Crystallisation of freeze-dried sucrose in model mixtures that represent the amorphous sugar matrices present in confectionery

Abstract

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 (T_g'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 T_g of amorphous sucrose was 58.3 ± 1.1 °C with a recrystallization enthalpy (ΔH_crys) of 72.8 ± 6.0 J g⁻¹. NaCl reduced both the T_g (54.8 ± 1.8 °C) and the ΔH_crys (35.7 ± 3.8 J g⁻¹) without affecting the onset temperature of sucrose's re-crystallization (T_crys, 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 T_g (52.3 ± 1.3 °C) and T_crys (85.1 ± 4.0 °C), as well as lowering ΔH_crys (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.