Effect of Humidity on Solid-state Isomerization of Various Kinds of Lactose During Grinding

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Abstract
The effect of humidity on isomerization during grinding of α-monohydrate, α-anhydrate and β-anhydrate of lactose was investigated. Samples were ground in an agate centrifugal ball mill at 270 rev min−1 at room temperature (21°C) and at 5 and 60% relative humidity. Crystallinity of the ground lactose was measured by Hermans' method from the powder X-ray diffraction profiles. The α- and β-lactose content of the ground lactose was measured by using angular rotation spectrophotometry. The crystalline lactose samples were transformed into noncrystalline solids by mechanical stress during grinding. After grinding, the adsorbed water content of all ground lactose samples increased, and the isomerized amount increased with increase of the water content during grinding at 5 and 60% relative humidity. The results suggest that the isomerization rate of α-monohydrate during grinding may depend on the crystallinity, but those of α-and β-anhydrate depend on the content of adsorbed water.