Kinetics of lactose crystallization and the effect on physical changes and reconstitution of milk powders

Lactose crystallization in dairy powders during storage is associated with impaired reconstitution performance. While this transformation from amorphous to crystalline lactose is known to accelerate above the glass transition temperature, its detailed kinetics and impact on reconstitution, particularly in complex milk powders, remain unclear. This study explores the crystallization behavior of lactose and its effect on reconstitution in three commercial milk powders (skim, whole, and fat-filled) stored at 33–75 % relative humidity (RH) and 25 °C for 50 days. Results show that anhydrous β-lactose forms between 43 % and 53 % RH, while at 64 % and 75 % RH, it further converts to α-lactose monohydrate. Crystallization rate increased with RH. Protein changes correlated to both the formation and phase conversion of crystalline lactose, whereas lipid movement became more pronounced when α-lactose monohydrate formed. Notably, and contrary to common assumptions, lactose crystallization at conditions near the glass transition (43 % RH at 25 °C) did not impair reconstitution, as determined by particle size distribution. This finding reveals a critical window for optimizing milk powder storage without compromising functionality, offering new insights for quality control and shelf-life management in the dairy industry.