Mobility of Lactose in Milk Powders

TL;DR

This study provides evidence that lactose in milk powders becomes mobile under humid storage conditions, which may influence physicochemical processes like crystallization and reactions with proteins, using advanced NMR techniques.

Contribution

The paper demonstrates lactose mobility in milk powders under humid conditions using NMR, filling a gap in understanding its role in physicochemical changes during storage.

Findings

Lactose exhibits mobility in milk powders under humid conditions.

NMR spectra show peaks indicating lactose mobility.

Lactose mobility correlates with storage humidity and affects physicochemical processes.

Abstract

Lactose is the major component of milk powders and is normally found to be in a glassy/amorphous state. During storage, lactose is known to participate in physicochemical processes, including crystallization on the surface and reaction with proteins such as $\beta$-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 $^{1}\mathrm{H}$ 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$-\delta(^{1}\mathrm{H})$ experiments under magic angle spinning. Furthermore, 2D $^{1}\mathrm{H}-^{13}\mathrm{C}$ HSQC magic angle spinning experiments of skim milk powder demonstrate the same fingerprint as that of lactose in the solution, confirming our observations.