Article

Thermal analysis of amorphous lactose and $\alpha$-lactose monohydrate

http://dx.doi.org/10.1051/dst:2008027

ABSTRACT It is common to find that some of the lactose in dairy powders and pharmaceutical tablets is present in the unstable amorphous state. If stored at inappropriate temperatures and humidities amorphous lactose is susceptible to crystallization. The integration of thermal gravimetric analysis (TGA) with single differential thermal analysis (SDTA) provided a descriptive method for a sequential and direct determination of surface water, water of crystallization and amorphous lactose in a single analysis on one sample. Peaks and mass changes on the TGA/SDTA thermograms characteristic of surface water, water of crystallization and amorphous lactose were identified. The content of water of crystallization was used to estimate $\alpha$-lactose monohydrate. The loss of surface water was indicated on the TGA/SDTA thermograms as weight loss between 40 and 130 °C and the loss of water of crystallization occurred at 153 °C. Amorphous lactose was indicated by an exothermic crystallization peak at 174 °C. The area under the exothermic crystallization peak was linearly related to the proportion of the amorphous lactose in mixtures with $\alpha$-lactose monohydrate ($r = 0.989$). This work presented the TGA/SDTA thermograms of lactose samples containing some crystalline forms of lactose and amorphous lactose. The study compared the methods for determining surface water and total water content of lactose accepted by official bodies worldwide with the TGA/SDTA approach. The potential of new methods for qualitatively detecting the amorphous and crystalline forms of lactose by thermochemistry and Fourier transform infra-red (FT-IR) was also explored and compared. Généralement une partie du lactose dans les poudres laitières et les comprimés pharmaceutiques se trouve à l'état amorphe instable. S'il est conservé à des températures et humidités inappropriées, le lactose amorphe peut cristalliser. Le couplage de l'analyse thermogravimétrique (TGA) avec l'analyse thermique différentielle (SDTA) a fourni une méthode descriptive pour la détermination séquentielle et directe de l'eau de surface, de l'eau de cristallisation et du lactose amorphe d'un échantillon en analyse. Les pics et les changements de masse sur les thermogrammes TGA/SDTA caractéristiques de l'eau de surface, de l'eau de cristallisation et la teneur en lactose amorphe ont été identifiés. La teneur en eau de cristallisation a été utilisée pour estimer le lactose $\alpha$ monohydrate. La perte d'eau de surface se traduisait sur les thermogrammes TGA/SDTA par la perte de poids entre 40 et 130 °C, et la perte d'eau de cristallisation avait lieu à 153 °C. Le lactose amorphe était indiqué par un pic de cristallisation exothermique à 174 °C. La surface sous le pic de cristallisation exothermique était linéairement reliée à la proportion de lactose amorphe dans les mélanges avec lactose $\alpha$ monohydrate ($r = 0,989$). Ce travail présente les thermogrammes TGA/SDTA d'échantillons de lactose contenant quelques formes cristallines de lactose et de lactose amorphe. L'étude a comparé les méthodes pour déterminer les teneurs en eau de surface et en eau totale du lactose acceptées par les organismes officiels internationaux avec l'approche TGA/SDTA. Le potentiel de nouvelles méthodes pour détecter qualitativement les formes amorphes et cristallines du lactose par thermochimie et spectroscopie infrarouge à transformée de Fourier a également été exploré et comparé.

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