spectroscopy indicated that the addition of covalently bound P to whole casein in any amount did not change the distribution of extended strand and sheet, helix, loop, and turns in their secondary structures. Study of the phosphates that were bound to casein and the influence of phosphates on casein interactions improves the understanding of how casein interacts in food systems