title: Kinetics of enthalpy relaxation of milk protein concentrate powder upon ageing and its effect on solubility.
abstract: Kinetics of enthalpy relaxation of milk protein concentrate (MPC) powder upon short-term (up to 67 h) storage at 25 °C and aw 0.85, and long-term (up to 48 days) storage at 25 °C and a range of aw values (0-0.85) were studied by differential scanning calorimetry (DSC). The short-term study showed a rapid recovery of enthalpy for the first 48 h, followed by a slower steady increase with time. The non-exponential β parameter was calculated using the Kohlrausch-Williams-Watts function and found to be 0.39. Long-term storage showed that enthalpy relaxation depends on both storage period and water activity. The enthalpy value was much less for lower moisture content (mc) (aw ≤ 0.23, mc ≤ 5.5%) than for higher mc (aw ≥ 0.45, mc ≥ 8%) samples for a particular storage period. The results suggest that the presence of more water molecules, in close proximity to the protein surface facilitates kinetic unfreezing and subsequent motion of molecular segments of protein molecules towards thermodynamic equilibrium. Although de-ageing of stored samples did not reverse storage-induced solubility losses, the timescale of enthalpy relaxation was similar to that of solubility loss. It is suggested that enthalpy relaxation within stored samples allows structural rearrangements that are responsible for subsequent solubility decreases.
authors.affiliation: School of Agriculture and Food Sciences, The University of Queensland, QLD 4072, Australia.
authors.forename: Enamul
authors.initials: E
authors.lastname: Haque
authors.affiliation: Centre for Magnetic Resonance and Australian Institute for Bioengineering and Nanotechnology (AIBN), University of Queensland, QLD 4072, Australia.
authors.forename: Andrew K
authors.initials: AK
authors.lastname: Whittaker
authors.affiliation: Centre for Nutrition and Food Sciences, The University of Queensland, QLD 4072, Australia.
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Gidley
authors.affiliation: School of Agriculture and Food Sciences, The University of Queensland, QLD 4072, Australia.
authors.forename: Hilton C
authors.initials: HC
authors.lastname: Deeth
authors.affiliation: School of Agriculture and Food Sciences, The University of Queensland, QLD 4072, Australia.
authors.forename: Kiki
authors.initials: K
authors.lastname: Fibrianto
authors.affiliation: School of Agriculture and Food Sciences, The University of Queensland, QLD 4072, Australia.
authors.forename: Bhesh R
authors.initials: BR
authors.lastname: Bhandari
mesh_terms: D000818:Animals; D002152:Calorimetry, Differential Scanning; D003611:Dairy Products / Q000032:analysis*; D005511:Food Handling; D007700:Kinetics; D008894:Milk Proteins / Q000737:chemistry*; D011208:Powders / Q000737:chemistry*; D012995:Solubility; D013696:Temperature; D013816:Thermodynamics; D014867:Water / Q000737:chemistry*
chemical_list: D008894:Milk Proteins; D011208:Powders; D014867:Water