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Multinuclear NMR study of the pH dependent water state in skim milk and caseinate solutions

Published online by Cambridge University Press:  01 June 2009

François Mariette ,
Charles Tellier ,
Gerard Brule  and
Philippe Marchal
François Mariette
Affiliation:
Cemagref, Division Technologie, 17 rue de. Cueillé, 35044 Rennes, France
Charles Tellier
Affiliation:
Laboratoire de RMN et Réactivité Chimique, CNRS URA 472, 2 rue de la Houssinière, 44072 Nantes Cedex 03, France
Gerard Brule
Affiliation:
ENSAR, 65 route de St Brieuc, 35000 Rennes, France
Philippe Marchal
Affiliation:
Cemagref, Division Technologie, 17 rue de. Cueillé, 35044 Rennes, France
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Summary

The effects of decreasing pH and micellar calcium concentrations of reconstituted skim milk and caseinate solution were studied by 1H and 17O NMR spectroscopy. The proton transverse relaxation rate 1/T2 of skim milk decreased as the pH decreased, reaching a minimum at pH 5·3. However, as the pH fell sodium caseinate solution showed a continuous increase in 1/T2, with no minimum. Analysis of proton relaxation as a function of the interpulse time in the CPMG (Carr-Purcell-Meiboom-Gill) sequence demonstrated that both the proton exchange mechanism and ‘bound’ water contributed to proton relaxation in skim milk. The study of 17O relaxation rate as a function of pH confirmed the change in protein hydration upon acidification. Increasing the amount of EDTA showed that the proton transverse relaxation rate of skim milk decreased until a plateau was reached when the micellar calcium was totally solubilized. With excess EDTA the relaxation rates of skim milk and caseinate solution were identical. A strong correlation was also found between the pH dependent relaxation rate and the solubilization of micellar phosphorus as detected by 31P NMR. Together, these results suggested that aggregation of caseins by calcium and colloidal calcium phosphate is mainly responsible for the excess hydration in skim milk micelles.

Type
Original Articles
Information
Journal of Dairy Research , Volume 60 , Issue 2 , May 1993 , pp. 175 - 188
Copyright
Copyright © Proprietors of Journal of Dairy Research 1993

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