Mobility of lactose in Milk powders

Lactose is the major component of milk powders and is normally found to be in an amorphous state. During storage, lactose is known to participate in physicochemical processes, including crystallization on the surface and reaction with proteins such as β-lactoglobulin. Lactose needs to be mobile to participate in such processes. However, there is a lack of evidence of its mobility in milk powders. In this study, we demonstrate that some of the lactose becomes mobile when milk powders are exposed to humid air - an inappropriate storage condition. This mobility is evidenced by peaks in magic angle spinning ¹H NMR spectra of milk powders in the range of 3.5 ppm to 4.0 ppm, which stem from lactose molecules displaying considerable rotational mobility. These signals have a longitudinal relaxation time constant T₁ similar to that of mobile water according to 2D T₁-δ_1H experiments under magic angle spinning - suggesting physical proximity and cross-relaxation between water and mobile lactose. Furthermore, 2D ¹H-¹³C HSQC magic angle spinning experiments of skim milk powder demonstrate the same fingerprint as that of lactose in the solution, confirming our observations.