A progressive increase in solubility index ofMMP may be attributed to the formation of water insoluble compounds caused by increased casein micelles size due to hydrogen bonding [24] consequent to protein–protein interactions or sugar–protein interaction [25].
The changes in protein structure due to Maillard reaction and protein– protein interaction during storage could be responsible for the loss of solubility [26]. Gradual increase in solubility index from 1.07 to 1.80 ml in WMP after storage for 6 months at 30 °C is reported by Mrithyunjaya and Bhanumurthi [15].
Solubility index of khoa powder [21] and kulfi mix powder also increased from 1.2 to 2.0 ml over a period of 8 months storage at 30 °C; in similar studies on dried milks [16,27,28] an increase in solubility index with the progress of storage time is also observed.