Lactose has two anomeric forms: α- and β-lactose. This study investigated the stability of solid lactose stored under high temperature and humidity conditions. Commercially available samples of α-lactose monohydrate (98% w/w α; 2% w/w β) and β-lactose (84 % w/w β; 16 % w/w α) were stored at 40 °C and 93% ± 3% relative humidity (RH) for up to one week and analyzed using proton nuclear magnetic resonance. The data show that the storage conditions can change the anomeric content and potentially affect the functionality of lactose as a pharmaceutical excipient.

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