title: Ageing-induced solubility loss in milk protein concentrate powder: effect of protein conformational modifications and interactions with water.
abstract: BACKGROUND
  Protein conformational modifications and water-protein interactions are two major factors believed to induce instability of protein and eventually affect the solubility of milk protein concentrate (MPC) powder. To test these hypotheses, MPC was stored at different water activities (a(w) 0.0-0.85) and temperatures (25 and 45 °C) for up to 12 weeks. Samples were examined periodically to determine solubility, change in protein conformation by Fourier transform infrared (FTIR) spectroscopy and water status (interaction of water with the protein molecule/surface) by measuring the transverse relaxation time (T(2) ) with proton nuclear magnetic resonance ((1) H NMR).
  
  
  RESULTS
  The solubility of MPC decreased significantly with ageing and this process was enhanced by increasing water activity (a(w) ) and temperature. Minor changes in protein secondary structure were observed with FTIR which indicated some degree of unfolding of protein molecules. The NMR T(2) results indicated the presence of three distinct populations of water molecules and the proton signal intensity and T(2) values of proton fractions varied with storage condition (humidity) and ageing.
  
  
  CONCLUSIONS
  Results suggest that protein/protein interactions may be initiated by unfolding of protein molecules that eventually affects solubility.
authors.affiliation: School of Agriculture and Food Sciences, University of Queensland, Brisbane, QLD 4072, Australia.
authors.forename: Enamul
authors.initials: E
authors.lastname: Haque
authors.forename: Bhesh R
authors.initials: BR
authors.lastname: Bhandari
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Gidley
authors.forename: Hilton C
authors.initials: HC
authors.lastname: Deeth
authors.forename: Andrew K
authors.initials: AK
authors.lastname: Whittaker
mesh_terms: D003611:Dairy Products / Q000032:analysis; D005511:Food Handling; D006358:Hot Temperature / Q000009:adverse effects; D008894:Milk Proteins / Q000737:chemistry*; D019906:Nuclear Magnetic Resonance, Biomolecular; D011487:Protein Conformation; D055550:Protein Stability; D058767:Protein Unfolding; D012995:Solubility; D017550:Spectroscopy, Fourier Transform Infrared; D013997:Time Factors; D014867:Water / Q000032:analysis*
chemical_list: D008894:Milk Proteins; D014867:Water