Isomerization of Lactose in Solid-state by Mechanical Stress During Grinding

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Abstract
Isomerization during grinding of solid-state 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 (20°C). The crystallinity of 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 obtained from the specific rotation measured by using angular rotation spectrophotometry. The crystalline lactose samples were transformed into non-crystalline solids by mechanical stress during grinding. After grinding, the water content of all ground lactose samples increased, and the samples had about 2 mol water per lactose after 10 h grinding. After 10 h grinding of α-monohydrate and α-anhydrate, 10 and 15%, respectively, of α-lactose was transformed into β-lactose by the mechanical treatment. After 10 h grinding of β-lactose, 20% of β-lactose was transformed into α-lactose. The results suggest that crystalline lactose was transformed into a non-crystalline solid, and water was adsorbed on the non-crystalline lactose. The non-crystalline solids of α-or β-lactose were then transformed into their counterparts by the mechanochemical effects of grinding.