Submitted to: Journal of Magnetic Resonance Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 25, 1996
Publication Date: N/A
Full Text Available
Interpretive Summary: Proteins undergo molecular changes during processes such as freezing, drying and aging, which influence their use as ingredients in dairy products and other foods. It is not known how these changes, which can affect functional performance in food systems, can be related to altered protein folding patterns formed during processing. In this study we applied a novel nuclear magnetic resonance (NMR) technique in conjunction with a new mathematical data analysis procedure (nonlinear regression) to detect, for the first time, molecular changes for proteins in freeze dried dairy products. Results showed changes in protein structure as a result of processes such as the pressing and aging of cheese and the heat treatment of milk in the manufacture of milk powder. An application of the technique to the potential detection of whey protein adulteration in milk powders was presented. The detection of protein adulteration could be advantageous to the baking and dairy product industry.
Technical Abstract: Solid-state 13-C and 31-P NMR spectroscopy was used to characterize individual milk components and freeze dried dairy products. Chemical shift correlations with secondary structure gave estimates of percent alpha-helix and beta-sheet structures for native caseins not available from crystallography. Gaussian/ Lorentzian lineshape analyses were used to observe the local conformational and motional characteristics of the materials. Phosphorylated serine residues have a high degree of motional freedom, while the protein backbones appear to be fairly rigid. Lactose in the powdered dairy products studied is in a rigid, crystalline environment as reflected in NMR lineshapes. Rotating frame relaxation measurements were used to characterize cation binding in metallated caseins. The effect of paramagnetic cations on proton T-1p indicate that binding to serine phosphates is favored over binding to carboxylate residues. The analytical methodology used to characterize milk fractions was successfully applied to observing bulk structural and chemical changes during the production of dried milk products and cheeses. 13-C and 31-P CP/MAS spectra were sensitive to structural changes such as protein denaturation and aggregation as a function of such industrial processes as heat treatment in the production of milk powders and aging and pressing procedures in the production of cheeses. An application of the technique to the possible detection of adulteration in commercially produced dairy solids is also presented.