The functional properties that govern milk protein concentrate (MPC) rehydration are not fully understood. To date, there are no published studies that have determined the influence of particle size and storage relative humidity (RH) on MPC rehydration using environmental scanning electron microscopy (ESEM).The focus of this research was to investigate a range of dynamic experiments involving the in situ hydration and dehydration of MPCs using ESEM.
Low-(MPC40 and 50), medium-(MPC65 and 75) and high-(MPC85) protein content MPC powders manufactured by Teagasc, Ireland were stored at 54% and 88% RH at both 4 oC and 20 oC. The rehydration of MPC powder particles were investigated using ESEM to examine the effects of temperature, RH and storage over three months. Surface composition of lactose, protein and fat was determined using X-ray photoelectron spectroscopy (XPS).
At month 0, MPC40 fully dissolved however, MPC solubility decreased dramatically with increasing protein content. At month 3, MPC40 and MPC50 stored at 4 oC in 54% and 88% RHs exhibited a weakening of the external framework leading to their subsequent collapse and in some cases full dissolution. MPC65 and MPC75 stored at 4 oC, 54% RH displayed large breaks on the external skin of powder particles. MPC85 expressed little dissolution.
MPC65, MPC75, and MPC85 stored at 4 oC and 88% RH showed a graded disruption of the external skin of the powder particles. MPC powders stored at 20 oC in 54% and 88% RHs displayed a marked decrease in their hydration profiles over three months.
XPS elemental analysis of the MPC powders indicated a trend towards increasing protein and decreasing lactose on the surface with increasing protein content in the MPC samples. During storage an increase in temperature enhanced lactose crystallisation in low protein content MPC40 and MPC55. Delayed crystallisation was evident in MPC75 and MPC85.