Influence of molecular weight on enthalpy relaxation and fragility of amorphous carbohydrates

Enthalpy relaxation and fragility of selected carbohydrates were correlated with their molecular weight. Anhydrous glucose and its oligomers (i.e. maltose and maltotriose) were selected to represent amorphous carbohydrates with increasing molecular weight. The enthalpy relaxation behavior of glucose, maltose and maltotriose was determined at 5 °C below their respective glass transition temperatures using differential scanning calorimetry (DSC). The non-exponential behavior of enthalpy relaxation was fitted with the Kohlrausch–Williams–Watts (KWW) equation. The mean relaxation time constants (τ) for glucose, maltose and maltotriose at (Tg − 5) were 16.9, 38.4 and 128.6 h, respectively. The fragility indices (m) of glucose, maltose and maltotriose, determined using the DSC glass transition width approach, were 105, 88.1 and 97.6, respectively. The glass transition temperature and mean enthalpy relaxation time increased with increasing molecular weight for the selected carbohydrates studied herein. The greater glass transition temperature and enthalpy relaxation time of maltotriose are attributed to the decreased specific volume, segmental mobility and increased entanglements resulting from its higher molecular weight. Maltotriose may be preferred as an encapsulant or food ingredient to reduce structural relaxation in food formulations during storage.

► The glass transition temperature increased with increasing molecular weight.
► Mean enthalpy relaxation time increased with increasing molecular weight.
► The KWW equation was able to describe enthalpy relaxation behavior of selected carbohydrates.
► Fragility analysis indicated maltose and maltotriose as strong systems.