Aging of amorphous raspberry powder: Enthalpy relaxation and fragility

Abstract

Enthalpy relaxation experiments were conducted in freeze-dried raspberry powder at 5, 10 and 15 °C below its onset glass transition temperature ( T gi ) using differential scanning calorimetry (DSC). The statistical differences in T gi for the raspberry powders aged at ( T gi − 5), ( T gi − 10), and ( T gi − 15) from 6 to 96 h were not significant ( p > 0.05). The non-exponential behavior of enthalpy relaxation in raspberry powder was fitted with the Kohlrausch–Williams–Watts (KWW) model. The mean relaxation time constant ( τ) (1017.4 h) for raspberry powder aged at the temperature ( T gi − 15) was greater than the τ (147.1 h) at ( T gi − 5). The KWW model parameter β values for freeze-dried raspberry powders were greater than the β values of pure food constituents. The activation energy for enthalpy relaxation in the glassy state was determined as 145.4 kJ/mol by approximating with the Arrhenius equation. The dependence of mean relaxation time on temperature followed an Arrhenius behavior indicating raspberry powder is a strong system. Several approaches were used to classify raspberry powder into strong/fragile system. There is a difference among fragility parameters of raspberry powders determined using selected methods.