Kinetics of enthalpy relaxation of milk protein concentrate powder upon ageing and its effect on solubility

Kinetics of enthalpy relaxation of milk protein concentrate (MPC) powder upon short-term (up to 67 h) storage at 25 °C and a _w 0.85, and long-term (up to 48 days) storage at 25 °C and a range of a _w 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) (a _w ≤ 0.23, mc ≤ 5.5%) than for higher mc (a _w ≥ 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.