A simple solution-based (1)H-NMR method for measurement of anomeric composition of lactose has been established. The solution β/α ratio at the time of drying is mirrored in the composition of the resulting solid amorphous material. In order to produce a consistent anomer composition within spray and freeze dried amorphous lactose, the standing time for the feed solution should be greater than 4 h, such that the most dynamic region of the mutarotation profile has been exceeded. If the amorphous material has been formed from a solution that has not been allowed to equilibrate for 4 h, the resulting solid will continue to undergo mutarotation if trace amounts of moisture are present, until the anomeric β/α ratio slowly approaches 1.7.

This article reports on the stereochemical aspects of the chemical stability of lactose solutions stored between 25 and 60 °C. The lactose used for the preparation of the aqueous solutions was α-lactose monohydrate with an anomer purity of 96% α and 4% β based on the supplied certificate of analysis (using a GC analytical protocol), which was further confirmed here by nuclear magnetic resonance (NMR) analysis. Aliquots of lactose solutions were collected at different time points after the solutions were prepared and freeze-dried to remove water and halt epimerization for subsequent analysis by NMR. Epimerization was also monitored by polarimetry and infrared spectroscopy using a specially adapted Fourier transform infrared attenuated total reflectance (FTIR-ATR) method. Hydrolysis was analyzed by ion chromatography. The three different analytical approaches unambiguously showed that the epimerization of lactose in aqueous solution follows first order reversible kinetics between 25 to 60 °C. The overall rate constant was 4.4 × 10(-4) s(-1) ± 0.9 (± standard deviation (SD)) at 25 °C. The forward rate constant was 1.6 times greater than the reverse rate constant, leading to an equilibrium constant of 1.6 ± 0.1 (±SD) at 25 °C. The rate of epimerization for lactose increased with temperature and an Arrhenius plot yielded an activation energy of +52.3 kJ/mol supporting the hypothesis that the mechanism of lactose epimerization involves the formation of extremely short-lived intermediate structures. The main mechanism affecting lactose stability is epimerization, as no permanent hydrolysis or chemical degradation was observed. When preparing aqueous solutions of lactose, immediate storage in an ice bath at 0 °C will allow approximately 3 min (180 s) of analysis time before the anomeric ratio alters significantly (greater than 1%) from the solid state composition of the starting material. In contrast a controlled anomeric composition (~38% α and ~62% β) will be achieved if an aqueous solution is left to equilibrate for over 4 h at 25 °C, while increasing the temperature up to 60 °C rapidly reduces the required equilibration time.

Recrystallisation occurs frequently in confectionery. More information on sucrose re-crystallisation will aid our understanding of popular foods like chocolate. However, progress has been limited due the lack of a robust method for the production of amorphous sucrose, with known purity. Poor control has led to the glass transition temperatures (Tg's) for amorphous sucrose varying between 48-78 °C in the literature. Our objective was to investigate the recrystallization of sucrose in the presence of lactose, NaCl and water. The purity of sucrose was confirmed by ion chromatography, polarimetry and differential scanning calorimetry. Amorphous sucrose was prepared by freeze-drying 10% w/v aqueous solutions. Fisher (99.7%) and Silver Spoon (98.4%) sucrose samples melted at 186 ± 0.6 °C & 189 ± 0.3 °C respectively. For the Fisher sample the absence of invert sugars and low mineral content allowed the observation of a small endotherm (≈ 150 °C). The Tg of amorphous sucrose was 58.3 ± 1.1 °C with a recrystallization enthalpy (ΔHcrys) of 72.8 ± 6.0 J g-1. NaCl reduced both the Tg (54.8 ± 1.8 °C) and the ΔHcrys (35.7 ± 3.8 J g-1) without affecting the onset temperature of sucrose's re-crystallization (Tcrys, 129.5 ± 6.9 °C), suggesting that a proportion of the sample remained amorphous. The presence of water (1.6 ± 0.07%) inside the hermetically sealed pans caused an earlier onset of Tg (52.3 ± 1.3 °C) and Tcrys (85.1 ± 4.0 °C), as well as lowering ΔHcrys (45.2 ± 2.4 J g-1) compared to samples contained in pin-holed pans (where evaporation was possible). The presence of lactose inhibited the crystallization of sucrose completely. On the basis of this study, it is apparent that sucrose crystallization is highly dependent on the presence of other common food ingredients within the matrix.