Evaluation of Lactose Crystalline Forms by Nondestructive Analysis

¹ Laboratoire de Physicochimie et Génie Alimentaires, Ecole Nationale Supérieure d'Agronomie et des Industries Alimentaires, 2, Avenue de la Foreêt de Haye, 54500 Vandoeuvre les Nancy, France
² Laboratoire de Chimie du Solide Minéral, URA 158, Faculté des Sciences, Université de Nancy I, 54500 Vandoeuvre les Nancy, France

Four methods of quantification of solid lactose using X-ray diffraction were tested. The diffracted peak areas of the two crystalline forms of lactose (-lactose monohydrate and ß-lactose) were compared. Linear and logarithmic relationships between these areas and the quantity of lactose forms were established; correlation coefficients were greater than 0.99. Three methods were applied successfully during hydration of amorphous lactose. However, the fourth method, which considered the peak of -lactose monohydrate corresponding to a diffraction angle of 2 = 12.4° as a reference, gave excessive values. Three of these methods were therefore applied to the hydration of skimmed milk powder. The weak excessive values observed may be the presence of mixed crystals of -and ß-lactose in milk powders. Gravimetry and X-ray diffraction of amorphous lactose, during water uptake under relative humidity of 53%, enhanced three periods: progressive water adsorption by amorphous lactose, water release because of crystallization of amorphous lactose, and water constant level with crystallized -lactose monohydrate and ß-lactose. Differential scanning calorimetry of the different forms of lactose (amorphous, partially crystallized and crystallized forms) were also done during the same period.

Key Words: differential scanning calorimetry • hydration • lactose crystalline forms • X-ray diffraction

Submitted on October 20, 1995
Accepted on August 28, 1996